i
KINETIC MODELING OF GASIFICATION
REACTIONS FOR LIGNITE COAL UNDER
HIGH PRESSURE
by
Imran Nazir Unar
Thesis submitted to MUET for the degree of
Doctor of Philosophy
in
Chemical Engineering
Directorate of Postgraduate Studies
Faculty of Engineering
Mehran University of Engineering and Technology, Jamshoro.
December 2018
ii
DEDICATION
To my beloved
FATHER and MOTHER
Who taught me the path of TRUTH
that I followed for my success
&
To my
WIFE & dauthers ABEEL and AIZA
Who helped me to follow the path of
TRUTH
iii
Certificate of Approval
This is to certify that the research work presented in this thesis, entitled "Kinetic
Modeling of Gasification Reactions for Lignite Coal under High Pressure" was
conducted by Engr. Imran Nazir Unar under the supervision of Prof. Dr. Abdul Ghani
Pathan, and approved by all the members of the thesis committee.
No part of this thesis has been submitted anywhere else for other degree. This thesis is
submitted to Department of Chemical Engineering in partial fulfillment of the
requirements for the degree of Doctor of Philosophy in the Field of Chemical
Engineering, and accepted by the Dean Faculty of Engineering of Mehran University
of Engineering and Technology.
Student Name: Engr. Imran Nazir Unar Signature:__________________
1. External Examiner: Dr. Abdul Waheed Bhutto Signature: _________________
Dean, Faculty of Engineering, Dawood UET, Karachi
2. Internal Examiner: Dr. Khadija Qureshi Signature: _________________
Professor, Chemical Engineering Department, MUET
3. Supervisor: Dr. Abdul Ghani Pathan Signature: _________________
Professor, Chemical Engineering Department, MUET
4. Co-Supervisor: Dr. Muhammad Aslam Uqaili Signature:_________________
Professor, Electrical Engineering Department &
Vice Chancellor of MUET Jamshoro
5. Co-Supervisor: Dr. Rasool Bux Mahar Signature: _________________
Co-Director, USPCAS-W, MUET, Jamshoro
6. Director: Prof. Dr. Khanji Harijan Signature:_________________
Postgraduate Studies, MUET, Jamshoro
7. Dean: Dr. Muhammad Moazam Baloch Signature: _________________
Faculty of Engineering, MUET, Jamshoro
Date: ___________________
iv
Author’s Declaration
I Engr. Imran Nazir Unar hereby state that my thesis titled "Kinetic Modeling of
Gasification Reactions for Lignite Coal under High Pressure" is my own work and has
not been submitted previously by me for taking any degree from Mehran University of
Engineering and Technology or any other Degree awarding institute and to the best of
my knowledge has not been submitted by any other scholar for the same purpose
anywhere else in the country/world.
At any time if my statement is found to be incorrect even after my graduation, Mehran
University of Engineering and Technology has the right to withdraw my PhD degree.
Name of Student: Engr. Imran Nazir Unar
Date: ______________________________
v
Plagiarism undertaking by the Scholar
I solemnly declare that research work presented in the thesis titled "Kinetic Modeling
of Gasification Reactions for Lignite Coal under High Pressure" is my research work
with no significant contributions from any other person. Small contribution/help
whenever taken has been duly acknowledged.
I understand the zero-tolerance policy of the Higher Education Commission and
Mehran University of Engineering and Technology towards plagiarism. Therefore, as
an author of the above titled thesis declare that no portion of my thesis has been
plagiarized and any material used as reference is properly referred /cited.
I undertake that if I am found guilty of any formal plagiarism in the above titled thesis
even after award of PhD degree, the university reserves the rights to withdraw/revoke
my PhD degree and that HEC and MUET has the right to publish my name on the
HEC/MUET website on which names of students are placed who submitted plagiarized
thesis.
Student Signature: __________________
Name: Engr. Imran Nazir Unar
vi
Copyright
© Copyright, 2018
Mehran University
of
Engineering and Technology
ALL RIGHTS RESERVED
vii
Acknowledgment
The work carried out was not possible without the support and guidance of many
individuals and organizations. First of all, I pay my sincere gratitude to Prof. Dr. Abdul
Ghni Pathan, my supervisor, who not only motivated me to initiate the work but always
a source of light, whenever I found myself in dark. I am thankful to my co-supervisor,
Prof. Dr. Muhammad Aslam Uqaili who remained a source of light and guidance
throughout the work and always put his all efforts to provide me the facilities. I would
also like to my heartiest thanks to Prof. Dr. Rasool Bux Mahar (co-supervisor), who
always encouraged me and give me new paths to find my way. I will not forget the
support and affection from all the faculty members in the department of chemical
engineering, especially Prof. Dr. Suhail Ahmed Soomro, Prof. Dr. Shaheen Aziz, Prof.
Dr. Khadija Qureshi, Engr. Zulfiqar Ali Bhatti and Engr. Zulfiqar Ali Solangi who
always supported and share their experiences.
I also pay thanks to Prof. Dr. Muhammad Aslam Uqalili, Vice Chancellor, Mehran
University of Engineering and Technology for his untiring efforts to provide research
faculties to the researchers. Prof. Dr. Khanji Harijan, Director, Post Graduate studies
was also a source of inspiration and much more friend who always motivated to
conclude my work. I would also appreciate the guidance provided by Prof. Dr. Wang
and Prof. Dr. Rudon Li for their continuous support during my stay in China.
It will be unfair if I will not pay tribute to my family for their sacrifice during my
research work and supported me throughout the period. Last but not the least; I am
thankful to all of my friends and well-wishers who not only supported me during my
work but always wished to complete work timely.
Finally, I acknowledge, Higher Education Commission, British Council, Shenyang
Aerospace University China and Mehran University of Engineering and Technology
for providing financial resources and research facilities during the research work.
viii
TABLE OF CONTENTS
Title Page i
DEDICATION ii
Certificate of Approval iii
Author’s Declaration iv
Plagiarism undertaking by the Scholar v
Copyright vi
Acknowledgements vii
Table of Content viii
List of Abbreviations
List of Figures
List of Notations
List of Tables
Abstract
CHAPTER 1 INTRODUCTION 1
1.1 NATIONAL AND STRATEGIC IMPORTANCE OF COAL 1
1.2 COAL GASIFICATION 2
1.2.1 Introduction 3
1.2.2 Thermochemistry of Gasification 3
1.2.3 Types of Gasifiers 5
1.3 ADVANCEMENT IN GASIFICATION SYSTEMS 8
1.4 IMPORTANCE OF COAL GASIFICATION IN RELATION TO THAR
COAL DEVELOPMENT 10
1.5 PROBLEM STATEMENT 12
1.6 OBJECTIVES 13
1.7 SCOPE AND OVERVIEW OF THE RESEARCH WORK 13
1.7 THESIS STRUCTURE 14
CHAPTER 2 KINETIC MODELING FOR COMBUSTION AND
GASIFICATION REACTIONS 16
2.1 GENERAL OVERVIEW 16
2.2 INTRODUCTION TO KINETIC MODELING 17
ix
2.3 LAWS FOR RATE DETERMINATION OF CHAR O2 (COMBUSTION)
REACTION AND MECHANISM 18
2.4 LAWS FOR RATE DETERMINATION OF CHAR-CO2 (GASIFICATION)
REACTION AND MECHANISM 20
2.5 LAWS FOR RATE DETERMINATION OF CHAR-H2O (GASIFICATION)
REACTION AND MECHANISM 21
2.6 KINETIC MODELS 22
2.6.1 Volumetric Model (VM) 24
2.6.2 Modified Volumetric Model (MVM) 24
2.6.3 Grain Model (GM) or Shrinking Core Model (SCM) 25
2.6.4 Random Pore Model (RPM) 25
2.7 REVIEW FOR WORK CONDUCTED ON KINETIC MODELING FOR
GASIFICATION AND COMBUSTION REACTIONS 26
2.8 SUMMARY 38
CHAPTER 3 LITERATURE REVIEW ON GASIFICATION TECHNOLOGIES
AND MODELING STUDIES
39
3.1 GENERAL DISCUSSION 39
3.2 HISTORICAL DEVELOPMENT OF GASIFICATION 39
3.3 COMMERCIAL GASIFICATION TECHNOLOGIES 42
3.4 FIXED-BED OR MOVING-BED PROCESSES 42
3.4.1 The Sasol-Lurgi dry bottom processes 43
3.4.2 British Gas Lurgi (BGL) 45
3.4.3 Multipurpose Gasifier (MPG) 45
3.5 FLUIDIZED BED PROCESSES 46
3.5.1 High-Temperature Winkler (HTW) Gasifier 47
3.5.2 HRL Process 48
3.5.3 BHEL Gasifier 49
3.5.4 Circulating fluidized-bed (CFB) processes 49
3.5.5 Kellogg Brown and Root (KBR) transport gasifier 51
3.5.6 U-Gas Process Gasifier 52
3.6 ENTRAINED FLOW PROCESSES 52
3.6.1 The Koppers-Totzek atmospheric process 53
3.6.2 Shell Coal Gasification Process (SCGP) 55
3.6.3 PRENFLO™ Gasifier/Boiler (PSG) 55
x
3.6.4 Siemens Gasifier 56
3.6.5 GE Energy Gasifier 57
3.6.6 ConocoPhillips E-Gas Gasifier 58
3.6.7 Mitsubishi Heavy Industries (MHI) gasifier 59
3.6.8 The EAGLE Gasifier 59
3.6.9 ICCT Opposite Multiple Burner (OMB) Process 60
3.7 CURRENT EXPERIMENTAL PRACTICES ON COMBUSTION &
GASIFICATION 61
3.7.1 Experimental work on Gasification Systems 61
3.7.2 Experimental work on Combustion related to Gasification Studies 66
3.7.3 Experimental work on Gasification Studies on Thar Lignite 67
3.8 MODELING AND SIMULATION WORK 67
3.9 RESEARCH ON FLAMELESS COMBUSTION/ GASIFICATION 83
3.10 SUMMARY 90
CHAPTER 4 EXPERIMENTAL WORK 92
4.1 GENERAL DESCRIPTION OF EXPERIMENTAL WORK 92
4.1.1 Sample Collection and Preparation 92
4.1.2 Proximate and Ultimate Analysis 93
4.1.3
TGA Analysis (Moisture removal, devolatization and Combustion
Study) 93
4.1.4 PTGA Analysis (Char Gasification Study) 95
4.1.5 Data analysis method 96
4.2 RESULTS AND DISCUSSION FOR EXPERIMENTAL WORK 101
4.3 RESULTS OF PROXIMATE AND ULTIMATE ANALYSIS 101
4.4 RESULTS FOR MOISTURE REMOVAL AND DEVOLATIZATION
KINETICS 103
4.4.1 Least square regression analysis for moisture removal 104
4.4.2 Least square regression analysis for devolatization 105
4.4.3 Rate constant “k” for drying and devolatization steps 112
4.5 RESULTS FOR COMBUSTION KINETICS 114
4.5.1 Rate constant “k” for the combustion reaction 118
4.6 RESULTS FOR COAL GASIFICATION KINETICS AT ATMOSPHERIC
AND ELEVATED PRESSURE 119
xi
4.6.1 Least square regression analysis for Char+CO2 reactions 121
4.6.2 Least square regression analysis for Char+H2O reaction 124
4.6.3 Rate constant “k” for gasification reactions 129
4.7 EFFECT OF PRESSURE ON GSIFICAITON KINETIC PARAMETERS 131
4.8 SUMMARY OF EXPERIMENTAL RESULTS 133
CHAPTER 5 CFD MODELING AND SIMULATION 134
5.1 CFD MODELING AND SIMULATION 134
5.2 CFD MODELING OF CHINESE COAL GASIFIER 135
5.2.1 Description of Physical system for Chinese Coal Gasifier 135
5.2.2 Development of computational domain for Chinese gasifier 136
5.2.3 Computational Models 137
5.2.4 Combustion/ Gasification Model 139
5.2.5 Boundary Conditions and Calculation Methods 142
5.3 CFD MODELING OF NEWLY DESIGNED COAL GASIFIER 142
5.3.1 Description of Physical system for Newly Designed Gasifier 142
5.3.2 Development of Computational Domain for Proposed Geometry 143
5.3.3 Probability Density Function (PDF) approach 144
5.4 MODEL DEVELOPMENT IN ASPEN PLUS®V10 FOR VALIDATION OF
MODIFIED GEOMETRY RESULTS 144
5.4.1 Coal Pyrolysis 146
5.4.2 Volatile combustion 147
5.4.3 Char gasification 147
5.5 MODELING AND SIMULATION RESULTS FOR CHINESE LIGNITE 148
5.5.1 Identification of Best Reaction Mechanism for Lignite Coal and
Validation of the CFD model 149
5.5.2 Effects of Coal/Oxygen Distribution on syngas composition 154
5.5.3 Effects on Char Conversion 155
5.5.4 Effects on Syngas Exit Temperature and Maximum inside
Temperature 156
5.5.5 Effects of coal distribution on particle trajectories 160
5.5.6 Effects on Turbulent Intensity 160
5.5.7 Heat generation and consumption analysis 161
5.6 MODELING AND SIMULATION RESULTS FOR THAR LIGNITE 163
xii
5.6.1 Effects of different models and O/C ratio 167
5.6.2 Effect of pressure on syngas composition and char conversion 168
5.6.3 Streamlines-Flow analysis for Multi-Opposite Burners 170
5.7 VALIDATION OF CFD RESULTS OF MODIFIED GEOMETRY WITH
ASPEN PLUS MODEL RESULTS 170
5.8 COMPARATIVE STUDY FOR NEWLY DESIGNED GASIFIER 172
5.9 THE FINAL PROPOSED SYSTEM 176
5.10 SUMMARY FOR CFD MODELING AND SIMULATION WORK 177
CHAPTER 6 CONCLUSION 178
6.1 CONCLUSIONS 178
6.1.1 Concluding remarks for Drying, Devolatization and Combustion
Steps 179
6.1.2 Concluding remarks for Gasification Reactions 179
6.1.3 Concluding remarks for CFD Modeling and Simulation Work 180
6.2 RECOMMENDATIONS FOR FUTURE WORK 181
REFERENCES 183
APPENDICES 211
A.1 TGA Model SDT Q600 211
A.2 Quartz fixed-bed reactor for char production 211
A.3 Thermax500 PTGA 211
A.4 List of Publications 212
xiii
List of Abbreviations
3D Three Dimensional
CFB Circulating Fluidized Bed
CFD Computational Fluid Dynamics
DDGS Dried Distiller’s Grains with Solubles
DPM Discrete Phase Model
EFR Entrained Flow Reactor
FCCCD Face Centered Central Composite Design
FVM Finite Volume Method
GM Grain Model
HAFTC High Ash Fusion Temperature Coal
IGCC Integrated Gasification Combined Cycle
LES Large Eddy Simulation
MILD Moderate and Intense Low-Oxygen Dilution
MSPV Multi Solids Progress Variables
MVM Modified Volumetric Model
NOx Nitrogen Oxides
O/C Oxygen to Carbon Ratio
OMB Opposite Multi Burner
PDF Probability Density Function
xiv
PHTER Pressurized High Temperature Entrained Flow
Reactor
PSDF Power Systems Development Facility
PTGA Pressurized Thermogravimetric Analysis
R&D Research and Development
ROM Reduced Order Model
RPM Random Pore Model
RSM Response Surface Methodology
SCM Shrinking Core Model
SOx Sulfur Oxides
TDL Tunable Diode Laser
TGA Thermogravimetric Analysis
TRF Turbulent Reacting Flow
UCG Underground Coal Gasification
UDF User Defined Function
VM Volumetric Model
xv
List of Figures
Fig 1.1: The moving-bed gasifier with various inlet and outlets 06
Fig 1.2: The fluidized-bed gasifier with various inlets and outlets 07
Fig 1.3: Fundamental Entrained flow gasifier with various inlets and outlets 08
Fig 1.4: The flow diagram of research activities 14
Fig 2.1: Directions of Literature Survey 16
Fig 3.1: Sasol-Lurgi dry bottom gasifier 44
Fig 3.2: British Gas Lurgi Gasifier 45
Fig 3.3: Lurgi Multipurpose Gasifier 46
Fig 3.4: High-Temperature Winkler Gasifier 48
Fig 3.5: IDGCC process of drying and Gasification developed by HRL 49
Fig 3.6: Lurgi circulating fluid-bed gasifier 50
Fig 3.7: KBR Transport Gasifier 51
Fig 3.8: U-Gas Process Gasifier 52
Fig 3.9: Koppers-Totzek gasifier 54
Fig 3.10: Shell Gasifier 55
Fig 3.11: PRENFLOTM 56
Fig 3.12: Siemens Gasifier 57
Fig 3.13: GE Energy Gasifier 58
Fig 3.14: Conoco Philips E-Gas Gasifier 58
Fig 3.15: MHI Gasifier 59
Fig 3.16: The EAGLE Gasifier 60
Fig 3.17: The ICCT Opposed Multiple Burner gasifier 61
Fig 4.1: Steps for Experimental Work 92
xvi
Fig 4.2: Schematic diagram of Thermax500 PTGA 96
Fig 4.3: Moisture removal at different heating rates 103
Fig 4.4: Volatiles removal at different heating rates 104
Fig 4.5: Linearity of Volumetric and Grain models for Moisture Removal at different
Heating Rate for Sample GT-01-443
106
Fig 4.6: Linearity of Volumetric and Grain models for Moisture Removal at different
Heating Rate for Sample GT-01-493
107
Fig 4.7: Linearity of Volumetric and Grain models for Devolatization at Low
Temperature with different Heating Rate for Sample GT-01-443
108
Fig 4.8: Linearity of Volumetric and Grain models for Devolatization at Low
Temperature with different Heating Rate for Sample GT-01-493
109
Fig 4.9: Linearity of Volumetric and Grain models for Devolatization at High
Temperature with different Heating Rate for Sample GT-01-443
110
Fig 4.10: Linearity of Volumetric and Grain models for Devolatization at High
Temperature with different Heating Rate for Sample GT-01-493
111
Fig 4.11: Rate constant (k) for drying step with (a) VM and (b) GM 113
Fig 4.12: Rate constant (k) for devolatization step with (a) VM-Low Temp_500°C
(b) VM-High Temp_900°C (c) GM-Low Temp_500°C (d) GM-High
Temp_900°C
114
Fig 4.13: Conversion of Char Samples from GT-01-443 in Oxygen environment at
different heating rates
115
Fig 4.14: Conversion of Char Samples from GT-01-493 in Oxygen environment at
different heating rates
115
Fig 4.15: Linearity of Volumetric and Grain models for Combustion of with different
Heating Rate for Coal Sample GT-01-443
116
Fig 4.16: Linearity of Volumetric and Grain models for Combustion of with different
Heating Rate for Coal Sample GT-01-493
117
Fig 4.17: Rate constant (k) for combustion step with (a) VM and (b) GM 119
Fig 4.18: Conversion of Char samples against Temperature at different pressures for
CO2 and H2O reacting gases
120
Fig 4.19: Least square regression analysis of Volumetric and Grain Models for
Char+CO2 reactions of sample S1
122
Fig 4.20: Least square regression analysis of Volumetric and Grain Models for
Char+CO2 reactions of sample S2
122
Fig 4.21: Least square regression analysis of Volumetric and Grain Models for
Char+CO2 reactions of sample S3
123
xvii
Fig 4.22: Least square regression analysis of Volumetric and Grain Models for
Char+H2O reactions of sample S1
125
Fig 4.23: Least square regression analysis of Volumetric and Grain Models for
Char+H2O reactions of sample S2
126
Fig 4.24: Least square regression analysis of Volumetric and Grain Models for
Char+H2O reactions of sample S3
127
Fig 4.25: Comparison of experimental and predicted conversion for Char-CO2/H2O
reactions of sample S1
129
Fig 4.26: Rate constant (k) for Char +CO2 reaction with (a) VM and (b) GM 130
Fig 4.27: Rate constant (k) for Char +H2O reaction with (a) VM and (b) GM 131
Fig 4.28: Effect of pressure on frequency factor (A) and activation energy (E) 132
Fig 5.1: (Left) Geometry of Gasifier with main inlets and outlets. (Right) The
Sections of gasifier at AA’ and BB’ level
136
Fig 5.2: Meshed computational domain of Chinese Coal Gasifier (Geometry-A) 137
Fig 5.3: Different views of newly designed gasifier operating conditions 143
Fig 5.4: Meshed computational domain of Proposed Gasifier (Geometry-B) 143
Fig 5.5: The process flow sheet for the simulation model of Entrained Flow Gasifier 146
Fig 5.6: The comparison of various preliminary simulation results with experimental
values for CO, H2 and CO2 mole fraction in Syngas (a) at O/C ratio=0.9, (b)
at O/C ratio=1.0 and (c) at O/C ratio=1.1.
150
Fig 5.7: The temperature profile for various preliminary simulated cases and
experimental work along central axis of gasifier (a) at O/C ratio=0.9, (b) at
O/C ratio=1.0 and (c) at O/C ratio=1.1
151
Fig 5.8: The velocity Vectors at the Sectional Planes of the gasifier (Case E) with a
close view of AA' and BB' planes.
153
Fig 5.9: The particles residence time at the Sectional Planes of the gasifier (Case E)
(for clarity only 200 particle tracks are shown with particle end time limited
at 0.8 sec)
153
Fig 5.10: The Mole % for CO, H2 and CO2 with variation in total Coal/ Oxygen % at
AA' Level
155
Fig 5.11: The Char Conversion with variation in Coal/Oxygen % at AA' Level 156
Fig 5.12: Syngas Average Exit Temperature (K) 157
Fig 5.13: Radial Temperature profiles for various simulation cases at different heights
of gasifier
158
Fig 5.14: Temperature Contours for Sectional Planes at AA' Level and BB' Level 159
Fig 5.15: The particles residence time at the Sectional Planes of the gasifier; Cases:
C30_O50, C50_O50 and C70_O50
160
xviii
Fig 5.16: Turbulent Intensity (%) Contours for selected cases 161
Fig 5.17: Net heat generated due to reactions (W) for selected cases 162
Fig 5.18: Heat consumption in different ways 162
Fig 5.19: The mole % of CO, CO2, and H2 at the exit of gasifier at different O/C
Ratios
168
Fig 5.20: The devolatization and Char conversion at different O/C Ratios 168
Fig 5.21: Effect of Pressure on the composition of CO, CO2 and H2 in syngas 169
Fig 5.22: Effect of Pressure on char conversion and exit gas temperature 169
Fig 5.23: Streamlines –Flow analysis for both the geometries 170
Fig 5.24: Comparison of CFD model results with Aspen Plus®V10 model results for
(a) CO, (b) CO2, (c) H2 and (d) Volatiles
171
Fig 5.25: Comparison of CFD model results with Aspen Plus®V10 model results for
(a) Devolatization (b) Char Conversion and (c) Syngas Exit Temperature
172
Fig 5.26: Comparison of temperature contours from both geometries 174
Fig 5.27: Comparison between the Original Geometry (A) and Modified Geometry
(B) through contours of important syngas components
175
Fig 5.28 Proposed system of coal gasification system with newly designed gasifier 176
Fig 5.29 Overall material balance of newly designed gasifier 176
xix
List of Notations
a Absorption coefficient
C Coefficient of Function for Linear-Anisotropic Phase
𝜌, 𝜌𝑝 Density, Density of particles (Kg/m3)
Dt Diffusion Coefficient for Turbulence (m2/sec)
𝐷𝑖 Diffusivity (m2/sec)
ε Dissipation Rate of Turbulence (m2/sec3)
DF Drag Force (Kg-m/sec2)
μ Dynamic Viscosity (N-sec/m2)
εw Emissivity
xF Force vector component along x-axis (Kg-m/sec2)
qr Heat Flux for Radiation Heat (J/m2-sec)
G Incident Radiation
k Kinetic Energy for Turbulence (m2/sec2)
Gk Mean Velocity Gradients
[C] Molar concentration of specie (Kmol/m3)
jC Mole fraction of species j
Mj Molecular weight of specie j
𝑤𝑗,𝑟 Net Production Rate of Species i through Chemical Reaction (Kmol/m3 s)
Prt Prandtl number for Turbulence
' Rate exponent for product reactant specie
'' Rate exponent for product specie
s Scattering coefficient (m-1)
Sct Schmidt number for Turbulence
mS , jS , hS , rS Source terms for mass, momentum, energy, and species
pc specific heat at constant pressure (J/kg-K)
σ Stefan–Boltzmann Constant
𝑣′′𝑗,𝑟 Stoichiometric Coefficient for Product j in Reaction r
𝑣′𝑖,𝑟 Stoichiometric Coefficient for Reactant i in Reaction r
xx
ij symmetric stress tensor
T Temperature (K)
μt Turbulence Viscosity
𝜆 Turbulent Thermal Conductivity (W/m-K)
u , pu Velocity, the velocity of particles (m/s)
Cμ Viscosity Constant
xxi
List of Tables
Table 1.1: World Energy Consumption by Fuel 01
Table 2.1 : Review of Kinetic Modeling of Carbon-based materials with various
techniques
27
Table 3.1: Sizes and capacities of Sasol-Lurgi dry bottom gasifiers 44
Table 3.2: Characteristics of Important Entrained Flow Gasifiers 53
Table 4.1 : Samples collected from Block-IX Thar Coal Field 93
Table 4.2: Experimental conditions in TGA for Moisture Removal and Devolatization 94
Table 4.3 : Experimental conditions in TGA for Combustion Reaction 95
Table 4.4: Proximate Analysis and Heating Value of Coal Samples 102
Table 4.5: Ultimate Analysis of Coal Samples 102
Table 4.6: Calculated kinetic parameters for moisture removal and devolatization steps 112
Table 4.7: Calculated kinetic parameters for char combustion step 118
Table 4.8: Arrhenius Parameters for Char+CO2 124
Table 4.9: Arrhenius Parameters for Char+H2O 128
Table 5.1: Properties of Chinese Coal 135
Table 5.2: Various preliminary cases to optimize the best reaction plan 141
Table 5.3: Selected kinetic parameters for Devolatization and Gasification/ Combustion
Reactions
141
Table 5.4: The chemical species used in the model 145
Table 5.5: Functions of each block used in Aspen Plus®V10 Model 147
Table 5.6: Operated parameters for Simulated Cases 149
Table 5.7: Simulation Cases in Block 1 (PDF model used to calculate the species) 163
Table 5.8: Simulation Cases in Block 2 (Species Model used with finite rate chemistry)
– Optimization of Oxygen-to-Carbon (O/C) Ratio
164
Table 5.9: Simulation Cases in Block 3 (Different Pressures at Optimized O/C ratio) 164
Table 5.10: Simulation Cases in Block 4 (Different feed rates with varying Pressures at
fixed Optimized O/C ratio) – Optimization of Feed Rate.
165
Table 5.11: Results for Simulation Cases in Block 1 (PDF model used) 166
Table 5.12: Results for Simulation Cases in Block 2 (Species model used) 166
Table 5.13: Results for Simulation Cases in Block 3 (Effect of Pressure on gasification) 166
xxii
Table 5.14: Results for Simulation Cases in Block 4 (Effect of Pressure on gasification) 167
Table 5.15: Comparative study for Original and Modified geometry of gasifier with two
different feedstocks
173
xxiii
Abstract
Pakistan is considered a coal-rich country after the discovery of Thar coalfield with the
total lignite reserves are estimated more than 175 billion tonnes. On the other side coal
contributes only about 7% of the total energy mix in Pakistan. The fundamental issues
of non-utilization of abundant coal reserves are lacks of financial resources, modern
technology, and expertise. A multi-pronged strategy, for the utilization of an indigenous
coal, is required to overcome the energy crisis in the country. Coal gasification is one
of the potential options available for the development of Thar coal. Lots of research has
carried out in rest of the world and researchers concluded that coal characteristics play
a vital role in gasification and hence the development of technology is based on the
characteristics of coal which will be used as feedstock in the technology. To design the
efficient gasification system, the kinetics of combustion and gasification reactions is an
essential part of the fundamental study.
In gasification process, solid or liquid hydrocarbon feedstock is converted into a
synthetic fuel gas that has the capability to produce electricity or could be utilized to
synthesize a raw material for the manufacture of chemicals, hydrogen, or transportation
fuels. The primary constituents of syngas are carbon monoxide and hydrogen. The
gasification process begins with a rapid removal of volatiles known as devolatilization
(or pyrolysis), leaving behind a char that is mainly composed of fixed carbon and ash.
The next step is the heterogeneous reactions of char with O2, CO2, and H2O (steam) to
produce the syngas. The reaction of char with oxygen (combustion reaction) is kept
controlled by having limited oxygen to produce sufficient heat for the endothermic
gasification reactions of char with CO2 and steam. The char reactivity with CO2 and
steam and their reaction kinetics is playing an important role and hence considered
essential for mathematical, physical process modeling, optimization and economical
operation of the gasifier.
The core objective of present research is to determine the kinetic parameters for the
overall gasification reactions including drying, devolatization, combustion and
reactivity of Thar chars with CO2 and H2O (steam) at atmospheric as well as elevated
xxiv
pressures, using TGA and PTGA. The present study also aims to find a suitable kinetic
model to explain the variation of char reactivity and its changes with the progress of the
CO2 and H2O gasification reactions individually. Key kinetic parameters like frequency
factor (or pre-exponential factor) and activation energy, are calculated, by means of two
standard kinetic models i.e. Volumetric Model and Grain Model and a comparative
study has been done on the basis of observed results.
After the experimental work, for a further better understanding of coal gasification with
indigenous coal parameters, the CFD modeling of entrained flow gasifier was done.
Initially, the Chinese coal gasifier with multi-opposite burners was modeled using
Chinese coal data. The Euler-Lagrangian mechanism was used. Various reaction
mechanisms were tested and the model results are validated on published experimental
results. After successful validation of the model, the geometry of the gasifier was
modified by introducing a neck section for Thar Lignite and simulations were carried
out using validated reaction mechanisms. Optimization was carried out for various
parameters like O/C ratio, pressure, and coal and oxidant feed rates.
Finally, it was concluded that the temperature and kinetics of gasification reactions
(reactions of char to CO2 and H2O) can be controlled with the optimized coal and
oxidant distribution between the two stages. These parameters are critical to the overall
performance of the gasifier, so coal and oxygen feedings must be optimized between
the two stages of the gasifier to get the optimized performance. Species Transport
approach shown good results as compared to Probability Density Function (PDF) in
terms of CO and H2 production. Geometry–A (original) shown good results with
Chinese coal whereas Geometry –B (modified with neck) was found best for Thar coal.
The maximum CO and H2 were observed 24.19% and 32.35% at O/C ratio of 1.881. At
maximum CO and H2 generation, the moisture removal was observed 97.7% whereas
char conversion was observed 98.24%. Lower and Higher (LHV) heating values of
syngas produced from modified geometry were calculated using Aspen HYSYS
software. It was observed that the syngas produced from Geometry-B has 12.27 MJ/Kg
lower heating value. The calculated ratio of LHV/coal heating value for Thar coal was
0.815 which was greater than Chinese coal with geometry-A (i.e 0.212).
1
CHAPTER 1
INTRODUCTION
1.1 NATIONAL AND STRATEGIC IMPORTANCE OF COAL
Pakistan is dependent majorly on fossil fuels for fulfilling its energy needs but
currently, the country is facing a massive challenge to overcome the serious energy
crisis. Social and industrial activities are severally affected due to huge electricity
shortfall and continuous depleting natural gas reservoirs. Coal is considered an
economic energy source and widely utilized throughout the world for generating power,
cement and in other process industries. Fuel wise distribution of world energy
consumption scenario is presented in Table 1.1 and discloses that coal is still sharing
major contribution (37%) for producing electricity in the world and it is on second place
(30.3%) for other primary energy utilization.
Table 1.1: Fuel wise distribution for Energy consumption in the World
(Petroleum, 2016)
Fuel/ Resource Primary Energy Consumption Electricity Consumption
Coal 30.3% 37%
Gas 23.7% 16%
Oil 33.1% 9%
Renewable 8.0% 21%
Nuclear 4.9% 17%
Sindh province of Pakistan is bestowed with huge lignite coal deposits i.e. more than
184 billion tons, which constitutes 99% of total coal deposits of the Country. Field wise
coal reserves of Sindh are as follows (Private Power & Infrastructure Board, 2008,
Petroleum, 2016):
2
a) Thar Coal Field = 175.60 billion tons
b) Lakhra Coal Field = 1.64 billion tons
c) Thatta – Sonda = 7.46 billion tons
Total = 184.70 billion tons
Thar coalfield, with the total reserves of more than 175 billion tons of lignite, has the
potential to fulfill the country’s energy requirements for more than a century.
It has been observed on practical backgrounds that despite economic development stage
of any coal-rich nations, their indigenous coal has been utilized to boost the electricity
sector of that nation. Pakistan spends upwards of US$ 2.00 billion annually of hard-
earned foreign exchange on the importation of fuel oil to support its current inventory
of oil-fired, electricity – generation installations. The dependency of imported oil for
fulfilling energy needs for the country like Pakistan who has proven potential of
generating electricity from indigenous coal for more than 100 years seems
inappropriate.
Since the time of discovery of Thar coalfield, its development has been a dream for the
Pakistani nation. Several efforts have made by Pakistan so far but unfortunately could
not achieve any remarkable result due to diverse reasons, including geopolitical,
technical, financial and law & order. Non-availability of fresh water at Thar, the
presence of highly saline groundwater, poor strata conditions, and high stripping ratio,
have made Thar coal projects less attractive for investors. Coal gasification through
gasifier is a potential and viable option for the development of Thar coal.
1.2 COAL GASIFICATION
Coal gasification is important process in which low-economic solid or liquid feedstocks
are converted into syngas. The details about gasification are discussed in subsequent
sub-sections.
3
1.2.1 Introduction
Gasification of coal is a complex chemical process in which flammable gases like CO
and H2 are being produced from coal or any other carbonaceous material in the presence
of oxygen/air and steam, at high temperature and pressures. The gas produced during
gasification is known as “syngas” which is rich in CO and H2 and can be utilized as
town gas for the household purpose, to generated electricity, to make methanol, liquid
fuels through the Fischer-Tropsch process, ammonia, urea etc. Air separation plant is
used to recover 95% pure oxygen which is used in the gasifier. Recovered nitrogen can
be used as a dilution in a gas turbine combustor and as feedstock for ammonia synthesis
process. The separated CO2 from syngas could be utilized as raw material for the
synthesis process of urea. Coal gasification is considered one of the environment
friendly technologies due to its capability of achieving extremely low NOx and SOx,
along with particulate emissions. The extraction of Sulphur in coal is possible during
the gasification process in the form of solid which then can be sold commercially. There
are 385 gasifiers of various designs currently operational worldwide. Total worldwide
gasifier capacity was expected around 75,000 MWth by the end of 2010 .
1.2.2 Thermochemistry for Gasification
Gasification is based on principal concept partially oxidization of carbonaceous
material like coal at high pressure and temperature with the limited amount of oxidant
like oxygen or air which triggers a number of complex elementary reactions producing
a mixture of gases primarily contain carbon mono oxide (CO) and hydrogen (H2) in
larger fractions. Apart from production of carbon monoxide and hydrogen, high
fractions of carbon dioxide (CO2) and water vapors (H2O) are also available in syngas
produced from gasification along with nitrogen (N2) and traces of methane (CH4)
depending on the process conditions. Few other chemical species like oxides of sulfur
and nitrogen (SO2, SO3, NO, and NO2), ammonia (NH3), hydrogen sulfide (H2S), and
hydrogen chloride (HCl) could also be available in the product syngas. The syngas also
contains Ash in addition to all gaseous products and the composition of ash dependent
on the nature of feedstock, conditions of the reactor and other parameters.
4
As per gasification theory, the occurrence of gasification is subdivided into different
zones or stages. These stages or zones may be the regions spatially localized inside of
the gasifier reactor vessel, as in moving-bed gasifiers, or may occur all the reactions in
each stage of the process throughout the entire volume of the reactor vessel, as happens
in fluidized-bed gasifiers. These stages, with their associated chemistry, can be
summarized as follows (Probstein and Hicks, 2006, Higman and Van Der Burgt, 2003,
Gautam et al., 2010).
1) Removal of moisture or Drying – The raw material of gasification or feedstock
undergoes drying process in which inherent moisture of feedstock evaporates due to
increasing temperature. The dry-feed is then ready for the upcoming process of
pyrolysis.
2) Pyrolysis or Devolatilization – The chemical bonds in the feedstock material starts
breaking due to heat produced during combustion reactions which result in the
generation of compounds with different molecular weights. The vaporization of lighter
compounds occurs from which some them remain in gaseous forms whereas rest of
compounds form a thick slurry known as tar. The char is produced from heavier
compounds.
3) Combustion – The flammable of combustible components like carbon, carbon
monoxide, hydrogen etc. react with oxygen, known as combustion process. In this
process, heat is released which act as a driving force for the entire gasification process.
COOC →+ 221 (ΔH= –111 KJ/mol) (1.1)
2221 COOCO →+ (ΔH= –283 KJ/mol) (1.2)
22 COOC →+ (ΔH= –394 KJ/mol) (1.3)
OHOH 222 21 →+ (ΔH= –242 KJ/mol) (1.4)
4) Gasification – During this, the carbon dioxide and steam react with the unburnt char
in the presence of heat to form carbon monoxide and hydrogen. This process is
endothermic in nature and requires high temperature to occur.
5
Water-Gas Reaction
22 HCOOHC ++ (ΔH= +131 KJ/mol) (1.5)
Boudouard Reaction
COCOC 22 →+ (ΔH= +172 KJ/mol) (1.6)
In addition, the other important reactions for gasification process are written as under:
Methane Combustion:
OHCOOCH 2224 22 +→+ (ΔH= –803 KJ/mol) (1.7)
Methanation (or Hydrogasification) Reaction:
422 CHHC + (ΔH= –75 KJ/mol) (1.8)
Methane Reforming Reaction:
224 3HCOOHCH ++ (ΔH= +206 KJ/mol) (1.9)
Water-Gas Shift Reaction:
222 HCOOHCO ++ (ΔH= –41 KJ/mol) (1.10)
Other Reactions:
222 22 HCOOHC ++ (ΔH= +90.1 KJ/mol) (1.11)
422 21
21 CHCOOHC ++ (ΔH= +7.7 KJ/mol) (1.12)
224 22
1 HCOOCH +→+ (ΔH= –36 KJ/mol) (1.13)
Boudouard, water-gas shift and methanation reactions where shown “fundamental”
reactions for the gasification process in previous work (Perry and Green, 1999).
1.2.3 Types of Gasifiers
Different types of gasifier are discussed as under.
1.2.3.1 Moving Bed
Fig.1.1 is showing the diagram of a generic moving bed gasifier. The flow arrangement
of feedstock and oxidant is countercurrent in moving bed gasifiers where coal is fed
from the top of the reactor and oxygen or air is blown from the bottom of the reactor.
The gasification of coal occurs in its traveling from top to bottom and remaining ash is
6
taken out from the bottom of the gasifier. The heat generated from combustion reactions
is utilized as pre-heating source of coal before its entering in gasification zone due to
countercurrent flow arrangement. Therefore, the temperature of outgoing syngas from
the gasifier is significantly lower as compared to the temperature required for complete
conversion of coal. The moving bed gasifier pertaining the coal residence time in the
order of hours within the gasifier.
Few salient features of moving bed gasifiers are as follows:
• It requires less oxidant;
• It produces the product syngas with a relatively higher fraction of methane;
• It produces hydrocarbon liquids, like the production of tars and other oils;
• It shows higher “cold gas” thermal efficiency including the heating values of the
hydrocarbon liquids;
• It has poor fines handing ability; and
• It requires special arrangements for managing caking of coal issues.
Fig. 1.1: The moving-bed gasifier with various inlet and outlets (Source: Higman
and Van Der Burgt (2008))
1.2.3.2 Fluidized Bed
Fig. 1.2 shows a generic fluidized bed gasifier diagram. A fluidized bed gasifier is a
well-stirred or back-mixed type of reactor in which the fresh coal particles are
continuously mixed with older, hot, partially and fully gasified particles. The uniform
temperature is maintained throughout the fluidized bed due to mixing. The flow of
7
steam, oxidant or recycled gas into the reactor must be maintained in such a range so
that coal particles remain in floating conditions within the bed and could not settle down
or skip out from the bed. However, the particles become smaller and lighter as they are
completely gasified and eventually entrained out of the gasifier. To avoid the
agglomeration of a particle within the bed, the temperatures of the bed should be well
below to the initial ash fusion temperature of coal. Usually, the larger particles that are
escaped out from the gasifier are captured through a cyclone downstream and fed back
into the bed as a recycled stream. Overall, fluidized bed pertains shorter residence time
of coal particles as compared with the residence time of moving bed gasifier.
The fundamental characteristics of fluidized bed gasifiers are as follows:
• It has extensive recycling of solid particles;
• It has a moderate temperature which is uniform throughout the bed; and
• Adequate requirements of steam and oxygen
Fig. 1.2: The fluidized-bed gasifier with various inlets and outlets (Source:
Higman and Van Der Burgt (2008))
1.2.3.3 Entrained Flow
Fig. 1.3 shows a diagram of generic entrained flow gasifier. The fine particles of coal
are inserted into the reactor co-currently with an oxidant. The rapid heat is transferred
from reactor environment into coal particles and reaction with oxidant started
immediately. The reaction occurs with a very short residence time of order a few
8
seconds within the entrained flow gasifier. The operating temperature of entrained flow
gasifier must be kept at higher ranges due to short residence time for achieving good
carbon conversion efficiency. Therefore, oxygen is used instead of air in most of the
entrained flow gasifiers and they are operated at the temperature which should be above
to the coal slagging point. Few salient features of entrained flow gasifiers are given
below:
• It is operated at high temperature, above to the coal slagging point;
• Raw syngas contained content of molten slag;
• It requires relatively high oxidant;
• Raw syngas contains a high amount of sensible heat; and
• It has the capability of gasifying all coal regardless of the amount of fines, caking
characteristics, and rank of coal.
