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Modeling a Three-Dimensional Hybrid Entrained-Flow/Fluidized-Bed Mild Coal Gasifier
You Lu and Ting Wang
Energy Conversion & Conservation Center University of New Orleans
New Orleans, LA 70148-222 504-208-8203, [email protected] and 504-280-7183, [email protected]
ABSTRACT The concept of the Integrated Mild Gasification
Combined Cycle (IMGCC) has been introduced as a potential
option for utilizing coal more cleanly, efficiently, and
economically, especially for retrofitting existing pulverized
coal power plants. However, one of the challenges in making
IMGCC work is designing an effective mild-gasifier. The
design presented in this paper is based on a new concept of
combining the speed of an entrained-flow gasifier to quickly
drive out most of the volatiles in the fuel with the efficiency of
a fluidized bed gasifier by cooking and cracking the remaining
volatiles to various degrees within a compact space. To help
the design process, a preliminary computational model has
been developed to simulate the reactive thermo-flow behavior
in a simplified 2-D gasifier. The objective of this paper is to
extend the previous 2-D model to a 3-D simulation model and
to further investigate several design options with various
operation parameters to achieve mild-gasification, including a
goal to sustain the fluidized bed at a constant height.
The Eulerian-Eulerian multiphase theory has been
selected to model mechanisms that happen within the primary
phase (hot gases) and secondary phase (coal particles). The
constitutive equations under the multiphase scheme, which are
derived from particle kinetic theory, are utilized for
calculating the effective shear viscosities, bulk viscosities, and
thermal conductivities of granular flows to simulate the
hydrodynamic and thermal interactions between the solid and
gas phases. The multiphase Navier-Stokes equations and
seven global reaction equations with associated species-
transport equations are implemented in order to simulate the
mild gasification process.
The results show that (a) most of the volatiles are
effectively driven out in the end of the draft tube, (b) most of
the volatiles are furthermore thermally cracked to lighter
carbon compounds, modeled as benzene and CO in this study,
i.e., a degree of mild gasification has been achieved, and (c)
the fluidized bed has been successfully sustained at the desired
height under the test operating conditions. Furthermore, the
results of this study provide useful information for
understanding the fundamental reactive granular thermo-flow
behavior in the gasifier, and this information will be used as
an essential guide to further improve the mild-gasifier design.
INTRODUCTION As the price of natural gas (NG) becomes cheap and
remains less than $4/MMBTU in the United States, coal-fed
power plants, including PC and IGCC plants, are facing steep
competition with NG-fired power plants. As such, alternative
options for utilizing coal more cleanly and economically need
to be explored. A new concept of the so-called “Integrated
Mild Gasification Combined Cycle” (IMGCC), introduced by
Khan and Wang [1], could be one such option. The mild
gasification concept was previously proposed by Wormser as
the Mild Air-Blown Gasification Integrated Combined Cycle
(MaGIC) [2, 3]. However, in MaGIC, the gasifier is operated
using an air-blown scheme without an air separation unit
(ASU); whereas, in an IMGCC system, either air-blown or
oxygen-blown operation can be implemented.
Figure 1 Schematic Diagram of an Integrated Mild Gasification Combined Cycle (IMGCC) System [1]
The concept of the Integrated Mild Gasification
Combined Cycle (IMGCC), as shown in Fig. 1, can effectively
and competitively (a) implemented to retrofit existing
pulverized coal power plants, (b) utilize low-rank coal (LRC),
(c) serve as an alternative to conventional IGCC plants, and (d)
incorporate the coal-biomass co-gasification process. The key
feature of IMGCC is that it preserves the volatiles with high
energy density, so the size of the piping and the gas cleanup
Proceedings of the 30th International Pittsburgh Coal Conference, Beijing, China, September 15 - 18, 2013
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system can be reduced compared to those of a fully gasified
system. Consequently, since mild gasification is applied, the
residence time is shorter than it is for full gasification, and the
product yield rate will be higher. Therefore, the mild gasifier’s
size will be significantly smaller than a full-blown gasifier.
The thermal energy saved from not cracking the heavy
volatiles is preserved as chemical energy in the fuel, which
partially contributes to the high-efficiency of IMGCC.
To explain what Mild Gasification is, Fig. 2 is introduced.
The gasification of coal particles involves three major steps: (a)
thermal decomposition (pyrolysis and devolatilization), (b)
thermal cracking of the volatiles, and (c) char gasification.
