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Evgeniia Komarova, Ziad Abosteif, Stefan Guhl, and Bernd Meyer Institute of Energy Process Engineering and Chemical Engineering, TU Freiberg 9th of June, 2015

1. Introduction: Background, Motivation and Objective

2. Experimental: Materials, Methods and Approaches

3. Results and Discussions

4. Summary: Conclusions and Achievements

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Outline

Lusatian Brown Coal

Pyrolysis

Sizing

200-315 µm

Gasification

Complete Conversion

Kinetics Evaluation

Structure Development

Introduction: Background, Motivation and Objective

Partial Conversion

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„Virtuhcon“ – Virtual High Temperature Conversion

Lusatian Brown Coal

Pyrolysis

Sizing

200-315 µm

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Experimental: Materials Pyrolysis conditions

Sample (bulk sample)

Ash, wt., % (db)

Volatile matter, wt., % (db)

Fixed Carbon, wt., % (db)

Lusatian brown coal 7.01 52.43 40.56

Sample (200-315µm)

Ash, wt., % (db)

Volatile matter, wt., % (db)

Fixed Carbon, wt., % (db)

Lusatian char 14.83 4.47 80.70

Before Pyrolysis:

After Pyrolysis:

Pyrolysis conditions: 1st step:

T = 600 °C (60 min); 2nd step:

T = 800 °C (60 min).

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Lusatian Brown Coal

Pyrolysis

Sizing

200-315 µm

Gasification

Complete Conversion

Kinetics Evaluation

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10 g char; T, °C - 800, 850, 900, 950; Inlet flow – N2/CO2;

C + CO2 = 2CO

The char conversion:

𝑋 𝑡 =𝑛CO t

𝑛CO total, -

Gasification reaction rate:

r =𝑑X

𝑑𝑡, 1/min

Specific reaction rate:

𝑟(𝑋) =1

1−𝑋

𝑑𝑋

𝑑𝑡*, g/g min

*Fan D, Zhu Z, Na Y, Lu Q. J Therm Anal Calorim 2013:599-607.

Experimental: Complete conversion experiments

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Experimental: Kinetic evaluation

Model Model equations Theoretical background Shortcomings

RPM*

𝑑𝑋

𝑑𝑡= 𝑘RPM 1 − 𝑋 [1 − 𝜓 ln 1 − 𝑋 ]

12

2

𝜓1 − 𝜓 ln 1 − 𝑋 − 1 = 𝒌𝑹𝑷𝑴t

𝜓Xmax=

2

2 𝑙𝑛 1 − 𝑋max + 1 ;

Particle – sphere;

Pores – cylinders;

Reaction – everywhere on the pore surface;

Pores enlarge and then coalescence;

Maximum reaction rate peak.

No pore is destroyed;

No new pore is created.

VM**

𝑑𝑋

𝑑𝑡= 𝑘VM(1 − 𝑋)

−𝑙𝑛 1 − 𝑋 = 𝒌𝑽𝑴𝑡

Particle – sphere;

Reaction – in all possible positions uniformly throughout the entire char.

No structure changes;

Monotonically decreasing reaction rate.

SCM***

𝑑𝑋

𝑑𝑡= 𝑘SCM(1 − 𝑋)

23

3(1 − (1 − 𝑋)1/3) = 𝒌𝑺𝑪𝑴𝑡

Particle – spherical;

Structure – aggregation of non-porous grains;

Reaction – everywhere on the outer surface of the grains.

No structure changes;

Monotonically decreasing reaction rate.

*Bhatia SK, Perlmutter DD. AlChE 1980:379-386. **Lu GQ, Do DD. Carbon 1993: 247-263. *** Szekely J, Evans JW. Chem Eng Sci 1971: 1901-1913.

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Results and Discussions Kinetic evaluation: Arrhenius Expression

Sample

Temp,°C

RPM VM SCM

𝑘RPM 𝜓Xmax

𝐸a,

kJ/mol

𝐴0, 1/min

𝑘VM 𝐸a,

kJ/mol 𝐴0,

1/min 𝑘SCM

𝐸a, kJ/mol

𝐴0, 1/min

Char

800 0.112 2.4

73.65 407

0.190

70.21 478

0.138

72.46 446 850 0.143 2.3 0.238 0.174

900 0.200 4.0 0.368 0.270 950 0.31 2.0 0.481 0.363

𝑘 𝑇 = 𝐴0𝑒−𝐸a/𝑅𝑇

Chemically controlled regime; Formal reaction order 𝑛 is 1. 𝐸a (29-280 kJ/mol) at low pressures and 800-1000 °C*

𝐸a (90-97 kJ/mol) using TGA at 800-950 °C**

*Ifran MF, Usman MR, Kusakabe K. Energy 2011: 12-40. ** Abosteif Z. Internal results of the research group (MPS).

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RMSE*: The RPM – 1.1 % The VM – 5.9 %

RMSE: The RPM – 5.7 % The VM – 8.1 %

RMSE: The RPM – 2.6 % The VM – 7.5 %

RMSE: The RPM – 6.1 % The VM – 10.3 %

Results and Discussions Kinetic evaluation: Application of the kinetic models

*RMSE – Root Mean Square Error

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Lusatian Brown Coal

Pyrolysis

Sizing

200-315µm

Gasification

Structure Development

Partial Conversion

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Results and Discussions: Partial gasification experiments§ Char structure characterization

5 char conversion degrees at 800, 850, 900, 950 °C; > 40 experiments; Partially gasified chars: N2 adsorption technique at 77 K*; Hg porosimetery analysis**; He pycnometery***; Dynamic image analysis1*.