Fig. 1.3: Fundamental Entrained flow gasifier with various inlets and outlets
(Source: Higman and Van Der Burgt (2008))
From various generic designs of gasifiers discussed above, entrained flow gasifier
shows the highest conversion rate in the range of 95-99% with large efficiency.
1.3 ADVANCEMENT IN GASIFICATION SYSTEMS
Moderate and Intense Low-Oxygen Dilution (MILD) combustion or more commonly
known as Flameless combustion is a novel and efficient combustion technology which
emits fewer oxides of nitrogen (NOx) and soot (though in some cases air is preheated
at very high temperatures) (Cavaliere and De Joannon, 2004). Currently, researchers
9
have deviated their focus to apply this latest novel technology for the combustion and
gasification of pulverized coal (Stadler et al., 2009b). This technology is based on state
of the art design of gas burners in the industries where heat treatment occurs. In this
technology, the emissions of NOx are in the lowest limit of detection. The reaction zone
in flameless combustion or gasification is prolonged throughout the large volume of
reactor or furnace. Therefore the inherent peaks of high temperature disappear which
cause the flame formation and produce a homogeneous distributions of species and
temperature fields. Flameless combustion or gasification occurs by significantly
diluting the reactants in the primary reaction zone through rapid and high recirculation
of exhaust (flue) gases. Experimental verifications have already been made for
reduction of NOx from coal combustion or gasification with modification and
advancement in burner designs (Ristic et al., 2008, Stadler et al., 2009a).
During the oxidization of most of the chars, the content of oxygen in the reaction zone
is in the higher range in formation of conventional flames. This oxidation or combustion
reaction is much faster in terms of magnitude than the gasification reactions of char
with water vapor and carbon dioxide. Therefore, during numerical investigations, those
gasification reactions are often neglected (Backreedy et al., 2006, Fiveland and Latham,
1993, Kim et al., 2007, Molina et al., 2000, Williams et al., 2002).
The fast dilution of oxygen (O2) in the primary reaction zone occurs during flameless
combustion through extensive recirculation of hot flue gases (combustion products).
Therefore the char oxidation follows a diverse parallel path due to higher partial
pressures of H2O and CO2 than O2 in the reaction zone. The distinct advantages of
flameless gasification over traditional combustion and gasification are (1) emitting
lower pollutants (2) saving of energy sources (3) producing lower noise (4) enhanced
thermal efficiency of combustion/gasification and (5) reduced cost of equipment due to
a reduction in the size of equipment (Tang et al., 2007).
10
1.4 IMPORTANCE OF COAL GASIFICATION IN RELATION TO THAR
COAL DEVELOPMENT
There are three potential options available for Thar coal development, viz. (a)
Integrated open pit mine and coal-fired power generation, (b) Underground coal
gasification (UCG) and (c) Open-pit coal mine and gasification through gasifier. Option
(a) requires a conventional strip mining method for the production of coal and its use
in the coal-fired boiler to run the steam turbine for power generation. Strata conditions
are very much favorable for the strip mining operation, but dewatering of three ground
aquifers is necessary around the mining operation to keep the pit dry. For which 25
million m3 of groundwater is to be pumped out per year for a pit having a production
capacity of 6 million tons. Since the groundwater is highly saline, so it can’t be used
for power plant without desalination. The additional cost of desalination plant will push
the power generation cost to a prohibitive limit. The government of Sindh has prepared
a scheme to bring 90 million m3 per year of fresh water from Nara canal to Thar
coalfield for power plants and mining projects, which will cost Rs. 27 billion.
Additional pumping and clarification cost of canal water is estimated as Rs. 125 million
per year. This option seems to be difficult to materialize.
In option (b) coal is converted in-situ to combustible gases (i.e. CO, H2, and CH4)
through complex reactions at high temperatures and pressures (say 5 MPa, 900oC)
where auto-thermal chemical equilibrium (ACE) conditions are approached. This is a
condition at which the heat value of the product gases and the conversion efficiency of
the gasified coal is a maximum, but carbon oxidation reactions dominate at lower
temperatures and pressures leading to a high CO2 content in the product gases and a
low heat value. Favorable hydro-geological conditions including strong roof and floor
rocks, deep coal seams (>300m), moisture content <40% and no water aquifer in the
vicinity of the coal seam, are essential for the underground coal gasification process.
Thar coalfield does not meet these conditions where the rock strata are mostly
composed of very weak clay-stone, siltstone, and sand. Rock mechanics investigations
carried out at Mehran University reveal that the average value of uniaxial compressive
strength of rock strata and coal is 2.25 MPa and 3.2 MPa respectively which shows that
the rock formation at Thar is even weaker than the coal. There are two major water
11
aquifers in the vicinity of coal seams, one is within the coal zone while the other is
below the coal seams. The bottom aquifer is 47m thick and is under pressure. When the
coal is ignited for UCG at Thar, the water from both the aquifers will rush into the
cavity and will not allow the temperature to rise thus producing a high concentration of
CO2. Due to a very weak rock formation above and below the coal seams, there will be
a substantial amount of gas and heat losses. There is a great chance of surface
subsidence as a result of a big cavity produced in a coal zone due to underground coal
conversion. Due to severe fracturing of overburden strata, the shallow water aquifer,
which is presently used by the local people, will be badly contaminated and there is also
a chance of disappearance of the aquifer around the area of UCG activity. On the basis
of technical reasons as discussed above, it can be concluded that underground coal
gasification is not a viable option for Thar coal.
In option (c) coal is first mined by open pit operation and then fed into a gasifier for
gasification. There are 385 gasifiers of various designs currently operational
worldwide. Total worldwide gasifier capacity is around 45,000 MWth and is expected
to some 75,000 MWth in near future. Various technologies of gasifiers are available
around the world but staged entrained flow gasifier has the highest efficiency with the
production of CO+H2 >80%. Syngas is used to make methanol, ammonia, oxo-alcohols,
liquid fuels via the Fischer-Tropsch process and power generation. IGCC technology is
mostly used to produce electricity from syngas. IGCC technology has a number of
advantages over conventional technologies. Its efficiency is 50% whereas that of the
sub-critical coal-fired power plant is 34%. Pollutant emissions are also significantly
reduced-even compared with advanced conventional technologies, with 33% less NOx,
75% fewer Sox, almost no particulate emissions and up to 90% of mercury emissions
can be captured. IGCC also uses 30-40% less water than a conventional plant. One of
the main barriers to the widespread uptake of IGCC has been its cost. The capital cost
of IGCC is $1850 per KW and operation & maintenance cost is $40 per KW whereas
the capital cost of the conventional coal-fired power plant is $1250 per KW for the sub-
critical system and $1300 per KW for super-critical. Operational & maintenance cost
of both sub-critical and super-critical is $24 per KW. A viable option for power
generation from syngas is through the combined cycle (gas turbine + steam turbine). It
12
requires only 25% of water as compared to the steam turbine. The waste heat of the
combined cycle operations can be utilized for drying of coal and desalination of
groundwater which will be available as a result of the mining operation at Thar coalfield
as mentioned above and we will not require to bring water from Nara Canal. The capital
cost of the gas turbine is $905/KW which is 27% less than a sub-critical steam turbine,
30% less than super-critical and 51% less than IGCC. Operation and maintenance cost
of the gas turbine is about $18 per KW. In order to make this program a real success,
strong R&D activity is required to support surface coal gasification and subsequent use
of syngas for the making of liquid fuels, methanol, ammonia etc. and power generation.
1.5 PROBLEM STATEMENT
As discussed in the previous section that surface coal gasification is the best option for
economic utilization of Thar lignite but the real challenge is the selection and proper
design of a gasifier which suites the characteristics of indigenous Thar lignite coal.
Selection of appropriate technology for any coal requires great knowledge and practical
experience but as per research Entrained Flow Gasifiers have shown great applications
for all types of feedstocks ranging from low-grade coals to biomass and solid waste and
also valid for all types of feeding shape of feedstocks like slurry or dry. But to design
an efficient technology for Thar lignite, strong research and development activity is
required for coal gasification and subsequent use of syngas for making of liquid fuels,
methanol, ammonia etc and power generation. As per previous research understanding
of the kinetics of the gasification reaction is essential for designing an efficient gasifier
(Bermúdez et al., 2011). Coal gasification is based on two fundamental reactions i.e.,
the reaction of char with CO2 and reaction of char with steam (H2O). These reactions
are highly dependent on the type and nature of coal. Lots of work have reviewed from
different parts of the world in which researchers have utilized basic techniques to
extract the kinetic parameters of those gasification reactions with specific to their types
of fuel. But very scant literature is available for the gasification of Pakistani coal
particularly Thar lignite gasification. So the aim of this research is to highlight
fundamental technical issues with Thar lignite gasification and extract the kinetic
parameters for gasification reactions at atmospheric and higher pressures.
13
1.6 OBJECTIVES
Looking at the versatility of coal gasification, Mehran University has devised a strategy
to establish a strong R&D base for the development of indigenous technology suitable
for local coal with the following objectives:
1. To examine the issues involved in the design of high-pressure double-stage
entrained flow coal gasifier under excess enthalpy combustion conditions
(flameless) using CFD simulations.
2. To conduct proximate and ultimate analysis of indigenous lignite coal.
3. Kinetic study of drying, devolatization and gasification reactions for indigenous
lignite using Atmospheric and Pressurized Thermogravimetric Analyzers (TGA
and PTGA).
4. CFD modeling of lignite gasification process under flameless combustion
conditions, using the data obtained from proximate and ultimate analysis and
kinetic study and its validation.
5. To optimize the performance of high-pressure double-stage entrained-flow
gasifier with numerical simulations for various important design parameters like
coal/oxygen distribution between the stages, Oxygen/Carbon ratio, the particle
size of coal, amount of steam, temperatures of feed streams etc.
6. Detailed design of a gasifier.
1.7 SCOPE AND OVERVIEW OF THE RESEARCH WORK
The present research study is on the development of kinetic models of indigenous coal
gasification reactions at high pressure. The working of the research project is divided
broadly into two parts. The first part is Experimental Work in which the samples of coal
collected from Thar Coal Field were analyzed for its proximate and ultimate analysis.
Then the thermogravimetric analysis (TGA) was made in different environments at
atmospheric and pressurized conditions. The TGA results were utilized to extract the
kinetic parameters of various intermediate gasification processes like drying,
devolatization, combustion, and gasification reactions.
14
The second part is regarding the computational fluid dynamics (CFD) modeling work.
In this part, the 3D CFD model was developed and simulated with Chinese coal. During
the simulations, the conditions were set as per published experimental work on double
stage entrained flow gasifier. The model was validated against published results. The
publications were made in the international conference and journal (Unar et al., 2014).
After this, the gasifier was modified and tuned for indigenous coal characteristics
through numerical simulations. Finally, the optimized designed double stage entrained
flow gasifier with multiple opposite burners was fabricated and experimentally verified
the applicability of kinetic models. The whole process is explained in Fig. 1.4.
Fig. 1.4: The flow diagram of research activities
1.8 THESIS STRUCTURE
A brief introduction regarding the Coal Gasification, its mechanism, various
technologies of gasification, Energy scenario in Pakistan and importance of coal
PhD Research Activities
Experimental Work CFD Modeling/ Simulation
Coal Sample Collection
Sample Preparation
Characterization
TGA Analysis
PTGA Analysis
Development of 3D Computational Domain
Selection of Governing Equations and Boundary Conditions
Development of CFD Model
Simulation and Validation
Modification of Geometry and Input Data
Optimization of modified CFD model on indigenous coal data
Proposed a New Optimized Designed of Gasifier
Proposed Process of Gasification system
Development of Kinetic Models
15
gasification research in Pakistan perspectives and objectives has discussed in present
Chapter.
Chapter 2 introduces the general aspects of kinetic modeling and various fundamental
kinetic models which have been used by numerous researchers. The rate determination
techniques of Char-CO2, Char-Steam, and Char-O2 are reviewed.
In Chapter 3, the history of gasification is discussed along with different commercial
designs of gasifiers. Further, the experimental and simulation & modeling work on
gasification are reviewed. The latest trends in gasification particularly the work
published in the area of flameless gasification is also reviewed.
Chapter 4, basically divided into two sections. The first section discusses the
methodology of the experimental work. The details of experimental work including
basic characterization of coal, TGA and PTGA experimental methodologies are
discussed. The second part discusses the results and discussion of experimental work.
The proximate and ultimate analysis of selected samples has tabulated. The kinetic
modeling parameters are extracted using volumetric and grain models at atmospheric
and pressurized conditions.
Similarly, Chapter 5 is divided into two parts. The first part explains the CFD modeling
strategies and methods used. The validation mechanism has also explained. Whereas
the second part discussed the results and discussion of modeling and simulation work.
The 3D CFD modeling results in the forms of contours, plots, and tables are also
presented and discussed in detailed.
The conclusion is given in Chapter 6 on the basis of the discussed results. Finally, the
future recommendations are presented.
16
CHAPTER 2
KINETIC MODELING FOR COMBUSTION AND
GASIFICATION REACTIONS
2.1 GENERAL OVERVIEW
An extensive literature review was conducted in different directions as shown in Fig.
2.1. The work conducted on the Kinetic Modeling of different gasification steps is
reviewed in this Chapter. Whereas the experimental and modeling work conducted on
current and advanced gasification systems like Flameless Combustion/ Gasification
will be reviewed in the next chapter.
Fig. 2.1: Directions of Literature Survey
Literature Survey
Kinetic Modeling
Experimental Work
Modeling and
Simulation
Flameless Combustion/Gasification
17
2.2 INTRODUCTION TO KINETIC MODELING
Reactivity of coal plays a significant role in understanding the coal gasification and
other coal conversion processes like carbonization, combustion, and liquefaction (Çakal
et al., 2007). A series of physical and chemical changes are taking place in coal on
heating it in an inert or oxidizing environment. Re-solidification, softening, the porosity
of the solid particle, surface morphology belongs to physical changes whereas
recombination and breakage of bonds come in the category of chemical changes
(Tromp et al., 1989, Elbeyli and Pişkin, 2006).
In coal combustion, moisture removal is the first step. After the removal of moisture,
coal converted into semi-coke and volatiles (gas and oil) in the pyrolysis process.
Pyrolysis is an important process for coal conversion and generated char
characterization. Ignition behavior and flame stability of coal are also affected by
Pyrolysis. The Combustion process takes place after char production in pyrolysis,
where char reacted with oxygen to produced combustion products like carbon dioxide,
water vapors, oxides of nitrogen and sulfur etc. Economic utilization of coal for power
generation needs an in-depth understanding of combustion characteristics of coal
(Azhagurajan and Nagaraj, 2009).
In gasification process, solid or liquid hydrocarbon feedstock (with low economic
value) is converted into a synthetic fuel gas that has the capability to produce electricity
or can be used as a raw material for the manufacture of chemicals, hydrogen, or
transportation fuels (Gary and Russell, 2011). The primary constituents of syngas are
carbon monoxide and hydrogen. Coal gasification technology significantly fulfills the
environmental control regulations (Us).
The gasification process begins with a rapid removal of volatiles known as
devolatilization (or pyrolysis), leaving behind a char that is mainly composed of fixed
carbon and ash. The next step is the heterogeneous reactions of char with O2, CO2, and
H2O (steam) to produce the syngas. The reaction of char with oxygen (combustion
reaction) is kept controlled by having limited oxygen to produce sufficient heat for the
endothermic gasification reactions of char with CO2 and steam. Therefore, these
18
gasification reactions have a strong influence on the char conversion efficiency under
specific process conditions (Wu et al., 2007a, Li et al., 2009).
2.3 LAWS FOR RATE DETERMINATION OF CHAR-O2 (COMBUSTION)
REACTION AND MECHANISM
The char combustion or the reaction of char with O2 is the fastest reaction among all
other reactions of char with gases occurring in a gasifier or combustor. Usually, the
reaction occurs at the external surface of char particles and reaction controlled by the
diffusion offered by a layer of ash formed on the surface. However, the reaction may
be controlled by gas-film diffusion at a higher temperature and/or increased particle
size. Moreover, chemical reaction controlled regime would be reached for the reaction
at considerable low temperature ranges and/or particle size, and the internal pore
surfaces of the particles are utilized for the occurrence of uniform reaction over there.
In the combustors where pulverized coal is used, the combustion is controlled through
chemical reaction kinetics for particles below 50 μm, whereas diffusion is dominated
during combustion for particles larger than 100 μm (Irfan et al., 2011).
For the reaction of carbon-oxygen, several mechanisms have been proposed during
research conducted over past forty years; however, Walker et al. (Walker et al., 1967)
rejected most of the proposed reaction mechanisms. For the combustion reaction, Elliott
(Elliott, 1981) predicted that at high temperature it shows a first-order mechanism due
to adsorption control of oxygen, and at low temperature, the combustion reaction
behaves like zero order reaction due to the desorption control of the product gases CO2
and CO. The extensive investigations were made for those carbon-oxygen reactions for
coal char (Laurendeau, 1978, Bews et al., 2001, Smith, 1982, Williams et al., 2001) but
still, poor understanding is there for lots of aspects. However, Hurt and Calo (Hurt and
Calo, 2001) summarized the mechanisms for combustion of coal char and is its
universal applications is assumed for the fuels originated from lignocellulosic materials.
The most extensively applied treatment is based on a global reaction of simple nature:
22 COOC →+ (2.1)
The power rate law form of this equation is as follows:
19
n
Oc PRTEAr2
)/exp(−= (2.2)
Where2OP =Partial Pressure of O2, A is the Arrhenius constant more commonly known
as Pre-exponential factor and E is known as the activation energy.
Actually, char-oxygen (C–O2) reaction comprises of a series of elementary reactions
based on adsorption and desorption processes, which includes following elementary
reactions:
)(221
2 OCOCk
f →+ (2.3)
COOCk2
)( → (2.4)
f
k
CCOOC +→ 2
3
)(2 (2.5)
)(/)( 22
4
OCCOCOOOCk
+→+ (2.6)
)(5
2 OCCOCOCk
f +→+ (2.7)
Where rate constants of Arrhenius nature are termed as k1 – k5. The oxygen
chemisorption on active sites are represented by reaction (2.3) and CO is formed by
desorption through reaction (2.4) (similarly described by the mechanisms of
gasification reactions, Cf represents available or active site). CO2 is formed either by
surface reaction (2.5) or through interaction gaseous oxygen with surface complexes
(2.6) through which new complex C(O) may or may not be generated on the product
side. The rate of reaction of carbon with CO2 (2.7) is much smaller than the rate of
reaction of carbon with O2. Therefore, at the time of carbon-oxygen reaction
consideration the reaction (2.7) is significantly excluded on usual basis.
Langmuir–Hinshelwood (L–H) form have been modeled as Semi-global mechanisms
of kinetic laws. For the reactions (2.3) and (2.4), its simplest forms is as under:
20
21
21
2
2
kPk
Pkkr
O
O
c+
= (2.8)
2.4 LAWS FOR RATE DETERMINATION OF CHAR-CO2
(GASIFICATION) REACTION AND MECHANISM
The reaction of char with CO2 (or more simply Char-CO2) is frequently used to check
the reactivities of various kinds of char produced from diverse parent coal through
different processes. This reaction shows a relatively slow rate of reaction, reactivity is
easy to measure and it is similar to the reaction of char with steam. The consumption
of CO2 is not industrial as much as steam in the activation or gasification processes but
at the laboratory level, it is used as a preferred agent due to high importance of C–CO2
reaction. Around the temperature of 723°C, the rate of C-CO2 reaction is slower and
gives better process control of the gasification and provides ease in the analysis of the
various important variables (Çakal et al., 2007). Various researchers (Nozaki et al.,
1992, Sha et al., 1990, Mühlen et al., 1985, Adánez et al., 1985, Kajitani et al., 2006,
Liu et al., 2000, Shufen and Ruizheng, 1994, Blackwood and Ingeme, 1960) extensively
investigated the pressure effects on gasification reaction of char with CO2 and proposed
the modern theories at the conditions of atmospheric pressure (Kapteijn et al., 1992,
Chen et al., 1993). The char-CO2 reaction usually occurs uniformly throughout the inner
surfaces of char particles and controlled by the rate of chemical reaction at lower
temperatures (less than 1000°C) with smaller sizes of char particles (<300 μm) (Sha et
al., 1990, Shufen and Ruizheng, 1994). However, diffusion through pore becomes
significant and important at high temperatures usually more than 1100°C and with
pulverized char having particle size less than 100μm (Liu et al., 2000). This also shows
that the reactivity of char is greatly influenced by temperature. The reactivity data of
high-pressure gasification at low temperature is extrapolated to high-temperature
conditions by Liu et al. (Liu et al., 2000) and a model was developed to estimate the
performance of entrained flow gasifier which is operated at high temperatures.
Kinetic data for coal and char gasification is necessary for the designing of coal
gasifiers. Various investigations have been conducted to determine the kinetics data for
21
coal gasification. The gasification of char with CO2 without a catalyst is a reaction with
endothermic nature:
mol
KJHCOCOC 7.15922 =+ (2.9)
The above reaction is often utilized to measure the rate of gasification due to its slower
rate and ease in measurement. The following mechanism of oxygen-exchange is used
to interpret the reaction [178]:
)(1
2 OCOCOi
+→ (2.10)
)()(3
OCCOOCj
f +→+ (2.11)
)(2
2
COCOi
j→ (2.12)
Where available active sites are shown by Cf and occupied sites are represented by
C(O). The reactivity, Rc is calculated through the following equation:
)1/(222 321 COCOCOCO PkPkPkR ++= (3.13)
Where rate constants are designated as k1, k2 and k3 and PCO2 and PCO are the partial
pressures of CO2 and CO respectively. The rate is approximate to first order at low-
pressure conditions with respect to CO2 concentration ignoring the CO delaying effect
but at a pressure above 1.52 MPa, it approaches to zero order (Dutta et al., 1977).
2.5 LAWS FOR RATE DETERMINATION OF CHAR-H2O
(GASIFICATION) REACTION AND MECHANISM
The mechanism of gasification reaction of char with stream is extensively investigated
through Langmuir–Hinshelwood kinetics in which influences of pressure are expressed
on the reactions of adsorption and desorption throughout gasification of char
(Woycenko et al., 1992, Ye et al., 1998, Beamish et al., 1998, Çakal et al., 2007,
Messenböck et al., 2000, Ochoa et al., 2001, Nozaki et al., 1992). The similar
22
mechanisms have been considered for C–CO2 reaction and C–H2O reaction (Woycenko
et al., 1992, Liu and Niksa, 2004). The proposed elementary reactions of adsorption-
desorption for Char-H2O are as follows:
)(() 22
4
4
OCHOHCk
k++
− (2.14)
())(5
CCOOCk
+ (2.15)
Where active sites of char are represented by C(). The rate equation for C-H2O reaction
has the following form:
OHH
OH
OHPkkPkk
PkR
22
2
2
5454
4
//1 ++=
−
(2.16)
2.6 KINETIC MODELS
The methods conventionally utilized for gasification rates measurement could roughly
be divided into two groups; (1) rapid single-point measurements, and (2) slow real-time
measurements. In rapid measurement technique, reactors are used with high rates of
mass and heat transfer to react the feedstock (coal or biomass) for a specified amount
of time, most of the order of 1 second to several minutes. Then the sample is removed
from the reactor and efforts are made to calculate the degree of conversion. Wire mesh
reactors (Messenböck et al., 1999) and drop tubes (Bryan Woodruff and Weimer, 2013,
Matsumoto et al., 2009, Simone et al., 2009, Biagini et al., 2005) are few examples of
such types of reactors. On one side this technique is efficient and effective to measure
the reactions taking place in the shorter time period but on the other side this technique
is limited for the only single extent of conversion per experiment and provides
difficulties for conducting extensive analysis over a range of gas concentrations,
temperatures, and conversion degrees.
Fixed beds (Smoliński et al., 2010, Luo et al., 2009, Ahmed and Gupta, 2011, Lussier
et al., 1998, Hüttinger and Merdes, 1992) and thermogravimetric analysis (TGA)
(Huang et al., 2010, Mühlen et al., 1985, Alevanau et al., 2011) are few examples of
systems for real-time measurement which have wide applications for gasification
23
studies of coal and biomass. These techniques are conventionally applied for reactions
occurring over 5 min to an hour.
In TGA measurements, usually, there are two methods for studying the kinetics of
gasification reactions. On in which the sample in the chamber heated at some higher
temperature in an inert environment (N2) and then keeping that temperature constant
the reacting gas is shifted. This method is called Isothermal Technique. While in
another method, the reacting gas is introduced from a lower temperature (usually room
temperature) and then at a constant heating rate, the sample is heated. This is called a
non-isothermal technique. In both the methods the weight loss in char due to the
reaction of reacting agent (O2, CO2 or Steam) is recorded. The ash-free basis conversion
(X) versus time profiles are usually calculated from the weight loss versus time data
obtained from the experiments using the following equation.
ashmm
mmX
−
−=
0
0 (2.17)
Where m shows the sample’s instantaneous mass, initial mass is represented by m0 is
the initial mass, and mash is the unconverted mass of sample (remaining mass) showing
the ash content. The derivative of conversion degree with respect to time, usually
detonated as dX/dt, is used to calculate the apparent rate of reaction. The standard
kinetic expression for calculating the overall rate of gas-solid reactions (like in
combustion/ gasification) is given by the following expression (Gil et al., 2012, Lu and
Do, 1994):
)(),( XfTPkdt
dXg= (2.18)
Where k is the apparent rate of combustion or gasification reaction including the effects
of the partial pressure of reacting gas (Pg) and temperature (T), and f(X) designates the
changes in the chemical and physical properties of the sample as the reaction of
combustion and/or gasification proceeds. With the assumption of the constant partial
pressure of reactive gas, the apparent rate of combustion or gasification reaction will
24
be dependent on temperature only and expression becomes the Arrhenius kinetic
equation, written as follows:
RTEAek /−= (2.19)
Where A is Arrhenius constant more commonly known as pre-exponential factor and E
is the activation energy. To describe the term f(x) in Eq. (2.18), various researchers
tested and proposed a number of kinetic models. Few of them are the most fundamentals
and are discussed briefly in subsequent paragraphs.
2.6.1 Volumetric Model (VM)
The volumetric model (VM) is the simplest model. It is assumed in this model that the
gas is diffused uniformly within the entire particle and consequently, reaction is
simplified through the assumption of homogeneous gas reaction with the char (Molina
and Mondragon, 1998, Fermoso et al., 2009, Murillo et al., 2004, Mianowski et al.,
2012). The Volumetric Model (VM) equation is given as under:
)1( Xkdt
dXVM −= (2.20)
The Eq. (2.20) is integrated and following integral form is achieved:
tkX VM=−− )1ln( , Where kVM is model-corresponding kinetic constant.
2.6.2 Modified Volumetric Model (MVM)
Kasaoka et al. (Kasaoka et al., 1985) introduced first the Modified Volumetric Model
(MVM) through modifying VM. However, the changing rate constant is assumed with
conversion of solid (X) during the proceeding of reaction (Wu et al., 2007a, Murillo et
al., 2004, Nowicki et al., 2011). The MVM equation is given as:
)1)(( XXkdt
dXMVM −= (2.21)
The Eq. (2.21) is integrated and converged in the following form:
25
batX =−− )1ln( (2.22)
Where kinetic constant corresponding to the model is represented by kMVM(X),
Meaningless empirical parameters are represented by a & b. Equation for kMVM(X) is:
b
bb
MVM XbaXk1
1
)1ln()(−
−−= (2.23)
The integration of Eq. (3.23) gives the mean value of kMVM(X) as follows:
=1
0
)()( dXXkXk MVMMVM (2.24)
2.6.3 Grain Model (GM) or Shrinking Core Model (SCM)
The Grain Model (GM) or more commonly known as Shrinking Core Model (SCM)
assumes that the outer surface of char particle is utilized for the occurrence of reaction
(Molina and Mondragon, 1998, Fermoso et al., 2009, Nowicki et al., 2011, Zou et al.,
2007, Ochoa et al., 2001). Reaction progressively travels inside the char particle leaving
the ash layer behind. The Shrinking core of unreacted solid phase is observed at the
particles’ intermediate conversion which reduces as reaction moves further. The
mathematical expression of GM can be written as follows:
3
2
)1( Xkdt
dXGM −= (2.25)
The integral form of Eq. (3.25) is:
( ) tkX GM=
−− 3
1
113 (2.26)
2.6.4 Random Pore Model (RPM)
Bhatia and Perlmutter (Bhatia and Perlmutter, 1980) developed the Random Pore
Model (RPM) by assuming the random overlapping of the pore surface during the
course of the reaction (Molina and Mondragon, 1998, Fermoso et al., 2009, Murillo et
al., 2004, Ochoa et al., 2001, Bhatia and Perlmutter, 1980, Liu et al., 2003, Wu et al.,
26
2009a). Hence, the reaction occurs on continuously changing surface area. Structural
parameter ‘ψ’, a characteristic of the model, is used to represent those changes. The
RPM expressed mathematically as follows:
)1ln(1)1( XXdt
dX−−−= (2.27)
And its integral form is:
( ) tkX RPM=−−− 1)1ln(12
(3.28)
To calculate the structural parameter, ψ, following equation is used:
2
0
00 )1(4
S
L
−= (3.29)
The estimation of ψ (at maximum X) can be given as:
1)1ln(2
2
max +−=
X (3.30)
2.7 WORK CONDUCTED ON KINETIC MODELING FOR
GASIFICATION AND COMBUSTION REACTIONS
A number of researcher has worked on the kinetic modeling of combustion and
gasification reactions. The most of the feed is carbonaceous materials like coal,
biomass, solid waste etc. Various types of equipment have used for getting the basic
time or temperature based data like weight loss. Thermogravimetric Analysis (TGA) is
the most common among all of those. The important operating parameters, model
equations used and calculated values of pre-exponential factor (A) and activation
energy (E) by various authors are reviewed in Table 2.1.
Table 2.1: Review of Kinetic Modeling of Carbon-based materials with various techniques
Sr.
No. Feedstock
Equipment
Used Objective of Research
Short Description for
Experimental Conditions
Rate Law/ Kinetic Model Used for
Calculation of A and E A E Ref.
1
New Mexico
bituminous coal,
Washington
subbituminous coal,
and North Dakota
lignite
TGA
Reactivities and surface areas
of coals which were gasified
in steam at high temperatures
were determined.
1 atm. 100 mesh size. 76 mole %
steam. Wt of Sample: 100 mg.
Constant heating Rate
Apparent Gasification Rate (Rc)
dtdWWo
dtdXRc −== /1
(24 to 9300
min-1)
(36.51 to
98.52
KJ/mol)
(Lee, 1
98
7)
2
Burner-jet
fine-grained coke
derived from Irsha-
Borodinsky and
Kuznetsky (II) coal
Fixed-Bed
Reactor
The chemical reactivity and
kinetics of nine Canadian
coal samples ranking from
lignite to
Semi-anthracite and one
wood sample were examined
in a fixed gasifier in the
presence of air and steam at
950-1000
1 atm. 50g sample. 6mm dia
opening wire,
Air+Steam=(2.0dm3/min
+3g/min-water rate). Temp.
Range=950-1000°C.
Gasification run=30 min,
Cooled with N2= 45cm3/min
dt
dc
WR =
1
Shrinking Core Model
( ) 3/111 X
tt −−=
t* is a complete conversion n
so KPrCt /=
Not Mentioned Not
Mentioned
(Fu
ng
and
Kim
, 19
90
)
3
Brown coal samples
(from Victoria) and
Bituminous coal (New
South Wales)
Rigaku-
Denki,
Type 8085
(Thermoflex)
TG-DTA
The pyrolysis of a suite of
brown coal samples and
bituminous coal maceral
concentrates is investigated
by non-isothermal TGA
Atmosphere: N2; Flow rate: 0.1
dm3/min; Sample size: 15 mg;
Heating rate: 10°C/min; Temp.
range: 20 to 950~ crucible:
platinum; TG range: 10 rag;
DTG range: 5 rag-rain -1. All
samples were air-dried and
pulverized to pass an 80 mesh
3.26 (Non-Isothermal)
1)1[(2
)1(33/1
3/1
−−
−=
−a
ak
dt
da
1.36×10-3 to
2.87×107 sec-1
(18.47to
168.56
KJ/mol)
(Ma et al., 1
99
1)
4 Four kinds of Chinese
coals TGA
The coal devolatization
process of different coals was
studied by means of TGA
method
Non-Isothermal; Heating Rates:
10 and 20°C/min; Enviro:N2,
Particle Size:74μm to 100μm;
Max: temp; 800°C
)/exp()( RTEKVVd
dV−−=
−−=−
P
P
RT
E
E
RTK
nVVV exp
1exp
2
2.27×10-8 to
3.26×10-8
s-1
165 to 253
KJ/mol
(Qiu
and
Liu
,
19
94)
27
5
Coal samples from
Soma, Tuncbilek and
Afsin Elbistan regions
PL-1500
TGA
TG/DTG was used to
determine the kinetic
parameters
of raw and cleaned coal
samples
Sample Size: 10mg;
Non-Isothermal;
Heating Rate:10°C/min;
Temp Range:20-900°C;
Air Flow:50ml/min;
Particle Size:<60mesh
nkdt
d
=
Coats and Redfern integral method
−
−=
−
−− −
RT
EERT
E
AR
nT
n
)/21(ln)1(
)1(1ln
2
1
Not calculated 2.8 to 45.7
KJ/mol
(Ozb
as et al.,
20
02)
6
5 coals (Alaska,
Cyprus, Drayton,
CNCIEC and
Denisovsky)
Pressurized
drop tube
furnace
(PDTF)
PTGA
The reactivity of char-CO2
gasification was investigated
with a PTGA in the temp.
range 850-1000° C and the
total pressure range 0.5-2.0
MPa
Sample Size: 10mg;
Non-Isothermal;
Heating Rate: 20°C/min;
Temp.Range:850-1000°C;
Env.CO2 balanced with N2
Shrinking Particle Model
3/2)1( xkdt
dxg −= , 3)
3
11(1 tkx g−−=
−=
RT
EAPkg n
CO exp2
5.17×104
to
1.0×108
min-1 MPa-1
149 to 223
KJ/mol
(Park
and
Ah
n, 2
00
7)
7 Chinese Binxian coal
Self-made
thermal
balance
(TGA)
The gasification activities of
three kinds of Binxian chars
with CO2 were studied at
1000–1300 °C and under
atmospheric pressure in self-
made thermal balance.
Sample Size: Not mentioned;
Isothermal; 3 chars prepared and
used in TGA; Temp: 1000,
1050,1100, 1150, 1200; 1250;
1300 C, CO2 rate: 200 ml/min
Random Pore Model
)1ln(1)1( xxss o −−−=
2
00 /)1(4 SLo −=
Not Calc. 160 to 180
KJ/mol
(Liu
et al.,
20
08)
8
Typical Chinese
bituminous coal from
Shenmu
Cahn
Thermax 500
PTGA
The gasification rates of
Shenmu coal chars with CO2
were experimentally studied
with a PTGA
Sample Size: 10.5 mg, Heating
Rate: 25°C/min (Room Temp to
750° C) and 2°C/min (from 750
to 1000 °C); Pressure: 0.1, 0.6,
1.6,3.1 MPa; Non-isothermal;
Environment CO2
dt
daR = ,
n
COPaRT
EaA
dt
da2
3/2)1(exp −
−=
2.89×1011 285.5
KJ/mol
(Wan
g et al.,
20
08)
9 Coal, Biomass and a
Pet-coke
Setaram TAG
24
The reactivity in the steam of
five different types of solid
fuels (two coals, two types of
biomass and a petcoke) has
been studied
Sample size: 6mg;
Isothermal Conditions; Temp.
Range: 998-1323 K;
Environment: Steam (at423K,
10-40Vol % with N2); flow rate
of reactive gas :150 cm3/min
Vol. Model: )1( Xkdt
dXVM −=
Grain Mod. or Shrinking Core Model:
3/2)1( Xkdt
dXGM −=
Random Pore Model:
)1ln(1)1( XXKdt
dXRPM −−−=
VM:2.97×103
to 2.07×107
GM: 2.22×103
to 1.64×107
RPM: 1.83×103
to 1.93×107
VM: 171-
238
GM: 171-
239
RPM: 171-
238
KJ/mol
(Ferm
oso
et al., 20
08
) 28
10 Biomass (Cynara
cardunculus) in
Down
flowing
Entrained
Flow Reactor
(EFR),
A complete set of
devolatilization and
combustion experiments
performed with pulverized
(∼500 μm) biomass in an
entrained flow reactor under
realistic combustion
conditions are presented.
Sample Size: Not mentioned;
realistic conditions; fuel
transported with air; Feed flow
40 g/h; heating rate104 K/s:
Combustion Conditions:1040,
1175 and1300°C (O2=4% molar)
Devolatization Conditons:800,
930, 1040, 1175°C.
One-step Devolatization Law:
Vkdt
dVv−=
Heterogeneous oxidation:
n
sOspc PkdNdt
dCm ,
2
2−==
•
)/exp( pvvv RTEAk −=
)/exp( pccs RTEAk −=
Av=47.17s-1
Ac=0.46
g/(m2.s.Pa)
Ev=11
KJ/mol
Ec=63
KJ/mol
(Jimén
ez et al., 200
8)
11
2 typical Chinese
coals,
Petroleum coke
and pitch coke
SETARAM
TG-
DTG/DSC
(TGA)
Investigation of physical
properties and gasification
reactivity of coal char and
petroleum coke
Sample Size: Not Mentioned;
Particle Size: <73μm;
Isothermal Method;
Environment: CO2 ashWWdt
dW
dt
dXR
−−==
0
1 Not Calculated
Not
Calculated
(Wu
et
al.,
20
09b
)
12 Australian coal
SETARAM
Setsys
Evolution
model
TGA
The reactivity of four
pulverized Australian coals
was measured under
simulated air (O2/N2) and
oxy-fuel
(O2/CO2) environments
Sample Size: 2-3mg;
Non-Isothermal, Temp. Range:
303-1474K; Heating Rate:
25K/min; Gas Flow rate: 20
ml/min; O2 conc. In N2/CO2: 2-
50%v/v basis
Char Reactivity:
dt
dm
mmR
ashco
cm)(
1
,
,−
−= s-1
Coal Reactivity: dt
dm
mR
o
pm)(
1,
−=
Not Calculated Not
Calculated
(Rath
nam
et al.,
20
09)
13
Woody Biomass
(Small chips of
Douglas-fir)
Pressurized
Thermobalan
ce ALVAC
9600
The effect of pyrolysis
conditions on char reactivity
has been studied using
Raman spectroscopy
Sample Size: Not mentioned;
Isothermal Conditions; Temp:
700, 800, 900, 1000 and
1100°C; Atmospheric
Conditions; Environment CO2
)(XfKdt
dXP=
Random Pore Model:
)1ln(1)1( XXKdt
dXP −−−=
Not Calculated Not
Calculated
(Ok
um
ura et
al., 20
09
)
14
Biomass char (a wood
portion of Japanese
cedar, Japanese cedar
bark, a
a mixture of hardwood,
and Japanese
lawngrass)
drop tube
furnace
(DTF)
The purpose of this study
was to investigate the
gasification kinetics of
biomass char, such as the
wood
Sample Size: 1-10g/h;
Residence Time: 0.5-3s;
Temp. Range: 900-1200°C;
Pressure:0.4MPa
Environment: H2O OR CO2 with
N2
Particle Size: 70-80 μm
)(XfKdt
dXR rg ==
−=
g
aa
n
grRT
EAPK exp
Random Pore Model:
)1ln(1)1( XXKdt
dXR rg −−−==
H2O
Gasification:
9.99×104
CO2
Gasification:
2.24×103
H2O
Gasification:
136 KJ/mol
CO2
Gasification:
93.9 KJ/mol
n=0.22
(Matsu
mo
to et al.,
20
09)
29
15 Wood Particle
(Biomass)
Thermo-
Balance
Reactor
A simple expression for the
apparent reaction rate of
large wood char gasification
with steam is proposed
Sample size: 500 mg
Particle size: Min.100μm, Large
particle size-14.3, and 21.1mm.
Reacting gas velocity: 5 m/s;
Temp.850-900°C; The partial
pressure of steam: 0.02-0.06
MPa.
)exp()(RT
EpXAg
dt
dXr n
A −==
Overlapped grain model
3/21
0
1
0
1
0
]])1(1ln[
)(ln1][)1(1[)(
X
XXg
−
−−
−−
+−−=
170.8 s-1 MPa-
0.35
103.3
KJ/Mol
n=0.35
εo=0.5
(Um
eki et al., 2
01
0)
16 Biomass (Pinus
densiflora)
Fixed Bed
Reactor
Carbon conversion was
calculated during CO2
gasification
Sample size: 1 gm;
Isothermal, Temp. Conditions:
850, 900, 950, 1000, 1050 °C,
CO2, flow rate of CO2: 0.5
lit/min; Particle size:250-300
μm.
)(),(2
XfpTkdt
dxCO=
Volume Reaction Model (VRM)
)1( xkdt
dxVRM −=
Shrinking Core Model (SCM)
3/2)1( xkdt
dxSCM −=
Random Pore Model (RPM)
)1ln(1)1( Xxkdt
dxRPM −−−=
VRM
9.03E+5
SCM
3.75E+7
RPM
1.51E+4
[min-1]
VRM
172 KJ/mol
SCM
142 KJ/mol
RPM
134 KJ/mol
(Seo
et al., 201
0)
17
PET from post-
consumer soft-drink
bottles
Thermobalan
ce (Setaram
TAG24)
The reactivity of the PET char
in CO2 was determined by
isothermal TGA at different
temperatures.