C
Moisture
Volatiles
Ash
C
Volatiles
Ash
C
Ash
Volatiles Heat
Ash
CO, CO2, H2
Heat
H2O
Thermal Cracking
Lighter products,
H2, CO, C2H2, etc.
Moisture Devolatilization
Full Gasification Mild Gasification Pyrolysis
Heat
Figure 2 Simplified global gasification processes of coal particles (sulfur and other minerals are not included in this figure). Heat can be provided externally or internally through combustion of char and volatiles.
Coal particles undergo pyrolysis when they enter the hot
combustion environment. Moisture within the coal boils and
when the particle temperature reaches the boiling point, it
leaves the coal’s core structure. The volatiles are then released
as the particle temperature continues to increase. This
volatile-releasing process is called devolatilization. The long
hydrocarbon chains are then thermally cracked into lighter
volatile gases such as H2, CO, C2H2, C6H6, CH4, etc. These
lighter gases can react with O2, releasing more heat, which is
needed to continue the pyrolysis reaction. With only char and ash left, the char particles undergo
gasification with CO2 or steam to produce CO and H2, leaving
only ash. The heat required for the pyrolysis and
devolatilization processes can be provided externally or
internally by burning the char and/or volatiles.
Devolatilization Devolatilization is a decomposition process that occurs
when, under heating, volatiles are driven out from a
hydrocarbon material (like coal). The rate of devolatilization is
influenced by temperature, pressure, residence time, particle
size, and coal type. The heating causes chemical bonds to
rupture and both the organic and inorganic compounds to
decompose. In a typical fluidized bed gasifier, the process
starts at a temperature of around 100C (212F) with
desorption of gases, such as water vapor, CO2, CH4, and N2,
which are stored in the coal pores. When the temperature
reaches above 300C (572F), the released liquid hydrocarbon
called tar becomes important. Gaseous compounds, such as
CO, CO2, and steam are also released. When the temperature
is above 500C (932F), the fuel particles are in a plastic state
where they undergo drastic changes in size and shape. The
coal particles then harden again and become char when the
temperature reaches around 550C (1022F). As heating
continues, H2 and CO are released through gasification. In an
entrained-flow gasifier, the residence time is short, and the
heating rate is much higher than in a fluidized bed, so the
afore-mentioned milestone temperatures will be higher.
The pyrolysis conditions affect the physical properties of
the char. It is reported that the heat transfer coefficient
decreases by a factor of 10 during the fast heating of the coal
particles mixed with a hot solid heat carrier. This reduced heat
transfer rate to the particle surface results in a temperature
plateau on the level of about 400C (752F) and lasts
throughout the devolatilization process.
In general, the larger the particle size, the smaller the
volatiles yield because in larger particles more volatiles may
crack, condense, or polymerize with some carbon deposition
occurring during their migration from inside the particle to the
particle surface. High pressure has a similar effect on the
devolatilization rate. Anthony et al. [4] reported that
devolatilization rates are higher at lower pressures. An
increase in pressure increases the transit time of volatiles
rising to the particle surface. This effect of pressure is usually
true for high rank coals but not always valid for low rank coals
because, at high pressures, volatiles yields of high-rank coals
decrease due to the low vapor pressure of tar. In contrast, low
rank coals do not show decreased volatiles yields with
increased pressure since these coals do not have as much tar.
Complete, Partial, Full, and Mild Gasification For clarification, the terms "complete, partial, full, and
mild" gasification are defined below and illustrated in Table 1:
Complete and Partial gasification describe how much char
is reacted. Complete Gasification implies that all the char is
completely gasified, while Partial Gasification indicates that
a portion of the char remains unreacted. The carbon
conversion rate (CCR), also called carbon conversion
efficiency, represents the fraction of carbon reacted and
formulated as:
CarbonTotalofAmount
actedReCarbonofAmountCCR (1) (1)
Full and Mild gasification describe the level (i.e., the
products' molecular weight or length of molecular chain) of
thermal cracking, which is typically affected by the
temperature level and residence time. Full Gasification
indicates that the feedstock undergoes complete de-
volatilization, gasification, and thermal cracking into a
composition consisting of light species like CO, H2, and CH4
as the major combustible components of the so-called syngas,
while Mild Gasification preserves the heavier volatiles
without further thermally cracking them into lighter
components. To be specific, the operation of "Mild
Gasification" refers to controlling the temperature and
residence time to achieve varying levels of gasification
between pyrolysis-only (0% gasification) and full gasification
(100% gasification). It is a comprehensive process to design
such a gasifier that can control the level of thermal cracking
and gasification that occurs.