§ -Komarova E, Guhl S, Meyer B. Fuel 2015: 38-47. * 3-Flex Surface area analyzer (Micromeretics) **AutoPore IV 9500 V1.09 ***AccuPyc 1330 V2.01 1* Camsizer XT

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Results and Discussions: Char structure characterization 𝐍𝟐 adsorption technique at 77 K

Adsorption isotherms*: Combination of Type I and Type IV Micropores Mesopores

Initial sample: Mostly microporous

Partially gasified chars: Mesopores develop as reaction

proceeds Hysteresis*: H3 and H4 Types Slit/needle-like pores

*The IUPAC – International Union of Pure and Applied Chemistry 13/25

Results and Discussions: Char structure characterization Interpretation of adsorption isotherms

Total SA Micro SA Meso SA

BET** (Type II and IV) or Langmuir method (Type I)

t-plot (based on Total SA)

BJH***- method

Pores in coals/chars are divided into*: Micropores (between 0 and 2 nm); Mesopores (between 2 and 50 nm); Macropores (wider than 50 nm).

* The IUPAC – International Union of Pure and Applied Chemistry **BET – Brunauer-Emmet-Teller *** BJH – Barrett-Joyner-Hallend 14/25

Results and Discussions: Char structure characterization Micro- and mesopore SA development

800 °C (blue line): Increase of mesopore SA with carbon conversion X

to a greater extent;

850, 900 and 950 °C (red line): Increase of mesopore SA with carbon conversion X to a lesser extent.

800 °C (blue line): Slight increase of Micro SA at early X. 850, 900 and 950 °C (red line): Steep decrease of Micro SA at early X.

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Results and Discussions: Char structure characterization Porosity distribution by Hg porosimetry

The share of micropores decreases; The share of mesopores increases; The share of macropores remains almost constant. Hg porosimetery analysis is consistent with the N2 adsorption technique results.

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Results and Discussions: Char structure characterization True and apparent density development

𝝆𝐭𝐫𝐮𝐞 , ash content is constant 𝝆𝐚𝐩𝐩 , the volume of voids increases

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Results and Discussions: Char structure characterization Total porosity and pore volume development

𝑉total =1

𝜌apparent−

1

𝜌true, cm3/g 𝜀 =

𝜌true−𝜌app

𝜌true∙ 100, %

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Results and Discussions: Char structure characterization Char particle size change

Char particle diameter remains constant up to 50 % of carbon conversion. 19/25

Initial sample

Results and Discussions: Char structure characterization

Image analysis, SEM at 900 °C

X=78 %

X=78 % Initial sample

x100

x1000

Small pieces

Cracks and ash layer

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Results and Discussions: Char structure characterization Kinetic models assumptions vs. experimental results

Properties Experimental RPM VM SCM

𝑟

Decreases as reaction proceeds with unclear maximum reaction rate peak

Decreases with or without maximum reaction rate peak

Decreases linearly Decreases linearly

𝑑

Constant until 50% of X, then decreases.

Constant until the end of the reaction

Constant until the end of the reaction

Decreases gradually as

reaction proceeds

𝜌

𝜌𝑡𝑟𝑢𝑒:decreases linearly

𝜌𝑎𝑝𝑝.:increases

linearly

Gradual change Gradual change No change

𝜀 Increases linearly Gradual change Gradual change No change

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Kinetics Evaluation

Structure Development

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Results and Discussions: Kinetics vs. surface area Correlation of the specific reaction rate to the specific SA

Intrinsic (normalized) reaction rate:

𝒓𝒊𝒏𝒕𝒓 =𝒓(𝑿)

𝑺(𝑿)

∗, g/min m2

rintr (total); rintr (micro); rintr (meso) - assumed to be constant** (all the variations in SA).

*Wang M, Roberts DG, Kochanek MA, Harris DJ, Chang L, Li C. Energy and Fuels 2014:285-290. **Gil MV, Fermoso J, Pevida C, Rubiera F. Fuel Proces Techn 2010:1776-1781.

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Results and Discussions: Kinetics vs. surface area Intrinsic reaction rate

Two competitive phenomena due to carbon conversion: 1.Slow reaction rate Carbon structure reordering (TT history). 2. Raise reaction rate Ash constituents (stronger catalytic activity). Possible dominating catalytic effect

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Summary Principal Conclusions and Achievements

Gasification experiments were performed in a fluidized-bed type reactor;

Brown coal char gasification kinetics was evaluated using the RPM, the

VM and the SCM;

Char structure development with carbon conversion was extensively

investigated:

Strong development of mesopores was observed;

Mesopores developed from micropores;

𝜀 and 𝑉 increased linearly – temperature independent;

𝑑 remained constant up to 50 % of char conversion – temperature

independent.

The RPM agrees best with the experimental results, because it

considers the main char structure changes;

Gasification reaction rate correlates best with the SA of the new-

developing mesopores SA.

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Acknowledgments This research has been funded by the Federal Ministry of Education and

Research of Germany in the framework of Virtuhcon (Project Number

03Z2FN12).

TU Bergakademie Freiberg

Institute of Energy Process Engineering and Chemical Engineering

09599 Freiberg – Germany

Phone: +49 3731 39-4217

Fax: +49 3731 39-4555

E-Mail: Evgeniya.Komarova@iec.tu-freiberg.de

Web: www.iec.tu-freiberg.de