Sample Size: 5 gm; Particle
Size: 0.5-1mm; Isothermal
conditions; Environment: CO2;
Flow of CO2: 50 N ml/min;
Temp. Conditions: 925, 975,
1025, 1075 and 1125°C;
Atmospheric pressure.
)(Xkfdt
dX=
Random Pore Model (RPM)
2/1)]1ln(1)[1( Xxkdt
dx−−−=
2
00 /)1(4 SLo −=
Not Mentioned Not
Mentioned
(Gil et al., 2
010
)
18 Food Waste Chamber
Reactor
Characteristics of syngas
from the pyrolysis and
gasification of food waste
have been investigated.
Sample Size: 35 g;
Isothermal Condition; Temp.
750, 800, 850, 900° C; steam
flow: 8 g/min; Inert Gas flow:
6.4 g of Ar.
dt
dm
mr
1−=
RTEkAer /−=
5028 to 67778
min-1
111 – 125
KJ/mol
(Ah
med
and
Gu
pta,
20
10)
30
19 Char from Lignite Coal
Pressurized
Thermogravi
metric
Analyzer
(PTGA,
Thermax
500)
Char–CO2 gasification
reactions in the presence of
CO and char–steam
gasification reactions in the
presence of H2 were studied
at the atmospheric condition
using a TGA
Sample Size:10 mg;
Isothermal Study; Temp.
Conditions: 1148, 1173 and
1198 K;
Environment: Mixture of CO2
and H2; Atmospheric
Conditions; Particle Size:
200μm
Langmuir–Hinshelwood (L–H) Model
For Char-CO2
2
2
)/()/(1 3132
1
COCO
CO
pkkpkk
pkr
++=
2
2
65
4
1 COCO
CO
pkpk
pkr
++=
For Char-Steam-CO2
222
22
6532
41
1 COCOOHH
COOH
pkpkpkpk
pkpkr
++++
+=
2
2
22
2
65
4
32
1
11 COCO
CO
OHH
OH
pkpk
pk
pkpk
pkr
+++
++=
k1=2.827E9
k2=1.598E-5
k3=2.25E4
k4=3.485E4
k5=3.458E-9
k6=1.38E-3
[bar/sec]
E1= 216
E2= -146
E3= 74
E4= 143
E5= -227
E6= -74
[KJ/mol]
(Hu
ang
et al., 201
0)
20
Carbonaceous waste
materials (Sewage and
industrial sludge, Fluff,
and Scrap Tire
Powder)
TGA
(TG, Netzsch
STA 409)
Synthesis gas production by
steam-gasification of
carbonaceous waste materials
was kinetically examined.
Sample Size: 53-161 mg;
Particle Size Range: 0.1-4.5
mm; Reactive Gas: H2O (8.5%
Vol in Ar); Heating Rate: 20 and
10K/min; Non-Isothermal from
room temp to 1476 K and then
Isothermal.
=
=N
i
iT XX1
, =
−=N
i
iiT Xk
dt
dX
1
' )1(
−=
)(exp
,
,0
'
tRT
Ekk
ia
ii
For Isothermal phase:
)1('
ii
i Xkdt
dX−= ,
=
= it
ii
C
tTCX
,
),(
0.1E+3
to20.2E9
SeWc-1
65.7E+3 to
185.4E+3
KJ/mol
(Piatk
ow
ski an
d
Stein
feld, 2
010
)
21
Two bio-oils produced
from
birch and aspen forest
and wood
TGA (TA
instruments
Q600)
The char produced by
pyrolysis of an 80 wt% bio-
oil/20 wt% char mixture at
heating rates of 100–10,000
°C/s was subjected to steam
gasification in a TGA
Sample size: 2 mg;
Drying at 110°C for 5 min;
Steam+N2 flow=200 ml/min;
Particle Size:0-250μm
Char Reactivity: dt
dX
XXr
−=
1
1)(
Langmuir-Hinshelwood kinetic Model
)1/(222 OHHOH bPaPkPr ++=
n
OHkPr2
= ,Where,
−=
RT
Ekk exp0
Wood:
9×104–5×106
Bio-Oil
4×104–3×106
[s-1Pa-1]
Wood:
235 KJ/mol
Bio-oil:
219 KJ/mol
(Sak
agu
chi et al.,
20
10)
31
22
Steam activated carbon
and Spectroscopically
pure graphite
TGA
(Setaram Co.,
France)
Non-isothermal TGA data
were used to evaluate the A
and E for the uncatalyzed
gasification by CO2 of two
carbons
Sample size: 8 mg; Non-
isothermal; Temp. Range: 383 K
to 1523 K; Heating Rates: 5, 10,
15, 20 K/min; Reacting Gas =
CO2; Reacting Gas
Flow=40ml/min; particle size:
40μm.
)(exp
)()(
fRT
EA
TkfdT
d
dt
d
−=
==
KAS/Vyazovkin linear method
RT
E
Eg
AR
T−=
)(lnln
2
Carbon:
7.06×105 to
2.83×106
Graphite:
2.51×105 to
1.78×106
[min-1]
Carbon:
145-155
Graphite:
153-169
[KJ/mol]
(Liu
et al., 20
11
)
23 Argentinean coal
TGA
(Model 2000,
Cahn
Instruments
Inc.)
The kinetics of Argentinean
asphaltite char gasification
using carbon dioxide as a
gasifying agent was studied
between 1048 and 1223 K
Sample Size:10 mg; Non-
isothermal, Heating Rate:
4°C/min, CO2/Ar Flow=15 lit/h;
PCO2=80KPa; Particle Size: 20-
150μm
dt
d
dt
dm
mmRate
c
=
−−=
0
1
)().().(2COPFTKG
dt
d
=
The Iso-conversional method
X
CO
a PRT
EBk
dt
d2
exp.)1( 0
3/2
−−=
1.1×105 185 KJ/mol
(Fo
ug
a et al., 20
11
)
24
Oil shale (from the
El-Lujjin deposit in
Jordan)
TGA
(Simultaneou
s Thermal
Analyzer
SDT Q600)
Thermogravimetric (TG)
data of oil shale obtained at
MI (Waste to Energy
laboratory) were studied to
evaluate the kinetic
parameters for El-Lujjun oil
shale samples
Sample Size: 8-15 g;
Non-isothermal; Heating rate: 5,
10, 15 and 20°C/min, Temp.
820°C; Particle Size: 50μm.
)(. XfKdt
dX=
Volumetric Model:
)1.( XKdt
dX−=
Drying:
1.6E-3 to
3.97E-1
Devolati:
2.6E+1 to
3.02E+2
CO2 Gasi:
4.2E+3 to
9.2E+6 [s-1]
Drying:
4.7 - 17
Devolati:
56.2 - 68
CO2 Gasi:
122.2-182.2
[KJ/mol]
(Sy
ed et al., 2
01
1)
25 Wood Chips (Yellow
pines)
Chamber
Reactor
Kinetics of woodchips char
gasification has been
examined through steam and
CO2 as gasifying
agents
Sample Size: 35 g;
Isothermal Condition;
Temp.900° C; Gasifying agent:
4.42 g/min of H2O (steam) or 5.4
g/min CO2. Gasifying agent
partial pressure: 1.5, 1.2, 0.9 and
0.6 bars. Total Pressure=2 Bar
dt
dXr =
Random Pore Model (RPM)
)1ln(1)1( Xxkdt
dx−−−=
Not calculated Not
Calculated
(Ah
med
and
Gu
pta, 2
01
1)
32
26 Sewage Sludge Char
Thermobalan
ce (Netzsch
STA 409 PG,
Germany).
Gasification of char derived
from sewage sludge was
studied under different
oxidizing atmospheres
containing CO2, O2 or H2O
Sample size: 20 mg; Isothermal
conditions; Total flow rate of
gasifying agent (O2 or CO2
mixed with H2O)=50
m3/min(with stram it is higher);
Particle size: 70μm; Different
Environments:
10% O2+Ar [450, 500, 550°],
16% H2O+Ar [750, 800, 850°C],
50% CO2+Ar [800, 850, 900° C]
)().,(
sg rTykdt
d=
First-order kinetics (Vol.Model)
)1(
−= vkdt
d
tkv=−− )1ln(
Shrinking Core Model
tks=−−= 3/1)1(1)(
O2:
1.16×104
H2O:
5.96×106
CO2:
1.09×106
[sec-1]
O2:
114 KJ/mol
H2O:
227KJ/mol
CO2:
193 KJ/mol
(No
wick
i et al., 201
1)
27
Solid
Waste Recovered
Fuels (SRF)
TGA
The gasification reactivity
data of some solid
waste recovered fuels (SRF)
obtained from
thermogravimetric
analysis (TGA) experiments
is presented
Sample Size: Not mentioned;
Isothermal; Temp. 700, 750, 800
and 850°C; Gasifying agent: H2O
and CO2 separately; atmospheric
conditions;
22
2
2
3
1
3
1
1
1 Hb
OH
f
OHf
OHC
Pk
kP
k
k
PkR
++
=−
COb
CO
f
COf
COC
Pk
kP
k
k
PkR
3
1
3
1
1
2
2
2
1 ++
=−
( )2
,modexp, −=N
j
CC jeljRRL
Steam
K1f:6.49E7
K1b:95.3
K3:1.64E9
CO2
K1f:1.64E7
K1b:4.59E2
K3:8.83E7
[bar-1s-1]
Steam
E1f:204
E1b:54.32
E3:243
CO2
E1f:188
E1b:88.27
K3:225
[KJ/mol]
(Ko
nttin
en et al., 2
012
)
28
Empty Fruit Bunches
(EFB)
[Seri Ulu Langat Palm
Oil
Mill, Dengkil,
Selangor, Malaysia]
TGA
(Model
Mettler
Toledo, TGA/
SDTA851,
USA)
Empty fruit bunches (EFBs),
a waste material from the
palm oil industry, were
subjected to pyrolysis and
gasification
Sample Size: 10 mg; Non-
isothermal; Heating rates: 10, 20
and 30°C/min; Max. Temp.
1000°C; Gasifying agent :Air;
flow: 100mL/min; Particle Size:
0.3-1 mm.
)()(
fTkdt
d=
n
iif )1()( −=
Cellulose
4E+3-6.5E+6
Lignin and
Char Comb:
2.95-16.62
[Sec-1]
Cellulose:
61.2-73.7
Lignin and
Char Comb:
40-48
[KJ/mol]
(Mo
ham
med
et al., 20
12
)
29 Thar Coal (Block V of
Thar coalfield) TGA
The concept of weighted
mean activation energy has
been used to assess the
reactivity of Thar coal in
terms of pyrolytic and
combustion behavior using
non-isothermal TGA
Non-isothermal study; For
Pyrolysis: N2 flow 30 ml/min
from 110-800°C@10°C/min. For
Combustion: O2 flow 20ml/min
from 110 to 1000°C @16°C/min;
particle size: 60 mesh
)(.exp xfRT
EA
dt
dx a
=−
First Order Kinetics (Volumetric
Model)
)1()( xxf −=
Not mentioned
Pyrolysis:
19.20–63.55
Combustion:
23.68–54.49
[KJ/mol]
(Sarw
ar et al.,
20
11)
33
30
Hard coal from the
“Janina” coal from
Poland
TGA
[Netzsch
STA 409PG
Luxx]
The kinetics of polish hard
coal and its char gasification
using CO2 as a gasifying
agent was
studied at atmospheric
pressure between 293 and
1223 K at different linear
heating rates.
Sample Size: 5 mg; Non-iso
conditions; (for Coal) Heating
Rate: 1, 3 and 10 K/min; Temp.
Range: 293-1273 K; Gaifying
Agent: N2 (10ml/min) CO2 (50
ml/min); (for Char) Heating
Rate: 1.25, 5 and 10 K/min;
Temp. Range: 293-1273 K;
Gaifying Agent: N2 (25 ml/min)
CO2 (50 ml/min);
)()(
fTkdt
d=
F1: First Order Kinetics
−= 1)(f
F2: Second Order Kinetics 2)1()( −=f
A2: Accidental Nucleation, Avrami-
Erofeev equation 2/1)]1ln()[1(2)( −−−=f
A3: Accidental Nucleation, Avrami-
Erofeev equation 3/2)]1ln()[1(3)( −−−=f
F1: 4.9
F2: 6.1
A2: 4.2
A3: 4.3
[LogA, 1/s]
F1: 172
F2: 193
A2: 158
A3: 160
[KJ/mol]
(Łab
ojk
o et al., 2
01
2)
31
Bituminous Coal (from
China) and Palm Shells
(from Malaysia)
TGA (Perkin
Elmer Pyris
STA 6000)
Kinetics of bituminous coal
and palm shells were
evaluated using TGA under
different environments
(N2/CO2/O2)
Sample Size: 10mg; particle size:
100-300 μm; Non-isothermal;
Heating rate: 10 K/min; Temp.
Room Temp to 1273 K;
Environments: O2/N2 and
O2/CO2; gas flow: 50 cm3/min
)/exp()1(
RTEA
dT
d−
−=
Integral form: )()1ln( xpR
AE
−=−
Doyle’s Approximation for P(x):
−−
=−
RT
E
R
AE052.133.5ln)]1ln[ln(
Coats-Redfern’s Approximation for
P(x): RT
E
R
AE
T−
=
−−
ln
)1ln(ln
2
Doyle’s:
1.57×10-7 to
6.13×10-4
Coats-
Redfern’s:
-1.82×10-5 to -
2.09×10-8
[sec-1]
Doyle’s:
32.11 –
82.84
Coats-
Redfern’s:
44.95 –
99.88
[KJ/mol]
(Irfan et al., 2
01
2)
32 Douglas fir chips
[Biomass]
TGA (TGD-
9600;
ALVAC)
The CO2 gasification
behavior of biomass chars
derived at 800 °C under
N2/CO2/O2 atmospheres was
investigated using the RPM
model at gasification temp.
of 800–1000 °C
Sample size: not mentioned;
Non-isothermal; Heating Rate:
10°C/min [upto 100°C with Ar]
and then 2°C/min from 800-
1000 °C; Environment: CO2,
Particle Size: 2-4mm.
)()( XfTkdt
dX=
Random Pore Model (RPM)
5.0)]1ln(1)[1( XXkdt
dXp −−−=
Not mentioned Not
mentioned
(Han
aok
a et al.,
20
12)
34
33
4 coals: a semi-
anthracite,
a medium-volatile
bituminous coal and
two high-volatile
Bituminous coals
Thermobalan
ce (Setaram
TAG24)
The thermal reactivity and
kinetics of four coal chars in
an oxy-fuel combustion
atmosphere (30%O2–
70%CO2) were studied using
a thermobalance
Sample Size: 5 mg; Isothermal
Conditions; 400, 450, 500, 550,
600° C; Gasifying agent (30%
O2, 70% CO2); Flow of gas: 50
Nml/min; Particle size: 75-150
μm.
)()( XfTkdt
dX=
Vol. Mod: )1( Xkdt
dXVM −=
Grain or Shrinking Core Model:
3/2)1( Xkdt
dXGM −=
Random Pore Model:
2/1)]1ln(1)[1( XXkdt
dXRPM −−−=
VM
3.55E4 to
4.09E5
GM
2.51E4 to
2.66E5
RPM
8.1E3 to
2.82E5
[s-1]
VM
118-128
GM
118-128
RPM
117-127
[KJ/mol]
(Gil et al., 2
012
)
34
Three types of Chinese
coals from Mulei,
Shenmu, and Shigouyi
TGA
(Netzsch
STA-449F3)
Comparative study on the
gasification reactivity
of the 3 types of Chinese coal
chars with steam and
CO2 at 850–1050° C was
conducted by isothermal
TGA
Sample size: 15 mg; Isothermal
conditions; Temp. 850, 900, 950,
1000, 1050° C; gasifying agent:
CO2 or Steam; Composition and
flow of gasifying gas: 3g/h
steam+100mL/min N2, 130
mL/min CO2
nth order DAEM model n
RTE
v
vvek
dt
vvd
−= −
*
**)/( /
0
RT
Ek
nt
n 1ln
1
)1(1lnln 0
1
+−
−
−−=
−
Not mentioned
Steam:
40-160
KJ/mol
CO2:
110-180
KJ/mol
(Fan
et al., 201
2)
35
Switchgrass (from
Colorado State
University)
[Biomass]
Downflow
fixed bed
reactor
A novel kinetics
measurement technique has
discussed based on a
modified fixed bed and data
collected solely
from a gas flow meter
Sample Size: 50mg and 150mg;
Isothermal; Temp. 1000°;
Gasifying Agent: Steam
'
tV
V
dt
dXr
•
==
Steam-char reaction
Langmuir–Hinshelwood Model
22
2
32
1
01 HOH
OH
pKpK
pKk
++=
k1: 2.51E3
k2: 6.74E-2
k3: 3.04E-1
[bars-1s-1]
E1: 112.6
E2: -37.3
E3: -36.6
[KJ/mol]
(Bry
an W
oo
dru
ff
and
Weim
er, 201
3)
36
Algal biomass
(Chlorella sp.)
And woody biomass
(commercial wood
mix)
TGA
(Model STA
449F3
Jupiter)
The gasification reactivity
measurement of an algal
biomass (Chlorella sp.) and
woody biomass (commercial
wood mix) char was
performed in a TGA at 3
different temperatures
Sample size: 4-5 mg; Isothermal
conditions; Temperatures: 800,
950, 1100° C; Environments:
CO2 and Steam; Gas
compositions: 20 mL/min
H2O/CO2 + 80mL/min N2;
ii
idt
dw
wR
−=
1 Not calculated
Not
calculated
(Kirtan
ia et al.,
20
14)
35
37 A Chinese lignite Coal
TGA
(Customer
Designed)
The chars were gasified
with CO2, H2O and their
mixtures in a TGA system to
investigate gasification
kinetics and derive the rate
expression
Sample size: 300 mg; Isothermal
Conditions; Temp; 1273 K;
Environments: CO2 and H2O
(Steam); Gas mixture flow:
3NL/min; Atmospheric
Conditions.
21 /1)/(1/122
kPkr COCO +=
43 /1)/(1/122
kPkr OHOH +=
Shrinking Core Model And Langmuir–
Hinshelwood Expression
1.564E-4 to
9.723E-4
[s-1]
Not
calculated
(Ch
en et al.,
20
13
a)
38 Coal (Lignite and
Bituminous)
TGA (Linseis
STA PT
1600) and
PTGA
(Linseis
GmbH in
Selb,
Germany)
The devolatilization rate of
two coals are measured in a
pressurized high temperature
entrained flow reactor at up
to 1600°C and 4.0 MPa and
in a wire mesh reactor
Sample size: 40-60 mg;
Isothermal; Temp. 1000° C; )1( xdt
dx
mdt
dmr
−=
=
Pyrolysis:
293 s-1
Char
Deactivation
23.4 s-1
Pyrolysis:
51 KJ/mol
Char
Deactivation
117 KJ/mol
(Trem
el and S
plieth
off,
20
13
a, Trem
el and
Sp
lietho
ff, 20
13b
)
39 Wood, Miscanthus,
and Straw [Biomass]
TGA (Hi-Res
TGA 2950;
TA
Instruments,
New Castle,
DE, USA)
The CO2 gasification kinetics
for each biomass sample was
established in the temp.
range from 800 °C to 1300
°C by the combination of
TGA and a novel aerosol-
based method
Sample size: 2.5 mg; Isothermal
Conditions; Temp. 800 to 1300°
C; Initially heated at 50°C/min
till temp with N2 then switch the
gas to CO2(33%) +N2 mixture;
Gas flow: 150 ml/min;
0
)(),(
S
XSTCK
dt
dXgas =
)/exp(),( RTEAaTCK gas −=
Random Pore Model
)1ln(1)1( XXkdt
dXRPM −−−=
Wood:
2.23×108
Miscanthus:
3.24×105
Straw:
4.48×104
[sec-1]
Mean Values
Wood:
240.5
Miscanthus:
187
Straw:
164.5
[KJ/mo]
Mean Values
(Lin
and S
trand
,
20
13)
40
Brown Coal
from Loy Yang
(Victoria, Australia)
TGA (TG-
DTA2000S,
MAC
Science)
The mechanisms and kinetics
model of the char
gasification and volatile–char
interactions were discussed
to describe quantitatively the
inhibition of char gasification
by volatiles
Sample size: Not mentioned;
Isothermal conditions; Temp:
800° C; Environment: Steam
(H2O).
cncchar kk
dt
dX+=
Non-catalytic Gasification
HncHncOHnc
OHnc
ncPKPKPK
PKk
432
1
22
2
1 +++=
Catalytic Gasification
HcHcOHc
OHc
cPKPKPK
PKk
432
1
22
2
1 +++=
Non-catalytic
Knc1=4.3E-4
Knc2=2.8
Knc4=200
Catalytic
Kc1=4.5E-3
Kc2=4.5
Kc4=45
[Diff. units]
Not
calculated
(Kajitan
i et al., 20
13
) 36
41
3 coals
(Spanish
anthracite, Spanish
lignite, and Russian
bituminous coal)
Entrained
Flow Gasifier
(EFR) at
LITEC’s
The results of an
experimental study on the
gasification kinetics of three
coals of different ranks under
realistic conditions in an EFR
are presented
Sample size: Not mentioned;
Particle size: 53-63 μm;
Environment: CO2, O2, and N2
with the different composition;
Temp. Range: 800-1450°C.
VRTEAVkdt
dVpvvv )/exp(−=−=
)/exp(, p
n
siii RTEPAk i −=
Devol:
9.2E4-1.85E8
Oxy:
1.03E-4-0.095
Gasification:
0.038-7.55 [s-1]
Devol:
92.3-184
Oxy: 58-108
Gasification:
100-148.5
[KJ/mol]
(Go
nzalo
-
Tirad
o et al.,
20
13)
42 Typical Chinese Coal
TGA
(NETZSCH
STA 409C,
Germany)
Char surface active sites
were first measured with the
help of the chemisorption
process of CO2 at 300 °C,
using a TGA
Sample size: 20 mg; Non-
isothermal; Heating rate: 5, 10,
20 K/min; Temp. 950-1300°C;
Environment: CO2
)()./exp( xgRTEAdt
dx−=
Ozawa Method:
)(log)/log(
315.2)/(4567.0log
m
m
xGRAE
RTE
−
+−−=
=x
xgdxxG0
)(/)(
Not calculated
Slow-
Pyrolysis
192 to 310
Rapid-
Pyrolysis
212 and 243
[KJ/mol]
(Xu
et al., 20
13
)
43
Beechwood chips
(Biomass) provided by
SPPS Company,
(France)
Macro TGA
Gasification reactivity of
high-heating-rate chars in
steam, CO2 and their
mixtures was investigated in
a new macro-TG
experimental device
Sample size: 0.1-1 g (for
pyrolysis); Isothermal
Conditions: Temps. 800, 850,
900, 950° C; Environment: H2O,
CO2, and Their Mixture
dt
dX
Xdt
dm
MR
t
t
t
t
X
)(
)(
)(
)(
)(1
11
−=−=
)()50( ),(),,( XFRRii PTXPT =
n
iTPT PkRi
= )(),()50(
Char+H2O
26.3×103
Char+CO2
55.18×103
[Sec-1bar-1]
Char+H2O
139 KJ/mol
Char+CO2
154 KJ/mol
(Gu
izani et
al., 20
13
)
44
3 coal samples:
lignite
coal from ‘‘Turów’’
mine, and 2 sub-
bituminous coals
from ‘‘Piast’’ and
‘‘Wieczorek’’ mines (
Poland)
TGA
(Netzsch
STA 409 PG
Luxx)
The gasification reactivities
of three char samples from
Poland toward CO2 were
investigated isothermally
using TGA
Sample size: 5 mg; Isothermal
Conditions; Temp. 900, 950,
1000°C; Particle Size: 200 μm;
Environment: CO2, Flow of CO2
)()( XfTkdt
dX=
Vol. Mod. (VM) )1( Xkdt
dXVM −=
Mod. VM.(MVM) )1( XXkdt
dXMVM −=
GM. 3
2
)1( Xkdt
dXGM −=
Random Pore Model (RPM)
)1ln(1)1( XXkdt
dXRPM −−−=
VM
6.87E7-1.22E9
MVM
1.76E7-6.1E8
GM
1.11E7-2.17E7
RPM
1.88E6-1.74E7
[min-1]
VM
199-246
MVM
187-240
GM
187-208
RMP
184-209
[KJ/mol]
(To
maszew
icz et al., 201
3)
37
38
2.8 SUMMARY
The gasification is a complex phenomenon in terms of elementary reactions and their
mechanism. Overall the whole gasification process has divided into four broad steps.
The first step is known as drying in which moisture is removed from the coal on the
increase of temperature. The second step is called devolatization or pyrolysis in which
volatiles are removed from the parent coal in the absence of oxygen. The third step is
char and volatiles combustion with releasing of CO2, H2O, and heat in the reactor.
Finally, the unburnt char is reacted with CO2 and/or H2O at a higher temperature to
produce syngas components like CO and H2. The fundamental kinetic mechanisms,
associated mathematical correlations (kinetic models), and experimental work
conducted to extract concerned kinetic parameters were reviewed in this chapter.
Among various kinetic models, Volumetric Model and Shrinking Core model (or more
commonly known as Grain Model) were proposed as most simple, accurate and easy in
calculations in the research. The solution of these models was carried out by either
direct plot method or integral method as per their simplicity and getting good accuracy.
A number of published literature is reviewed regarding the experimental work
conducted in the quest of kinetic modeling. In this regard, various types of feedstocks
like coal, biomass, waste, tires, etc. were used. Most promising devices for kinetics
modeling of these fuels were lab-scale reactors, wire-mesh reactors, drop-tube furnaces,
and thermogravimetric analyzers. Among these TGA has most significant applications
in this field and provides best and accurate results with all types of feedstocks. The
prime objective of most of the research was to calculate the important kinetic
parameters like activation energy (E) and pre-exponential factor (A). From the review,
it was concluded that each material has distinct behavior towards the kinetics of
combustion and gasification reactions. Keeping in view the importance of kinetics of
coal gasification, the literature on kinetic modeling for indigenous coal was surveyed
but few published literature specific to combustion was found. No literature for
indigenous coal regarding gasification kinetics has searched. So there is the great
necessity of conducting research on indigenous lignite coal for extracting kinetic
mechanism for the whole gasification process.
39
CHAPTER 3
LITERATURE REVIEW ON GASIFICATION TECHNOLOGIES
AND MODELING STUDIES
3.1 GENERAL DISCUSSION
The kinetic modeling of various gasification processes is reviewed in detail in the
previous chapter. The standard kinetic models and extraction of kinetic parameters
through experiments were also discussed. In this chapter, the literature survey is mainly
focused on the Gasification Technologies and general practices for experiments on
gasifiers and combustors along with simulation and modeling strategies. Apart from
this, the latest advancement in gasification system will also be reviewed.
3.2 HISTORICAL DEVELOPMENT OF GASIFICATION
Humans used wood as a fuel at first, and the practice of using wood as a fuel is still
continued by a huge population of the world for heating the homes and cooking meals.
But wood is also used for producing furniture, in the construction of buildings and in
industrial processes as a raw material in the form of charcoal like reduction of ore.
Hence the increasing demand of wood made the shortage of this fuel and this shortage
led to investigate some other fuel instead of wood and found ‘Coal’ as a suitable
alternative (Higman and Van Der Burgt, 2008).
Coal production and utilizing are not new in the human civilization history but its
production reached at peak level in the second half of the 18th century with the industrial
revolution in England. The development of coke oven was an initial stage to fulfill the
requirements of metallurgical industry for providing coke as an alternative to costly
charcoal. Gas was produced from coal through pyrolysis process up to the end of 18th
century on a slightly larger scale. Finally, gasification turns into the commercial process
with the foundation of London Gas, Light and Coke Company in 1812. Since after,
gasification pertains major role in the development of various industrial processes.
40
Town gas was used as the most significant gaseous fuel during the first century of
industrial development. Two processes i.e., pyrolysis and water gas process were used
to produce town gas. In pyrolysis, coke along with gas of relatively high heating value
(20,000–23,000 kJ/m³) is produced in ovens operated in batch mode. Whereas coke is
converted into a mixture of hydrogen and carbon monoxide in water gas process which
is also batch mode operation. The gas produced in water gas process categorizes as
medium BTU gas with is approximately 12,000 kJ/m³ heating value.
Illumination was the first application of industrial gas produced from coal. The scope
of applications of gas further expanded for heating, utilization in the chemical industry
as raw material and recently generation of electric power. Initially cooking and heating
were the only applications of town gas due to its expensive production from
gasification. For these applications, town gas had advantages over the alternatives
options like coal and candles. But with the advent of electric bulbs around 1900 gas lost
its worth for lightening purpose its only utilization remained for space heating.
Producer gas and blast furnace gas were the only gases produced through continuous
process till the end of the 1920s. Coke’s partial oxidation with humid air was the key
process to produce Producer gas. However, the application of both gases was restricted
in the vicinity of their production due to their low heating values (3500–6000 kJ/m³).
The partial oxidation of solid fuels for the production of gases has advantages of ease
in handling of gases and obtaining a fraction of hydrogen which remained a clean and
high heating value gas. Though the start of gasification was based on producing gas for
lighting and heating purpose, but from 1900 and onward gasification become an
important source for producing raw material for other chemical industries due to an
equal amount of hydrogen and carbon monoxide with water gas process.
After the availability of pure oxygen from Carl von Linde commercialized cryogenic
air separation process during the 1920s, synthesis gas along with hydrogen was
produced from gasification processes in complete continuous mode with an oxygen
blast. At that time some most important processes were developed which became the
precursors of several important processes of the current era. Like in 1926 Winkler fluid-
bed process was developed, in 1931 Lurgi moving-bed pressurized gasification process
41
was invented and Koppers-Totzek entrained-flow process came into existence in 1940.
These processes became the foundation for the technological progress for the
gasification of solid fuels in the upcoming 40 years. However, the capacity of the
gasifier was the basic need in the quest of gasification technology innovation due to
synthetic fuels program of Germany during wartime and worldwide expansion of the
ammonia development industry.
South African Coal, Oil, and Gas Corporation, nowadays more commonly known as
Sasol was also established at that time. The Fischer-Tropsch synthesis is the foundation
of this process to produce synfuels complex from coal gasification. Sasol becomes the
world’s biggest gasification hub after its extensions at the end of 1970.
The importance of coal gasification along with its technological progress declined in
the 1950s due to the advent of plentiful quantities of natural gas and naphtha. But the
scope of syngas applications got enhanced with the increasing demand for ammonia for
the expansion of the fertilizer industry. The increased demand could only be met with
the steam reforming of naphtha and natural gas. The shell and Taxaco (formally known
as GE) processes for gasification of oil were developed in the 1950s. They fulfilled the
demand of ammonia production from syngas at the locations with a shortage of naphtha
or natural gas.
The real gasification importance revived for generating liquid and gaseous fuel in the
early 1970s due to first oil crisis along with the enhanced cost of natural gas. The
investigations started for developing new technologies with appreciable investments.
Initially, efforts were made for developing coal hydrogenation via either direct
liquefaction or through hydro-gasification in which coal is converted directly into
methane and known as a substitute natural gas (SNG). Several processes for hydro-
gasification were investigated through demonstration units but the process could not
get commercial viability due to the thermodynamic constraint of high pressure (Speich,
1981). In reality, moving-bed gasification technology based plant of SNG with oxygen-
blown mode was to be built for providing synthesis gas as an alternative step of
methanation (Dittus and Johnson, 2001).
42
Few older processes were developed further with from fuel research programs by
various investors. Existing technology of Lurgi was modified in a slagging version with
the partnership between British Gas and Lurgi (BGL) (Brooks et al., 1984). Koppers-
Totzek gasifier was modified in its pressurized versions by a joint venture of Shell and
Koppers and produced Shell Coal Gasification Processes (SCGP) and Prenflo
technologies (Van Der Burgt and Kraayveld, 1978). A modified version of Winkler
fluidized-bed process was developed by Rheinbraun known as High Temperature
Winkler (HTW) process (Speich, 1981), and coal slurry feeding system was introduced
in to oil gasification process by Texaco (Schlinger 1984). However, the importance of
coal gasification and liquefaction and its research decreased in the 1980s with a new
oversupply of oil.
3.3 COMMERCIAL GASIFICATION TECHNOLOGIES
A wide range of reactor types is in use since long for carrying gasification processes
considering practical constraints. Those types of reactors are broadly grouped into one
of three renowned classifications: (1) fixed-bed or moving-bed gasifiers, (2) fluid-bed
gasifiers, and (3) entrained-flow gasifiers. The classification is based on certain
characteristics of gasifiers fall in each class that differentiates the gasifier from one
class to other. The classification of commercial gasifiers based on these three broad
types is discussed in subsequent sections.
3.4 FIXED-BED OR MOVING-BED PROCESSES
In fixed-bed gasifiers (sometimes called moving-bed gasifiers) a fixed bed of coal
moves slowly downward direction due to gravity during its gasification by a blast
mostly counter-current to the coal flowing direction. The downward moving coal is
pyrolyzed and preheated by hot synthesis gas, produced in the gasification zone during
counter-current flow. This method is economical in the consumption of oxygen, but
produced synthesis gas is contaminated with pyrolysis products. The exiting syngas is
usually at lower temperatures but even with high temperature, the core of the bed
reached hardly at slagging temperatures. A lump of coal is used in moving-bed
processes. The coal caking property with the presence of excess fines produces
43
blockage of passing syngas in an upward direction. Few commercially developed
technologies are discussed as under.
3.4.1 The Sasol-Lurgi dry bottom processes
The Lurgi dry bottom process was patented in 1927 which also known as “coal pressure
gasification”, later in 1931, the existing technology was modified by Lurgi with the
cooperation of Technical University in Berlin, in a pressurized version with oxygen
blast for lignite coal gasification (Gmbh, 1970, Küffner, 1997). The first modification
was commercialized in 1936 (Rudolf, 1984). Two large-scale plants, Brüx (Czech
Republic) and Böhlen (Saxony) came into existence in 1944 for the production of town
gas. The technological developments like automation, scale-up, and optimization of
operation for only pressurized Lurgi dry bottom gasifier were continued for several
years. A joint venture between Lurgi and biggest technology tycoon Sasol further
developed an advanced version of gasifier known today as Sasol-Lurgi Dry Bottom
Gasifier and made efforts for its marketing.
The reactor is the heart of the Lurgi process where down coming coal feedstock meets
with upflowing blast and syngas counter-currently (Fig. 3.1). The coal is inserted into
the reactor in a cyclic manner using a lock-hopper mechanism for maintaining the
pressure of reactor vessel. The boiling water is filled in an annular space between the
two walls of the dual walled jacked reactor vessel. The exothermic heat from the reactor
core is transferred into the boiling water and converts it into steam. Even distribution
of coal from the lock-hopper in the reactor is maintained through a mechanical
distribution device. The slowly down-flowing coal undergoes the processes of moisture
removal (drying), devolatilization, combustion, and gasification. The rotating grate is
used to remove the ash from the reactor through ash lock-hopper. The precooling of ash
to 300–400°C is taking place at grate zone through entering blast of steam and oxygen.
The grate is also used for even distribution of entering blast in the reactor bed. The hot
ash preheats the upward flowing blast which reaches at combustion zone and the
reaction of char with oxygen produces CO2 and heat and then gasification reactions
start in the presence of steam, coal, and CO2 producing CO, H2 and fraction of CH4.
Table 3.1 summarizes the standard sizes of reactors.
44
Fig. 3.1: Sasol-Lurgi dry bottom gasifier (source: (Gmbh, 1970)).
Table 3.1: Sizes and capacities of Sasol-Lurgi dry bottom gasifiers (Rudolf, 1984,
Higman and Van Der Burgt, 2008)
Type
Nominal Vessel
Diameter
(cm)
Coal feeding rate
(tons / hour)
Production of Dry Gas
(103 Nm3/day)
MK-III 300 20 1000
MK-IV 400 40 1750
MK-V 500 60 2750
The biggest complex for gasification in the world is installed at South Africa operated
by Sasol synthetic fuel company which produces 55 million Nm3/day syngas from 13
MK-III, 83 MK-IV, and 1 MK-V reactors. About 170 000 bbl/day F-T liquid fuels are
being produced from the generated synthesis gas (Erasmus and Van Nierop, 2002).
45
3.4.2 British Gas Lurgi (BGL)
The modified version of Lurgi gasifier with slagging capabilities, known as British Gas
Lurgi (BGL) gasifier (Fig. 3.3), was developed by British Gas from the period of 1958
to 1965 in the Research Station at Gas Council Midlands. The feeding capacity of 13ft
developed BGL gasifier was 100 t/day (Seed et al., 2007). The BGL gasifier contains
refractory-lined walls with dry feeding and oxygen-blowing characteristics. This
gasifier shows good performance with different types of coals along with the capability
of acceptance for blends of various solid fuels like wood waste, RDF and tires. The
demonstration plant of this gasifier is operated from 1986 to 1990 in North America
with 500 TPD capacity. Its first commercial version has been operated with lignite from
2000 – 2005 at Schwarze Pumpe Power Station, Germany.
Fig. 3.2: British Gas Lurgi Gasifier (Source: (Higman and Van Der Burgt, 2008))
3.4.3 Multipurpose Gasifier (MPG)
The Multipurpose Gasifier (MPG) technology was developed by Lurgi on the basis
gasification process with fixed-bed configuration. It is refractory-lined gasifier with
down fired and oxygen-blown mechanisms and considered a good choice for a wide
46
range of raw materials used as feedstocks including solid waste, coal slurries, and
petroleum. Coal and/or petcoke are used in this with a quench configuration. The
demonstration plant by Lurgi is in operation since 1968 at Schwarze Pumpe Power
Station, Germany.
Fig.3.3: Lurgi Multipurpose Gasifier (Source: (Breault, 2010))
3.5 FLUIDIZED BED PROCESSES
In Fluid-bed gasifiers, the mass and heat transfer are promoted with excellent mixing
between oxidant and solid feed. The wastage of unreacted fuel with ash removing could
be minimized with material’s even distribution in the bed and hence carbon conversion
is limited on this even distribution characteristic in the operation of the fluid-bed
gasifier. As ash slagging creates hindrances in the fluidization process so the
temperature of fluidized bed gasifier usually restricted below the ash softening point
during its operation. Investigative tries were made to operate the gasifier at controlled
and limited ash softening zone for improving carbon conversion efficiency. Particle
size is the critical parameter in the fluidized bed operation as fine particles could be
entrained with syngas and become wastage of fuel. Cyclones are used to reduce the
wastage of unreacted fuel in the product gases. Biomass and low-rank coals are suitable
47
for fluidized bed gasifiers as it is usually operated at low temperatures with limited
conversion efficiencies. Few commercial technologies are discussed as under.
3.5.1 High-Temperature Winkler (HTW) Gasifier
The first oxygen-blown modern continuous gasification process was Winkler
atmospheric fluid-bed process. The patent of the process was granted in 1922 whereas
the first unit was constructed in 1925. More than 70 reactors since then are
commercialized till to date with about 20 million Nm³/d of total capacity (Bögner and
Wintrup, 1984).
Any kind of fuel can be gasified with the Winkler process. Bituminous, Sub-
bituminous and brown coal along with coke have been used in commercial plants. Less
than 10 mm particle size is usually required with no drying of coal containing less 10%
moisture. The screw conveyor is used to feeding the fuel into the gasifier. The blast
enters from the conical shaped grate from the base which maintains the fluidization of
bed. To reduce the tar content in the syngas and recover the entrained fuel particles,
feeding of an additional quantity of blast is maintained at above the bed level. The
refractory is used on the reactor walls. Commercial gasifiers are operated at the
temperature range of 950 - 1050°C which well below to ash melting point. Gas velocity
is maintained at 5 m/s for catering maximum load in Winkler Gasifier (Higman and
Van Der Burgt, 2008).
The “High-Temperature Winkler” is the modification of the original Winkler process
developed by Rheinbraun. The phrase “High-Temperature” in the name is actually a
misnomer as the modification mainly in the increase of pressure from the original
design and a demonstration unit has operated 30 bar. Fig. 3.4 shows a High-
Temperature Winkler Gasifier with fluidized bed configuration. It could be operated
either with air or oxygen blown mode, utilizes dry feed and producing dry bottom ash.
The development of technology was based on lignite coal but later a wide range of
feedstocks could be gasified efficiently. A demonstration plant installed in 1977,
operated for 20 years for producing about 800,000 metric tons methanol by consuming
1.6 million metric tons of dry-lignite (Higman and Van Der Burgt, 2008).
48
Fig. 3.4: High-Temperature Winkler Gasifier (Source:(Higman and Van Der
Burgt, 2008))
3.5.2 HRL Process
The HRL Process based on the fluid-bed configuration shown in Fig. 3.5 was initially
developed by the State Electricity Commission of Victoria and then modified by HRL
Limited during the time period of 1989–1998. The process was specifically designed
for the brown coal found in Victoria, Australia containing high-moisture. The most
vibrant characteristic of HRL process is the utilization of syngas coming out from
circulation fluidized bed gasifier at high temperature as a drying medium for high
moisture (about 60-67%) feed coal. The demonstration unit of 24t/d capacity of brown
coal has been operated for about 1200 hours. Typhoon gas turbine of 5 MW capacity
has been energized from the syngas generated at 25 bar pressure from the gasification
plant at Morwell, in Latrobe Valley coal fields of Victoria. A demonstration unit of 400
MW has also been commissioned at brownfield site of Loy Yang, near Morwell in 2010
with a joint venture between Harbin Power Engineering and HRL (Anon, 2007).