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Table 1 Illustration of relationship of Complete, Partial, Full, and Mild Gasification
Level of gasification
% of char reacted
Low (Partial) High(Complete)
Temperature
Low (Mild, heavy)
High ( Full, light)
mild-partial mild- complete
Full- partial full- complete
DESCRIPTION OF THE STUDIED MILD GASIFIER
The studied gasifier (Fig. 3), a variation derived from
Wormser's design [2], combines the features of both entrained-
flow and fluidized-bed gasifiers. The entrained-flow feature is
characterized by the centralized draft tube. Through the draft-
tube's bottom inlet, coal and combusted gases are introduced
to produce heat, which is used to drive out the volatiles during
the journey upward through the draft tube. Surrounding the
draft tube is the fluidized bed. The fast flow speed and high
temperature in the draft tube can quickly drive out the
volatiles with hot gases, and, due to the short residence time,
the thermal-cracking process can be minimized to avoid full
gasification taking place. When the coal particles are deflected
by the deflector suspended above the draft tube and fall down
to the fluidized bed, further thermal cracking and mild
gasification can be more conveniently controlled and fine
tuned in a relative low temperature environment. The
inclusion of the draft tube reduces the size of the fluidized bed
and separates the hot gases from the warm fluidized-bed area.
In addition, the inclusion of the fluidized-bed allows the
operator to control the temperature and residence time to
achieve varying levels of gasification between pyrolysis-only
(0% gasification) and full gasification (100% gasification). It
is a comprehensive process to design such a gasifier that can
control the level of thermal cracking and gasification that
occurs to the desired degree.
The studied gasifier is based on a lab model (Fig. 4) with
a coal feeding rate of 0.636 metric tons per day and a capacity
of approximately 183kW. For 2-D simulations, the capacity is
converted to 9.92 MW/m. It has a draft tube to prevent hot
gases from directly contacting particles in the fluidized bed,
but this setup allows heat to be transferred from the draft tube
wall to the fluidized bed. Above the draft tube, a deflector is
installed to block the particles from being entrained out of the
fluidized bed. The height and width of the studied mild
gasifier are 34.25 inches (87 cm) and 18 inches (45.75 cm),
respectively. There are four outlets: two for char and two for
the produced syngas. The char outlets are inclined 45 degrees.
To determine the most effective location to extract the desired
char, three pairs of inclined char chutes are included in the test
model, although only one pair will be used for each
experiment. The fluidized bed is 10 inches (0.254 m) deep.
The studied gasifier has three velocity inlets. The first
inlet is located on the bottom of the draft tube, and is used for
coal injection. The second inlet is designed for the hot
combusted gases with an annular passage surrounding the
central tube fed from the first inlet. These hot gases provide
the energy needed for driving the devolatilization process and
part of mild gasification.
Figure 3 Schematic diagram of the cold-flow model of the conceptual Mild Gasifier, a variation derived from Wormser's design [2]
Fluidization
Air Inlet
Coal Inlet
Hot
Gas
Inlet
Front View
Draft
Tube
Syngas
Outlet
Char
Outlet
Inside
Construction
Deflector
Bottom View
Figure 4 Schematic of the 3-D simulated hybrid gasifier
Coal is transported by warm gases to the center pipe and
is entrained by the hot combusted gases from the annular
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passage into the mild gasifier. The third inlet, consisting of
horizontal and inclined perforated plates, is used for providing
fluidization gas. For the 2-D geometry, a perforated surface
with a total of 28 equally spaced round holes are created. For
the 3-D geometry, the horizontal perforated plates have too
many holes to be meshed and simulated; hence they are
simplified as eight trapezoidal open slots as shown in Fig. 4.
On the inclined, perforated surface, there are eighty-four holes. MOTIVATION AND OBJECTIVE
Mazumder, Wang, and Khan [5, 6] introduced a 2-D
numerical model to simulate the coal gasification process
inside the studied mild gasifier. Their results provided a
preliminary overall view of the thermal-flow and gasification
process of in the studied mild gasifier. Furthermore, Khan and
Wang [1] improved the 2-D model by successfully
implementing a demoisturization and devolatilization sub-
model, which is not available for multiphase modeling using
the Eulerian-Eulerian approach in Fluent. The propose of the
above two studies was to establish and improve the
appropriate computational model for the studied mild gasifier,
so no additional parametric study or further optimization for
achieving sustained mild gasification was performed.