49
Fig. 3.5: IDGCC process of drying and Gasification developed by HRL (Source:
(Johnson, 2001))
3.5.3 BHEL Gasifier
Bharat Heavy Electricals Limited (BHEL) developed a pressurized gasifier with fluid-
bed configuration based on the local coal characteristics and conditions. India possesses
huge coal reserves with high ash content ranges in 40% except for few lignite coal fields
in the south, hence system with non-slagging features could be a better choice for
India’s high ash coal reserves. IGCC test facility of 6.2 MWe was developed at initial
stage by BHEL using moving bed gasifier at Tiruchirapalli site and tested the operation
of IGCC using 18t/d Indian sub-bituminous coal with high ash content and then the unit
was modified into air-blown fluidized be gasification system of 165 t/d capacity. The
unit was tested for 980-1050°C temperature and in the range of 3-10 bar pressure
(Viswanathan et al., 2006). A 125 MWe IGCC unit has also been commissioned by
BHELat Auraiya in Uttar Pradesh.
3.5.4 Circulating fluidized-bed (CFB) processes
Several advantages of the transport reactor and fluidized bed system are merged in the
circulating fluidized bed (CFB) process. Excellent mixing is achieved through high slip
velocities and that promotes promote tremendous mass and heat transfer. Particles with
smaller size are either converted 100% in one go or separated from the product gas and
50
recycled back into the reactor. Internal recycling occurs again and again of larger
particles which take some time before converted into a smaller size and then recycled
from the outgoing product gas. The rate of circulation in CFB is much higher than a
classical stationary bed system hence a higher rate of heat transfer has been observed
from entering particles of feed. The tar formation is reduced during the process of
heating from that rapid heat transfer. The salient features of circulation fluid bed
systems are high conversion rate, high heat and mass transfer and less sensitive to the
feedstock nature of feed particle size. That’s why this technology is equally popular
among gasification of solid waste, biomass, and low or high-rank coals.
The circulating fluid-bed gasifier developed by Lurgi is shown in Fig. 3.6. The system
of CFB contains a chamber of the reactor, an interconnected cyclone for recycling and
a seal pot. Most of the heavier particles are entrained with the high velocities (usually
from 5-8 m/s) of gas and evacuated from top of the reactor chamber. The recycling of
separated solids from the cyclone is carried out from seal pot. Air is used usually as
gasifying agent inserted into the reactor at separate nozzles categorized as primary and
secondary air. The particle size for biomass materials should be reduced up to 25–50
mm (Greil et al., 2002).
Fig. 3.6: Lurgi circulating fluid-bed gasifier (Source: (Greil and Hirschfelder,
1998))
51
3.5.5 Kellogg Brown and Root (KBR) transport gasifier
Kellogg Brown and Root or shortly KBR transport gasifier, shown in Fig. 3.7, is
classified as high-velocity regime of fluid-bed configuration with air or oxygen-blown
arrangements. The gas velocities in the rise section of KBR gasifier are usually reported
in the range of 11–18 m/s (Smith et al., 2002). The modification of in the shape of KBR
in conventional circulating fluidized bed arrangement was primarily done in higher
rates of circulations, riser densities and increased velocities which gave better
throughput and enhanced heat transfer and mixing rates. The design of KBR became
reliable based on the experience of years for construction and designing FCC units of
the petroleum industry. The KBR gasifier is designed for feeding coarse low rank coals,
without burners and with non-slagging characteristics. Currently, Mississippi Power
Company is the owner of a 560 MWe IGCC in Kemper County.
Fig. 3.7: KBR Transport Gasifier (Source: (Smith et al., 2002, Higman and Van
Der Burgt, 2008))
52
3.5.6 U-Gas Process Gasifier
Fig. 3.8 shows a dry-feed U-Gas gasification process based on fluidized bed
configuration. All types of coal and blends of biomass with coals can be fed in this unit.
It is a non-slagging gasifier produces dry bottom ash and work efficiently with both
either oxygen or air-blown configuration. Synthesis Energy Systems (SES) currently
hold license agreement of 30 years. The systems are installed and working for more
than 20 years in Finland, Shanghai, and Hawaii. Presently 520 MWth syngas is being
produced from two plants (Higman and Van Der Burgt, 2008).
Fig. 3.8: U-Gas Process Gasifier (Source: (Breault, 2010, Higman and Van Der
Burgt, 2008))
3.6 ENTRAINED FLOW PROCESSES
Entrained flow processes are beneficial for handling any type of coal or biomass as
feedstock, producing tar-free clean syngas and frit or inner slag form of ash. These
benefits of the entrained-flow process are being enjoyed on the compromise of high
consumption of oxygen, especially with high moisture or ash content feedstocks or
slurry feedings along with the additional feed preparation steps.
53
Different internal configurations are designed for achieving maximum contact of the
gasifying agent with feedstock based on entrained flow process and are summarized in
Table 3.2, in which characteristics and features of important processes based on
Entrained-flow are outlined. Slagging gasifiers with entrained-flow configurations
remained most successful systems for coal gasification, developed after 1950 which are
being operated at high temperatures usually above 1400°C and at the pressures ranges
between 20–70 bar (Higman and Van Der Burgt, 2008). Most commercial IGCC plants
are equipped with entrained-flow gasifiers in which hard coals are being preferred. The
technologies mentioned in Table 3.2 are discussed briefly in subsequent paragraphs.
Table 3.2: Characteristics of Important Entrained Flow Gasifiers (Source:
(Higman and Van Der Burgt, 2008))
Type of
Process
No. of
Stages
Nature
of Feed
Flow
direction
Type of
Reactor
Wall
Cooling System of
Syngas
Type of
Oxidant
Koppers-
Totzek 1 Dry Up Jacket Syngas Cooler Oxygen
Shell SCGP 1 Dry Up Membrane Gas quench and
syngas cooler Oxygen
Prenflo 1 Dry Up Membrane Gas quench and
syngas cooler Oxygen
Siemens 1 Dry Down Membrane Water quench
and/or syngas cooler Oxygen
GE Energy 1 Slurry Down Refractory Water quench
and/or syngas cooler Oxygen
E-Gas 2 Slurry Up Refractory Two-stage
gasification Oxygen
MHI 2 Dry Up Membrane Two-stage
gasification Air
Eagle 2 Dry Up Membrane Two-stage
gasification Oxygen
OMB
Process 2 Dry Down Refractory
Two-stage
gasification Oxygen
3.6.1 The Koppers-Totzek atmospheric process
The first slagging gasification process based on entrained-flow configuration was
operated at atmospheric pressure, similar to the start of fixed-bed and fluid-bed systems.
54
The development of Koppers-Totzek (KT) process operated at atmospheric pressure
was carried out in the 1940s, and its commercial units were installed mostly for
manufacturing ammonia at Greece, Finland, Zambia, Turkey, South Africa, India, etc.
About 95% conversion of feedstock is reported for the South African unit (Krupp-
Koppers, 1996). Installation of new units of this type of gasifier is seized in present
years.
In the KT reactor, burners are mounted on sides for the oxygen and coal feeding, the
gas outlet is at top and slag outlet is available at the bottom as shown in Fig. 3.9. The
initial units were designed with 2 opposed burners along with the total capacity of 5000
Nm³/h whereas the later revised units were improved in the capacity up to 32,000 Nm³/h
using four burners. The temperature of the exiting gas from the top is decreased from
1500ºC to 900°C through quenching with water near the reactor’s top to condense the
slag and then it is used for the production of steam through syngas cooler with water
tubes (Higman and Van Der Burgt, 2008).
Fig. 3.9: Koppers-Totzek gasifier (Source: (Higman and Van Der Burgt, 2008))
55
3.6.2 Shell Coal Gasification Process (SCGP)
The development of Shell gasifier has started in 1956 and its first demonstration unit
came into existence in1974 (Mark, 2009). Dry feeding of crushed coal is used as
feedstock in Shell gasification process. Water wall, oxygen-blown shell gasifier is
shown in Fig. 3.10. The issues regarding the durability of refractory have been
eliminated with water all lining in the gasifier. This is suitable for various types of
feedstocks, like all types of coals from low rank to high rank, pet coke biomass, waste
etc. The alliance of Shell, Black & Veatch, and Uhde work together on commercial
terms in which technology of gasification is being provided by shell whereas EPC is
carried out by the rest of two companies. About 8500 MWth syngas is being produced
from 26 plants up to 2010 (Higman and Van Der Burgt, 2008).
Fig. 3.10: Shell Gasifier (Breault, 2010)
3.6.3 PRENFLO™ Gasifier/Boiler (PSG)
Uhde marketed the technology of pressurized entrained flow gasifier along with
generation of steam, known as PRENFLO™ Gasifier-cum-Boiler shown in Fig. 3.11.
It is a membrane wall, oxygen-blown, dry feed gasifier in which extensive variety of
solid fuels including anthracite, lignite, hard coal, refinery residue, biomass etc., could
be gasified. The gasification at 48 TPD rate is being carried out in demonstration unit
in Fürstenhausen, Germany. Biggest IGCC Plant of the world based on solid-feedstock
at Puertollano, Spain is equipped with this technology.
56
Fig. 3.11: PRENFLOTM (Breault, 2010)
3.6.4. Siemens Gasifier
Deutsches Brennstoffinstitut developed Siemens gasifier for the gasification of solid
waste and low-rank coal in 1975 at Freiberg, Germany. Its first demonstration unit of
200 MW thermal capacity was installed in 1984 at Schwarze Pumpe (Higman and Van
Der Burgt, 2008). Noell grouped initially marketed the technology with the name GPS
and then name of Future Energy but later in 2006, it was purchased by Siemens. Fig
3.12 shows the Siemens gasifier which is an oxygen-blown, dry feed, a top fired reactor
equipped with water wall screens. Feedstocks of a wide variety can be gasified in this
gasifier ranging from low-rank to bituminous coals. A power block with Gasification
Island is provided by Siemens. China granted a contract of $39 million to Siemens for
installation of two gasifiers with 500MW capacity of each for their Shenhua DME
Project (Higman and Van Der Burgt, 2008). Currently, 787 MWth syngas is being
produced from one plant operation.
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Fig. 3.12: Siemens Gasifier (Breault, 2010, Higman and Van Der Burgt, 2008)
3.6.5 GE Energy Gasifier
Taxaco developed a gasifier initially which was named The Chevron-Texaco gasifier
after the merging of both companies. GE purchased the technology and named GE
Energy gasifier as shown in Fig.3.13. The gasifier is oxygen-blown slagging gasifier
with entrained flow configuration, fed with a slurry of coal and water along with
refractory-lined reactor. The company offers two versions of the gasifier: gasifier
equipped with a radiant cooler and a gasifier with full quench arrangements. Pet coke,
bituminous coal or blends of low/rank coals with pet coke are suitable feedstocks for
this type of gasifier. GE Energy works in a commercial alliance with Bechtel where
gasification technology would be provided by GE whereas EPC for IGCC Plant would
be tackled by Bechtel. More than 15000 MWth syngas is being produced by 64
operational plants in 2010 (Breault, 2010).
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3.6.6 ConocoPhillips E-Gas Gasifier
DOW Chemicals developed Conoco Philips E-gas Gasifier as shown in Fig.3.14 its
demonstration was carried from 1987 to 1995 at Louisiana Gasification Technology
Inc. (LGTI). It is a double-stage gasifier in which feed’s 80% is inserted from the first
stage located at a lower level of the gasifier. The oxygen-blown gasifier is being
injected with coal-water slurry and lined with refectory. It is also equipped with the
continuous systems for removal of slag and dry particulate matter. A wide range of
coals including bituminous, PRB, petcoke and their blending are suitable for
gasification through the E-Gas process. Gasification technology is being provided by
ConocoPhillips on commercial basis whereas EPC alliances are to be made with other
organizations for the development of combined power plant cycles. A plant of 590
MWth syngas production capacity is in operation whereas planning of 6 more plants is
in progress (Higman and Van Der Burgt, 2008).
Fig. 3.13: GE Energy Gasifier
(Breault, 2010)
Fig. 3.14: Conoco Philips E-Gas
Gasifier (Breault, 2010)
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3.6.7 Mitsubishi Heavy Industries (MHI) gasifier
The Combustion Engineering and Mitsubishi Heavy Industries (MHI) mutually
developed a slagging, air-blown based on Combustion Engineering as shown in Fig.
3.15. A system of dry feed is available in this gasifier and low-rank coals containing
high moisture are found suitable for this technology. It is double-stage entrained flow
gasifier equipped with the water-wall membrane. 250 MWe power is being produced
from a single demonstration unit installed in 2007 at Nakoso, Japan (Higman and Van
Der Burgt, 2008).
Fig. 3.15: MHI Gasifier (Breault, 2010)
3.6.8 The EAGLE Gasifier
Electric Power Development Company developed a two-stage, oxygen-blown, dry feed
reactor in Japan, known as The EAGLE gasifier as shown in Fig. 3.16. Its commercial
facility was started in March 2002 after successful trails using 150 t/d pilot plant
(Tajima and Tsunoda, 2002).
The high oxygen-to-fuel ratio is maintained at first stage with about 1600°C
temperature. In the second stage, the temperature is reduced up to 1150°C from
endothermic reactions of char and coal with carbon dioxide in lean oxygen conditions.
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The tangential direction of firing is maintained to increase the residence time for
particles of coal within the reactor. Equal quantities of total coal are being injected from
both stages whereas the oxygen rates are used to control the reactor.
Fig. 3.16: The EAGLE Gasifier (Tajima and Tsunoda, 2002)
3.6.9 ICCT Opposite Multiple Burner (OMB) Process
In 1995, East China University of Science and Technology at Shanghai started the
development of a new concept for the gasification process in its Institute of Clean Coal
Technology (ICCT). Initially, the design was based on opposed multiple burners
(OMB) with coal-water slurry feeding arrangements in the reactor vessel lined with
refractory as shown in Fig. 3.17. A successful operation of the pilot plant with 22 t/d
capacity was carried out in 2000. The OMB gasifier of 750 t/d capacity was operated
at 65 bar on a commercial scale in 2004 at Dezhou. In 2005 two more units of 1150 t/d
capacity were commissioned in the power plant of Yankuang near Lunan. Both plants
gave 98% conversion of carbon. Further modifications were made in the design to cater
dry feed with carrier gases like CO2 or N2 by ICCT and membrane walls with water
cooling systems (Yu et al., 2007, Zhou et al., 2006). Licenses for another 7 plants were
granted in 2007 (Yu et al., 2007).
The conventional horizontal ball mill has used for preparing slurry and membrane
piston pump is used for feeding. The down-flow OMB reactor possesses a distinctive
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feature of having four burners mounted on opposite sides, which promotes high
efficiency for carbon conversion. The efficient quenching design of the reactor for
cooling the syngas consumes less water. The quench’s water bath is used to collect the
slag and lock-hopper is used to discharge the collected slag (see Fig. 3.17). Depending
on the pressure of operation, the gas from quench water system is cooled up to the range
of 220–250°C. Finally, a cyclone, a jet mixer, and water scrubber are used for the
removal of particulate matter.
Fig. 3.17: The ICCT Opposed Multiple Burner gasifiers (Zhou et al., 2006)
3.7 CURRENT EXPERIMENTAL PRACTICES ON COMBUSTION AND
GASIFICATION
Various researchers worked on lab-scale or pilot scale combustors/ gasifiers for a better
understanding of the processes. A selected work has been reviewed here.
3.7.1 Experimental work on Gasification Systems
Choi et al. (2001c) conducted experiments on an entrained flow gasifier with a capacity
of 1 ton/day feeding of coal-water slurry (at 65% concentration) with oxygen blown
conditions at above 1300 °C temperature. The slag formation and its control were
investigated in the temperature range of gasification reactions by characterizing the
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fusion temperature of ash through CaO addition as flux material. The effects for the
ratio of O2 to coal feed rate on the composition of product gas, syngas calorific value,
cold-gas efficiency and temperature of gasifier were assessed to characterize the
gasification performance. Guo et al. (2007) investigated the performance of the pilot
scale unit of a pulverized coal fed entrained-flow gasifier operated at 1-3 MPa and 30-
45 tons/day capacity. Coal was inserted into the gasifier using pneumatic conveying
mechanism with the help of CO2 and N2 as carrier gases. The effects of steam-to-carbon
and Oxygen-to-carbon ratios on gasification performance were evaluated. Choi et al.
(2007) investigated the performance of the gasification process of vacuum residue (VR)
using entrained flow gasifier at the Korea Institute of Energy Research (KIER) with
oxygen-blown conditions. The temperature and pressure of reaction were maintained
in the range of 1200-1250°C and 1.0 Kg/cm2 respectively during experiments along
with 0.4-0.7 steam-to-fuel ratio and 0.8-1.2 oxygen-to-fuel ratio. As per results, the
syngas was produced with a combined fraction of CO and H2 in the range of 77-88%
along with its heating value found in the range of 2300-2600 kcal/Nm3. The cod gas
and carbon conversion efficiencies were found in the ranges of 68-72% and 95-99%,
respective.
Yun and Chung (2007) utilized pilot-scale of fluidized bed pressurized gasifier with a
dry-feeding system for gasification of subbituminous coal from Indonesia. A syngas
was produced with CO, H2, and CO2 in the ranges of 36-38%, 14-16%, and 5-8%
respectively. Metal filers were used to remove particulates from syngas up to 99.8% at
200-250°C temperature. Desulfurization of syngas was carried out up to 0.5% using Fe
chelate for removing compounds containing sulfur like COS and H2S. Niu et al. (2008)
performed the atmospheric gasification of diesel oil on lab-scale entrained flow gasifier
with two impinging burners installed on opposite sides. The composition of important
constituents of syngas (Like CO, H2, CO2, O2, CH4) was monitored at various O/C
ratios ranging from 1.48 to 2.36 using mass spectrometry. The results utilized to clarify
the reaction and mixing characteristics within the gasifier, understanding the
combustion and gasification mechanism and provide a foundation for mathematical
modeling.
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The gasification characteristics of high ash fusion temperature coal (HAFTC) was
investigated by Wu et al. (2008a) in a lab-scale gasifier with the downward flow for
designing new technology of entrained flow gasifier for using HAFTC as feedstock in
it with dry slag. According to the results, the high conversion of carbon was achieved
with high temperature with higher fractions of H2O and CO2 due to a higher amount of
oxygen. The optimized temperature range was found from 1300° to 1350°C as beyond
this upper-temperature limit the decrease in cold gas efficiency was observed. Font et
al. (2010) investigated the effects of feed conditions (blend of pet-coke with coal with
equal ratio and addition of limestone) and separation of trace elements on the
performance of entrained flow gasifier installed at Puertollano IGCC power plant
(Spain) having a capacity of 335 MW. Liang et al. (2011) studied the fuel conveying
characteristics at 4 MPa pressure in lap-scale entrained flow gasifier and effects of
moisture content on those characteristics of conveying. Lignite and soft coal were used
for the experiments with similar particle size and density. The decrease of mass flow
rate was observed for lignite on increasing moisture from 3.24% to 8.18%, but with
similar operating conditions, the mass flow rate of soft coal was first increased and then
decreased with the increase of moisture from 0.4% to 6.18%. The flowing
characteristics of lignite were found better than soft coal.
Gonzalo-Tirado et al. (2012) investigated the devolatization, oxidation, and gasification
of pulverized sub-bituminous coal from Indonesia with CO2 through entrained flow
reactor. The kinetic parameters were derived using obtained data from experiments for
all three mentioned processes. Li and Whitty (2012) studied char-slag transition
phenomena during gasification for three types of coals using entrained flow-reactor
operated with laminar flow. Particles with different conversions were oxidized partially
at above ash fusion temperatures. Finally, physical characterization like particle size,
density, morphology, and internal surface area, was carried out for prepared particles
of char and slag. Preliminary lab-based characterization of Australian coals’ suite was
carried out by Roberts et al. (2012a) (2012b) to assess the performance under the
conditions of practical entrained flow gasifier and then tested on a pilot scale entrained
flow gasifier having a capacity of 5MWth. The data generated from laboratory and pilot
scale experiments on a suite of coals provides a fundamental understanding and
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correlations between the measurements taken in laboratory gasification experiments
and behavior of coal gasification under realistic practical conditions.
Pyrolysis and gasification behavior of German anthracite, bituminous and lignite from
Rhenish was analyzed by Tremel et al. (2012) at the conditions of commercial-scale
entrained flow gasifier. A pressurized high temperature entrained flow reactor (PiTER)
along with wire mesh reactor were used to quantify the yield of volatiles at high
pressure and high temperature. Sun et al. (2013) used first time the tunable diode laser
(TDL) absorption technique for measuring the temperature of gas and concentration of
species during the experiments on pilot-scale entrained-flow slagging, oxygen-blown
high-pressure coal gasifier. The operating parameters were 18 atm pressure and 1800
K temperature. Continuous solid feed fitted high-pressure gasifier with fixed bed
configuration was used by Fermoso et al. (2010) to perform coal gasification
experiments using steam and oxygen as agents for gasification. Coal gasification at high
pressure was assessed on the basis of important operating parameters including the
concentration of steam and oxygen and temperature.
Bläsing and Müller (2011) provided details of the effect of pressure on the release of
key chemical species, e.g. sodium, potassium, sulfur, and chlorine during coal
gasification. A total of 19 different coals were investigated in lab-scale gasification
experiments in an electrically heated pressurized furnace at absolute pressures of 2, 4,
and 6 bar in an atmosphere of He/7.5v% O2 at 1325 °C. Mass spectroscopy with
molecular beam was used for analysis of hot gases. Butterman and Castaldi (2011)
characterized the pore structure of char produced during thermogravimetric analysis of
lignin with both N2/H2O and CO2 environments using SEM (Hitachi 4700). The
complete conversion of lignin into volatiles was found CO2 environment during thermal
processing as compared to a N2/H2O gasification medium due to more porous surface
and intricate channel structure by CO2 thermal treatment during pyrolysis. Fushimi et
al. (2011) investigated the effect of hydrogen and tar on the reaction rate of woody
biomass char in steam gasification through varying the concentrations in a rapid-heating
thermo-balance reactor. It was observed that the gasification of biomass char through
steam could be separated into two periods.
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Zhang et al. (2011) investigated the influences of interactions between char and
volatiles on the promotion of char structure during the steam gasification of brown coal
from Victoria. An innovative single-stage fixed-bed/fluidized-bed quartz reactor was
used to conduct the experiments in the absence and presence of interaction between
volatile and char. As per results, features of char structure were significantly influenced
by the interactions between volatile and char. Biomass char was used by Yang et al.
(2012) produced during the pyrolysis of Dunaliella salina feedstock at a temperature of
500°C. The fixed-bed reactor was used to conduct the reaction study between biomass
char and CO2 for investigating the effect of steam and temperature on the yield of CO,
conversion of CO2 and composition of syngas. According to the results, the yield of CO
observed 61.84% and conversion of CO2 was found to be 0.99 mol/(mol of CO2) at
800°C without using catalyst and steam. High conversion of CO2 could be achieved via
high temperature and Steam.
Gao et al. (2013) pyrolyzed the Huolinhe lignite at a fast heating rate in the CO2
atmosphere using fixed bed reactor attached with thermo-balance and investigated the
influence of CO2 on the behavior of pyrolysis. The gas composition, yield of char and
its physical properties including FT-IR spectra, surface structure, element etc. were
analyzed. As per results, the gasification of nascent char with CO2 increase the H
radical generation. Li (2013) provided an overview of the research work conducted on
Volatile-Char interaction so far and examined the importance and mechanisms for the
interaction of char with volatiles during the gasification process of fuels categorized as
low-ranked fuels. Stelzner et al. (2013) aimed to determine the best fuel dilution
configuration for studying the partial oxidation reactions through calculating
experimentally the impacts of various fuel dilutions on exit temperature, kinetics, and
elementary combustion. Moreover numerical and experimental investigations were also
conducted to evaluate the phenomena of thermochemical reactions in the flame and
post-flame zones. Tay et al. (2013) investigated the variations in the reactivity and
structure of char during the gasification process of brown coal from Victoria.
Gasification of brown coal from Loy Yang was carried out in an innovative
fixed/fluidized bed reactor at 800 °C with different gasification atmospheres. TGA was
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used to measure intrinsic kinetics of char reacted with air at low temperatures ranges
from 380 or 400 °C.
3.7.2 Experimental work on Combustion related to Gasification Studies
The combustion reaction is important during the gasification process as it gives the
primary heat for the occurrence of endothermic char reactions with CO2 and Oxygen.
A hefty research has already been conducted on combustion of coal with different
operating conditions. Among those, few are discussed here which are important for
gasification scenario.
Bejarano and Levendis (2008) conducted a fundamental investigation on the single
particle combustion of various ranked coals (like bituminous or lignite) along with
synthetic chars with different particle sizes at varying mole fractions of O2 balanced
either by CO2 or N2 gases. Drop-tube furnace was used in laboratory and the
experiments were conducted in the temperatures range from 1400 to1600 K. O2/N2
environment was found favorable and gave higher combustion reaction rate during
combustion reactions as compared to an O2/CO2 environment with a same mole fraction
of O2. Dhaneswar and Pisupati (2012) investigated the characteristics of the rank of
coal during combustion in the O2/CO2 environment using four suites of coals. Drop-
tube reactor was used to conduct the combustion experiments in oxy-fuel and air
atmospheres. TGA was used to measure the intrinsic kinetic rate parameters.
Combustion characteristics were assessed by Khatami et al. (2012) for single particles
(75-90 μm particle size) of lignite, high-volatile bituminous, sub-bituminous and
sugarcane bagasse in O2/CO2 and O2/N2 environments with 20-100% mole fraction of
oxygen. The experiments were conducted in electrically-heated, transparent, bench-
scale drop-tube furnace at 1400 K temperature. The time history of burnout and
temperature were measured high-resolution, high-speed cinematography whereas
combustion was monitored through optical pyrometry of three colors.
Karlström et al. (2013) examined the oxygen concentration effects on the rate of
oxidation within the temperature range of 1223–1673 K for 5 types of char produced
from anthracite coals. Plug flow reactor (4 meters long) was used in experiments with
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isothermal conditions and fast heating rate as per realistic situation (104-105K/s) for
the determination of burnout profiles. Finally, the multivariable optimization method
was used to determine the kinetic parameter and order of reaction.
3.7.3 Experimental work on Gasification Studies on Thar Lignite
Thar lignite has explored about two decades ago and lots of fundamental studies like
its chemical and physical analysis has carried out by several researchers (Choudry et
al., 2010). But scant literature is available on the kinetic studies of char combustion and
gasification reactions for Thar coal. For instance, Anila S. et al (Sarwar et al., 2011,
Sarwar et al., 2014) studied the kinetics of pyrolysis and combustion of Thar coal using
Thermo-gravimetry. Reactivity of catalytic gasification of Thar lignite chars with steam
at atmospheric pressures was studied by Jaffri and Zhang (Jaffri and Zhang, 2009). But
no study is available for the kinetic modeling of Thar coal with CO2 and steam at high
pressure.
3.8 MODELING AND SIMULATION WORK
The kinetic modeling of reactors and its applications for designing the commercial
gasification reactors are limited today due to the involvement of extreme mathematical
complications in current chemical reactors like plug flow reactors (PFR) or continuous
stirred tank reactors (CSTR). However, the development of sophisticated models is
possible with a deep understanding of chemical and physical processes regarding coal
combustion and gasification along with the availability of powerful computers and
high-performance computing machines (Williams et al., 2002). In this regard,
computational fluid dynamics (CFD) has proved its robust applied benefits for several
emerging technologies of this era. Among those technologies, coal combustion
remained the most important applications which have taken the advanced shape after
the utilization of complex coal combustion kinetics through CFD. Though CFD has
provided an advanced platform in designing of coal combustion devices like furnaces
or combustors, demand has increased for getting quantitative analysis rather than results
with qualitative results (Williams et al., 2002).
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Since the past two decades, the focus is more diverted towards the application of CFD
for the advancement of gasification of coal and other fuels (like biomass and waste).
CFD has provided great benefit in the development of gasification technology like
utilizing CFD computations directly to industrial scale avoiding the scale-up issues
from laboratory data. Some previous work on CFD modeling of different types of
gasification systems has been reviewed here.
Korchevoi et al. (1996) developed mathematical models, algorithms and programs for
the thermochemical conversion of single particles of coal with high-ash at raised
pressures. The installation and operation issues of circulating fluidized bed were
investigated. Fletcher et al. (2000) used the Lagrangian approach in the development of
a detailed CFD model on CFX package for simulating the reaction and flow in a
biomass-fed entrained flow gasifier. Chen et al. (2000) developed a 3D CFD model for
entrained-flow coal gasifiers consisted of conventional combustion sub-models for coal
in pulverized form. Multi Solids Progress Variables (MSPV) method was used as an
extension in coal-gas mixture fraction model to simulate the gasification reactions and
product mixing. The turbulence impacts on the properties of gases were calculated via
PDF model clipped with the function of Gaussian distribution.
Choi et al. (2001a) predicted the process of coal gasification of an entrained-flow
gasifier with a slurry feeding system through numerical computations. The numerical
model for coal gasification was developed by dividing the process of complicated coal
gasification into various simplified steps like evaporation of slurry, volatiles removal
from coal (or devolatilization), turbulent flow carrying homogeneous and
heterogeneous reactions and heat transfer occurrence between both phases. Gas phase
turbulence was embedded via the k-ε turbulence model while the behavior of
homogeneous and heterogeneous reactions was calculated using Random-Trajectory
model. Choi et al. (2001b) numerically simulated the gas flow behavior in an entrained-
flow gasifier for describing the process of coal gasification and investigated the effects
of diameters and angel of gas injecting nozzles, the velocity of incoming gas, extension
in the length of burner and geometry of gasifier. Turbulence was calculated with the
standard k-e turbulence model whereas velocity and pressure were coupled with a
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SIMPLE algorithm. The results depicted that parabolic distribution of flow pattern is
insensitive with respect to variation in selected parameters.
Bockelie et al. (2002) have presented initial results for various entrained-flow coal
gasifiers, illustrating, for example, performance variations when using different
feedstocks. This work was the part of a program to investigate generic improvements
for operation and design of such gasifiers. Vicente et al. (2003) used a numerical model,
based on the Euler-Euler framework, for the simulation of coal gasification process in
an entrained flow gasifier. Separate continuity equations were solved for both
particulate and gas phases with the Eulerian mechanism. Wang et al. (2004) performed
three-dimensional numerical simulations for non-premixed combustion turbulent
mixing processing in the high temperature combustion furnace. The flow, combustion,
heat transfer and NOx turbulent formation were simulated. The distributions of mixture
fraction and its turbulent fluctuation were predicted under three different inlet air
temperature conditions in the combustion furnace. The comparison between simulation
and experimental results are in good agreement.
The performance of entrained flow gasification was evaluated by Wu et al. (2004)
through mathematical calculations for various gasification parameters. The studied
gasification parameters included gasifier species and its ratio to gasifying medium,
gasification temperature and pressure and residence time. As per results O2/H2O and
O2/ CO2 as gasifying medium separately have their own advantages. Gasification
temperature was a key factor on the gasification process and syngas composition, while
the pressure in gasification has significant impact on gasification process but it possess
a small influence on syngas composition near equilibrium. Bockelie et al. (2005)
described a modeling approach using CFD for simulating oxygen-blown, pressurized
entrained flow coal gasifiers. The coal gasification reaction kinetics at high fuel loading
rate, high pressure and slagging walls were calculated through sub-models. The
comparison of results obtained from CFD computations and experimental work by
other research shown satisfied agreement. Though the model was developed on oxygen-
blown conditions but air-blown conditions could also be simulated through developed
model.
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Shi et al. (2005) and (2006) developed a CFD model of an oxygen-blown, coal-slurry,
double-stage entrained-flow gasifier for performing simulations of advanced power
plant. The flow of coal slurry was simulated by discrete phase model (DPM). The user-
defined functions (UDFs) were used to calculate the chemical and physical processes
during coal slurry gasification. The predicted syngas composition was compared with
the model developed in Aspn Plus® platform based on equilibrium reactor assumptions
and found good similarity. 100% char conversion was observed from first stage whereas
86% char conversion was achieved from second stage. Zitney and Guenther (2005)
developed CFD models of two commercial-scale gasifiers for simulating power plant
of advanced nature. The first gasifier was the pressurized, oxygen-blown, coal-slurry
two-stage entrained-flow gasifier, modeled in FLUENT. Two phases were modeled
through Eulerian-Lagrangian framework where solid phase was modeled as discrete
phase using discrete phase model (DPM) and gas phase was treated as continuous
phase. The scale-up of the transport gasifier in advanced power plant at Alabama was
the second gasifier. The CFD multiphase code based on transient Eulerian-Euerian
MFIX model was used to model the complicated processes of gasification and solid-
gas hydrodynamic.
Hla et al. (2006) used FLUENT as CFD platform to develop the relation for
determination of rate of gasification at high pressure and temperature as a function of
various physical and reactor parameters. The “effectiveness facror” was applied in the
development of correlation. Watanabe and Otaka (2006) predicted the performance of
gasification in an entrained flow gasifier and modeled the reactions occur in the process
of coal gasification. The research was mainly aimed to develop a technique based on
numerical simulation for evaluating the design performance and optimization of
gasification system, and to adopt the best model validation method. Hla et al. (2007)
developed a model for coal gasification conversion that could be used as predictive tool
for assessing the gasifier performance with various types of feedstock compositions and
operating conditions. Intrinsic data for chemical kinetics for char-gas reactions were
measured at lower pressure and temperature levels as compared to realistic conditions
entrained flow combustors/gasifiers.
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A novel concept of pressurized gasifier based on fluidized bed configuration known as
Power High-Temperature Winkler gasifier (PHTW gasifier) was modeled by Gräbner
et al. (2007). A 4800 t/day with 1000 MW power capacity gasifier in a power plant with
oxygen/steam blown and fueled by lignite coal was simulated numerically at 33 bar
pressure. Wu et al. (2007b) used FLUENT software for simulating the Texaco coal
gasifier with a robust model including various sub-models. PDF model was used to
define the chemical process by defining coal-slurry as fuel stream and oxygen as an
oxidizer stream. Turbulence in flow was calculated by Realizable k-ε model whereas
radiative heat transfer was calculated through the P-1 model. User-defined functions
were used to incorporate the heterogeneous reactions. Dai et al. (2008) developed a
novel type of technology for pressurized entrained flow gasifier with the pulverized
coal feeding system. The special feature of the gasifier was the installation of symmetric
four nozzles attached on the upper portion of the gasifier. The model of gasifier was
based on the principle of Gibbs energy minimization technique. The results obtained
from simulations were compared with pilot trial data and found good agreement.
Entrained flow gasifier could be designed, assessed and improved through this model.
The oxidization zone of the double-stage gasifier with downdraft mechanism was
investigated by Gerun et al. (2008) through the CFD model with 2D axisymmetric
dimensions. Sharma (2008) presented kinetic and thermodynamic modeling for
reduction reactions of char in a biomass-fed, downdraft gasifier. Gas composition along
with its heating value, un-reacted char and fuel conversion efficiency, the temperature
of leaving gas and output power of gasifier were predicted by coupling of energy and
mass balances with parameters of kinetic rate and equilibrium relations. Validation was
carried out for the model results by comparing those with experimental data. The effects
of temperature of reduction zone reactions and length of char bed were investigated on
the sensitive parameters of gasification. Tinaut et al. (2008) presented a stationary one-
dimensional model for downdraft fixed-bed biomass gasifier. Sub-processes of biomass
gasification like drying of biomass, devolatization, volatile and char oxidation, char
gasification reactions and reforming of hydrocarbon were incorporated in the model.
Experimental validations were conducted for Fuel/air equivalence ratios, combustion
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rate of biomass and temperature field predictions and found good agreement between
predicted and experimental results.
Silaen and Wang (2008) used FLUENT software to perform numerical simulations for
the oxygen-blown generic entrained-flow gasifier. Navier-stokes equations along with
transport model equations for seven species were solved. Finite rate chemistry was used
to solve three heterogeneous reactions whereas the minimum value between eddy-
breakup combustion model and the finite rate was used as a solution of two
homogenous reactions. Xiu et al. (2008) experimentally validated the fast pyrolysis
kinetic parameters of corn stalk for the prediction of the behavior of its gasification in
the horizontal entrained-flow reactor (HER). Experiments were conducted with a
uniform feeding rate of 0.3 Kg/h, and in the temperature range from 792-1031 K.
Particle image velocimetry (PIV) was used to validate the CFD modeling results and
then prediction of corn stalk pyrolysis in HER was carried out using previous fast
pyrolysis kinetic data for corn stalk. The comparison of pyrolysis results obtained from
model and experiments shown good similarity and found capable to predict the
laboratory-scale, pilot-scale or full commercial scale units. Ajilkumar et al. (2009)
performed numerical simulations in FLUENT software to evaluate the performance of
an air-blown entrained flow gasifier at laboratory scale. An Eulerian-Lagrangian
modeling approach was used for simulating gas and particle phases. The gas-phase
turbulence was calculated using k–ε model along with stochastic tracking model was
used to predict the particle dispersion in the gas phase. Devolatization, combustion of
char and volatiles and gasification reactions were incorporated in the model developed
for coal gasification. The performance of gasification was evaluated on varying air
ratios along with varying inlet steam and air temperatures. Steam/air pre-heating found
directly proportional relation to the gasifier inside temperature and thus its increase
raise the rate of various reactions to occur in the gasifier.
Barranco et al. (2009) formulated a novel kinetic model of char combustion for coal in
pulverized form. The rate of nth–order global chemical reaction was considered as a
function of the mass of fuel and intrinsic reactivity of coal. Rayleigh method of
dimensional analysis was used to develop the rate of reaction equation for the
combustion of char. The reactivity of char was found dependent on various parameters
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related with coal like the composition of minerals in coal, apparent density and surface
area of char, activation energy, the temperature of reaction and time taken for
devolatilization. The empirical validation has described by Rojas et al. (2012) in its
part-2 series. Chui et al. (2009a) developed a CFD model of a pressurized pilot-scale
entrained flow coal gasification system. The simulations were carried out keeping the
aim for advancement in the coal gasification technology at commercial scale. The
performance of gasifier and IGCC system at various operating conditions were
investigated by Gnanapragasam et al. (2009) through gasifier’s sensitivity analysis.
Variations in the oxygen/coal and steam/coal ratios, thermal conditions during
operation of the IGCC system, and type of fuel were investigated during the sensitivity
analysis.
Jaojaruek and Kumar (2009) established an important model specifically for the
pyrolysis zone of a downdraft gasifier based on finite computation method. Implicit
finite difference method was used to solve the mass and energy conserving equations.
Heat transfer considered convection, conduction, and the influence of solid radiation
components. Chemical kinetics concept was also adopted to simultaneously solve the
temperature profile and feedstock consumption rate on the pyrolysis zone.
Experimental and numerical investigations were carried out for Opposed Multi-Burner
(OMB) gasifier having an internal diameter of 1 m at high pressure and temperature for
studying the flow field of the particle-gas regime by Ni et al. (2009). The behavior of
particle-gas flow was simulated using an Eulerian-Lagrangian framework based 3-D
numerical model. The dispersion of ash/slag particles due to their collision was
predicted. Experimental data was used to validate the simulated results and found good
similarity between both. According to the results, the section height above to burner
increased the residence time of material and the flux of deposition raised with the
velocity of inlet streams.
Papadikis et al. (2009) developed the CFD model based on the Eulerian-Lagrangian
framework for fast biomass pyrolysis in an entrained flow reactor. The discrete particle
of biomass is thermally degraded into biomass char and releasing tar and gases as per
formation of the model. Semi-global model with two stages was used to define the
chemical reactions. The composition of gases produced during the pyrolysis process in
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the radial direction was predicted along with their influence on the properties of
particles. Simone et al. (2009) coupled experimental investigations with CFD modeling
for extraction of important global chemical kinetic parameters that could be used in the
biomass applications like co-combustion and gasification. The experiments were
conducted in an advanced lab-scale drop tube furnace at varying heating rates, particle
size within the temperature range of 400 –800°C. CFD was used as a tool for diagnosis
and prediction using extracted global kinetic parameters experimentally. Wang et al.
(2009) developed a 3D comprehensive CFD model for simulating fluidized bed gasifier
for gasification of coal. Homogeneous/heterogeneous chemical reactions along with
solid-gas flow were considered in the development of the model.
Numerical simulations were carried out by Chyou et al. (2010) in FLUENT software
for the process of coal gasification in an oxygen-blown cross-type double-stage gasifier
(like E-Gas) for getting more insight regarding gasification mechanism in such type of
gasifiers. Mixing of the species at the molecular level was assumed. Species transport
equations with an eddy-breakup model for reaction were solved along with 3-D Navier-
Stokes governing equations. The effects of the concentration of coal-to-water ratio in
coal-slurry and O2/coal ratio on the performance of gasification were investigated.
Dong et al. (2010) developed a CFD model of PC boiler of 600 MWe, fueled by co-
firing of coal with syngas produced from gasification of biomass. Emun et al. (2010)
developed a model in Aspen Plust® software for IGCC containing a Texaco gasifier
along with its related process units. The model was used to enhance the efficiency of
IGGC process along with the performance of environmental constraints through
utilizing Pinch analysis techniques, Process integration and parametric studies. The
varying parameters were considered in the air separation unit, coal preparation, cleaning
of gas cleaning, gas turbine, recovery of sulfur, steam turbine and recovery of heat.
Gerber et al. (2010) discussed the approach for multiphase modeling through Eulerian
framework for wood gasification occurs in fluidized bed taking. The gasification model
was based on sub-processes like pyrolysis of wood, gasification of char and
homogeneous reactions occurring gas phase. Montagnaro and Salatino (2010) analyzed
the conversion and trajectory of the carbon particles in the slagging regime of
gasification with entrained-flow configuration. The concept for the segregation
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framework of carbon particle was used to develop the simplified one-dimensional
model of a gasifier with entrained-flow configuration. The influence of various patterns
for segregation of carbon particle on the extent and conversion of carbon particle.
Nguyen et al. (2010a) (2010b) used FLUENT for the development of a three
dimensional CFD model for predicting coal gasification performance in an entrained-
flow gasifier. The model consisting sub-models of devolatization (pyrolysis),
gasification of char and homogeneous reactions in the gas phase. The particulate phase
was modeled using discrete phase model (DPM) whereas the combustion and
gasification of carbon/char were calculated using Multiple Surfaces Reaction (MSR)
model. The parameters studied were temperature, flow field, and distribution of species
composition inside the gasifier. The results from published literature were used for
comparing with the CFD model results. Sadhukhan et al. (2010) analyzed the
combustion of a high-ash single-particle coal char at high pressure. The combustion of
coal char with high ash was characterized by a complete dynamic shrinking core model
including simplified kinetics. The kinetics of the reaction, mass and heat transfer
phenomena along with details of intra-particle forces were included in the model. The
transient governing differential equations of the model were solved through the finite
volume method (FVM). Time taken for complete conversion of char particle and
weight-loss profile could be predicted from the combustion model of char at varying
concentration of oxygen and temperature.
Silaen and Wang (2010) numerically simulated the generic oxygen-blown entrained-
flow gasifier. The Navier-Stokes equations along with dynamics of particles were
solved by Eulerian-Lagrangian technique. Equations for species transport along with
homogeneous and heterogeneous reactions were incorporated in the model. The
heterogeneous reactions were solved by finite rates method. Slezak et al. (2010)
numerically simulated commercial-scale single-stage down flow and double-stage
upward flow entrained-flow gasifiers for investigations the influence of the size of
particle and density on the overall performance of gasifier. Zhang et al. (2010) carried
gasification of carbon and coal char by CO2 in the presence of potassium and calcium
catalysts and usual theoretical models for the kinetics of heterogeneous reactions where
reviewed. The conversion based reactivities calculated from random pore model (RPM)
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and those extracted from experiments were compared and found significant deviations
at high or low levels of conversion as theoretical predictions.
Álvarez et al. (2011) investigated the combustion of coal contain different volatiles
composition, at varying atmospheres of O2/CO2 in a reactor equipped with entrained
flow configurations. The predictions of rates of drying, profiles of temperature, time
for complete conversion and major species composition like CO, CO2, and O2 were
carried out and compared with literature, found a satisfactory resemblance. Chejne et
al. (2011) developed a mathematical model for coal gasification with the capability to
predict conversion, temperature, composition of produced gas, velocity, distribution of
solid particle size and other fluid-dynamic related parameters in pressurized fluidized
bed gasifier. Model validation was done from published experimental results. Cornejo
and Farías (2011) developed a 3D-CFD model of fluidized bed coal gasifier using
FLUENT, consisting of moisture removal (drying), pyrolysis (devolatilization),
combustion and gasification sub-processes. The euler-lagrangian framework was used
to describe the gas phase and particulate phase. Homogenous and heterogeneous
reactions were also incorporated.
Gordillo and Belghit (2011) developed a numerical model of biomass-fed solar
downdraft gasifier utilized steam kinetics. The model possesses the capability for
predicting steady state and dynamic profiles of species concentration and temperature,
based on the balances of heat and mass. The radiative energy transfer into the bed was
calculated by the Rosseland equation. The varying variable was char reactivity factor
(CFR). The effects of dynamic heat transfer in the bed along with the velocity of steam
were investigated. The comparison between model predictions and experimental results
from literature confirmed the validity of the model. Kempf et al. (2011) illustrated
stability problems, the need for consistent modeling in premixed and non-premixed
combustion, and showed how RANS models that would have frequently been applied
in an LES context can lead to strong conceptual errors. The application of the error
landscape approach to a complex non-premixed flame was outlined and investigated
several error indicators that could have been developed for situations where no
experimental reference data was available.
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The generation of electricity and production of fuel were thermally improved by Kiso
and Matsuo (2011) through gasification with dry feeding of coal. They proposed a new
process in which water was injected at the outlet of the gasifier and was vaporized to
enhance the extent of the shift reaction. This process utilized the high temperature of
the syngas, which was sufficient for the shift reaction to occur without a catalyst. A
model was developed that incorporated the shift reaction velocity to evaluate the
proposed process. Nayak and Mewada (2011) simulated the coal gasification process
through Aspen Plus for presenting its overview. The influence of flow rate of oxygen
and steam-to-coal ratio on the composition of produced gas was investigated. Seo et al.
(2011) investigated computationally the operating parameters like transporting gas/coal
ratio, O/C ratio, reaction pressure and temperature for the gasification of Adaro coal
and the results were compared with experiments. 82.19% cold gas efficiency was
estimated from the optimized parametric conditions. Unar (2011) performed the CFD
simulations on downdraft gasifier with entrained-flow configuration for evaluating the
gasification process of Thar lignite. The FLUENT software was used for the
simulations in which effects of coal compositions and form of coal (dry and slurry)
were investigated. Navier-Stokes equations were solved for nine species and
homogeneous reactions were treated with eddy-dissipation combustion model.
Xu et al. (2011a) and (2011b) conducted the experiments for steam gasification of
biomass and coal and observed the fundamental difference between the gasification of
those two fuels based on the differences of microstructures in the fuels. A char
gasification model was developed on the basis of reaction kinetics and steam and gas
transportation mechanisms. Álvarez et al. (2012) investigated the emissions of NO from
gasification of high volatile bituminous and anthracite coals in an entrained flow reactor
with oxygen and air conditions through experiments and numerical simulations. A CFD
model was used with three assumptions (1) all of the nitrogen in the fuel had been
converted to HCN (2) all of the nitrogen in the volatile would evolve as HCN, and (3)
a conversion factor used to calculate the No formed by char–N reaction. Botero et al.
(2012) and (2013). studied numerically (using 1-D RO model) the influence of slurry
feeding of CO2 on the kinetics of Illinois coal gasification and eventually on conversion
of carbon along with consumption of oxygen in a single-stage gasifier with the
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entrained-flow configuration at high pressure. According to results, CO2 slurry feeding
system has great influence on the gasification of coal and produces almost double CO
as compared to conventional oxidizing agents. On further investigations, it was
observed that the slurry of coal with water decreases the rate of gasification up to 60%.
Chen et al. (2012) predicted the gasification of coal in an entrained flow gasifier using
a numerical method. The effects of injection patterns along with the ratio of steam-to-
coal were investigated on the production of syngas and composition of H2 in the syngas.
The developed numerical method predicted the composition of syngas with satisfaction.
Lee et al. (2012) investigated different angles for oxygen injectors and various types of
burners for gasification in a lab-scale entrained flow gasifier. The experiments were
conducted to determine the impacts of the amount of oxygen, coal burner’s angel and
location of oxygen in the gasifier on the composition of syngas and reactor temperature.
The optimization of operating parameters was carried out using simulations in Aspen
Plus software by inserting the data obtained from experiments. The impinging zone for
industrial scaled Opposed Multi-Burner (OMB) gasifier was modeled in 3D by Li et al.
(2012). The eulerian-lagrangian framework was used to treat the particulate and gas
phases and the realizable k-ε model was used to calculate the turbulence in the gas
phase. Assuming particles as the hard spheres, the modified Nanbu method and
Simulation Monte Carlo (DSMC) method were used to calculate the collisions of
particles. The laboratory experiments conducted on an equipment with two jets on
opposite side were used to validate the model.
Meng et al. (2012) developed a 3D model of thermogravimetric (TG) furnace using
COMSOL Multiphysics Software for a better understanding of velocity and
temperature profiles inside the furnace. The model results were compared with
experiments and found a good agreement. Monaghan and Ghoniem (2012a) described
the development of a dynamic reduced order models (ROM) model for entrained flow
gasifier in the first part while second part presented its validation for four designs of
entrained flow gasifiers and conducted the sensitivity analysis (Monaghan and
Ghoniem, 2012b). Murgia et al. (2012) developed a CFD model of updraft air-blown
coal Wellman-Galusha gasifier and simulated gasification process through the
developed model. The Euler-Euler framework was used to simulate the multiphase
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regime due to the presence of a high volume fraction of solid phase. No-continuous
feeding of coal was considered along with the extraction of ash and aimed to
characterize the behavior of process based on time and space.
Abani and Ghoniem (2013) investigated reacting multi-phase flow in a lab-scaled
entrained flow gasifier of axial-flow type fed with coal using Reynolds-average Navier
Stokes (RANS) and large-eddy simulations (LES) models. The multiphase scenario
was modeled using the Euler-Lagrangian Framework. RANS and LES models were
compared on the basis of performance and found LES better for capturing the structure
of unsteady flow within the gasifier. Chen et al. (2013b) established a 3D CFD model
of slag for conducting the study of slag/ash behavior like burning of char at the wall,
deposition of ash or char, the flow of molten slag, the formation of a layer of solid slag
on the walls of the reactor. The pilot-scale combustion unit for coal slagging was
simulated using the developed model. Chen et al. (2013c) evaluated numerically and
compared the gasification potential of raw and terrified bamboo along with bituminous
coal containing high volatile in an oxygen-blown entrained-flow gasifier. More than
90% carbon conversion was achieved with all the cases. Franchetti et al. (2013) applied
Large Eddy Simulation (LES) to study the jet flame produced by pulverized coal. The
LES framework under investigations consisted a set of models like combustion of coal,
transport of particles through the Lagrangian approach and heat transfer by radiations.
Non-reactive and reactive experimental results were used to validate the model and
found satisfactory results.
Holkar and Hebbal (2013) used FLUENT software to compare the two models for
radiative heat transfer, i.e., P1 Model and Discrete ordinates (DO) Model. The furnace
of boiler operated by pulverized coal and over-fire air (OFA) ports were evaluated for
fitting temperature profile. The accuracy of DO and P1 models was checked by
comparing numerical results with experimental investigations. Janajreh and Al Shrah
(2013) developed a CFD model for gasification of wood based on their experimental
setup of an air blown, lab-scale downdraft gasifier. The particle phase was model
through Lagrangian framework whereas gas phase turbulence was calculated through
the k-ε model. The computed composition of species and temperature profile based on
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equilibrium and zero-dimension approach was compared with experimental results and
found a reasonable match.
Janajreh et al. (2013) developed conventional gasification method with air and plasma
gasification method for gasifying numerous feedstock like coal, waste tire, plywood,
oil shale, pine needles, algae, municipal solid waste (MSW) and untreated/treated
wood. Both methods were founded on the Gibbs energy minimization approach. Steam
or air was used in plasma gasification whereas only air was used in conventional
gasification as an oxidizing agent. The efficiency of process and composition of syngas
were the performance evaluation parameters. As per results, the gasification of waste
could be possible through plasma gasification. Kumar and Ghoniem (2013)
implemented a validated model of entrained flow gasifier for investigating the influence
of particle size on the overall performance of GE and MHI design of gasifiers. The
limitations from either boundary layer or intrinsic kinetic were observed for carbon
conversion due to variation in particle size. Lu and Wang (2013a) concentrated their
focus on rates of water gas shift (WGS) reaction in the presence and absence of
catalysts. Initially, three published WGS reaction rates were modified to reduce the
difference in prediction as compared to experimental values. Later in their second part
of study (Lu and Wang, 2013b) simulations were performed for Japanese research
gasifier (CRIEPI) with published rates of WGS reactions and modified rates as per
findings of part 1. 3D Navier-Stokes equations were solved in the developed CFD
model along with species transport model equations for selected species. The
devolatization of fuel was described by Chemical Percolation Devolatilization (CPD)
model.
Luan et al. (2013) developed a 3D CFD model of oxygen blown, pressurized E-Gas
entrained flow gasifier. 3D Navier-Stokes equations were solved with Eulerian-
Lagrangian Framework and chemical reactions were calculated through Finite-
Rate/Eddy-Dissipation Model. The investigations proved that the chemical reactions
are affected through finite-rate chemistry. The performance of gasifier was successfully
evaluated through CFD simulations. CFD simulations were carried out by Singer et al.
(2013) for the combustion of lignite coal in an oxyfuel pilot-scale test facility with a
feed containing 29% oxygen for investigating concerned regions of char burning flame.
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Trajectories of coal particles were examined to calculate typical transient boundary
conditions at the time of exposure of char particles along the traveling in the furnace.
Char conversion then evaluated based on the local environment using those accurate
boundary conditions in the model of single particle combustion. Stöllinger et al. (2013)
simulated the combustion process of pulverized coal in a partially industrial scale
furnace using established model probability density function (PDF). The results
obtained from simulations were found sensitive to the selected model due to the transfer
of mass from solid to gas phase through combustion of char and devolatilization.
A framework of the model was presented by Tremel and Spliethoff (2013c) that
described the kinetics for reactions occurring during gasification in an entrained flow
gasifier. Numerous sub-models were incorporated in the model like devolatilization of
fuel, gasification of char, evolution of surface, pore diffusion, cooling of char, diffusion
through boundary layer, and particle density and size variations. Experimental
verifications were made from the date of devolatization of fuel and kinetics of lignite
gasification. Xiangdong et al. (2013a) developed an equilibrium model in Aspen Plus
software for gasification of coal in the Texaco type coal gasifiers to calculate the
product gas composition, conversion extent of carbon and temperature of the gasifier.
The gasification process was divided into the three stages in the model: (1) combustion
and pyrolysis stage, (2) Heterogeneous reactions (char-gas) stage and (2) Homogeneous
reactions (gas phase) stage. The results obtained from simulations were in good
agreement from experimental values obtained from the literature. A fluidized bed
gasifier was numerically simulated by Xie et al. (2013) through a comprehensive 3D
model. The multiphase particle-in-cell model (MP-PIC) was used in the model where
fluid phase was described by Eulerian method and particle phase was calculated
through discrete particle method. Heat and mass transfer, the flow of dense particulate
matter, chemistry for heterogeneous and homogeneous within the mixture of fluid were
considered. The particle distribution function (PDF) transport equation was solved to
calculate the particle dynamics. The flow pattern formation, particle species profile, and
composition of the gas, reaction rates distributions and carbon consumption were
examined at various conditions of operations. The predicted composition of product gas
was compared with experimental data and found in good agreement.
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Vascellari et al. (2013a) developed a CFD model of entrained flow gasifier using sub-
models of advanced nature for conversion of coal and results were compared with data
extracted from experiments. The investigations were mainly focused to model pyrolysis
process more accurately. A technique based on iterative solution was proposed and
validated to reduce the gap between calculations from various pyrolysis models like
FG-DVC, FLASHCHAIN, CPD and single-or multisite empirical kinetic models
available in CFD. The simplified Single Nth-Order Reaction (SNOR) kinetic model
was calibrated through CBK/G and CBK/E models and found appropriate for CFD
calculations (Vascellari et al., 2014). Unar et al. (2014) developed an Euler-Lagrangian
framework based 3D CFD model for two-stage entrained flow dry feed coal gasifier
with multi opposite burners (MOB) equipped with tangential and impinging nozzles.
Investigations were made through numerous numerical simulations for studying the
effects of distributions for fuel and oxidant between the two gasifier’s stages. Finite
rate/eddy dissipation model was used to define heterogeneous and homogeneous
chemical reactions. Published kinetic data was used in the reaction. Turbulence was
incorporated thorough realizable k–ε turbulent model. Reaction mechanisms were
created with different reaction schemes and validated through published experimental
results.
Bi et al. (2015) developed a heat transfer with slag flow model coupled with a capturing
of particles via sub-model in a 3-D gasifier model for describing the characteristics of
slag and process of gasification in an entrained flow gasifier. Halama and Spliethoff
(2015) presented a 3D-CFD model of a pressurized entrained flow gasifier for
gasification of Rhenish lignite. The model was validated against experimental data
obtained from a pilot-scale unit. A good correlation was obtained between simulations
and experimental data. Labbafan and Ghassemi (2016), simulated numerically a three-
dimensional oxygen-blown, two-stage E-Gas entrained flow gasifier. A coal containing
high ash was characterized for gasification at high pressure.
Wang et al. (2017) numerically simulated a 2D double-stage entrained flow gasifier
with dry feed. The performance of gasifier was investigated at various oxygen to carbon
ratios. Moreover, the different reaction schemes were validated from date published in
the literature. As per results, The molar composition of syngas particularly CO and H2
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increased and optimized at 0.8 O/C ratio whereas temperature gradually decreases from
2nd injection level due to the water-shift reaction. Gao et al. (2018) developed Euler-
Lagrangian based biomass gasification model in an Entrained flow gasifier. The
submodels of char-gas reactions were inserted with intrinsic biomass kinetic data for
calculating the rate of heterogeneous reactions. The simulations were carried out at
different temperatures and equivalence ratios (ERs).
3.9 RESEARCH ON FLAMELESS COMBUSTION/GASIFICATION
Excess enthalpy combustion, known as FLAMELESS COMBUSTION, is, technology
has popularized and developed rapidly since the 1990s (Tang et al., 2007, Runge, 1993).
Since then various countries have developed their own technologies characterized, few
examples described, like, HiTAC (Japan), MILD-Moderate and Intensive Low
Oxidation Dilution (Italy), FLOX-Flameless Oxidation (Germany), and LNI-Low NOx
Injection (America) (Qi et al., 2003).
Firstly, Weinberg (1971) brought an idea of excess enthalpy combustion in 1971. In
this technique, the heat of flue gases is recycled, which was originally used to preheat
reactants through the heat exchanger, regenerative heater or others. Many researchers
have numerically analyzed MILD combustion/Flameless combustion, both at industrial
scale (Galletti et al., 2007, Parente et al., 2011, Mancini et al., 2007) and lab-scale
(Galletti et al., 2009, Coelho and Peters, 2001, Christo and Dally, 2005, Christo and
Dally, 2004, Kim et al., 2005, Frassoldati et al., 2010, Ihme and See, 2011, Mardani
and Tabejamaat, 2010, Mardani et al., 2010) furnaces. To define and develop suited
CFD sub-models, some effective efforts were made for this kind of combustion regime
using a huge variety of sub-models in CFD. Few examples are given, like, schemes of
global kinetics ranging from (Galletti et al., 2007) and detailed and reduced (Galletti et
al., 2009, Christo and Dally, 2005, Frassoldati et al., 2010, Mardani and Tabejamaat,
2010, Mardani et al., 2010) that were employed. A lot of combustion models were taken
into an account similarly and were investigated the interaction of the turbulence
chemistry. Some researchers also worked on Flameless gasification system utilizing the
excess enthalpy combustion (Flameless combustion) mechanism (Kobayashi et al.,
1999, Yashikawa, 1999, Tingyu et al., 1998, Tingyu et al., 1999a, Tingyu and Yang,
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1999, Tingyu et al., 1999b, Li and Jiazhou, 2008). A selected literature review is
presented here.
Jiang et al. (2000) successfully developed the honeycomb ceramic regenerator that was
cheap and can recover waste heat effectively, which could reduce the production cost
of high -temperature air, promotes the development of new technology. Principle and
critical technology of high-temperature air, high-temperature air flameless combustion,
and high-temperature air gasification were analyzed to create conditions for developing
high-temperature air resources and applying new type combustion and gasification
technology. Golovitchev and Jarnicki (2001) provided the understandings of the
processes chemically and physical fundamentally that occurred in high temperature
during combustion in air or exhaust gases with low usage of oxygen content using
approach chemistry in the detailed. O'neal (2004) aimed to develop in a project was
continuous advanced MILD gasification products and processes for processes
upgrading that will be eventually capable of commercialization the system. The
program core task was to a bench-scale investigated data to generate the design of mild
gasification reactor for a larger scale and study of a bench-scale of upgrading of char
products to add the value.
Gupta (2006) demonstrated the benefits and applications of flameless oxidation. It was
described that the flameless oxidation of fuels or high-temperature combustion is new
and innovative means for the conversion of chemical energy to the thermal energy of
fuels. Flameless or colorless oxidation of fuels could be obtained using high-
temperature combustion air at low oxygen concentration (with heat and gas
recirculation) incorporated in the basic design principles of High-Temperature Air
Combustion (HiTAC) technology. Gasification using HiTAC offers clean
transformation of solid and liquid fuels into clean syngas that can be used for cleaner
combustion. Murer et al. (2006) characterized the natural gas experimentally with
flameless combustion and with FLUENT by CFD modeling. And in a laboratory scale
performed the measurements furnace are used as boundary conditions and validation
data for the various models tested. Turbulence and combustion were being modeled
respectively the Eddy-Dissipation model and with k-ε standard model, and model of
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Finite Rate was combined. And according to the chamber temperature, the combustion
of furnace presents 2 regimes, numerically and experimentally.
Onwudili and Williams (2008) discussed that in water about major two organic
reactions under high temperatures and high pressures, hydrothermal oxidation, and
gasification. These methods possess great potential towards the thermochemical
treatment of moisture content present highly in wastes of organic with sources and
varieties differently. The application of these hydrothermal processes to a range of
model compounds and biodegradable waste samples were described to illustrate the
potential of these novel organic waste treatment technologies i.e. flameless oxidation.
Water-cooled sampling tube and gas purification analytic system were used by Guo et
al. (2008) in a hot-state experimental study of the gas concentration distribution of a
nozzle plane in a gasification furnace. Through an image processing, the flame image
was divided into three portions along the direction of a gas sampling tube: namely, a
flame impinging zone, a transition zone, and a flameless zone. Test results showed that
the gas constituents of the flame impinging plane in the gasification furnace are closely
related with the flame shape, and the gas concentration in the transition zone has the
greatest changes.
Wei et al. (2008) summarized the development of coal MILD gasification. The catalyst
type, the mechanism and reaction kinetics of catalytic gasification were discussed. The
effect of the factors on gasification reactivity and the product were analyzed, such as
coal structure, catalyst, gasification temperature, and atmosphere. Numerical
simulations for different working condition of MILD combustion were executed by
(Calchetti et al., 2009) using solid fuel and slurry form in FLUENT. The combustion
was modeled with eddy dissipation conceptual model and adopted the P1 radiation
model for radiative heat transfer. The realizable k-ε model was used to calculate
turbulence in the flow. A special and effective mixing nozzle was developed by
Masashi et al. (2009) for creating a homogeneous mixture of air and fuel by mixing
rapidly, thus this rapid-mixing nozzle was thereafter applied to a burner of Bunsen-type
to observe characteristics of combustion of the rapid-mixture. Finally, in conclusion,
the rapid-mixture combustion of exhibited the structure of same flame and
characteristics of combustion prepared perfectly flame was premixed, for such purpose
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rapid-mixing nozzle the mixing time was short extremely such as a few milliseconds.
In this paper, the rapid-mixing nozzle was used for creating premixed preheated flames
and also there mixing time was less than that of the fuel, the delay ignition time.
The IFRF experiments were analyzed by Schaffel et al. (2009) used the mathematical
model that is CFD-based. Both models such as one is the Chemical-Percolation-
Devolatilization (CPD) model and another one is model with the char combustion
intrinsic reactivity were adapted for combusted of Guasare coal. As well as, the flow-
field of the temperature and the fields of the oxygen were predicted accurately by the
model that is CFD-based. The temperature predicted and composition of the gas fields
was uniformed and to demonstrate which slowdown combustion occurred volume of
the whole furnace. And the predictions on CFD-based was highlighted the reduction
potential of NOx combustion of MILD according to the following mechanism in the
system. A numerical study was presented by Stadler et al. (2009b) on the importance
of char reaction with CO2 and H2O formation on pollutant in the flameless combustion.
And the models numerically designed that was being used against experimental data
for validation. The wall temperature by varying and of air ratio the excess of the burner
and investigated other various cases in which considered the impact of gasification and
was assessed on the NO formation prediction. Conclusively, the investigated within
ranges given parameters the char fraction increased up to 35% when that was being
gasified.
Szego et al. (2009) described characteristics its stability and performance of a parallel
jet MILD, Intense or Moderate Low Oxygen Dilution, the system of combustion of
burner in a laboratory-scale furnace and on the same wall the exhaust and reactants
ports all were mounted. The measurement of thermal field for cases was presented
without and with combustion air preheat, additionally measurements for a range of
equivalence ratio to global emission and temperature, extraction of heat, fuel dilution
and preheating of air levels. Caprariis et al. (2010) analyzed the coal devolatilization
process that took place in an oxy-combustion reactor working under pressure and in
flameless condition. The work was focused on the coal devolatilization study and
research, and the combustion process was entirely influenced. The devolatilization
process analysis was performed FG-DVC (Functional Group-Depolymerization
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Vaporization Cross-Linking model) know as dedicated scientific software and
experimental tests supported with TGA. And the models like Kinetic models were
developed of the pyrolysis and a CFD code (FLUENT) was implemented into that and
analyzed the behavior of the burner. Ecological and technical aspects of High-
Temperature Air Combustion was implemented to power station boilers fired with
pulverized coal was considered by Schaffel-Mancini et al. (2010). Numerical
simulations CFD-based were performed for determination of its dimensions and the
boiler shape, both the distance between locations block of the burner and burners was
optimized. Finally, that HTAC technology was concluded that for pulverized coal-fired
boilers it could be a realizable, effective, clean and efficient technology.
Tang et al. (2010a) (2010b) (2010d) analyzed a novel dry pulverized coal gasifier based
on of formation mechanism, realization conditions, and implementation was
approached to the new technology of flameless oxidation, which was applicable to
double-high coal. A test system for the pressurized coal gasifier was described, and the
study processes and results of the test were given. The feasibility of flameless
gasification inside the gasifier was validated from the gasification reaction images,
which accord with the characteristic of flameless oxidation. Tang et al. (2010c) studied
visually gasification flow characteristics of a new gasifier put forward which was based
on technology flameless oxidation. Numerical simulation 3D for high ash was
conducted of the gasifier melting point of the pulverized coal. And results shown in the
gasifier, that the temperature, inner velocity, and species concentrations, it was able to
investigate that in the gasifier temperature field became much possible an increase in
average temperature, the intensity of the gasification would be accordingly boosted up,
based on flameless oxidation spatial gasification reactions in the gasifier able to
achieve.
Numerical study results were presented by Danon et al. (2011) in a furnace for four
type of burner configurations equipped with three pairs in burners of flameless
combustion firing Dutch natural gas. And results of the simulations were also validated
in this research work against experimental work published previously. The CFD was
used through utilizing its commercial code Fluent 6.30. (EDC), Eddy Dissipation
Concept, for turbulence model was used for interaction of the chemistry in combination
88
with the realizable k-ε model. And also due to relatively of the low Reynolds numbers
it was found that in the flow of cooling air in the annulus of cooling tubes and it was
predicted that rates of extraction of heat were improved of these cooling tubes as
laminar by treating the cooling tubes flow. Khoshhal et al. (2011) numerically
investigated fuel temperature influence on the formation of NOx. Emission of NOx was
developed CFD modeling an experimentally equipped furnace with high-temperature
system combustion of air (HiTAC). And results were shown better when compared
experimental values with the predicted results, in which the suitability was suggested
of combustion of adoption and formation of NOx models to predict the flow
characteristics, heat transfer, combustion, and emissions of NOx in the chamber of
HiTAC. Li et al. (2011) worked for the research since many years and development of
combustion of intense or moderate low oxygen dilution (MILD). The combustion of
MILD and its requirements for establishing were observed than conventional systems
was more relaxed. Also, it was revealed that of different type of combustion, i.e.,
partially premixed, fully premixed, and non-premixed by firing various fuels could be
achieved (i.e., liquid fuels, solid fuels, and gaseous fuels). The significant expansion of
MILD combustion could be predicted by analyzing its commercial applications.
Investigations were made systematic numerically by Mi et al. (2011) for the influence
of studying conditions of the initial injection of reactants on characteristics of flame
from a parallel multi-jet burner in a laboratory-scale furnace. Particularly,
characteristics varying from the visible flame to invisible combustion of Intense or
Moderate Low-oxygen Dilution (MILD) was explored. Parameters were examined like
different initial separation of fuel and air streams (S), fuel nozzle diameter (Df), air
nozzle diameter (Da), and air preheat temperature (Ta). Qualitatively the present
simulations agree with experimental measurements work. A modeling study was
reported of Reynolds averaged Navier-Stokes (RANS) by Wang et al. (2011) that was
investigated the effects circulating of heat extracted (Qout/Qin) through the furnace wall
and the recirculation rate of internal exhaust gas on the premixed combustion
performance. And suggested the results that the ratio Qout/Qin and recirculation rate of
the exhaust gas played remarkable roles in the combustion of premixed flameless while
establishing the system. Aminian et al. (2012) numerically studied the burner of jet-in-
89
hot co-flow (JHC) emulating MILD conditions of combustion in the 2D axisymmetric
domain by solving the Reynolds Averaged Navier-Stokes equations. And for the
turbulence-chemistry interaction treatment, the Eddy Dissipation Concept (EDC) was
used. And systematic methodology to analyze sources all possible of discrepancies was
used and observed the difference between numerical and experimental data, and tried a
lot when shaded light of specific models on the suitability for combustion of MILD.
De Joannon et al. (2012) characterized in a steady laminar the reactive structures and
on a dense grid unidimensional mixing layer of parameters conditions in MILD
combustion with diluted and hot fuel. Regarding in terms of the temperature were
studied the structures and profiles of heat release for various ranges in a mixture fraction
space of rates of stretch and for two reference pressures, ranging from 1.00 and 10.00
bar, and used another standard code and also kinetic scheme with the standard.
Vascellari and Cau (2012) deeply investigated the turbulence-chemistry influence
interaction on pulverized coal combustion of MILD and to reproduce the process
discussed the accuracy and suitability of models. Two models were analyzed of
turbulence-chemistry interaction models, such as finite rate chemistry Eddy Dissipation
Concept and fast chemistry Eddy Dissipation Model. While advanced turbulence-
chemistry models resulted of comparison that with numerical results used with complex
kinetic mechanisms given the best agreement results.
Experiments and numerical simulations were conducted by Cao et al. (2012) of staged
entrained-flow gasifier for dry pulverized coal which was proposed based on flameless
oxidation technology. The influence of changing mole ratio of O to C and feeding
condition on syngas components concentration of CO, H2, and CO2, the calorific value
of syngas, carbon conversion were analyzed. Simulations results were in good
agreement with the experimentally measured results. Vascellari et al. (2013b)
investigated a newly developed sub-model application and analyzed for gasification
and char particle combustion. For this model, the distinguishing feature representation
was detailed of convection processes and the diffusion as well as reactions with the
homogeneous nature around the char particle in the boundary layer. Ansys FLUENT
was used as commercial CFD code. Coupled solver was used for simulating the IFRF
full scale pulverized of coal combustion MILD furnace, and experimental data was
90
detailed and for evaluation of the model were available. A good and reliable agreement
was given for the new model as compared to the standard modeling approach with
measured data.
3.10 SUMMARY
The historical development of gasification process, commercial gasification
technologies and experimental and modeling work on gasification technologies have
reviewed in this chapter. Gasification remained an old technique which was initially
utilized to produce a syngas for further synthesis of chemicals. Later the syngas being
used as the source of power generation. There are several gasification technologies have
developed considering different types of feedstocks. All those technologies are grouped
in three configurations i.e., Fixed Bed or Moving Bed, Fluidized Bed and Entrained
Bed. Among these, entrained bed configuration gave maximum efficiency and versatile
nature of feed capacity. Wide experimental work was conducted on all three types of
configurations. The main focus of all the studies was to design an efficient gasifier for
a particular type of feedstock like low, moderate or high-grade coals, the biomass of
different types, solid waste including hospital waste, waste tires, a slurry form of feed
etc. Numerous types of oxidizers have tested like oxygen, air, steam, CO2, and mixture
of these. Various operating parameters were optimized for individual configurations
like fuel feeding rate, oxidizer feeding rate, oxygen-to-fuel ratio, oxygen-to-carbon
ratio, the pressure of gasifier, the temperature of feed streams, fuel composition etc.
The performance of gasifiers were evaluated mostly on the basis of the quantity of
syngas produced, the composition of syngas, the heating value of syngas, fuel
conversion efficiency, the temperature of exiting syngas, the temperature at inside the
gasifier, the efficiency with whole IGCC system, slagging or ash fusion issues etc.
Among various types of entrained flow configurations, entrained flow gasifier with
opposite multi-burners (OMB) got special attention by the researchers in recent years.
The opposite injectors for fuel or oxidizer produce impinging flows inside the gasifier
and flameless combustion conditions are being produced which reduces the NOx
formation and enhance the system efficiency. OMB has shown good performance for
91
low-grade coals due to workability at low temperatures. The slag formation and their
concern issues could be avoided in this type of gasifier.
Computational fluid dynamics (CFD) proved a simple and robust tool to design
complex chemical systems like reactors, gasifier etc. The gasification technology
emerged due to CFD modeling and simulation as a number of researchers developed
various models of all types. Usually, multiphase flow modeling is being carried out
with either Euler-Euler framework considering both phases as the continuous phase or
Euler-Lagrangian framework in which solid phase is being treated with discrete phase
model (DPM) whereas gaseous phase is being calculated through continuity equations.
The finite rate chemistry has proved a better option in CFD modeling of the gasification
process. Turbulent flows are being captured by a k-ε turbulence model. As per an
extensive literature review, it is concluded that CFD plays a vital role in designing a
gasifier based on local coal chemistry and characteristics. Hence in this research, the
CFD has used as a basic tool to develop an indigenous gasification technology for local
coal based on entrained flow gasifier with OMB.
92
CHAPTER 4
EXPERIMENTAL WORK
4.1 GENERAL DESCRIPTION OF EXPERIMENTAL WORK
Main purpose of the experimental work is to determine the kinetic parameters pre-
exponential factor ‘A’ and activation energy ‘E’ for Thar lignite at atmospheric and
elevated pressures. Overall experimental work is divided into four steps as shown in
Fig.4.1. In the first step, coal samples were collected and prepared for the upcoming
analysis. After the sample preparation, proximate and ultimate analysis tests were
conducted, then the experiments were performed on TGA for extraction of kinetic
parameters of coal drying, devolatization, and combustion at atmospheric pressure.
Then PTGA experiments were conducted on prepared char for studying coal
gasification kinetics with CO2 and steam (H2O). These three steps are further described
in more details in the next sections.
Fig. 4.1: Steps for Experimental Work
4.1.1 Sample collection and preparation
The samples of Thar lignite were collected from available drill holes GT-01 and GT-
02 of Block IX of Thar coalfield. The details about the sample ID, the thickness of the
geotechnical layer, depth of sample location etc., are given in Table 4.1.
After the collection of samples, the samples were properly preserved by waxing. Then
they were brought to the laboratory where they were taken out from the waxed layered
shell. The samples were ground and converted into powder form in conventional lab
scale grinder. The coal powder was passed through 100 mesh size sieve (150μm).
Minus 100 mesh size sieve powder was collected and stored in plastic bags for
experimental work.
Sample Collection and
Preparation
Proximate and Ultimate Analysis
Experiments on TGA
Experiments on PTGA
93
4.1.2 Proximate and Ultimate Analysis
The prepared samples were used for conducting proximate and ultimate analysis tests.
The ASTM standard methods were used for these tests. For Proximate Analysis, ASTM
D 3172 method was used. Moisture was measured using ASTM D3173-03, Volatiles
were determined with ASTM D3175-02, Ash was analyzed with ASTM D3174-02,
Fixed Carbon was determined using ASTM D3174-02 and Ultimate Analysis was
conducted by ASTM D3176. Finally, the Gross Calorific Value was calculated using
ASTM D5865 standard.
Table 4.1: Samples collected from Block-IX Thar Coal Field
Sr# Sample ID From (m) To (m) Thickness (m)
01 KTN-GT01-123 229.66 229.83 0.17
02 KTN-GT01-138 249.43 249.77 0.34
03 KTN-GT01-139 250.33 250.57 0.24
04 KTN-GT01-140 250.57 250.76 0.19
05 KTN-GT-02-627 203.42 203.53 0.11
06 KTN-GT-02-632 204.26 204.47 0.21
07 KTN-GT-02-642 205.82 205.92 0.10
08 KTN-GT-02-644 206.11 206.33 0.22
09 KTN-GT-02-691 215.17 215.33 0.16
10 KTN-GT-02-697 216.17 216.34 0.17
11 KTN-GT01-443 229.43 229.66 0.23
12 KTN-GT01-493 249.89 250.33 0.44
4.1.3 TGA Analysis (Moisture removal, devolatization, and Combustion Study)
Thermogravimetric analyzer (TGA) SDT Q600 (Appendix A.1) was used to study the
kinetic of moisture removal, devolatization, and combustion of collected samples. The
non-isothermal technique was used in TGA analysis. The individual samples weighing
30-40 mg was heated from room temperature to 110° C at different heating rates for
moisture removal study. The sample kept in that condition for 5 minutes and then again
94
heated from 110°C to 900°C at three different heating rates for devolatization study and
kept there for 5 minutes. The inert environment was maintained in the combustion
chamber of TGA by flowing N2 at 100 ml/min. The char prepared after devolatization
was cooled with N2 below 100°C and then again heated from 200°C to 900°C in an
Oxygen environment at three different heating rates. All the experimental conditions of
moisture removal, devolatization, and combustion analysis are given in Table 4.2 and
Table 4.3. The weight loss was recorded with time and temperature and then recorded
weight loss data was utilized in the extraction of kinetic parameters of these processes.
Table 4.2: Experimental conditions in TGA for Moisture Removal and
Devolatization
Experimental Conditions Values/Information
Technique Non-Isothermal (Dynamic heating rate)
Weight of Sample 30 - 40 mg
The particle size of Sample 150 μm (100-mesh)
Method Set in TGA
1 Select gas N2
2 Ramp 10°C/min, 20°C/min, 30°C/min to 110.00 °C
3 Equilibrate at 110.00 °
4 Isothermal for 5.00 min
5 Ramp 20°C/min , 30°C/min, 40°C/min to 900.00 °C
6 Equilibrate at 900.00 °C
7 Isothermal for 5.00 min
8 Cooling
Flow-rate of N2 100mL/min
Signals Weight Loss, time, Temperature.
95
Table 4.3: Experimental conditions in TGA for Combustion Reaction
Experimental Conditions Values/Information
Technique Non-Isothermal (Dynamic heating rate)
Weight of char sample 10 - 20 mg
The particle size of Sample 150 μm (100-mesh)
Method Set in TGA
1 Select gas N2
2 Ramp 20 °C/min, 30 °C/min, 40 °C/min to 200.00 °C
3 Equilibrate at 200.00 °
4 Isothermal for 2.00 min
5 Select gas 2 (O2)
6 Ramp 20°C/min , 30°C/min, 40°C/min to 900.00 °C
7 Equilibrate at 900.00 °C
8 Isothermal for 5.00 min
9 Mark end of cycle 0
Flow-rate of N2 ,O2 100mL/min
Signals Weight Loss, time, Temperature.
4.1.4 PTGA Analysis (Char Gasification Study)
Prior to the gasification experiments, coal char was prepared in a quartz fixed-bed
reactor in a nitrogen environment at an atmospheric pressure (Appendix A.2). The
lignite sample, weighing 10±0.05 gram, was heated from room temperature to 900° C
at the rate of 20° C min−1 and kept for 20 minutes. Then the devolatilized char was
cooled with nitrogen to room temperature and was ground to -100 µm size.
Prepared char was used for kinetic study using Cahn Thermax500 Pressurized
Thermogravimetric Analyzer (PTGA) (Appendix A.3). Fig. 4.2 shows the schematic
diagram of Thermax500 PTGA, which consists of three parts: pressurized balance,
pressurized furnace, and control system. A ceramic extension wire, hanging on one arm
of a balance, extends into the quartz reaction tube. A platinum sample pan was hung on
96
the end of the extension wire. A set of thermocouples is installed about 8 mm below
the sample pan to measure the temperature of the reaction zone. The reactant gas flows
through the sample pan in an upward direction with the controlled flow rate. A manual
back pressure regulator was applied to minimize the fluctuation of pressure.
Three lignite char samples having IDs KTN-GT01-123, KTN-GT01-140 and KTN-GT-
02-627 (As per list is given in Table 4.1) were tested in PTGA (Thermax 500) at three
pressures viz: 1 atm, 5 atm, and 10 atm. Each sample, weighing 10±0.5 mg, was loaded
into the sample pan, and then the system was pressurized with the reacting gas either
CO2 or H2O (steam) in a pure state to the set pressure. Then the system was heated from
room temperature to 1000° C, at a constant heating rate of 10° Cmin−1. The reaction
was completed before the temperature reaches to 1000° C. At each pressure, trial
experiments were done to minimize the external diffusion resistances by adjusting the
flow rate of reacting gas.
Fig. 4.2: Schematic diagram of Thermax500 PTGA
4.1.5 Data analysis method
The conversion of moisture removal, devolatization, combustion, and char-gasification
was calculated from the recorded weight loss curves of individual steps using the
relationship:
97
1000
0 −
−=
WW
WWX (4.1)
Where W is the instantaneous coal/char weight, W∞ is the weight of residual ash and
W0 is the weight of initial coal/char. The apparent reaction rate was defined as the
change rate of conversion X. A general kinetic expression for the overall reaction rate
may be written as follows (Lu and Do, 1994).
( ) )(, XfTCkdt
dXg= (4.2)
where k is the apparent gasification reaction rate, which is a function of temperature (T)
and the effect of the gasifying agent concentration (Cg) and f(X) describes the changes
in physical or chemical properties of the sample as the gasification proceeds. Assuming
that the concentration of the gasifying agent remains constant during the process, the
apparent gasification reaction rate is dependent on the temperature and can be expressed
as follows, using the Arrhenius Equation with the reaction order (n) which represents
the effect of partial pressure of reactant gas:
RT
E
neAPk−
= (4.3)
Where, A and E are the pre-exponential factor (Arrhenius constant) and activation
energy, respectively. In the present study, n was taken as 0.1.