Consequently, both of these two above studies showed two
undesired results: (a) the fluidized-bed couldn’t be sustained at
its desired height and was eventually blown away; and (b) the
fluidization process and the velocity in the draft tube were not
optimized, so the coal was fully gasified instead of being
mildly gasified. To continue the previous studies, the
objectives of this paper are to (a) expand the 2-D model to 3-D,
(b) perform a parametric investigation to achieve a certain
degree of mild gasification, and (c) maintain the fluidized-bed
at a desired height.
HYBRID GASIFIER DESIGN CONSIDERATION In this study, in order to only reach mild and partial
gasification but not overachieve to reach full or complete
gasification, the particle residence time needs to be
meticulously controlled. By using the existing design, the
height of the mild gasifier is fixed at 34.25 inches (87cm).
However, the particle velocity and gasifier power could vary
with different geometric designs for the fuel and combusted
gas inlets, respectively. Also, the exact power capabity of this
mild gasifier is not fixed in this study, since the syngas mass
flow rate could vary by changing fuel injection velocity and
injection area.
The initial residence time is set at 3 seconds. With the
selected initial particle residence time of 3 seconds, many
parameters could be then selected accordingly for a
preliminary design of the mild gasifier.
The studied mild gasifier is designed to deliver
accumulated char from the gasifier to the boiler for use in a
steam cycle through the use of the char chutes. However, the
2-D study from Mazumder, Khan, and Wang [6] shows that
the char is removed too quickly through these chutes, so the
height of the packed char within the fluidized bed can’t be
maintained. This problem is caused by simplifying a 3-D
gasifier with 2-D geometry. In a 2-D model, the geometry
does not represent all of the real characteristics of the actual 3-
D gasifier unless all of the inlets and outlets are
axisymmetrically placed. For example, if a circular pipe in a
3-D configuration is represented as an opening with a width
having the same dimension as the pipe diameter, then this
opening actually represents a rectangular slot which occupies
a greater percentage of the surface area than does the original
circular cross-section in the 3-D configuration. Under this
circumstance, the 2-D computation will over-predict the char
removal rate through the char chutes. In order to solve this
problem, the pressure at the char chutes is intentionally
increased to reduce the char removal rate. But the next
question is “What is the appropriate pressure that should be
assigned at the char chute exit?” Based on the fact that the
char is removed through gravity, the char chute exit pressure is
then calculated to be 498.3 Pascals above the furnace pressure
in the boiler.
Hybrid Bed Design Changes
This study modifies the preliminary mild-gasifier
configuration used in the previous study of Mazumder, Wang,
and Khan [5 and 6] with the following changes:
1. Change the shape of the deflector from a flat plate to an
arc in order to reduce deposition of char on the top of
the deflector.
2. Adjust the diameters for the coal inlet and syngas
outlets to satisfy the design power.
3. Close the entraining openings between the draft tube
and the fluidized bed to prevent the fluidized bed from
being blown away by the strong flow coming from the
draft tube through this entraining opening.
Computational Adjustment
In the previous 2-D design, coal particles unrealistically
fell through the perforated openings even though the diameter
was less than that of the coal particles due to the adoption of
the volume fraction method, which does not actually simulate
the true particle sizes. In order to resolve this problem, the
bottom boundary of the computational domain is moved from
the actual gasifier’s outer casing to the perforated plate surface.
By doing this, two fluidization air inlet zones are outside the
computational domain. Considering that this study is focused
on the reaction areas inside the gasifier rather than the
fluidization air flow field in the plenum before enters the
fluidized bed, this modification compromise is adopted.
COMPUTATIONAL MODEL Using a Computational Fluid Dynamics (CFD) simulation
is an economical and effective tool in studying coal
gasification. A 2-D multiphase reactive model was established
by Mazumder and Wang [3, 6] to simulate the studied mild
gasifier. In that model, the volatiles were provided as part of a
coal but were injected into the gasifier as an independent
component by assuming that the volatiles were outside the
coal, i.e. no devolatilization process was modeled. In this
study, the devolatilization process is to be implemented in the
model. Coal gasification is a multiphase reactive flow
phenomenon: it is a multiphase problem between gases and
coal particles and is also a reactive flow that involves
homogeneous reactions between gases and heterogeneous
reactions between coal particles and gases. The Eulerian-
Eulerian (E-E) method is adopted in this study because the
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concentrations of coal particles are dense in the fluidized bed
and tracing each particle with the Lagrangian method is not
realistic. Inside the draft tube, the conditions are similar to an
entrained-flow gasifier, so the Lagrangian-Eulerian method
could be used here. However, since the Lagrangian-Eulerian
method can't be used to obtain a solution within the fluidized
bed, while Eulerian-Eulerian can be used in both the
entrained-flow and fluidized bed portions of the gasifier, the
Eulerian-Eulerian method is adopted in this study. This means
that both the gas phase (primary phase) and coal phase
(secondary phase) are solved by using the Eulerian method.