In this work, two models, Volumetric Model (VM) and Grain Model (GM), have been
used to describe the reactivity of the chars. These models give different formulations of
the term f(X). The VM assumes a homogeneous reaction throughout the particle and a
linearly decreasing reaction surface area with conversion (Ishida et al., 1971). The
overall reaction rate is expressed by:
( )Xkdt
dX−= 1 (4.4)
The Grain Model or Shrinking Core Model, proposed by Szekely and Evans (1971),
assumes that a porous particle consists of an assembly of uniform nonporous grains and
98
the reaction takes place on the surface of these grains. The space between the grains
constitutes the porous network. The shrinking core behavior applies to each of these
grains during the reaction. In the regime of chemical kinetic control and assuming the
grains have a spherical shape, the overall reaction rate is expressed in these models as:
( )3
2
1 Xkdt
dX−= (4.5)
This model predicts a monotonically decreasing reaction rate and surface area because
the surface area of each grain is receding during the reaction.
For constant heating rate (β), a linear relationship between time and temperature can be
written as,
dt
dT= (4.6)
Now, substituting Eqs. 4.6 and 4.3 in Eqs. 4.4 and 4.5, rearranging gives:
dTeAP
X
dXRT
En−
=− )1(
(4.7)
( )dTe
AP
X
dXRT
En−
=
− 3
2
1
(4.8)
The above equations can be used to evaluate E and A at constant heating rate using TG
data. Unfortunately, the right-hand side of Eq. 4.7 and Eq. 4.8 has no definite integrals
which make it difficult to find the exact solution. Therefore, several procedures are
devised to estimate the value of the temperature integral. In the following sections, two
different methods are discussed which can be used to evaluate the value of Arrhenius
parameters using Eqs. 4.7 and 4.8.
4.1.5.1 Integral method
This method is based on the approximation of temperature integral made by Coats and
Redfern (1964), and it is widely used and accepted for the calculation of kinetic
99
parameters. For each discrete event of gasification, Eqs. 4.7 and 4.8 are integrated with
temperature limits from T0 to T while the conversion factor (X) has the limits 0 to X:
For Volumetric Model: −
=−
T
T
RT
EnX
dTeAP
X
dX
001
(4.9)
For Grain Model:
( )
−
=
−
T
T
RT
EnX
dTeAP
X
dX
00 3
2
1
(4.10)
Integration of left side of Eqs. 4.9 and 4.10 are:
)1ln(1
0
XX
dXX
−−=−
(4.11)
( )
−−=
− 3
1
0 3
2)1(13
1
X
X
dXX
(4.12)
As the right-hand side of Eq. 4.9 and 4.10 have the same expression with no definite
integral, so assumptions are made to solve this integral. The first assumption is to take
T0 = 0, as no reaction is taking place at T0 and also X is zero at T0. Therefore the right
hand side of Eq. 4.8 becomes:
−−
=
T
RT
EnT
T
RT
En
dTeAP
dTeAP
00
(4.13)
Coats and Redfern (1964) has used series of expansion method to solve the right-hand
side of the above integral which results in the following:
RT
EnT
T
RT
En
eE
RT
E
RAPTdTe
AP −−
−=
212
0
(4.14)
Substituting Eqs. (4.11) and (4.12) (for left side integrals) and Eq. (4.14) (for right side
integrals) in Eqs. (4.9) and (4.10), re-arranging and taking natural log on both sides
gives:
100
For Volumetric Model: RT
E
E
RT
E
RAP
T
X n
−
−=
−− 21ln
)1ln(ln
2
For Grain Model: RT
E
E
RT
E
RAP
T
Xn
−
−=
−−
21ln
)1(13
ln2
3
1
The above equations can be more simplified by assuming E » 2RT. Which is a
reasonable assumption, as the value of activation energy (E) is normally 5–10 order of
magnitude more than the product 2RT.
For Volumetric Model: E
RAP
RT
E
T
X n
ln
)1ln(ln
2+−=
−− (4.15)
For Grain Model:
E
RAP
RT
E
T
Xn
ln
)1(13
ln2
3
1
+−=
−−
(4.16)
The above equations represent the equations of a straight line having a slope of -E/R
and an intercept of ln(APnR/βE). Using the values of X and T from TGA and PTGA
experiments, graphs of left-hand sides of Eqs. (4.15) and (4.16) can be plotted against
1/T. The plots should result in a series of data points close to a straight line. Regression
analysis with the least square fitting method is used to find the equation of the straight
line and plot it to evaluate the values of E and A.
4.1.5.2 Direct Arrhenius plot method
Eqs. (4.7) and (4.8) can be modified by taking natural log on both sides, and
rearranging:
For Volumetric Model:
nAP
TR
E
dT
dX
Xln
1
1
1ln +
−=
− (4.17)
For Grain Model:
( )
nAP
TR
E
dT
dX
X
ln1
1
1ln
3
2+
−=
−
(4.18)
101
Where 12
12
TT
XX
dT
dX TT
−
−=
The value of the above differential can be taken from the DTG curve. Putting the above
value in Eqs. (4.17) and (4.18),
For Volumetric Model:
nTT AP
TR
E
TT
XX
Xln
1
1
1ln
12
12 +
−=
−
−
− (4.19)
For Grain Model:
( )
nTT AP
TR
E
TT
XX
X
ln1
1
1ln
123
2
12 +
−=
−
−
−
(4.20)
Similar to the Integral method, plots of the left-hand sides of Eqs. (4.19) and (4.20) are
obtained and least square regression analysis is used to find E and A for gasification
reactions.
4.2 RESULTS AND DISCUSSION FOR EXPERIMENTAL WORK
As mentioned earlier that lignite samples of main coal seam were obtained, at different
depths, from drill holes GT-01 and GT-02 of Block-IX at Thar coalfield. These samples
were prepared in the laboratory and proximate and ultimate analysis tests were
conducted as per ASTM standard methods. Then experiments were conducted on TGA
and PTGA for kinetic analysis. The results are discussed in the subsequent sections.
4.3 RESULTS OF PROXIMATE AND ULTIMATE ANALYSIS
The results of the proximate and ultimate analysis are given in Table 4.4 and Table 4.5
respectively. It was observed from the proximate analysis of Thar lignite that moisture
content is ranging from 14 to 45 % which confirms the previous studies finding
(Choudry et al., 2010). It was also noticed that the fixed carbon content is less than the
volatiles which shows the basic lignite characteristics. The amount of sulfur is up to 2%
which indicates the good characteristics in terms of less SOX production during
combustion or gasification. The heating value of the collected samples was found in the
range of 5156.54 to 6476.23 Btu/lb. The values were compared with the values obtained
102
by other researchers (Jaffri and Zhang, 2009, Choudry et al., 2010, Sarwar et al.,
2014)and found in similar ranges.
Table 4.4: Proximate Analysis and Heating Value of Coal Samples
Sr.
# Sample ID
Proximate Analysis Heating Value
(Btul/lb) Moisture
(%)
Volatiles
(%)
Ash
(%)
Fixed Carbon
(%)
1 KTN-GT01-123 14.18 38.27 20.54 27.01 6339.56
2 KTN-GT01-138 35.65 33.8 4.24 26.31 6093.99
3 KTN-GT01-139 39.16 31.12 4.87 24.84 5417.51
4 KTN-GT01-140 30.04 38.31 9.78 21.87 5876.11
5 KTN-GT-02-627 41.61 30.91 3.52 23.95 5286.04
6 KTN-GT-02-632 45.88 28.4 3.3 22.43 5156.54
7 KTN-GT-02-642 26.58 36.86 5.23 31.32 6352.16
8 KTN-GT-02-644 20.77 49.87 4.23 25.12 6387.24
9 KTN-GT-02-691 29.82 40.01 6.34 23.82 6476.23
10 KTN-GT-02-697 27.49 42.14 10.16 20.21 6084.62
Average 31.12 36.97 7.22 24.69 5947
Standard Deviation 9.68 6.33 5.27 3.1 492.13
Table 4.5: Ultimate Analysis of Coal Samples
Sr. # Sample ID
Ultimate Analysis
C
(%)
H
(%)
N
(%)
O
(%)
Total S
(%)
1 KTN-GT01-123 32.34 6.8 0.32 58.44 2.1
2 KTN-GT01-138 30.32 9.55 0.34 58.29 1.5
3 KTN-GT01-139 34.24 6.76 0.62 57.65 0.73
4 KTN-GT01-140 32.12 6.54 0.32 59.61 1.41
5 KTN-GT-02-627 35.32 6.17 0.35 56.43 1.73
6 KTN-GT-02-632 34.21 7.12 0.76 57.12 0.79
7 KTN-GT-02-642 31.23 6.87 0.43 60.23 1.24
8 KTN-GT-02-644 30.87 7.23 0.88 59.93 1.09
9 KTN-GT-02-691 32.65 8.54 0.73 57.43 0.65
10 KTN-GT-02-697 33.21 7.12 0.43 58.46 0.78
Average 32.65 7.27 0.52 58.36 1.20
Standard Deviation 1.61 1.01 0.21 1.25 0.48
103
4.4 RESULTS FOR MOISTURE REMOVAL AND DEVOLATIZATION
KINETICS
Coal gasification comprises four fundamental processes, (1) drying or removal of
moisture (2) devolatization or removal of volatiles (3) char combustion and (4) char
gasification (means reaction with CO2 or H2O). One of the fundamental objectives of
the present research was to conduct the kinetic modeling of coal gasification processes.
In this regard, the TGA analysis was carried out and the data were evaluated for
extracting the kinetics of each process. First, the coal samples were heated under N2
environment from room temperature to 110°C with different heating rates. Then the
dried coal was further heated, at fixed heating rates, to 900°C under the same inert
environment for removal of volatiles. The detailed experimental conditions are already
explained in Table 4.2. Two prepared coal samples (i.e., GT-01-443 and GT-01-493)
were run on TGA for moisture and volatiles removal. The results of moisture and
volatiles removal were calculated using Eq. 4.1 and shown in Fig. 4.3 and 4.4. It is
observed from these figures that the removal of moisture or volatiles is delayed with
the decrease in heating rate. At higher heating rates i.e. 30°C / minute for moisture
removal study and 40°C / minute for devolatization, the removal occurs fast enough
and all moisture and volatiles are removed before 40 minutes of heating. The similar
trend was obtained from research conducted by other researchers (Sarwar et al., 2011,
Sarwar et al., 2014).
(a) Sample: GT-01-443 (b) Sample: GT-01-493
Fig. 4.3: Moisture removal at different heating rates
104
(a) Sample: GT-01-443 (b) Sample: GT-01-493
Fig. 4.4: Volatiles removal at different heating rates
It was further observed from the devolatization removal trends of Fig. 4.4 that the
removal of volatiles is fast enough in the lower ranges (from 110 to 500°C) but it
becomes slower in higher ranges of temperature (>500°C) as explained by (Sarwar et
al., 2011). This is due to the removal of lighter volatiles at lower temperature whereas
the heavier volatiles at higher temperatures. This is the reason that the kinetic modeling
of devolatization step was carried out in two steps categorized with lower and higher
temperature ranges.
The kinetic modeling and Arrhenius parameters (A and E) calculations for removal of
moisture and devolatization were carried out using two standard models i.e.,
Volumetric Model (VM) and Grain Model (GM). The mathematical description has
already been discussed in the previous chapter. Integral solution method was used for
both models.
4.4.1 Least square regression analysis for moisture removal
The TGA data has been used to conduct the least square regression analysis for Eqs.
4.15, and 4.16. These equations are based on the integral method strategy for solving
Volumetric and Grain Model differential equations and are standard forms of straight
line with ln(-ln(1-X)/T2) as an ordinate for Volumetric Model whereas ln(3(1-(1-
X)(1/3))/T2) as an ordinate for Grain Model and 1/T as abscissa. The TGA experiments
produced the weight-loss data against temperature ‘T’ which is used in Eq. 4.1 to
105
calculate corresponding conversion ‘X’ against ‘T’. The calculated ordinates for VM
and GM were then plotted against 1/T as per Eq.4.15 and Eq. 4.16. The graphs are
shown in Fig. 4.5 (a-f) and 4.6 (a-f) for the moisture removal of sample GT-01-443 and
GT-01-493 respectively. The R2 was found in the range of 0.97-0.99 which is
considered as good linearity. Hence it is summarized that volumetric and grain models
showed good results with all hearing rates.
4.4.2 Least square regression analysis for devolatization
The removal of volatiles was studied using TGA within the temperature range of 110°C
to 900°C with three different constant heating rates. During this study, it was noticed
that the volatiles removal followed dual kinetic rates due to the presence of heavy and
lighter volatiles, as observed by Sarwar et al. (2011). So the kinetics study was divided
into low and high-temperature ranges. The linear graphs for devolatization at low-
temperature ranges are shown in Figs. 4.7 and 4.8, whereas devolatization for high-
temperature ranges are shown in Fig. 4.9 and 4.10 for samples GT-01-443 and GT-01-
493. Linear regression for best fit straight was carried out in all graphs. It was observed
that at high or low temperature ranges both the models (Volumetric and Grain model)
show a good linearity with R2 in the range of 0.92-0.99.
Overall it is summarized that volumetric and grain models showed good results with all
hearing rates. The calculated Arrhenius parameters for moisture removal and
devolatization are tabulated in Table 4.6. The values of moisture removal and
devolatization kinetics are in accordance with low-grade coals or low-calorific fuels
from previous research (Jiménez et al., 2008, Okumura et al., 2009, Piatkowski and
Steinfeld, 2010).
106
(a) Heating Rate 10°C/min, Vol. Model (d) Heating Rate 10°C/min, Grain Model
(b) Heating Rate 20°C/min, Vol. Model (e) Heating Rate 20°C/min, Grain Model
(c) Heating Rate 30°C/min, Vol. Model (f) Heating Rate 30°C/min, Grain Model
Fig. 4.5: Linearity of Volumetric and Grain models for Moisture Removal at
different Heating Rate for Sample GT-01-443
y = -5821.5x + 4.1686
R² = 0.9967
-14.5-14
-13.5-13
-12.5-12
-11.5-11
-10.5-10
0.0
02
6
0.0
02
7
0.0
02
8
0.0
02
9
0.0
03
0.0
03
1
ln(-
ln(1
-x)/
T2)
1/T
y = -5997.9x + 3.9804
R² = 0.9952
-14.5
-14
-13.5
-13
-12.5
-12
-11.5
0.0
02
65
0.0
02
7
0.0
02
75
0.0
02
8
0.0
02
85
0.0
02
9
0.0
02
95
0.0
03
ln(-
ln(1
-x)/
T2)
1/T
y = -7482.9x + 13.326
R² = 0.9691
-9.5-9
-8.5-8
-7.5-7
-6.5-6
-5.5-5
0.0
02
65
0.0
02
7
0.0
02
75
0.0
02
8
0.0
02
85
0.0
02
9
0.0
02
95
0.0
03
0.0
03
05
ln(-
ln(1
-x)/
T)
1/T
y = -5125.1x + 2.0674
R² = 0.9936
-14.5-14
-13.5-13
-12.5-12
-11.5-11
-10.5-10
0.0
02
6
0.0
02
65
0.0
02
7
0.0
02
75
0.0
02
8
0.0
02
85
0.0
02
9
0.0
02
95
0.0
03
0.0
03
05
0.0
03
1
ln(3
[1-(
1-X
)(1/3
) ]/T
2)
1/T
y = -5617.8x + 2.8483
R² = 0.997
-14.5
-14
-13.5
-13
-12.5
-12
0.0
02
65
0.0
02
7
0.0
02
75
0.0
02
8
0.0
02
85
0.0
02
9
0.0
02
95
0.0
03
ln(3
[1-(
1-X
)(1/3
) ]/T
2)
1/T
y = -6654x + 10.852
R² = 0.9819
-9.5-9
-8.5-8
-7.5-7
-6.5-6
-5.5-5
0.0
02
65
0.0
02
7
0.0
02
75
0.0
02
8
0.0
02
85
0.0
02
9
0.0
02
95
0.0
03
0.0
03
05
ln(3
[1-(
1-X
)(1/3
) ]/T
2)
1/T
107
(a) Heating Rate 10°C/min, Vol. Model (d) Heating Rate 10°C/min, Grain Model
(b) Heating Rate 20°C/min, Vol. Model (e) Heating Rate 20°C/min, Grain Model
(c) Heating Rate 30°C/min, Vol. Model (f) Heating Rate 30°C/min, Grain Model
Fig. 4.6: Linearity of Volumetric and Grain models for Moisture Removal at
different Heating Rate for Sample GT-01-493
y = -7335.4x + 14.395
R² = 0.9998
-10
-9
-8
-7
-6
-5
0.0
02
7
0.0
02
8
0.0
02
9
0.0
03
0.0
03
1
0.0
03
2
0.0
03
3
ln(-
ln(1
-x)/
T2)
1/T
y = -7102.7x + 12.544
R² = 0.9858
-10-9.5
-9-8.5
-8-7.5
-7-6.5
-6-5.5
0.0
02
65
0.0
02
7
0.0
02
75
0.0
02
8
0.0
02
85
0.0
02
9
0.0
02
95
0.0
03
0.0
03
05
0.0
03
1
0.0
03
15
0.0
03
2
ln(-
ln(1
-x)/
T2)
1/T
y = -6912.5x + 11.802R² = 0.9754
-10-9.5
-9-8.5
-8-7.5
-7-6.5
-6-5.5
0.0
02
65
0.0
02
7
0.0
02
75
0.0
02
8
0.0
02
85
0.0
02
9
0.0
02
95
0.0
03
0.0
03
05
0.0
03
1
0.0
03
15
ln(-
ln(1
-x)/
T2)
1/T
y = -6668.1x + 12.298
R² = 0.9993
-10
-9
-8
-7
-6
-5
0.0
02
7
0.0
02
8
0.0
02
9
0.0
03
0.0
03
1
0.0
03
2
0.0
03
3
ln(3
[1-(
1-X
)(1/3
) ]/T
2)
1/T
y = -6481.5x + 10.641
R² = 0.9932
-10-9.5
-9-8.5
-8-7.5
-7-6.5
-6-5.5
0.0
02
65
0.0
02
7
0.0
02
75
0.0
02
8
0.0
02
85
0.0
02
9
0.0
02
95
0.0
03
0.0
03
05
0.0
03
1
0.0
03
15
0.0
03
2
ln(3
[1-(
1-X
)(1
/3) ]
/T2)
1/T
y = -6319.3x + 9.9918
R² = 0.9858
-10
-9.5
-9
-8.5
-8
-7.5
-7
-6.5
0.0
02
65
0.0
02
7
0.0
02
75
0.0
02
8
0.0
02
85
0.0
02
9
0.0
02
95
0.0
03
0.0
03
05
0.0
03
1
0.0
03
15
ln(3
[1-(
1-X
)(1/3
) ]/T
2)
1/T
108
(a) Heating Rate 20°C/min, Vol. Model (d) Heating Rate 20°C/min, Grain Model
(b) Heating Rate 30°C/min, Vol. Model (e) Heating Rate 30°C/min, Grain Model
(c) Heating Rate 40°C/min, Vol. Model (f) Heating Rate 40°C/min, Grain Model
Fig. 4.7: Linearity of Volumetric and Grain models for Devolatization at Low
Temperature with different Heating Rate for Sample GT-01-443
y = -3284.9x - 8.9313
R² = 0.993
-15
-14.5
-14
-13.5
-13
0.0
01
2
0.0
01
3
0.0
01
4
0.0
01
5
0.0
01
6
0.0
01
7
0.0
01
8
0.0
01
9
ln(-
ln(1
-x)/
T2)
1/T
y = -2965.1x - 9.4983
R² = 0.9874
-15.2
-14.8
-14.4
-14
-13.6
-13.2
0.0
01
2
0.0
01
3
0.0
01
4
0.0
01
5
0.0
01
6
0.0
01
7
0.0
01
8
0.0
01
9
ln(3
[1-(
1-X
)(1/3
) ]/T
2)
1/T
y = -3568.9x - 8.5644
R² = 0.9976
-15.5
-15
-14.5
-14
-13.5
-13
-12.5
0.0
01
2
0.0
01
3
0.0
01
4
0.0
01
5
0.0
01
6
0.0
01
7
0.0
01
8
0.0
01
9
ln(-
ln(1
-x)/
T2)
1/T
y = -3247.6x - 9.1282
R² = 0.9956
-15.2
-14.8
-14.4
-14
-13.6
-13.2
0.0
01
2
0.0
01
3
0.0
01
4
0.0
01
5
0.0
01
6
0.0
01
7
0.0
01
8
0.0
01
9
ln(3
[1-(
1-X
)(1/3
) ]/T
2)
1/T
y = -3814.5x - 1.61
R² = 0.9222
-10
-9
-8
-7
-6
-5
-4
0.0
01
2
0.0
01
3
0.0
01
4
0.0
01
5
0.0
01
6
0.0
01
7
0.0
01
8
0.0
01
9
0.0
02
0.0
02
1
0.0
02
2
ln(-
ln(1
-x)/
T2)
1/T
y = -3631.6x - 1.9823
R² = 0.9265
-10.5
-9.5
-8.5
-7.5
-6.5
-5.5
0.0
01
2
0.0
01
3
0.0
01
4
0.0
01
5
0.0
01
6
0.0
01
7
0.0
01
8
0.0
01
9
0.0
02
0.0
02
1
0.0
02
2
ln(3
[1-(
1-X
)(1/3
) ]/T
2)
1/T
109
(a) Heating Rate 20°C/min, Vol. Model (d) Heating Rate 20°C/min, Grain Model
(b) Heating Rate 30°C/min, Vol. Model (e) Heating Rate 30°C/min, Grain Model
(c) Heating Rate 40°C/min, Vol. Model (f) Heating Rate 40°C/min, Grain Model
Fig. 4.8: Linearity of Volumetric and Grain models for Devolatization at Low
Temperature with different Heating Rate for Sample GT-01-493
y = -5895.1x + 2.0292R² = 0.9774
-10
-9
-8
-7
-6
-5
-4
0.0
01
2
0.0
01
3
0.0
01
4
0.0
01
5
0.0
01
6
0.0
01
7
0.0
01
8
0.0
01
9
0.0
02
ln(-
ln(1
-x)/
T2)
1/T
y = -5636.5x + 1.5594R² = 0.98
-10
-9
-8
-7
-6
-5
-4
0.0
01
2
0.0
01
3
0.0
01
4
0.0
01
5
0.0
01
6
0.0
01
7
0.0
01
8
0.0
01
9
0.0
02
ln(3
[1-(
1-X
)(1/3
) ]/T
2)
1/T
y = -6214.1x + 2.0309R² = 0.9733
-9
-8
-7
-6
-5
0.0
01
2
0.0
01
3
0.0
01
4
0.0
01
5
0.0
01
6
0.0
01
7
0.0
01
8
ln(-
ln(1
-x)/
T2)
1/T
y = -5882.5x + 1.4496R² = 0.9761
-9
-8
-7
-6
-50
.00
12
0.0
01
3
0.0
01
4
0.0
01
5
0.0
01
6
0.0
01
7
0.0
01
8
ln(3
[1-(
1-X
)(1/3
) ]/T
2)
1/T
y = -523.17x - 7.8065
R² = 0.9687
-9.2
-9.1
-9
-8.9
-8.8
-8.7
0.0
01
9
0.0
02
0.0
02
1
0.0
02
2
0.0
02
3
0.0
02
4
0.0
02
5
0.0
02
6
0.0
02
7
ln(-
ln(1
-x)/
T2)
1/T
y = -512.99x - 7.8525
R² = 0.9689
-9.3
-9.2
-9.1
-9
-8.9
-8.8
-8.7
0.0
01
9
0.0
02
0.0
02
1
0.0
02
2
0.0
02
3
0.0
02
4
0.0
02
5
0.0
02
6
0.0
02
7
ln(3
[1-(
1-X
)(1/3
) ]/T
2)
1/T
110
(a) Heating Rate 20°C/min, Vol. Model (d) Heating Rate 20°C/min, Grain Model
(b) Heating Rate 30°C/min, Vol. Model (e) Heating Rate 30°C/min, Grain Model
(c) Heating Rate 40°C/min, Vol. Model (f) Heating Rate 40°C/min, Grain Model
Fig. 4.9: Linearity of Volumetric and Grain models for Devolatization at High
Temperature with different Heating Rate for Sample GT-01-443
y = -981.63x - 12.167
R² = 0.9912
-13.18
-13.16
-13.14
-13.12
-13.1
-13.080
.00
09
3
0.0
00
95
0.0
00
97
0.0
00
99
0.0
01
01
0.0
01
03
0.0
01
05
ln(-
ln(1
-x)/
T2)
1/T
y = -125.69x - 13.326
R² = 0.995
-13.456
-13.455-13.454
-13.453
-13.452
-13.451
-13.45
0.0
00
99
0.0
00
99
5
0.0
01
0.0
01
00
5
0.0
01
01
0.0
01
01
5
0.0
01
02
0.0
01
02
5
0.0
01
03
ln(3
[1-(
1-X
)(1/3
) ]/T
2)
1/T
y = -869.65x - 12.267R² = 0.9871
-13.18
-13.16
-13.14
-13.12
-13.1
-13.08
0.0
00
94
0.0
00
95
0.0
00
96
0.0
00
97
0.0
00
98
0.0
00
99
0.0
01
0.0
01
01
0.0
01
02
0.0
01
03
0.0
01
04
ln(-
ln(1
-x)/
T2)
1/T
y = -432.22x - 12.861
R² = 0.9716
-13.425
-13.42
-13.415
-13.41
-13.405
-13.4
-13.395
0.0
01
24
0.0
01
25
0.0
01
26
0.0
01
27
0.0
01
28
0.0
01
29
0.0
01
3
ln(3
[1-(
1-X
)(1/3
) ]/T
2)
1/T
y = -4368.1x - 0.7394
R² = 0.9987
-5.25-5.2
-5.15-5.1
-5.05-5
-4.95-4.9
-4.85
0.0
00
94
0.0
00
95
0.0
00
96
0.0
00
97
0.0
00
98
0.0
00
99
0.0
01
0.0
01
01
0.0
01
02
0.0
01
03
0.0
01
04
ln(-
ln(1
-x)/
T2)
1/T
y = -3567.1x - 1.8566
R² = 0.9991
-5.55
-5.5
-5.45
-5.4
-5.35
-5.3
-5.25
-5.2
0.0
00
94
0.0
00
95
0.0
00
96
0.0
00
97
0.0
00
98
0.0
00
99
0.0
01
0.0
01
01
0.0
01
02
0.0
01
03
0.0
01
04
ln(3
[1-(
1-X
)(1/3
) ]/T
2)
1/T
111
(a) Heating Rate 20°C/min, Vol. Model (d) Heating Rate 20°C/min, Grain Model
(b) Heating Rate 30°C/min, Vol. Model (e) Heating Rate 30°C/min, Grain Model
(c) Heating Rate 40°C/min, Vol. Model (f) Heating Rate 40°C/min, Grain Model
Fig. 4.10: Linearity of Volumetric and Grain models for Devolatization at High
Temperature with different Heating Rate for Sample GT-01-493
y = -3982x - 0.2997
R² = 0.9942
-6
-5.5
-5
-4.5
-4
-3.5
-3
0.0
00
8
0.0
00
9
0.0
01
0.0
01
1
0.0
01
2
0.0
01
3
0.0
01
4
ln(-
ln(1
-x)/
T2)
1/T
y = -3468x - 1.1136
R² = 0.9963
-6
-5.5
-5
-4.5
-4
0.0
00
8
0.0
00
9
0.0
01
0.0
01
1
0.0
01
2
0.0
01
3
0.0
01
4
ln(3
[1-(
1-X
)(1
/3) ]
/T2)
1/T
y = -3877.4x - 0.7973
R² = 0.9975
-6
-5.5
-5
-4.5
-4
0.0
00
9
0.0
00
95
0.0
01
0.0
01
05
0.0
01
1
0.0
01
15
0.0
01
2
0.0
01
25
0.0
01
3
0.0
01
35
ln(-
ln(1
-x)/
T2)
1/T
y = -3345.4x - 1.6564
R² = 0.9986
-6.5
-6
-5.5
-5
-4.5
-4
0.0
00
9
0.0
00
95
0.0
01
0.0
01
05
0.0
01
1
0.0
01
15
0.0
01
2
0.0
01
25
0.0
01
3
0.0
01
35
ln(3
[1-(
1-X
)(1/3
) ]/T
2)
1/T
y = -3714x - 1.1373
R² = 0.9972
-6.5
-6
-5.5
-5
-4.5
-4
0.0
00
9
0.0
00
95
0.0
01
0.0
01
05
0.0
01
1
0.0
01
15
0.0
01
2
0.0
01
25
0.0
01
3
0.0
01
35
ln(-
ln(1
-x)/
T2)
1/T
y = -3221.6x - 1.9425
R² = 0.998
-6.5
-6
-5.5
-5
-4.5
-4
0.0
009
0.0
009
5
0.0
01
0.0
010
5
0.0
011
0.0
011
5
0.0
012
0.0
012
5
0.0
013
0.0
013
5
ln(3
[1-(
1-X
)(1/3
) ]/T
2)
1/T
112
Table 4.6: Calculated kinetic parameters for moisture removal and
devolatization steps
Step Model Heating Rate A E (KJ/Mol)
GT-01-443 GT-01-493 GT-01-443 GT-01-493
Moisture
Volumetric
10°C/min 3.762×103 1.309×108 48.4 60.99
20°C/min 6.422×103 3.983×107 49.87 59.05
30°C/min 1.376×108 2.769×107 62.21 57.47
Grain
10°C/min 4.051×102 1.462×107 42.61 55.44
20°C/min 1.939×103 5.420×106 46.71 53.89
30°C/min 1.031×107 4.142×106 55.32 52.54
Devolatization
(Low Temp)
Volumetric
20°C/min 8.684×103 8.970×102 27.31 49.01
30°C/min 2.043×102 1.421×103 29.67 51.66
40°C/min 3.050×101 8.519×103 31.71 4.35
Grain
20°C/min 4.446×103 5.361×102 24.65 46.86
30°C/min 1.058×102 7.520×102 27.00 48.91
40°C/min 2.001×101 7.978×103 30.19 4.26
Devolatization
(High Temp)
Volumetric
20°C/min 1.021×104 5.902×101 8.16 33.11
30°C/min 1.227×104 5.241×101 7.23 32.24
40°C/min 8.341×101 4.764×101 36.32 30.88
Grain
20°C/min 4.101×106 2.278×101 1.045 28.83
30°C/min 3.368×105 1.915×101 3.59 27.81
40°C/min 2.229×101 1.847×101 29.66 26.78
4.4.3 Rate constant “k” for drying and devolatization steps
The rate constant “k” as explained by Eq. 4.3 was calculated using extracted kinetic
parameters (activation energy E and pre-exponential factor A) for drying and
devolatization steps. Due to the exponential nature of values, the values of ln(k) are
plotted for selected samples with different heating rates. Fig. 4.11 shows the ln(k)
values for drying step whereas Fig. 4.12 shows the ln(k) values for devolatization step.
113
For drying, 100°C temperature was taken as constant temperature. Rate constant was
observed increasing with the increase in the heating rate for sample GT-01-443 as per
observation of Tremel and Spliethoff (2013a) but it is slightly decreased on increasing
heating rate for another sample i.e., GT-01-493. The probable reason is the difference
of moisture quantity with the in the coal as explained by Ma et al. (1991). An almost
similar trend was observed with both the models but VM showed the little higher
prediction of k as compared to GM.
The devolatization step is further sub-divided into low (500°C) and high temperature
(900°C) volatiles. The reverse trend as compared to drying was observed for
devolatization rate constant. Again the reason is the amount of volatiles available in
each sample (Qiu and Liu, 1994).
(a) Volumetric Model @100°C (a) Grain Model @100°C
Fig. 4.11: Rate constant (k) for drying step with (a) VM and (b) GM
02468
101214161820
10°C/min 20°C/min 30°C/min
Ln
(k)
Heating Rate
GT-01-443
GT-01-493
02468
1012141618
10°C/min 20°C/min 30°C/min
Ln
(k)
Heating Rate
GT-01-443
GT-01-493
114
(a) Volumetric Model (Low Temp_500°C) (c) Grain Model (Low Temp_500°C)
(b) Volumetric Model (High Temp_900°C) (d) Grain Model (High Temp_900°C)
Fig. 4.12: Rate constant (k) for devolatization step with (a) VM-Low
Temp_500°C (b) VM-High Temp_900°C (c) GM-Low Temp_500°C (d) GM-
High Temp_900°C
4.5 RESULTS FOR COMBUSTION KINETICS
Combustion is an important step in the gasification process (Rathnam et al., 2009).
After the removal of moisture and volatiles, the char is formed which contains the fixed
carbon and ash of the coal. The char reacts with available limited oxygen and
combustion takes place. This combustion produces the necessary heat for the
occurrence of endothermic reactions of gasification (char reactions with CO2 and H2O).
During TGA analysis, after the drying and devolatization step, the car was formed and
that was cooled in N2 environment down to room temperature. Then re-heating of the
chamber was done with three different constant heating rates i.e., 20°C, 40°C and 50°C
with the O2 environment. The conversion of the char samples, obtained from GT-01-
443 and GT-01-493, with O2 was calculated using Eq. 4.1 and shown in Fig. 4.13 and
4.14 respectively.
0
2
4
6
8
10
10°C/min 20°C/min 30°C/min
Ln
(k)
Heating Rate
GT-01-443
GT-01_493
0
2
4
6
8
10
10°C/min 20°C/min 30°C/min
Ln
(k)
Heating Rate
GT-01-443
GT-01_493
0
2
4
6
8
10
10°C/min 20°C/min 30°C/min
Ln
(k)
Heating Rate
GT-01-443
GT-01_493
0
5
10
15
20
10°C/min 20°C/min 30°C/min
Ln
(k)
Heating Rate
GT-01-443
GT-01_493
115
Fig. 4.13: Conversion of Char Samples
obtained from GT-01-443, in Oxygen
environment at different heating rates
Fig. 4.14: Conversion of Char Samples
obtained from GT-01-493, in Oxygen
environment at different heating rates
It was observed that the combustion of char started after 300°C and it showed a slower
conversion rate with higher heating rates as observed by other researchers (Rathnam et
al., 2009, Irfan et al., 2012). After the complete combustion, a drop in temperature was
noticed due to the little cooling by the incoming cool oxygen in the chamber.
The least square regression analysis was done in a similar way as carried out for
moisture removal and devolatization steps. Both Volumetric and Grain models fit well
for combustion of char for all three tested heating rates, as shown in Fig. 4.15 and Fig.
4.16 for char samples obtained from GT-01-443 and GT-01-493 respectively. The
calculated kinetic parameters for combustion reaction are tabulated in Table 4.7. The
data is in good agreement as compared with previous studies (Irfan et al., 2012). It is
observed from this Table that the Arrhenius parameter (A) is higher for the sample GT-
01-443 than the sample GT-01-493. It means that the combustion reaction of GT-01-
443 is faster than other sample. Activation energy (E) of sample GT-01-443 is also
higher which affects the combustion start-up and this is also proven from the conversion
curves shown in Fig. 4.13 and 4.14, in which the sample GT-01-443 shows start-up of
combustion after 350°C whereas the sample GT-01-493 was ignited just near at 300°C.
Moreover, it was also noticed that the heating rate is inversely proportional to the
kinetic parameters for the combustion as explained by Sakaguchi et al. (2010).
116
(a) Heating Rate 20°C/min, Vol. Model (d) Heating Rate 20°C/min, Grain Model
(b) Heating Rate 30°C/min, Vol. Model (e) Heating Rate 30°C/min, Grain Model
(c) Heating Rate 40°C/min, Vol. Model (f) Heating Rate 40°C/min, Grain Model
Fig. 4.15: Linearity of Volumetric and Grain models for Combustion of with
different Heating Rate for char sample obtained from GT-01-443
y = -16669x + 9.2334
R² = 0.948
-16.5
-16
-15.5
-15
-14.5
-14
-13.5
-13
-12.5
-12
0.0
01
3
0.0
01
35
0.0
01
4
0.0
01
45
0.0
01
5
0.0
01
55
ln(-
ln(1
-x)/
T2)
1/T
y = -14815x + 6.54
R² = 0.9704
-16.5
-16
-15.5
-15
-14.5
-14
-13.5
-13
-12.5
-12
0.0
01
3
0.0
01
35
0.0
01
4
0.0
01
45
0.0
01
5
0.0
01
55
ln(3
[1-(
1-X
)(1/3
) ]/T
2)
1/T
y = -13644x + 4.5642
R² = 0.9957
-16.5-16
-15.5-15
-14.5-14
-13.5-13
-12.5-12
0.0
01
2
0.0
01
25
0.0
01
3
0.0
01
35
0.0
01
4
0.0
01
45
0.0
01
5
0.0
01
55
ln(-
ln(1
-x)/
T2)
1/T
y = -12033x + 2.2587
R² = 0.9985
-16.5-16
-15.5-15
-14.5-14
-13.5-13
-12.5-12
0.0
01
2
0.0
01
25
0.0
01
3
0.0
01
35
0.0
01
4
0.0
01
45
0.0
01
5
0.0
01
55
ln(3
[1-(
1-X
)(1/3
) ]/T
2)
1/T
y = -11739x + 1.6927
R² = 0.9972
-16.5-16
-15.5-15
-14.5-14
-13.5-13
-12.5-12
0.0
01
15
0.0
01
2
0.0
01
25
0.0
01
3
0.0
01
35
0.0
01
4
0.0
01
45
0.0
01
5
0.0
01
55
ln(-
ln(1
-x)/
T2)
1/T
y = -10294x - 0.3552
R² = 0.992
-16.5-16
-15.5-15
-14.5-14
-13.5-13
-12.5-12
0.0
01
15
0.0
01
2
0.0
01
25
0.0
01
3
0.0
01
35
0.0
01
4
0.0
01
45
0.0
01
5
0.0
01
55
ln(3
[1-(
1-X
)(1/3
) ]/T
2)
1/T
117
(a) Heating Rate 20°C/min, Vol. Model (d) Heating Rate 20°C/min, Grain Model
(b) Heating Rate 30°C/min, Vol. Model (e) Heating Rate 30°C/min, Grain Model
(c) Heating Rate 40°C/min, Vol. Model (f) Heating Rate 40°C/min, Grain Model
Fig. 4.16: Linearity of Volumetric and Grain models for Combustion of with
different Heating Rate for char sample obtained from GT-01-493
y = -11708x + 3.9525
R² = 0.9672
-16.5-16
-15.5-15
-14.5-14
-13.5-13
-12.5-12
0.0
01
4
0.0
01
45
0.0
01
5
0.0
01
55
0.0
01
6
0.0
01
65
0.0
01
7
ln(-
ln(1
-x)/
T2)
1/T
y = -10512x + 2.028
R² = 0.9765
-16.5
-16
-15.5
-15
-14.5
-14
-13.5
-13
-12.5
-12
0.0
01
4
0.0
01
45
0.0
01
5
0.0
01
55
0.0
01
6
0.0
01
65
0.0
01
7
ln(3
[1-(
1-X
)(1/3
) ]/T
2)
1/T
y = -6956.4x - 5.6591
R² = 0.9868
-16.5-16
-15.5-15
-14.5-14
-13.5-13
-12.5-12
0.0
01
0.0
01
1
0.0
01
2
0.0
01
3
0.0
01
4
0.0
01
5
0.0
01
6
ln(-
ln(1
-x)/
T2)
1/T
y = -6199.9x - 6.6993
R² = 0.9956
-16.5
-16
-15.5
-15
-14.5
-14
-13.5
-13
0.0
01
0.0
01
1
0.0
01
2
0.0
01
3
0.0
01
4
0.0
01
5
0.0
01
6
ln(3
[1-(
1-X
)(1/3
) ]/T
2)
1/T
y = -9251.9x - 0.6908
R² = 0.9952
-16.5-16
-15.5-15
-14.5-14
-13.5-13
-12.5-12
0.0
01
25
0.0
01
3
0.0
01
35
0.0
01
4
0.0
01
45
0.0
01
5
0.0
01
55
0.0
01
6
0.0
01
65
ln(-
ln(1
-x)/
T2)
1/T
y = -8191.7x - 2.3196
R² = 0.997
-16.5-16
-15.5-15
-14.5-14
-13.5-13
-12.5-12
0.0
01
25
0.0
01
3
0.0
01
35
0.0
01
4
0.0
01
45
0.0
01
5
0.0
01
55
0.0
01
6
0.0
01
65
ln(3
[1-(
1-X
)(1/3
) ]/T
2)
1/T
118
Table 4.7: Calculated kinetic parameters for char combustion step
Sample ID Model Heating Rate A E (KJ/Mol)
GT-01-443
Volumetric
20°C/min 3.412×106 138.59
30°C/min 3.929×104 113.44
40°C/min 2.552×103 97.60
Grain
20°C/min 2.051×105 123.17
30°C/min 3.455×103 100.04
40°C/min 2.887×102 85.58
GT-01-493
Volumetric
20°C/min 1.219×104 97.34
30°C/min 7.274×101 57.84
40°C/min 1.855×102 76.92
Grain
20°C/min 1.913×103 87.40
30°C/min 2.291×101 51.55
40°C/min 3.221×101 68.11
4.5.1 Rate constant “k” for the combustion reaction
The rate constant “k” was calculated from Eq. 4.3 for the combustion step at different
heating rates from both VM and GM models at constant temperature i.e., 800°C, as
shown in Fig. 4.17 From the figure, it was revealed that the rate constant is decreasing
with increasing heating rate for both the samples. GT-01-443 shows higher rate constant
as compared to GT-01-493 and a probable reason is of higher carbon and less moisture
content in that sample (Konttinen et al., 2012). The heating rate has a vital impact on
rate constant values as lower heating rate means high residence time and hence higher
rate constant similar to findings of Sarwar et al. (2014).
119
(a) Volumetric Model @800°C (a) Grain Model @800°C
Fig. 4.17: Rate constant (k) for combustion step with (a) VM and (b) GM
4.6 RESULTS FOR COAL GASIFICATION KINETICS AT
ATMOSPHERIC AND ELEVATED PRESSURE
As mentioned earlier in this chapter that three lignite char samples having IDs KTN-
GT01-123, KTN-GT01-140 and KTN-GT-02-627 (As per list is given in Table 4.1)
were tested in PTGA (Thermax 500) at three pressures 1 atm, 5 atm, and 10 atm
respectively. For ease in writing the samples, IDs were marked as S1, S2, and S3 for
GT01-123, KTN-GT01-140, and KTN-GT-02-627 respectively. The external diffusion
due to high pressures as explained by Wang et al. (2008) was minimized by testing the
different flow rates of CO2 and H2O at 5 atm and 10 atm. The coal char conversion has
calculated, using Eq. 4.1 and the results are plotted in Fig. 4.18. It is observed that the
conversion of char started at from 750 to 800°C for all the samples with both reacting
gases i.e. CO2 and H2O at 1 atm pressure. This range is similar to other studies
(Tomaszewicz et al., 2013, Lin and Strand, 2013, Chen et al., 2013a). At higher
pressures, the char conversion is little earlier than atmospheric pressure following the
findings by Wang et al. (2008). Fig. 4.18 shows that the increasing pressure slightly
increases the apparent reaction rate which confirms the observations of earlier studies
(Park and Ahn, 2007, Wang et al., 2008, Fermoso et al., 2009, Botero et al., 2013). The
fundamental reason for this behavior is the increase of density of reacting gases at
0
2
4
6
8
10
12
14
16
10°C/min 20°C/min 30°C/min
Ln
(k)
Heating Rate
GT-01-443
0
2
4
6
8
10
12
14
10°C/min 20°C/min 30°C/min
Ln
(k)
Heating Rate
GT-01-443
120
higher pressures due to which it diffuses faster into the solid char particle and enhance
the reactivity with it.
(a) S1_CO2 (b) S1_H2O
(c) S2_CO2 (d) S2_H2O
(e) S3_CO2
(f) S3_H2O
Fig. 4.18: Conversion of Char samples against Temperature at different
pressures for CO2 and H2O reacting gases
121
It is observed from Fig. 4.18 (a – f) that the slope of the curve, after the initiation of
reactions, at atmospheric pressure is less than the slopes of curves at higher pressures
(5 atm and 10 atm). It means the overall rate of reaction, at higher pressure, is higher
and starts little earlier than atmospheric pressure which confirms the previous findings
(Wang et al., 2008, Park and Ahn, 2007).
The kinetic modeling and Arrhenius parameters (A and E) calculations were carried out
by the same standard models i.e., Volumetric Model (VM) and Grain Model (VM), as
already applied for drying, devolatization, and combustion steps. Here for the
gasification reactions, two solution strategies, Integral Method and Direct Plot Method
were used for both models.
4.6.1 Least square regression analysis for Char+CO2 reactions
Char reactions with CO2 have been evaluated on the basis of volumetric and grain
model using equations 4.15, 4.16, 4.19 and 4.20. The representative data for selected
samples are presented in Fig. 4.19, 4.20 and 4.21 (a-d) for the Char-CO2 reactions with
respect to 1/T. Linear regression analysis has been done through data points in the range
of 10-90% conversion, as shown in Fig. 4.19 to 4.21. It is observed from these graphs
that at atmospheric pressure both the models with each method show a good linearity
with R2>0.99. At higher pressures (5 atm and 10 atm) the direct method gives linearity
in satisfactory limits, whereas, the integral method shows a good linearity. All the
samples show good linearity with either model at any pressure but Volumetric Model
with the integral method shown best performance with R2>0.999.
(a) CO2_Volumetric_Direct
(b) CO2_Volumetric_Integral
122
(c) CO2_Grain_Direct
(d) CO2__Grain_Integral
Fig. 4.19: Least square regression analysis of Volumetric and Grain Models for
Char+CO2 reactions of sample S1
(a) CO2_Volumetric_Direct
(b) CO2_Volumetric_Integral
(c) CO2_Grain_Direct
(d) CO2__Grain_Integral
Fig. 4.20: Least square regression analysis of Volumetric and Grain Models for
Char+CO2 reactions of sample S2
123
(a) CO2_Volumetric_Direct
(b) CO2_Volumetric_Integral
(c) CO2_Grain_Direct
(d) CO2__Grain_Integral
Fig. 4.21: Least square regression analysis of Volumetric and Grain Models for
Char+CO2 reactions of sample S3
From the above discussion, it can be concluded that the integral method gives better
results for both the models i.e., Volumetric and Grain Models at atmospheric as well
higher pressures. The calculated Arrhenius parameters are summarized in Table 4.8
which is in good agreements with previous studies (Blackwood and Ingeme, 1960, Sha
et al., 1990, Yun and Lee, 1999, Park and Ahn, 2007, Wang et al., 2008, Long et al.,
2012).
124
Table 4.8: Arrhenius Parameters for Char+CO2
Samples Pressure Arrhenius
Parameters
Volumetric Model Grain Model
Direct Integral Direct Integral
S1
1 atm
A 2.181×109 2.729×106 3.027×106 1.078×105
E (KJ/mol) 251.9308 274.0045 189.9 244.6395
R2 0.9945 0.9982 0.9648 0.9927
5 atm
A 2.657×109 4.329×107 1.116×106 9.859×105
E (KJ/mol) 267.47 294.997 195.396 261.101
R2 0.9895 0.9999 0.9382 0.9974
10 atm
A 8.677×109 7.849×108 2.134×107 3.992×107
E (KJ/mol) 269.581 311.999 215.84 286.135
R2 0.996 0.9999 0.9795 0.9991
S2
1 atm
A 1.134×1011 1.906×109 1.069×107 2.174×107
E (KJ/mol) 272.816 317.6031 189.343 278.5356
R2 0.9724 0.9964 0.8703 0.9884
5 atm
A 9.202 ×1010 3.145×1010 1.936×106 1.832×108
E (KJ/mol) 282.917 336.451 187.747 292.254
R2 0.9936 0.9999 0.9008 0.9963
10 atm
A 1.385×1012 9.939×1011 3.923×107 6.509×109
E (KJ/mol) 298.19 356.671 208.399 314.652
R2 0.995 0.999 0.9272 0.9971
S3
1 atm
A 3.951×1021 7.99×107 9.848×1017 1.44×106
E (KJ/mol) 488.7884 280.9134 416.4316 247.0339
R2 0.9904 0.9959 0.9968 0.9895
5 atm
A 2.701×1010 1.399×109 1.077×107 3.083×107
E (KJ/mol) 264.701 300.235 197.956 268.675
R2 0.9919 0.9999 0.9484 0.9979
10 atm
A 9.671×1010 4.527×1010 7.232×106 4.674×108
E (KJ/mol) 284.854 341.339 200.933 302.048
R2 0.9597 0.9999 0.8648 0.997
4.6.2 Least square regression analysis for Char+H2O reaction
Char reactions with H2O have been evaluated on the basis of Volumetric and Grain
Models using equations 4.15, 4.16, 4.19 and 4.20. Evaluated values of char reactions
are plotted against 1/T, as shown in Fig. 4.22, 5.23 and 5.24 (a-d) for selected samples.
Best fit straight lines are also plotted, through the data points, in all graphs.
125
(a) H2O_Volumetric_Direct
(b) H2O _Volumetric_Integral
(c) H2O _Grain_Direct
(d) H2O _Grain_Integral
Fig. 4.22: Least square regression analysis of Volumetric and Grain Models for
Char+H2O reactions of sample S1
126
(a) H2O_Volumetric_Direct
(b) H2O _Volumetric_Integral
(c) H2O _Grain_Direct
(d) H2O _Grain_Integral
Fig. 4.23: Least square regression analysis of Volumetric and Grain Models for
Char+H2O reactions of sample S2
127
(a) H2O_Volumetric_Direct
(b) H2O _Volumetric_Integral
(c) H2O _Grain_Direct
(d) H2O _Grain_Integral
Fig. 4.24: Least square regression analysis of Volumetric and Grain Models for
Char+H2O reactions of sample S3
It is observed from the analysis, that the reactions of char with steam (H2O), for both
the models, with each solution method, show a satisfactory linearity at all pressures in
all samples. As discussed earlier the Integral method has given good linearity with all
the cases. Volumetric Model with integral method proved best for the reactivity
calculations as this model R2>0.999.
From the above discussion, it can be concluded that the integral technique yields better
results for both the models i.e, volumetric and grain models at atmospheric as well
higher pressures. The calculated Arrhenius Parameters for char+H2O are summarized
in Table 4.9 which are in good resemblance from literature (Hecht et al., 2012, Bryan
Woodruff and Weimer, 2013, Kajitani et al., 2013). The calculated values of A and E
for steam gasification of char are in similar range as calculated by Jaffri and Zhang
(Jaffri and Zhang, 2009) for Thar lignite.
128
Fig. 4.25 (a to f) shows a representative comparison of experimental data for sample 1
with calculated data from volumetric and grain models using the integral method. Good
predictions of char conversion have observed at all pressures for both models. Hence
the validity of the volumetric model and grain model confirms for the samples selected
in this research.
Table 4.9: Arrhenius Parameters for Char+H2O
Samples Pressure Arrhenius
Parameters
Volumetric Model Grain Model
Direct Integral Direct Integral
S1
1 atm
A 2.134×107 4.172×104 2.792×104 1.626×103
E (KJ/mol) 204.0588 231.4701 142.3773 202.5872
R2 0.9919 0.9963 0.9306 0.9867
5 atm
A 1.918×1010 7.555×108 8.304×106 1.824×107
E (KJ/mol) 284.488452 320.388304 213.985732 287.032536
R2 0.998 0.9999 0.9651 0.9979
10 atm
A 7.228×1012 1.540×1012 9.819×108 2.069×1010
E (KJ/mol) 339.244456 390.999106 256.977426 352.164412
R2 0.9972 0.9999 0.9574 0.9977
S2
1 atm
A 8.024×1010 8.301×1011 2.112×106 4.467×109
E (KJ/mol) 287.881 375.9923 192.2031 329.542
R2 0.9628 0.9403 0.9628 0.9139
5 atm
A 3.015×1013 5.897×1013 5.510×108 3.188×1011
E (KJ/mol) 327.438576 395.339014 232.966594 351.191674
R2 0.9893 0.9999 0.9186 0.9969
10 atm
A 5.047×1014 1.156×1015 2.696×109 3.485×1012
E (KJ/mol) 357.950956 428.345594 250.783496 378.220488
R2 0.9856 0.9999 0.9247 0.997
S3
1 atm
A 2.274×1010 4.567×105 1.014×108 3.139×104
E (KJ/mol) 275.56753 240.2247 228.186 217.6522
R2 0.979 0.9871 0.9316 0.9913
5 atm
A 1.775×109 1.337×107 2.892×106 3.334×105
E (KJ/mol) 264.401828 284.613162 205.098066 251.565012
R2 0.9904 0.9999 0.9628 0.9971
10 atm
A 6.935×1011 2.130×1010 1.273×108 3.302×108
E (KJ/mol) 294.56502 325.343448 220.853096 290.624184
R2 0.997 0.9515 0.8992 0.9202
129
(a) S1_CO2_1 atm
(b) S1_H2O_1 atm
(c) S1_CO2_5 atm
(d) S1_H2O_5 atm
(e) S1_CO2_10 atm
(f) S1_H2O_10 atm
Fig. 4.25: Comparison of experimental and predicted conversion for Char-
CO2/H2O reactions of sample S1
4.6.3 Rate constant “k” for gasification reactions
Similar to drying, devolatization and combustion steps, the rate constant “k” was
calculated for gasification reactions i.e., reactions of char with CO2 and H2O. The
130
values of A and E calculated through integral method (see Table 4.8 and 4.9) were used
in the calculation of rate constant (k). The calculations were made at a fixed temperature
of 950°C. Fig. 4.26 shows the calculated values of ln(k) for Char+CO2 reaction with all
three samples i.e., S1, S2, and S3 at 1, 5 and 10 atm pressures. Similarly Fig. 4.27 shows
the calculated values of ln(k) for Char+H2O reaction with all three samples i.e., S1, S2,
and S3 at 1, 5 and 10 atm pressures. From these figures, it was observed that with
increasing pressure the rate constant is increasing, with both the models. The amount
of increase is highly dependent on the nature of the sample due to variation in its
composition as explained by (Fan et al., 2012). Overall it was observed that the rate
constant of the Char+CO2 reaction is higher than Char+H2O reaction at the respective
sample and pressure conditions in accordance with previous studies (Fan et al., 2012,
Kajitani et al., 2013, Kirtania et al., 2014).
(a) Volumetric Model (Integral) @950°C (b) Grain Model (Integral) @950°C
Fig. 4.26: Rate constant (k) for Char +CO2 reaction with (a) VM and (b) GM
(a) Volumetric Model (Integral) @950°C (b) Grain Model (Integral) @950°C
Fig. 4.27: Rate constant (k) for Char +H2O reaction with (a) VM and (b) GM
0
5
10
15
20
25
30
S1 S2 S3
Ln
(k)
Different Samples
1 atm 5 atm10 atm
0
5
10
15
20
25
S1 S2 S3
Ln
(k)
Different Samples
1 atm 5 atm
0
5
10
15
20
25
30
35
40
S1 S2 S3
Ln (
k)
Different Samples
1 atm 5 atm 10 atm
0
5
10
15
20
25
30
35
S1 S2 S3
Ln
(k)
Different Samples
1 atm 5 atm
131
4.7 EFFECT OF PRESSURE ON GASIFICATION KINETIC
PARAMETERS
Reaction study of Thar lignite char with CO2 and H2O reveals that the frequency factor
‘A’ and activation energy ‘E’ increase on increasing the pressure as shown in Fig. 4.28
(a-c). This is in accordance with earlier work (Wang et al., 2008, Park and Ahn, 2007).
Increase in frequency factor indicates the higher rate of reaction and this can be verified
from the increasing trend in the slope of reactivity curves at higher pressures (Fig. 4.18).
High Activation Energy means the reactions require more heat energy to initiate and it
can be verified from the reactivity curves, which clearly indicate that the reaction of
char with CO2 and H2O starts at higher temperatures as pressure is increased but due to
higher frequency factor at that pressure the conversion archives faster than lower
pressure.
132
(a) Sample S1
(b) Sample S2
(c) Sample S3
Fig. 4.28: Effect of pressure on the frequency factor (A) and activation energy
(E)
133
4.8 SUMMARY OF EXPERIMENTAL RESULTS
• The proximate and ultimate analysis of collected samples confirms the composition
of Thar lignite as reported in the literature.
• The Volumetric Model and Grain Model fit well for Moisture Removal,
Devolatization, Combustion and Gasification Processes.
• The Devolatization step was broken into low and high-temperature ranges to
increase the linearization of the models. The low-temperature range was selected
from room temp. to 500°C whereas high-temperature range was from 501 - 900°C.
• At atmospheric pressure, the inherent kinetics of Char-CO2 or Char-H2O reactions
is dominant and drives the overall reaction. At high pressures, the diffusion of
reacting gas in solid particle plays an important role and diffusion rate is dominant
over inherent kinetics of those heterogeneous reactions.
• Volumetric model and grain model have shown satisfactory results in the reactivity
study of Char-CO2 or Char-H2O reactions at atmospheric or high pressures with the
integral method. Among both Volumetric model remained best for Thar lignite
samples at all the pressures.
• Increasing pressures affect the overall kinetics of the heterogeneous reactions in
terms of increase in frequency factor and activation energy.
• Overall it was observed that the rate constant of the Char+CO2 reaction is higher
than Char+H2O reaction at the respective sample and pressure conditions
• The frequency factor (A) and activation energy (E) for moisture removal,
devolatization and combustion, and gasification steps were calculated from
Volumetric Model and Grain Model. This is important data and could be utilized as
a base for future work.
134
CHAPTER 5
CFD MODELING AND SIMULATION
5.1 INTRODUCTION
For a successful and efficient coal gasification, a deep understanding of all physical and
chemical changes involved in the process is required. Computational Fluid Dynamics
(CFD) provides an easy, cost-effective and reliable way, to study the effect of various
controlling parameters, like coal composition, oxidant to fuel ratio, residence time of
fuel particles in the system, temperature and pressure of the gasifier, rate of chemical
reactions, etc. on the gasification process. Since last 20 years, the CFD has been an
effective tool to simulate and visualize the gasification process (Chen et al., 2001,
Tominaga et al., 2000, Wu et al., 2008b). But unfortunately, this robust tool has not
been utilized for designing a gasification technology which suits the characteristics of
indigenous coal. Hence in the present research, an attempt is made to develop a generic
gasification technology using CFD that gives maximum efficiency with local coal. It is
observed from literature that Entrained flow gasifiers with multi-opposite burners are
efficient even for low-grade coals. So this type of gasifier is selected for our case.
In this study, there are two phases of CFD modeling and simulation. In the first phase,
the CFD model was developed for lab scale double stage entrained flow coal gasifier
with multiple opposite burners. The numerical simulations were performed on the
Chinese coal data and results were verified from published experimental work. In the
second phase, the modified geometry was modeled using CFD techniques for Thar coal
data. The kinetic parameters for Thar Coal Gasification were taken from the
experimental work on TGA and PTGA as discussed in Chapter 4. The commercial CFD
code ANSYS FLUENT®14.0 was used for all computations. The validation for CFD
modeling results with Thar Coal and modified geometry was carried out with the model
developed in AspnPlus®V10 software following the strategy described by Xiangdong
et al. (2013b). The individual geometries (computational domains) and numerical
setup along with boundary conditions are discussed in the subsequent sections.
135
5.2 CFD MODELING OF CHINESE COAL GASIFIER
To understand the strategies of modeling of an entrained flow gasifier, the Chinese
double stage entrained flow gasifier was selected to simulate numerically. The physical
description of the system and mathematical models used are described as under.
5.2.1 Description of Physical system for Chinese Coal Gasifier
An oxygen-blown, entrained flow coal gasifier for Chinese high-ash lignite coal is used
in this study as shown Fig. 5.1. The proximate, ultimate analysis of coal used in the
model is listed in Table 5.1. The particle diameters are fitted to the Rosin-Rammler
distribution (minimum dia is 4μm, maximum dia is 125μm, and mean dia 45.6 μm).
The dimensions and coal feeding rate (7.2kg coal/hour) are taken from earlier research
work (Tang et al., 2010c). The inner radius of the experimental gasifier is 0.32 m and
its height is 0.55 m. Four nozzles are installed at two levels of gasifier chamber named
as AA’ Level (Top) and BB’ Level (Bottom). Two opposite nozzles, exactly at the
central axis line for each level, are impinging in nature whereas the other two nozzles
are slightly at a side of the axis and produced a swirl in the flow.
Table 5.1: Properties Of Chinese Coal (Tang et al., 2010c)
Proximate Analysis (w/%) Ultimate Analysis (war/%)
Moisture 1.16 C 63.51
Ash 23.10 H 4.19
Volatile 27.03 O 6.39
Fixed Carbon 48.21 N 1.02
S 0.63
Heating Value 32.91 MJ/Kg
136
Fig. 5.1: (Left) Geometry of Gasifier with main inlets and outlets. (Right) The
Sections of gasifier at AA’ and BB’ level
5.2.2 Development of computational domain for Chinese gasifier
A 3D computational domain was developed in Ansys Meshing®14.0 with a total of
135934 tetrahedral cells. Near the burners, the high density of the grid was applied due
to high turbulence and combustion zones. Fig. 5.2 shows the developed grid and
zoomed view of nozzles where a high density of grid was applied. The minimum
orthogonal quality of the grid was 0.64.
137
Fig. 5.2: Meshed computational domain of Chinese Coal Gasifier (Geometry-A)
5.2.3 Computational Models
In present work, a numerical study was carried out with 3D, steady and incompressible
turbulence flow with heterogeneous and homogeneous reactions. Therefore, time-
averaged- steady-state Navier–Stokes, mass momentum and energy and species
equations have solved. The governing equations for the system are as follows (Silaen
and Wang, 2010).
mij
i
Sux
=
)(
jjiij
ii
jji
i
Suuxx
Pguu
x+−
+
−=
)()(
_____''
hip
ii
ip
i
STucx
T
xTuc
x++
−
=
_____' ')(
rji
i
j
i
i
ji
i
SCux
CD
xCu
x+
−
=
_____' ')(
where ij is the symmetric stress tensor and
_____''
jiuu is the Reynolds stress. The
realizable k –ε turbulence model is also employed to solve the turbulent flow. The
turbulence kinematic viscosity is calculated by:
(5.1)
(5.3)
(5.2)
(5.4)
138
/2kCt =
Where, C is the viscosity constant, k the turbulence kinetic energy and ε the turbulence
dissipation rate. K and ε are calculated from the following transport equations (Jones
and Launder, 1972, Launder and Spalding, 1974):
−+
+
=
k
ik
t
i
i
i
Gx
k
xku
x)(
kGC
kGC
xxu
xkk
i
t
i
i
i
2
21)(
−+
+
=
where Gk is the generation of turbulence kinetic energy due to mean velocity gradients.
The turbulence heat conductivity(λ) and diffusion coefficient(D) in Eqs. (5.3) and (5.4)
are given by:
it
tp
i
ipx
Tc
x
TTuc
=
−=
Pr'
_____'
i
j
t
t
i
j
ijix
C
Scx
CDCu
−=
−=
_____' '
Where, Prt (=0.85) is the turbulence Prandtl number and Sct (=0.7) is the turbulence
Schmidt number.
The Lagrangian approach has been used to calculate the motion of particles by using
the Discrete Phase Model (DPM). The trajectories of coal particles are predicted in
DPM by integrating the force balance on the coal particle when they move through the
continuous phase of the fluid (Silaen and Wang, 2010). This force balance equates the
coal particle inertia with the forces acting on the coal particle and can be written (for
the x-direction in Cartesian coordinates) as.
x
p
p
xpD
pFguuF
dt
du+
−+−=
)(
(5.5)
(5.6)
(5.8)
(5.9)
(5.10)
(5.7)
139
The interaction between the discrete phase and the continuous phase is also taken into
account by treating the heat and mass losses of the particles as the source terms in the
governing equations. The P-1 model has adapted to calculate the radiant heat in the
gasifier (Wu et al., 2008b, Gerun et al., 2008). In the P1 model, the local radiation
intensity is calculated by the equation
44 TaGaGqr −=−
Where
GCa
qss
r −+
−= )(3
1
Where a is the absorption coefficient, σs is the scattering coefficient, G is the incident
radiation, C is the linear anisotropic phase function coefficient, and σ is the Stefan-
Boltzmann constant.
5.2.4 Combustion/ Gasification Model
Species transport model (Eq. 5.4) is used for the gasification reactions chemistry. This
modeling approach gives an option to define the important reactions and set their kinetic
parameters. During the high-temperature environment of coal gasification, the coal will
be decomposed into volatiles, char, and ash (Chen et al., 2007). The compositions
released from the coal can be expressed by the following equilibrium equation (Wen
and Chaung, 1979).
Coal ➔ α1Volatiles + α2 H2O + α3Char + α4Ash
After undergoing fast heating, the hot flow around coal particles will trigger a number
of physical and chemical reactions (Du et al., 2007). The reactions include the
devolatilization of coal, the combustion of volatiles and unburned char as well as the
gasification of the char. In present work, the volatiles are lumped into one volatile gas
species C1.37H4.58O0.44 and are calculated from proximate and ultimate analysis of coal.
The volatiles release is described by a two-step devolatilization model (Du and Chen,
2006) and it is given as follows:
VolatileYCharYCoal lll
kl
+−→ )1( (for low temperature)
(5.11)
(5.12)
(5.13)
(5.14)
140
VolatileYCharYCoal hhh
kh
+−→ )1( (for high temperature)
Where Y is the stoichiometric coefficient. At low-temperature Eq. (5.14) is dominated
whereas Eq. (5.15) shows a higher rate at high temperature. The reaction kinetic
equations are as follows:
CoalYkYkdt
dVhhl )( 1 +=
)/exp( plll RTEAk −=
)/exp( phhh RTEAk −=
Where V denotes the mass fraction of volatiles, k is the reaction rate constant, A the pre-
exponential factor, TP the coal particle temperature and E the activation energy of the
reaction. The values of Yl, kl, Yh, kh, El and Eh are obtained from previous studies (Du
and Chen, 2006, Ubhayakar et al., 1977) and they are listed in Table 5.2.
When char is produced from coal devolatilization, CO and H2 can be generated from
char gasification. There are various reactions selected by different researchers to define
the gasification reaction mechanism (Silaen and Wang, 2010, Vicente et al., 2003,
Gerun et al., 2008, Bouma et al., 1999, Choi et al., 2001a, Watanabe and Otaka, 2006,
Fletcher et al., 2000, Ajilkumar et al., 2009, Silaen and Wang, 2012, Chui et al., 2009b).
First, the preliminary simulations were carried out to find the best reaction-plan among
different reactions. The details of those cases are tabulated in Table 5.2.
The various reaction mechanisms are modeled to involve the following chemical
species: C(s), O2, N2, CO, CO2, H2O, H2, and Volatiles. Finite rate/Eddy dissipation rate
model has been used to calculate the rate of formation for each species and update the
source term rS in Eq. (5. 4) by the following expression.
=
=N
j
rjjr wMS1
,
( )
−−=
==
rr N
ieq
N
i
frjrjrj CK
Ckvvw11
'
,
''
,,
''''
][1
][
)/( RTEB
faeATk
−=
(5.16)
(5.17)
(5.18)
(5.15)
(5.19)
(5.20)
(5.21)
141
In the above equation, the forward reaction rate constant fk is established based on
Arrhenius law; A is the pre-exponential factor, B the temperature exponent and Ea the
activation energy of the reaction. The values of A, B, and Ea for various reactions are
obtained from earlier studies and given in Table 5.3.
Table 5.2: Various preliminary cases to optimize the best reaction plan
Reactions Simulation Cases
A B C D E F Vol + 2.295 O2 → 1.37 CO2 + 2.29 H2O
(Volatiles complete Combustion)
Vol + 1.61 O2 → 1.37 CO + 2.29 H2O
(Volatiles partial Combustion)
C<s> + 0.5 O2 → CO (Char partial combustion)
C<s> + O2 → CO2 (Char complete combustion)
C<s> + CO2 → 2CO (Gasification, Boudourad reaction)
C<s> + H2O → CO + H2 (Gasification)
CO + 0.5 O2 → CO2 (CO combustion)
H2 + 0.5 O2 → H2O (H2 combustion)
CO + H2O ↔ CO2 + H2 (Watershif Reaction)
Table 5.3: Selected kinetic parameters for Devolatization and
Gasification/Combustion Reactions
Devolatization (Du and Chen, 2006, Ubhayakar et al., 1977) Yl ; Yh 0.3; 1
kl ; kh (s-1) 2 ×105; 1.3 ×107
El; Eh (KJ mol-1) 104.6; 167.4
Combustion/Gasification Reactions (Chen et al., 2012) A B Ea(J Kmol-1)
Heterogeneous Reactions (Solid-Gas Phase) C(s) + O2 → CO2 0.002 0 7.9×107
C(s) + 0.5O2 → CO 0.052 0 6.1×107
C(s) + CO2 → 2CO 242 0 2.75×108
C(s) + H2O → CO + H2 426 0 3.16×108
Homogeneous Reaction (Gas Phase) CO + 0.5O2 → CO2 2.239×1012 0 1.7×108
H2 + 0.5 O2 → H2O 6.8×1015 0 1.68×108
CO +H2O ↔ CO2+ H2 (WGS Reaction) f 2.75×1010 0 8.38×107
b 2.65×10-2 0 3.96×103
C1.37H4.58O0.44(Volatile)+1.61 O2→1.37 CO+2.29 H2O
(Volatile Partial Combustion)
2.119×1011 0 2.027×108
C1.37H4.58O0.44(Volatile)+2.295 O2→1.37 CO2+2.29 H2O
(Volatile Complete Combustion)
2.119×1011 0 2.199×1011
142
5.2.5 Boundary Conditions and Calculation Methods
The mass-flow inlet and pressure-outlet boundary conditions were used for all
input/output streams. Buoyancy force has considered in the present model. Water
cooled walls were assumed to be at constant temperature at 800 K. No-slip state (zero
velocity) is applied on the surfaces of walls. Steady-state simulations were carried out
with an implicit pressure-correction scheme (pressure-based solver) by decoupling
energy and momentum equations. For coupling the velocity and pressure, a SIMPLE
algorithm was followed. Convective terms were spatially discredited by the second-
order-upwind scheme. The values for temperature dependent properties were calculated
using piecewise-polynomial equations for all gas and solid species. Convergence of the
solution was achieved when the mass, turbulent kinetic energy and momentum
residuals satisfied at 10-3 and residuals for energy and radiation at 10-6. Parallel
processing was used for computation. The cold flow simulations were converged first
in all the cases then the reacting flows were solved by activating all reactions along
with the injection of coal with an ignition temperature of 2000 K.
5.3 CFD MODELING OF NEWLY DESIGNED COAL GASIFIER
After the simulations on Chinese coal gasifier with Chinese coal data, the geometry of
the gasifier was modified. The details of geometry and other physical features are
discussed in the following section.
5.3.1 Description of Physical system for Newly Designed Gasifier
The geometry of oxygen-blown, entrained flow coal gasifier with multiple opposite
burners (shown in Fig. 5.1) was modified by implying a throat section. The basic reason
for doing this is to increase the residence time of the coal particle in the upper section.
Fig. 5.3 shows the 3 views of new geometry along with its dimension. The proximate,
ultimate analysis of Thar coal was used in the model and given in previous Chapter
(Table 4.4 and 4.5). The particle diameters are fitted to the Rosin-Rammler distribution
(minimum dia is 4μm, maximum dia is 125μm, and mean dia 45.6 μm).
143
Fig. 5.3: Different views of newly designed gasifier operating conditions
5.3.2 Development of Computational Domain for Proposed Geometry
3D computational domain was developed in Ansys Meshing®14.0 with a total of
126275 tetrahedral cells. Similar to the previous geometry case, near the burners the
high density of grid was applied due to high turbulence and combustion zones. Fig. 5.4
shows the developed grid and zoomed view of nozzles where a high density of grid was
applied. The minimum orthogonal quality of grid was achieved 0.53 which usually
come in excellent criteria.
Fig. 5.4: Meshed computational domain of Proposed Gasifier (Geometry-B)
144
5.3.3 Probability Density Function (PDF) approach
Under species modeling approach as discussed in section 5.2.4, there are few other
options available to calculate the species chemistry apart from the chemical reactions.
Among those Non-premixed modeling is prominent which utilize Probability Density
Function (PDF) approach. Non-premixed modeling involves the solution of transport
equations for one or two conserved scalars (the mixture fractions). Equations for
individual species are not solved. Instead, species concentrations are derived from the
predicted mixture fraction fields. The thermochemistry calculations are preprocessed
and then tabulated for look-up in ANSYS FLUENT. Interaction of turbulence and
chemistry is accounted for with an assumed-shape Probability Density Function (PDF)
(Ansys, 2011).
The basis of the non-premixed modeling approach is that under a certain set of
simplifying assumptions, the instantaneous thermochemical state of the fluid is related
to a conserved scalar quantity known as the mixture fraction, f. The mixture fraction
can be written in terms of the atomic mass fraction as (Sivathanu and Faeth, 1990)
oxifueli
oxii
ZZ
ZZf
,,
,
−
−=
Where Zi is the elemental mass fraction for the element, i. The subscript ‘ox’ denotes
the value at the oxidizer stream inlet and the subscript ‘fuel’ denotes the value at the
fuel stream inlet. In this research, few simulations are carried out using the PDF
approach with geometry B. Then the results are compared with species transport
approach.
5.4 MODEL DEVELOPMENT IN ASPEN PLUS®V10 FOR VALIDATION
OF MODIFIED GEOMETRY RESULTS
The CFD modeling results with modified geometry and Thar coal feedstock were
verified and validated by conducting a comparative study between CFD results and the
results extracted from the model developed in Aspen Plus®V10 software. The modeled
entrained flow gasifier in Aspen Plus was initially developed by Aspen (2010) and then
(5.22)
145
modified by Xiangdong et al. (2013b). The following are modified from previous work
(Aspen, 2010, Xiangdong et al., 2013b).
i. The properties of components like the composition of feed coal along with its
calorific value.
ii. The composition and quantity of pyrolysis gases, char, and tars produced during
devolatilization stage.
iii. Gasification operating and design parameters like flowrates of coal, oxygen and
steam, temperature and pressure, the diameter of gasifier and height of gasifier.
iv. Indigenous kinetics of reactions was inserted through FORTRAN coding.
The chemical species present in the process are tabulated in Table 5.4. In the list C6H6
represents tar, Char 1 represents the pyrolyzed coal at 1 atm whereas the Char2 is the
corrected solid phase of coal. The composition of Thar lignite (calculate from this
research) was inserted through the Coal stream.
Table 5.4: The chemical species used in the model
Symbol Category Title
CO2 Conventional Carbon Dioxide
CO Conventional Carbon monoxide
H2 Conventional Hydrogen
O2 Conventional Oxygen
N2 Conventional Nitrogen
H2S Conventional Hydrogen sulfide
H2O Conventional Water
CH4 Conventional Methane
C6H6* Conventional Benzene
C Solid Carbon graphite
S Solid Sulfur
COAL Non-Conventional -----
CHAR1* Non-Conventional -----
CHAR2* Non-Conventional -----
ASH Non-Conventional -----
Fig. 5.5 displays the model of the gasification process in terms of a process flow
diagram created in Aspen Plus®V10. The quenching system of hot gases released from
146
the gasifier is not included in the developed model. Table 5.5 highlights important
blocks used in the model along with its core function. The process of coal pyrolysis is
simulated through PYROLYS and PRESCORR blocks. The combustion process of
volatiles is modeled by COMBUST block. The block of GASIFIER is used to simulate
the gasification process of char. Rest of blocks are helping aid in the construction of
whole simulation process.
Fig. 5.5: The process flow sheet for the simulation model of Entrained Flow
Gasifier (Aspen, 2010, Xiangdong et al., 2013b)
As per general facts, gasification process is based on three fundamental stages, so
accordingly during modeling of the gasification process in Aspen Plus, these three
stages are simulated via three kinds of unit operations. These are explained as follows:
5.4.1 Pyrolysis of Coal
PRESCORR and PYROLYS blocks are two RYield reactors which are used in the
current model to simulate the process of coal pyrolysis. The PYROLYS block being
first RYield reactor is simulating the pyrolysis of coal at 1 atm based on devolatization
experimental work. Then PRECORR block (second RYield reactor) is correcting the
obtained yield of individual species released during pyrolysis on the basis of operating
pressure of gasifier. A user-defined function USPRES is automatically correct the yield
calculations within the model.
147
5.4.2 Combustion of Volatiles
The combustion kinetics of volatiles is not considered in the model as the time-scale of
gas combustion is too short and all the combustible gasses converted in a shorter time.
The combustion of volatiles is simulated through the COMBUST block based on
RStoic reactor tool available in Aspen Plus by setting fractional conversions at 1.0 for
all combustible gases.
5.4.3 Char gasification
The process of char gasification is modeled using RPlug reactor and the block named
as GASIFIER. This block performs the gasification reaction calculations based on two
important factors i.e., gasification reaction kinetics (which has been calculated in during
experimental work in present research) and residence time of char within the gasifier.
Based on these two factors, the composition and yield of syngas are calculated.
TABLE 5.5: Functions of each block used in Aspen Plus®V10 Model
Model/Tool Block Function of Block
RYield PYROLYS The pyrolysis of coal is simulated based on experimental
work for devolatization at atmospheric conditions (1 atm).
RYield PRESCORR Pressure corrections are being made in this block for the
yield and composition of volatiles produced during
pyrolysis from 1atm to gasifier’s operating pressure.
Sep2 SEPSG Solid char and gases are being separated in this block
RStoic COMBUST The combustion of violates is simulated through this block
RStoic SEPELEM Char species is decomposed into its elementary
components (like C, H2, O2, N2, S, and ash) for defining
heterogeneous reactions easily in GASIFIER block.
Mixer MIXER The feedstock is being mixed for inserting in the
GASIFIER block.
RPlug GASIFIER Model the char gasification process
Calculator SPELMCAL Stoichiometric coefficients of C, O2, H2, S, N2, and ash are
determined for solving reaction in SEPELEM block.
Calculator GASIFCAL Calculate the residence time of solid char which is then
used in GASIFIER block.
148
5.5 MODELING AND SIMULATION RESULTS FOR CHINESE LIGNITE
After the ending of experimental investigations, the second phase of research was
started in which the performance of entrained flow gasifier with multiple burners
(injectors) was evaluated through numerical simulations using commercial CFD code
ANSYS FLUENT®14.0. The reaction and fluid dynamics was calculated with the help
of numerical computations. For making best CFD modeling strategy, initially, a
published experimental work was selected on a working entrained flow coal gasifier
with multiple burners at China whose detailed physical description along with geometry
is given in section 5.2 (Fig. 5.1). The Chinese coal was used as feedstock of the gasifier.
The properties of Chinese coal are given in Table 5.1. The gasifier is a double stage,
where the upper stage is referred as AA’ level and the lower level is referred as BB’
level (Fig. 5.1).
Various cases were simulated by varying the mass fraction (%) of total coal or total
oxidant at both injection levels. The effects of coal and oxidant distributions for two
stages were investigated by maintaining the overall oxygen/coal ratio at a constant
value of 1 as per available experimental condition (Tang et al., 2010c). The names of
simulation cases referred to the percentages of coal and oxygen at top level (AA'-
Level). Table 5.6 describes parameters of simulated conditions of all cases.
149
Table 5.6: Operated parameters for Simulated Cases
Case
Name
1st Level (Upper Level, AA'-Level) 2nd Level (Down Level, BB'-Level)
Fraction
of Total
Coal
Mass
Flow of
Coal
Fraction
of Total
Oxygen
Mass
Flow of
Oxygen
Fraction
of Total
Coal
Mass
Flow of
Coal
% of
Total
Oxygen
Fraction
of Total
Oxygen
% kg/sec % kg/sec % kg/sec % kg/sec
C30_O40 30 0.0006 40 0.0008 70 0.0014 60 0.0012
C40_O40 40 0.0008 40 0.0008 60 0.0012 60 0.0012
C50_O40 50 0.001 40 0.0008 50 0.001 60 0.0012
C60_O40 60 0.0012 40 0.0008 40 0.0008 60 0.0012
C70_O40 70 0.0014 40 0.0008 30 0.0006 60 0.0012
C30_O50 30 0.0006 50 0.001 70 0.0014 50 0.001
C40_O50 40 0.0008 50 0.001 60 0.0012 50 0.001
C50_O50 50 0.001 50 0.001 50 0.001 50 0.001
C60_O50 60 0.0012 50 0.001 40 0.0008 50 0.001
C70_O50 70 0.0014 50 0.001 30 0.0006 50 0.001
C30_O60 30 0.0006 60 0.0012 70 0.0014 40 0.0008
C40_O60 40 0.0008 60 0.0012 60 0.0012 40 0.0008
C50_O60 50 0.001 60 0.0012 50 0.001 40 0.0008
C60_O60 60 0.0012 60 0.0012 40 0.0008 40 0.0008
C70_O60 70 0.0014 60 0.0012 30 0.0006 40 0.0008
Note: The highlighted case (C50_O60) is the case at original experimental conditions (Tang et al., 2010c)
and the results of that case will be used to validate the model.
5.5.1 Identification of Best Reaction Mechanism for Lignite Coal and Validation
of the CFD model
The reaction mechanism is an important aspect to understand gasification modeling.
Researchers proposed various mechanisms of gasification among which few most
commonly applied mechanisms are highlighted in Table 5.2. Fig.5.6 shows the
comparison between the experimental results (Tang et al., 2010c) and various
preliminary simulation cases from A to F mentioned in Table 5.2. In all those cases the
coal or oxygen distribution between the two stages is kept constant as per experimental
conditions (Coal 50% (of total) at AA' level and O2 60% (of total) at AA' level). The
oxygen/carbon (O/C) ratio was taken 0.9, 1.0 and 1.1. The temperature profiles along
the axis of the gasifier for all simulated cases and experimental observations are shown
in Fig. 5.7 at different O/C ratios.
150
(a) (b)
(c)
Fig. 5.6. The comparison of various preliminary simulation results with
experimental values (Tang et al., 2010c) for CO, H2 and CO2 mole fraction in
Syngas (a) at O/C ratio=0.9, (b) at O/C ratio=1.0 and (c) at O/C ratio=1.1.
151
(a) at O/C ratio=0.9 (b) at O/C ratio=1.0
(c) at O/C ratio=1.1
Fig. 5.7: The temperature profile for various preliminary simulated cases and
experimental work along central axis of gasifier (a) at O/C ratio=0.9, (b) at O/C
ratio=1.0 and (c) at O/C ratio=1.1
It is clear from Fig. 5.8 for all O/C ratio cases, that there is a great impact for reaction
sets on the overall composition of syngas. Case A follows the most common reaction
sets i.e. the partial combustion of char and volatiles and the involvement of CO
combustion in the bulk gas phase. But from Fig. 5.8 (a to c), it shows that it predicts
quite less CO and high CO2 as compared to experimental values for this particular type
of gasifier with different arrangements of injecting nozzles. The possible cause is that
the well-mixed pattern of particles with gas due to impinging and tangential injection
152
patters of fuel and oxygen. The velocity vectors and particles are shown in Fig. 5.8 and
Fig. 5.9 respectively that gives a clear idea of the flow behavior of gas inside the gasifier
along with the 250 collisions of particles. The Fig. 5.9 shows the particles' residence
time for 200 tracks only with limited end time i.e. 0.8sec. Fig. 5.9 confirms the idea that
there is a greater chance to have the particle combustion as compared to CO combustion
due to homogeneous temperature mixing but in case A the false unrealistic prediction
is visible due to rapid CO combustion and increasing the temperature of overall gas
mix. The excess temperature rise can also be verified in Fig. 5.9 for that case. Rest of
the cases (from B to F) shows a good agreement of H2 mole % but there are significant
differences for CO and CO2 mole %. In this regard, Case E shows the best results
compared with experimental values for prediction of syngas composition and
temperature at different O/C conditions. The reaction mechanism of case E contains the
complete combustion of volatiles and char and ignoring the CO combustion reaction.
The values of CO and H2 mole % for this case are in good agreement and show less
than 1% error while the prediction of CO2 is moderate but good as compared to other
cases. The one reason for not predicting the good CO2 is the assuming no formation of
other species like CH4 etc. Temperature prediction through case E is also in satisfactory
limits (less than 1% error) so the Case E has been validated and the solution strategies
and reaction mechanism of Case E has followed to conduct the rest of studies.
153
Fig. 5.8. The velocity Vectors at the Sectional Planes of the gasifier (Case E) with
a close view of AA' and BB' planes.
Fig. 5.9. The particles residence time at the Sectional Planes of the gasifier (Case
E) (for clarity only 200 particle tracks are shown with particle end time limited
at 0.8 sec).
154
5.5.2 Effects of Coal/Oxygen Distribution on syngas composition
15 different cases were simulated with coal/oxygen distribution variations at both the
levels as per Table 5.5. The effects on the mole % of CO, CO2, and H2 by varying the
total coal % at AA' level are shown in Fig. 5.10. As per Fig. 5.10(a), the mol% of CO
shows first increasing and then decreasing trend by increasing coal % at AA' level with
40 and 50% oxygen at the same level. This behavior is slightly changed with 60%
oxygen case where it is almost of increasing order. The optimized conditions for
maximum CO production (52.59%) was found at 50% Coal and oxygen at AA' level
(C50_O50 case). The minimum CO mol % was observed 50.123% in C30_O60 case.
Exactly inverse trends can be seen for CO2 mol % with respective cases (Fig. 5.10(b)).
A plausible cause for these trends is the variation of Oxygen/Coal ratio at local levels
and the rate of mixing due to vortex formation with the tangential nozzles (either at AA'
level or BB' level) as explained by Seo et al (Seo et al., 2011). The maximum and
minimum for CO2 mol % were observed 19.98% and 18.39% in C30_O60 and
C50_O40 cases respectively.
The production for H2 was found in the range of 27 to 28% in all the cases as per Fig.
5.10(c). In this narrow range (27-28%) it shows a bit increasing and then decreasing
trend with increasing coal at AA' level for 50% oxygen; a decreasing trend with 40%
oxygen and almost increasing behavior with 60% at AA' level. The reason for this
behavior of H2 production in these various cases is the non-availability of abundant
water because all the gasification simulations were carried out without steam. Further,
there is also less moisture (1.16%) present in the coal as per its proximate analysis (refer
to Table 5.1). The less presence of water has a great influence on the water-shift reaction
as per earlier studies (Lu and Wang, 2013a, Lu and Wang, 2013b).
155
(a) CO Mol % (b) H2 Mol %
(c) CO2 Mol %
Fig. 5.10: The Mole % for CO, H2, and CO2 with variation in total Coal/Oxygen
% at AA' Level
5.5.3 Effects on Char Conversion
The coal/oxygen variation at both distributions has an impact on its local ratio
distribution and that has also impact on the overall char conversion. This impact is
shown in Fig. 5.11 for various simulated cases. In general, an increase can be seen in
conversion by increasing the coal at the top level (from 30% to 50%) with all oxygen
cases. But conversion decreases by increasing coal from 50% to 70% at the top level
(AA') with 40% oxygen at that level. The other two conditions (50% and 60% Oxygen
at the top) show a continual increase in conversion by increasing coal from 50% to
70%. This behavior is due to greater residence time of the coal injected at top-level
(AA' level) as compared to the coal injected at the bottom level (BB' level) according
156
to the conclusions of earlier studies (Guo et al., 2007, Watanabe and Otaka, 2006). So
the higher amount of coal injected at AA' level with sufficient oxygen has enough
residence time to reach at maximum conversion, usually more than 99%. The minimum
char conversion i.e., 95.45% was found with 30% coal and 40% oxygen at top level
case (C30_O40) whereas the maximum char conversion was found 99.7% in C60_O50
and C70_O60 both the cases.
Fig. 5.11: The Char Conversion with variation in Coal/Oxygen % at AA' Level
5.5.4 Effects on Syngas Exit Temperature and Maximum inside Temperature
The temperature plays an important role in the gasification system. The variations in
the coal and oxygen at both the injection levels actually affect local oxygen/coal ratio
as discussed in previous sections and hence the defined combustion/gasification
reactions occurred with different rates. Because of the difference of those exothermic
and endothermic reaction rates, there is an increase or decrease in the overall
temperatures. Fig. 5.12 shows an overall picture for the syngas exit temperatures for
simulated cases. It is clear that in all the cases the exit temperature for syngas is in the
range from 1250 K to 1450 K. Chao Li (Li et al., 2012) observed that in the center of
impinging zone there is an increase of particle cohesion and agglomeration by resulting
a high particle concentration region. The rapid deceleration of particles near the center
157
of impinging zone improves the performance of gasifier in terms of heat and mass
transfer.
Fig. 5.12: Syngas Average Exit Temperature (K)
The radial temperature profiles from the center can be seen in Fig. 5.13 for three coal
distribution scenarios. It is evident that there is a great impact for the local temperature
distribution due to variation in coal or oxidant between two stages. Further, the results
can be verified by examining the temperature profile contours of the top and bottom
injection sectional planes as given in Fig. 5.14.
It is apparent from those temperature contours that the inside temperature increases by
increasing the amount of coal at any level with appropriate amount of oxygen. The
maximum temperature was observed 2027 K at AA' level with 70% of total Coal and
60% of total oxygen (Case: C70_O60, Fig.5.30(c)).
158
Fig. 5.13: Radial Temperature profiles for various simulation cases at different
heights of gasifier
159
(a) 40% Oxygen at AA' Level
(b) 50% Oxygen at AA' Level
(c) 60% Oxygen at AA' Level
Fig. 5.14: Temperature Contours for Sectional Planes at AA' Level and BB'
Level
160
5.5.5 Effects of coal distribution on particle trajectories
The particle residence time is key parameters for the conversion of char and occurrence
of different reactions with their specific rates, as explained in an earlier section. The
effects on the particle residence time can be depicted from Fig. 5.15, where particles
trajectories are shown for variation of coal distribution with fixed 50 % oxygen
distribution at both the levels. For clarity, the trajectories are limited to 20 particle
streams with maximum of 0.2 seconds. The variation in coal at any injection level with
fixed oxygen actually impacted on the local particle/gas volume ratio and hence
ultimately impacted on the particle trajectories. It is observed that the less particle/gas
ratio increases the particles residence time and confirms the previous studies (Steiler et
al., 1996).
Fig. 5.15: The particles residence time at the Sectional Planes of the gasifier;
Cases: C30_O50, C50_O50, and C70_O50
(Note: For clarity, only 20 particle tracks are shown with particle end time limited at 0.2 sec).
5.5.6 Effects on Turbulent Intensity
Turbulent intensity has predicted by solving the equations of the reliable k-ɛ model from
Eq. 5.5 to 5.9. Due to impinging and tangential nozzles at AA' and BB' levels, the
major turbulence has observed on the sectional planes on these levels. The effects of
coal and oxygen distribution on Turbulent Intensity can be visualized from Fig. 5.16.
161
The turbulent intensity of 108% to 300 % was observed at the main reaction zone on
both the planes as per previous observation (Vascellari and Cau, 2012). The obvious
reason is the high mixing rate at these sectional planes for injectors.
(a) at constant 50% coal at both injection levels
(b) at constant 50% Oxygen at both injection levels
Fig. 5.16: Turbulent Intensity (%) Contours for selected cases
5.5.7 Heat generation and consumption analysis
The overall energy balances accounted through Eq. (5.3). The primary source for
generation of heat energy is from exothermic reactions. Then the heat is consumed by
endothermic reactions and remaining heat is either absorbed by walls which are
assumed to be at a constant temperature of 800 K (water cooled system) or taken out
with syngas. A fraction of heat is also consumed by the solid fuel particles to raise their
temperature for drying, devolatization and then reactions. The net heat generated
162
through reactions has observed in the range of 3000 W and shown in Fig. 17 for a few
selected cases. The maximum heat consumed by the solid fuel particles and observed
in the range of 18700 W (Fig. 5.17). Heat lost through walls is found in the range of
9000 W whereas the heat carried away by syngas is found in 2000 to 26000 W (Fig.
5.18).
Fig. 5.17: Net heat generated due to reactions (W) for selected cases
Fig. 5.18: Heat consumption in different ways
163
5.6 MODELING AND SIMULATION RESULTS FOR THAR LIGNITE
After the successful validation of CFD modeling and simulation results from available
Chinese coal data, the geometry of the gasifier was partially modified by introducing a
throat section (with shorter diameter) between two stages. The geometry details are
given in section 5.3 and Fig. 5.3.
Various cases were simulated using modified geometry and then compared the results
with the previous geometry results. All the simulation cases are grouped into four major
blocks as described below:
Block 1: Simulations with Probability Density Function (PDF) Method for optimization
of O/C ratio.
Block 2: Simulations with Species Transport Model (Kinetic Approach) for
optimization of O/C ratio.
Block 3: Simulations at different Pressures with Optimized Kinetic Model and O/C
ratio.
Block 4: Simulations at different Pressures with Increased Feed Flow.
The case information is group-wise tabulated in Table 5.7 to 5.10.
Table 5.7: Simulation Cases in Block 1 (PDF model used to calculate the species)
Sr.
No. Case Name Geometry
Type
of
Coal
Pressure
Coal
Feed
Rate
O2 Feed
Rate O/Coal
Ratio
O/C
Ratio
atm (Kg/sec) (Kg/sec)
3 A_2.963OC_PDF Previous
Geometry
Thar
Coal 1 0.0028 0.003 1.0714 2.963
4 A_2.53OC_PDF Previous
Geometry
Thar
Coal 1 0.0028 0.0024 0.8571 2.53
5 A_2.963OC_PDF Previous
Geometry
Thar
Coal 1 0.028 0.03 1.0714 2.963
6 B_2.963OC_PDF
Modified
Geometry
with Neck
Thar
Coal 1 0.0028 0.003 1.0714 2.963
7 B_2.819OC_PDF
Modified
Geometry
with Neck
Thar
Coal 1 0.002 0.002 1 2.819
164
Table 5.8: Simulation Cases in Block 2 (Species Model used with finite rate
chemistry) – Optimization of Oxygen-to-Carbon (O/C) Ratio
Sr. No.
Case Name Geometry Type
of Coal
Pressure Coal Feed
Rate O2 Feed
Rate O/Coal Ratio
O/C Ratio
Atm (Kg/sec) (Kg/sec)
1 A_2.097OC_Kin Previous Geometry Thar Coal
1 0.0028 0.0018 0.6429 2.097
2 B_2.097OC_Kin Modified Geometry
with Neck Thar Coal
1 0.0028 0.0018 0.6429 2.097
3 B_1.665OC_Kin Modified Geometry
with Neck Thar Coal
1 0.0028 0.0012 0.4286 1.665
4 B_1.881OC_Kin Modified Geometry
with Neck Thar Coal
1 0.0028 0.0015 0.5357 1.881
5 B_2.314OC_Kin Modified Geometry
with Neck Thar Coal
1 0.0028 0.0021 0.75 2.314
6 B_1.89OC_Kin Modified Geometry
with Neck Thar Coal
1 0.002 0.00108 0.54 1.89
Table 5.9: Simulation Cases in Block 3 (Different Pressures at Optimized O/C
ratio)
Sr. No.
Case Name Geometry Type
of Coal
Pressure
Coal Feed Rate
O2 Feed Rate
O/Coal Ratio
O/C Ratio
atm (Kg/sec) (Kg/sec)
1 B_1.881OC_Kin_P2 Modified Geometry
with Neck Thar Coal
2 0.0028 0.0015 0.5357 1.881
2 B_1.881OC_Kin_P3 Modified Geometry
with Neck Thar Coal
3 0.0028 0.0015 0.5357 1.881
3 B_1.881OC_Kin_P4 Modified Geometry
with Neck Thar Coal
4 0.0028 0.0015 0.5357 1.881
4 B_1.881OC_Kin_P5 Modified Geometry
with Neck Thar Coal
5 0.0028 0.0015 0.5357 1.881
5 B_1.881OC_Kin_P7 Modified Geometry
with Neck Thar Coal
7 0.0028 0.0015 0.5357 1.881
6 B_1.881OC_Kin_P10 Modified Geometry
with Neck Thar Coal
10 0.0028 0.0015 0.5357 1.881
7 B_1.881OC_Kin_P20 Modified Geometry
with Neck Thar Coal
20 0.0028 0.0015 0.5357 1.881
165
Table 5.10: Simulation Cases in Block 4 (Different feed rates with varying
Pressures at fixed Optimized O/C ratio) – Optimization of Feed Rate.
Sr. No.
Case Name Geometry Type
of Coal
Pressure Coal Feed
Rate O2 Feed
Rate O/Coal Ratio
O/C Ratio
atm (Kg/sec) (Kg/sec)
1 B_1.88OC_Kin_P5_2 Modified Geometry with Neck
Thar Coal
5 0.0056 0.003 0.5357 1.8809
2 B_1.88OC_Kin_P5_1 Modified Geometry with Neck
Thar Coal
5 0.0028 0.0015 0.5357 1.8809
3 B_1.88OC_Kin_P10_1 Modified Geometry with Neck
Thar Coal
10 0.0028 0.0015 0.5357 1.8809
4 B_1.88OC_Kin_P1_2 Modified Geometry with Neck
Thar Coal
1 0.0056 0.003 0.5357 1.8809
5 B_1.88OC_Kin_P10_2 Modified Geometry with Neck
Thar Coal
10 0.0056 0.003 0.5357 1.8809
6 B_1.88OC_Kin_P5_2 Modified Geometry with Neck
Thar Coal
5 0.0056 0.003 0.5357 1.8809
7 B_1.88OC_Kin_P10_2
@ 10 atm kin
Modified Geometry with Neck
Thar Coal
10 0.0056 0.003 0.5357 1.8809
8 B_1.88OC_Kin_P1_3 Modified Geometry with Neck
Thar Coal
1 0.0084 0.0045 0.5357 1.8809
9 B_1.88OC_Kin_P5_3 Modified Geometry with Neck
Thar Coal
5 0.0084 0.0045 0.5357 1.8809
10 B_1.88OC_Kin_P10_3 Modified Geometry with Neck
Thar Coal
10 0.0084 0.0045 0.5357 1.8809
11* B_1.88OC_Kin_P5_3 Modified Geometry with Neck
Thar Coal
10 0.0084 0.0045 0.5357 1.8809
The above-mentioned cases were simulated using the validated numerical schemes and
reaction mechanism. After getting the converged solution of each simulated case, the
results regarding the volatile conversion, char conversion, syngas composition, and
outlet temperature are tabulated from Table 5.10 to 5.13.
166
Table 5.11: Results for Simulation Cases in Block 1 (PDF model used)
Sr. No.
Case Name Devolatization
Char Conv.
Syngas Analysis (mol %) Exit
Temp
% % CO CO2 H2 H2O O2 Vol (K)
1 A_2.963OC_PDF 100 58.69 6.651 37.48 1.834 53.3 0 - 2196
2 A_2.53OC_PDF 100 34.56 9.982 34.29 3.96 51.5 0 - 1823
3 A_2.963OC_PDF 100 56.34 5.299 39 1.742 53.8 0 - 1910
4 B_2.963OC_PDF 100 60.03 8.561 35.5 2.378 52.6 0 - 2262
5 B_2.819OC_PDF 100 44.6 7.388 36.85 2.181 53.2 0 - 2105
Table 5.12: Results for Simulation Cases in Block 2 (Species model used)
Sr. No.
Case Name Devolatization
Char Conv.
Syngas Analysis (mol %) Exit
Temp
% % CO CO2 H2 H2O O2 Vol (K)
1 A_2.097OC_Kin 100 99.67 13.6 37.51 35.85 0 0 12.43 1400
2 B_2.097OC_Kin 100 100 13.96 37.17 35.9 0 0 12.8 1322
3 B_1.665OC_Kin 90.27 74.19 17.57 30.85 33.01 0 0 18.4 974
4 B_1.881OC_Kin 97.7 98.24 24.19 27.54 32.35 0 0 15.76 1065
5 B_2.314OC_Kin 100 100 1.5 48.22 38.92 0.98 0 10.25 1589
6 B_1.89OC_Kin 90.25 92.12 18.98 31.62 34.24 0 0 15.09 996.6
Table 5.13: Results for Simulation Cases in Block 3 (Effect of Pressure on
gasification)
Sr.
No. Case Name
Devolatization Char
Conv. Syngas Analysis (mol %)
Exit
Temp
% % CO CO2 H2 H2O O2 Vol (K)
1 B_1.881OC_Kin_P2 98.25 93.75 22.85 28.39 33.26 0 0 15.38 999.9
2 B_1.881OC_Kin_P3 96.35 88.56 20.67 30.31 33.7 0 0 15.23 995.1
3 B_1.881OC_Kin_P4 95.82 87.74 19.43 31.25 34.25 0 0 14.95 962.8
4 B_1.881OC_Kin_P5 95.34 86.57 18.81 31.65 34.54 0 0 14.8 946.4
5 B_1.881OC_Kin_P7 94.26 84.71 14.8 34.8 35.14 0 0 14.9 938.7
6 B_1.881OC_Kin_P10 94.77 84.52 6.67 29.2 27.6 0 0 11.53 931.2
7 B_1.881OC_Kin_P20 92.21 83.11 5.5 36.5 23.4 0 0 12.2 923
167
Table 5.14: Results for Simulation Cases in Block 4 (Effect of Pressure on
gasification)
Sr.
No. Case Name
Devolatization Char
Conv. Syngas Analysis (mol %)
Exit
Temp
% % CO CO2 H2 H2O O2 Vol (K)
1 B_1.88OC_Kin_P5_2 100 99.99 25.8 26.2 32.6 0 0 15.3 1079
2 B_1.88OC_Kin_P5_1 96.58 89.39 17 33 35.4 0 0 14.5 954.7
3 B_1.88OC_Kin_P10_1 98.57 92.32 7.77 31.1 28.9 0 0 12.36 1087
4 B_1.88OC_Kin_P1_2 99.83 98.99 24.76 27 32.3 0 0 15.7 1244
5 B_1.88OC_Kin_P10_2 98.55 95.33 23.12 28.1 33.9 0 0 14.7 1003
6 B_1.88OC_Kin_P5_2 99.97 99.9 24.58 27.12 33.14 0 0 14.8 1086
7 B_1.88OC_Kin_P10_2
@ 10 atm kin 95.67 85.15 24.9 26.5 32.7 0 0 15.7 994
8 B_1.88OC_Kin_P1_3 99.71 98.78 26.4 25.7 32.6 0 0 15.2 1276
9 B_1.88OC_Kin_P5_3 100 100 25.6 26.6 32.5 0 0 15.3 1183
10 B_1.88OC_Kin_P10_3 100 100 25.2 26.7 32.7 0 0 15.1 1129
11* B_1.88OC_Kin_P5_3 99.95 99.8 26 26.02 32.6 0 0 15 1112
5.6.1 Effects of different models and O/C ratio
From the results, it was observed that PDF model predicts less molar composition of
syngas components (Table 5.11) as compared to the species transport model (Table
5.12) due to the involvement true kinetic expression during the computations. CO is
coming 6-7% whereas less than 2% H2 is achieved through PDF model with modified
geometry.
The results regarding the mole % of CO, CO2, and H2 at the exit of gasifier during the
simulations for various O/C are plotted in Fig. 5.19 whereas the devolatization and char
conversion against O/C ratios are plotted in Fig. 5.38. From Fig. 5.19 it can be seen that
the increase in O/C ratio initially increases the CO mole % but then decreases. Its
optimized value achieved about 24% at 1.881 O/C ratio. At this O/C ratio, the minimum
CO2 was coming out from the gasifier. There is a minute increase can be seen in H2 but
168
that is not very much significant. Further, the char conversion was also achieved
maximum at this ratio (Fig. 5.20) so 1.881 O/C ratio is considered as the optimized ratio
for further simulations.
Fig. 5.19: The mole % of CO, CO2, and H2 at the exit of gasifier at different O/C
Ratios
Fig. 5.20: The devolatization and Char conversion at different O/C Ratios
5.6.2 Effect of pressure on syngas composition and char conversion
The simulations were carried out at various pressure from 1 atm to 20 atm as per Table
5.13. The variations in the syngas composition (CO, CO2, and H2) are shown in Fig.
5.21 whereas the char conversion along with exit temperature are shown in Fig. 5.22
169
with respect to pressure. It is evident from these figures that pressure has inverse effects
on the syngas composition. An increase of pressure decreases the percentage of CO and
H2 components in the syngas whereas CO2 content increase with increase of pressure.
This is due to the kinetics effects already discussed in the kinetic experimental
discussion at high pressure. The same behavior is witnessed from the Char conversion
history at various pressures (Fig. 5.22). Char conversion decreases with increasing
pressure due to diffusional resistances increases on the char particles. The exit gas
temperature decreases as less amount of char converted in the chamber.
Fig. 5.21: Effect of Pressure on the composition of CO, CO2, and H2 in the syngas
Fig. 5.22: Effect of Pressure on char conversion and exit gas temperature
170
5.6.3 Streamlines-Flow analysis for Multi-Opposite Burners
The designed gasifier is entrained flow gasifier with multi-opposite burners. The
Chinese gasifier was modified for meeting the Thar lignite requirements by introducing
a neck section between the two stages. The streamlines-flow analysis after simulation
was conducted for both the geometries and compared as shown in Fig. 5.23. It is seen
that the without throat (Geometry-A) there is less residence time and flow is smooth
going down. This is good for the coal with high fixed carbon and less moisture but with
high moisture, there must be some delay for complete drying between 1st and 2nd stage.
This constraint is removed by having a throat which restricts the smoothness of the flow
and creates some positive disturbance. One can observed this turbulence through stream
flow analysis of Geometry-B (modified with neck section).
(a) Geometry–A (b) Geometry – B
Fig. 5.23: Streamlines –Flow analysis for both the geometries
5.7 VALIDATION OF CFD RESULTS OF MODIFIED GEOMETRY WITH
ASPEN PLUS MODEL RESULTS
The CFD modeling of modified geometry with Thar coal feedstock was carried out and
the results discussed in detailed in previous sections. Due to absence of physical model,
the CFD results were validated with Aspen Plus®V10 model results. The model of
171
entrained flow gasifier was developed as per guidelines are given in previous work
(Aspen, 2010) and then simulations were carried out at similar O/C ratios as per CFD
analysis. The results of important syngas components like CO, CO2, H2 and Volatiles,
Devolatization, char conversion, and syngas temperature were compared from both
modeling approaches. The Fig. 5.24 (a-d) shows the comparison of important
components’ composition in outlet syngas. The comparison of devolatization, char
conversion, and syngas temperature are shown in Fig. 5.25 (a-c). From the comparison,
it can be observed that CFD results showing good agreement with Aspen Plus model
results. An insignificant error has observed in the CO, CO2, H2 and Volatile percentages
in the syngas. Moreover, less than 5% error is being observed in devolatization, char
conversion, and syngas temperature. The similar situation can be seen in previous work
conducted by Xiangdong et al., (2013) (Xiangdong et al., 2013b). Hence the CFD
model is validated and could be used for further analysis.
(a) CO (b) CO2
(c) H2 (d) Volatiles
Fig. 5.24: Comparison of CFD model results with Aspen Plus®V10 model results
for (a) CO, (b) CO2, (c) H2 and (d) Volatiles
0
5
10
15
20
25
30
1.665 1.881 1.89 2.097 2.314
Mol
%
O/C Ratio
CO - CFD Model
CO - Aspen Model
0
10
20
30
40
50
60
1.665 1.881 1.89 2.097 2.314
Mol
%
O/C Ratio
CO2 - CFD Model
CO2 - Aspen Model
0
5
10
15
20
25
30
35
40
45
1.665 1.881 1.89 2.097 2.314
Mo
l%
O/C Ratio
H2 - CFD Model
H2 - Aspen Model
0
2
4
6
8
10
12
14
16
18
20
1.665 1.881 1.89 2.097 2.314
Mol
%
O/C Ratio
Volatiles - CFD Model
Volatiles - Aspen Model
172
(a) Devolatization (b) Char Conversion
(c) Syngas Exit Temperature
Fig. 5.25: Comparison of CFD model results with Aspen Plus®V10 model results
for (a) Devolatization (b) Char Conversion and (c) Syngas Exit Temperature
5.8 COMPARATIVE STUDY FOR NEWLY DESIGNED GASIFIER
The newly designed gasifier was tested with Thar lignite as feedstock and then its
performance was evaluated and discussed in detail in the above sections. Finally, both
the gasifier were evaluated on the basis of the heating value of syngas produced and
other parameters tabulated in Table 5.15. The syngas heating value was calculated using
standard processing modeling tool Aspen HYSYS®V10.
From the information given in Table 5.15, it is verified that the Chinese coal is good in
heating value i.e. 32.91 MJ/Kg whereas Thar lignite is much below to this value and
lies only the range of 15.06 MJ/Kg. But after gasification, the syngas heating value is
almost in equal range that of produced from Chinese coal. The lower heating value
(LHV) of syngas produced from Thar lignite with modified geometry is 12.27 MJ/Kg
84
86
88
90
92
94
96
98
100
102
1.665 1.881 1.89 2.097 2.314
%
O/C Ratio
Devolat-CFD Model
Devolat-Aspen Model
0
20
40
60
80
100
120
1.665 1.881 1.89 2.097 2.314
%
O/C Ratio
CharConv.-CFD Model
CharConv.-Aspen Model
0
500
1000
1500
2000
1.665 1.881 1.89 2.097 2.314
Tem
p. (K
)
O/C Ratio
Temp-CFD Model
Temp-Aspen Model
173
(with considering volatiles) and 7.506 MJ/Kg (without volatiles inclusion) whereas the
Chinese coal gives syngas of 8.96MJ/Kg LHV. Similarly, the higher heating value
(HHV) of syngas produced from Thar lignite with modified geometry is 14.43 MJ/Kg
(With volatiles) and 8.77 MJ/Kg (without volatiles) is in comparison with 10.08 MJ/Kg
HHV of Chinese coal syngas. The ratio of LHV to coal heating value or HHV to coal
heating value for Thar lignite is 0.815 and 0.958 respectively which is much greater
than 0.212 and 0.306 for Chinese coal. The Char conversion is also in satisfactory limits
of above 98%. The coal feeding rate 10.08 KG/hr is also higher for modified geometry
as there is less fixed carbon in Thar lignite.
Table 5.15: Comparative study for Original and Modified geometry of gasifier
with two different feedstocks
Sr.
No. Parameter
Modified Geometry with Thar
Coal Feedstock
Original Geometry with
China Coal Feedstock
1 Coal Heating Value 15.06 MJ/Kg 32.91 MJ/Kg
2 Lower Heating
Value of Syngas
12.27 MJ / Kg (With Volatiles)
7.506 MJ / Kg (Without Volatiles) 8.95 MJ / Kg
3 Higher Heating
Value of Syngas
14.43 MJ / Kg (With Vol.)
8.777 MJ / Kg (Without Vol.) 10.08 MJ / Kg
4 Ratio - LHV/Coal
Heating Value
0.815 (With Vol.)
0.498 (Without Vol.) 0.212
5 Ratio - HHV/Coal
Heating Value
0.958 (With Vol.)
0.583 (Without Vol.) 0.306
6
Devolatization
(Removal of
Volatiles from
Coal)
97.7% 100%
7 Fixed Carbon
Conversion 98.24% 98.6%
8 Geometry
Difference With Throat Without Throat
9 Coal Feeding Rate 10.08 Kg/hr 7.2 Kg/hr
174
The comparison of simulation results based on Temperature and mole fraction of CO,
CO2, and H2 from both geometries is carried out by the development of contours of
those variables. For comparison, the optimized cases were selected from simulated
cases conducted on both geometries. The temperature contours at sectional planes from
both the geometries are shown in Fig. 5.26 whereas the contours of mole fractions of
important syngas components like CO, CO2, and H2 are shown in Fig. 5.27. It is evident
from the figures that the temperature from geometry A is higher with Chinese coal
feeding as compared to the temperature obtained from Thar lignite with geometry-B.
The fundamental reason is the higher carbon percentage in Chinese coal as compared
to Thar lignite. Similarly, the less CO is being observed from Thar lignite due to same
reason but a good amount of H2 is produced from Thar lignite due to higher moisture
content.
Fig. 5.26: Comparison of temperature contours from both geometries
175
(a) Original Geometry (Geometry-A)
(b) Modified Geometry (Geometry-B)
Fig. 5.27: Comparison between the Original Geometry (A) and Modified
Geometry (B) through contours of important syngas components
176
5.9 THE FINAL PROPOSED SYSTEM
The complete system including the coal feeding system, validated new gasifier, ash
collection system, and syngas cleaning system is proposed and shown in Fig. 5.28. The
overall material balance for gasifier is shown in Fig. 5.29. According to overall material
balance, the design gasifier possesses the maximum capacity of 0.0028 Kg/sec (10 Kg/
hr) and can produce up to 0.00356 Kg/sec syngas (12.816 Kg/hr) containing 24% CO
and 32% H2.
Fig. 5.28: Proposed system of coal gasification system with newly designed
gasifier
Fig. 5.29: Overall material balance of newly designed gasifier
177
5.10 SUMMARY FOR CFD MODELING AND SIMULATION WORK
CFD model was developed for lab scale double stage entrained flow coal gasifier with
multiple opposite burners available at China which is referred here as Geometry A. The
numerical simulations were performed on the Chinese coal data and results were
verified from published experimental work. After this, the modified geometry (referred
as Geometry-B) was modeled using CFD techniques for Thar coal data. The kinetic
parameters for Thar Coal Gasification were taken from the experimental work on TGA
and PTGA. The commercial CFD code ANSYS FLUENT®14.0 was used for all
computations. The validation for CFD modeling results with Thar Coal and modified
geometry was carried out with the model developed in AspnPlus®V10. The
summarized points are as under:
• Geometry A showed good results with Chinese Coal whereas for Thar Coal the
Geometry –B (with neck) was found best.
• The maximum CO and H2 were observed 24.19% and H2 was 32.35% at O/C ratio
of 1.881 with Geometry B with Thar Coal Data.
• With Geometry B and Thar Coal at maximum CO and H2 generation, the moisture
removal was observed 97.7% whereas char conversion was observed 98.24%.
• Lower and Higher (LHV) heating values of syngas produced from modified
geometry were calculated using Aspen HYSYS software. From that, it was
observed that the syngas produced from Geometry-B has 12.27 MJ/Kg lower
heating value.
• The calculated ratio of LHV/coal heating value for Thar coal is 0.815 which is
greater than Chinese Coal with geometry-A (i.e., 0.212).
• Less than 1 % different was observed in syngas composition and char conversion
with and without Sulfur inclusion for CFD modeling.
• Good agreement was found between the results of the CFD model and Aspen Plus
model at different O/C ratios.
• Detailed design of optimized gasifier was proposed along with complete
gasification system. Overall material balance of the proposed gasification system
was also presented.
178
CHAPTER 6
CONCLUSION
6.1 CONCLUSION
Pakistan is considered among coal-rich countries but at the same time facing a major
energy crisis. The fundamental reason for not utilizing indigenous coal is the
unavailability of technology and basic coal characteristics which are essential to design
the advanced energy harvesting systems like gasification. Kinetics of gasification
reactions is one important aspect which is a basic hurdle to utilize the indigenous lignite
coal. The prime aim of this research to extract the kinetic parameters of fundamental
steps involved in the gasification process and then evaluate those kinetic parameters
using CFD modeling and Simulation. In this regard, the thermogravimetric analysis
(TGA) was utilized to study thermal degradation of Pakistani lignite under different
environments and then the data is used to calculate the Arrhenius parameters for various
steps of gasification processes.
The sample characterization started from the proximate and ultimate analysis. The
proximate and ultimate analysis was carried out for few samples to see the composition
of Thar lignite. The moisture lies in the range of 14 to 46%, volatiles in 28% to 50%,
Ash in 3 to 20% whereas fixed carbon lies in the range of 20 to 31% for selected
samples. After this, the reactivity study was conducted for Thar Coal Samples for
Moisture removal, Devolatization, Combustion and Gasification Reactions. At the first
stage, the moisture removal and devolatization studies were carried out at different
heating rates on TGA. Then Gasification reactions of Thar Chars were studied with
CO2 and H2O reacting gases at three pressures i.e, 1, 5 and 10 atm. Pressurized
Thermogravimetric Analyzer (PTGA) was used for this study. All the experiments
were conducted at non-isothermal conditions. Two standard kinetic models i.e,
Volumetric Model and Grain Model were used to evaluate the experimental data and
values of Arrhenius parameters (A and E) were calculated by adopting the direct plot
method and integral method. The least square regression technique was used to analyze
the models.
179
6.1.1 Concluding remarks for Drying, Devolatization and Combustion Steps
The Volumetric Model and Grain Model fit well for drying, devolatization, combustion
and Gasification Processes. The Devolatization step was broken into low and high-
temperature ranges to increase the linearization of the models. The low-temperature
range was selected from room temperature to 500°C whereas high-temperature range
was from 501 - 900°C. The range of calculated values of frequency factor (A) and
activation energy (E) for moisture, devolatization, and combustion are as follows:
Step Model A E (KJ/mol)
Min Max Min Max
Moisture Removal Vol. Model 3.6×103 1.38×108 48.4 62.2
Gr. Model 4.05×102 1.46×107 42.6 55.3
Devolatization
(Low Temp)
Vol. Model 8.52×103 8.68×103 27.7 31.7
Gr. Model 4.45×103 7.98×103 24.7 30.2
Devolatization
(High Temp)
Vol. Model 4.76×101 1.02×104 7.23 36.3
Gr. Model 1.28×101 4.10×106 1.04 2.97
Combustion Vol. Model 1.855×102 3.412×106 57.83 138.58
Gr. Model 3.221×101 2.051×105 51.5 123.17
6.1.2 Concluding remarks for Gasification Reactions
At atmospheric pressure, the inherent kinetics of Char-CO2 or Char-H2O reactions is
dominant and drives the overall reaction. At high pressures, the diffusion of reacting
gas in solid particle plays an important role and diffusion rate is dominant over inherent
kinetics of those heterogeneous reactions. The high pressure of reacting gas increases
the overall rate of reaction of gasification. Volumetric model and grain model are
showing satisfactory results in the reactivity study of Char-CO2 or Char-H2O reactions
at atmospheric or high pressures with the integral method. The direct method is showing
good results at atmospheric pressure but it is invalid to be used at high pressures.
Increasing pressures affect the overall kinetics of the heterogeneous reactions in terms
of increase in frequency factor and activation energy.
180
The calculated values of frequency factor (A) for Char-CO2 reaction lies in the range
of 2.729×106 to 7.99×107 and 1.078×105 to 2.174×107 from the volumetric model and
grain model respectively at atmospheric pressure using the integral method. The range
lies between 7.849×108 to 9.939×1011 and 3.992×107 to 6.509×109 from the volumetric
model and grain model respectively at 10 atm using the integral method. Similarly, the
calculated values of frequency factor (A) for char-H2O reaction lies in the range of
4.172×104 to 8.301×1011 and 1.626×103 to 4.467×109 from the volumetric model and
grain model respectively at atmospheric pressure using the integral method. The range
lies between 2.13×1010 to 1.156×1015 and 3.302×108 to 3.485×1012 from the volumetric
model and grain model respectively at 10 atm using the integral method.
The activation energy (E) calculated in the range of 244.64 - 317.6 KJ/mol at
atmospheric pressure whereas it is lying in the range of 286.13 – 356.67 KJ/mol at 10
atm pressure for Char-CO2 reaction. Similarly, it is found 202.58 – 376 KJ/mol and
290.6 – 428.34 KJ/mol at atmospheric and at 10 atm respectively for char-H2O reaction.
6.1.3 Concluding remarks for CFD Modeling and Simulation Work
Finally, the CFD modeling was done for developing Entrained Flow Gasification
System with Multi-opposite Burners. The complete combustion of char and volatiles
reaction mechanism was used in the modeling which could predict the mol% of CO,
H2, and CO2 with less than 1% error. Kinetic Parameters of devolatization and
gasification reactions played a vital role in the simulation of real gasification process.
The variation of oxygen or coal at any injection level basically impacted on the local
oxygen/coal ratio and the gas-solid mixing pattern. This impact is transferred in the
variation of char consumption rates with combustion and gasification reactions and that
plays a key role in the variation of syngas components and temperature. The production
for H2 was found in the range of 27–28 mol% in all the cases. [Maximum CO mol% =
52.59% for 50% coal and 50% oxygen at upper injection level whereas the minimum
CO mol% = 22.37% for 30% coal and 70% oxygen at upper level]. The exit temperature
for syngas was found in the range from 1250 K to 1450 K. The maximum temperature
was observed 2027 K at AA’ level with 30% of total coal and 70% of total oxygen at
the upper level.
181
The maximum char conversion was found 99.79% with coal 60% and oxygen 50% of
the total at AA’ level. The minimum char conversion was observed 95.45% at 30% coal
with 40% oxygen at AA’ level. In general with oxygen/coal above or equal to 50% of
total at upper injection level has shown an optimized performance. Overall it was
concluded that the coal and oxygen distribution has great effect on the syngas
composition, char conversion, and exit syngas temperature.
The temperature and kinetics of gasification reactions (reactions of char to CO2 and
H2O) can be controlled with the optimized coal and oxidant distribution between the
two stages. These parameters are critical to the overall performance of the gasifier, so
coal and oxygen feedings must be optimized between the two stages of the gasifier to
get the optimized performance. Species Transport approach shows good results as
compared to Probability Density Function (PDF) in terms of CO and H2 production.
Geometry–A shows good results with Chinese Coal whereas for Thar Coal the
Geometry –B (with neck) was found best. The maximum CO and H2 were observed
24.19% and H2 was 32.35% at O/C ratio of 1.881. At maximum CO and H2 generation,
the moisture removal was observed 97.7% whereas char conversion was observed
98.24%. Lower and Higher (LHV) heating values of syngas produced from modified
geometry were calculated using Aspen HYSYS software. From that, it was observed
that the syngas produced from Geometry-B has 12.27 MJ/Kg lower heating value. The
calculated ratio of LHV/coal heating value for Thar coal is 0.815 which is greater than
Chinese Coal with geometry-A (i.e 0.212).
Further, comparison of CFD results with Aspen Plus®V10 Model results confirmed a
good agreement between both the strategies. The insignificant error was observed
between the results obtained from the CFD model and Aspen Plus model.
6.2 RECOMMENDATIONS FOR FUTURE WORK
The work could be extended in the following directions for further investigations.
• Detailed kinetic modeling would be recommended for a large number of samples
for Pakistani lignite with further advanced standard kinetic models like Random
Pore Model.
182
• The Online syngas analyzer should be used during the gasification experiments
during TGA for better understanding the behavior of reactions.
• Drop Tube Furnace would be used for gasification studies to see the intrinsic
kinetics of feedstocks during actual gasification environment. The exit gas analysis
should be mandatory in those experiments.
• Transient CFD modeling of a gasification system with extracted kinetic data will
be recommended in future.
• The validation of CFD model with the actual system will give high understanding
and confidence about the modeling results.
183
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APPENDICES
A.1 TGA Model SDT Q600
A.2 Quartz fixed-bed reactor for char
production
A.3 Thermax500 PTGA
212
A.4 LIST OF PUBLICATIONS
• Unar, I. N., Wang, L., Pathan, A. G., Mahar, R. B., Li, R., & Uqaili, M. A.
(2014). “Numerical simulations for the coal/oxidant distribution effects
between two-stages for multi opposite burners (MOB) gasifier”. Energy
Conversion and Management, Elsevier (IF=2.77) Vol. 86, pp. 670-682 .
• Wang, L., Jia, Y., Kumar, S., Li, R., Mahar, R.B., Ali, M., Unar, I.N., Sultan,
U. and Memon, K., 2017. Numerical analysis on the influential factors of coal
gasification performance in two-stage entrained flow gasifier. Applied Thermal
Engineering, 112, pp.1601-1611.
• A.G. Pathan, M.A. Uqaili, F.I. Siddiqui and I.N. Unar, (2015). “Sustainable
Development of Thar Coal, Pakistan”, Presented at The 7th International
Conference on Mining Science and Technology (ICMST), China, April 2015.
• Imran Nazir Unar, Lijun Wang, Abdul Ghani Pathan, Rasool Bux Mehar,
Rundong Li and M. Aslam Uqaili (2013) "Study the Coal/Oxidant Distribution
Effects in a Two-stage Dry-Feed Coal Gasifier with Numerical Simulations",
Presented at 1st International Coal Conference, held and organized by MUET
Jamshoro under INSPIRE Project of HEC from November 7-9 November 2013.