In the fluidized-bed portion of the gasifier, all of the solid
particles are placed side by side inside the gasifier like a bed
of granular material, and the gas phase is passed up through
this bed, converting this granular material from a static solid-
like state to a dynamic fluid-like state. This process is known
as "fluidization." In the draft tube, on the other hand, the coal
phase is transported by hot gases (either air or carbon-dioxide
and water vapor), absorbing heat from the hot gases and
releasing the volatiles and water vapor into gas phases. This is
how the devolatilization and demoisturization processes are to
be modeled in this study. The gasification processes involves
both homogeneous (gas-gas) reactions and heterogeneous
(gas-solid) reactions.
Physical Characteristics of the Problem The physical characteristics of the problem are modeled
as follows:
1. The gas species involved in this study are Newtonian
fluids with variable properties as functions of
temperature. These variable properties are calculated by
using a piecewise-polynomial method.
2. A mass-weighted mixing law for specific heat and the
incompressible, ideal gas law for density are used for
gas species mixtures.
3. All of the outside walls are impermeable and adiabatic,
but the draft tube's wall is set as a "coupled" condition
with zero thickness (called "shell wall") so the heat
transfer can be computed across the shell wall by
imposing the same heat flux on both sides of the wall.
4. The no-slip condition (zero velocity) is imposed on all
wall surfaces.
5. The gravitational force is considered.
Multiphase Flow Regimes Based on the E-E model, all of the phases involved in this
study are treated as interpenetrating continua, mathematically.
Also, a phasic volume fraction is defined as a basic concept in
this model, which assumes that space and time have a sum
always equal to one as a continuous function. The derived
governing equations for each phase are resulted from
conservation equations, therefore, sharing the similar structure.
In terms of applying kinetic theory or granular flow theory,
constitutive relations are provided to allow the model to
achieve closure. There are two major phases that exist in this
study: the primary phase (referred to as the gas phase,) which
consists of all gases, e.g. O2, N2, H2, CO, CO2, H2O vapor,
C6H6, and volatiles; and the secondary phase (referred to as
the coal phase), which consists of char (pure carbon), H2O
vapor, and volatiles. The devolatilization model disengages
the H2O vapor and the volatiles from the coal phase and places
them into the gas phase.
The E-E model allows for the modeling of multiple,
separate, interacting phases. It solves a set of "n" momentum
and continuity equations for each phase, where n is the
number of phases. The pressure and inter-phase exchange
coefficients are coupled in this model. Since a comprehensive
description of the E-E model could refer to a significant
number of equations, only the most important ones are
presented below. Any interested reader could find more details
from previously published works [5, 7].
Governing Equations The conservation equations of mass, momentum, and
energy from the Eulerian multiphase model for unsteady state
are presented below:
The continuity equation for phase "q" is:
q
n
1p
qppqqqqqq Smmvρρt
(2)
Where,qv
= the velocity of phase "q"
pq= the mass transfer from phase "p" to "q"
qp= the mass transfer from phase "q" to "p"
εq =the volume fraction of phase "q"
Sq =the source term of phase "q"
The momentum balance for phase "q" is:
qvm,qlift,q
1
qppqpqqq
qqqqqqqqq
FFFvvRgρ
τpvvρvρt
n
p
qppq mm
(3)
where q, is the stress-strain tensor of phase "q," which is
given by:
Iv3
2vv qqqq
T
qqqqq
(4)
pqR
= Inter-phase force. pqR
depends on the friction, pressure,
cohesion, and other effects.
q = the shear viscosity of phase "q"
q = the bulk viscosity of phase "q"
qF
= an external body force of phase "q"
qlift,F
= lift force acting on secondary phase "p" in a primary
phase
qvm,F
= inertia of the primary phase mass encountered by the
accelerating particles, droplets, or bubbles exerted by a
''virtual mass force'' on the particles
p = the pressure gradient shared by all phases
pqv
= the inter-phase velocity between the phase p and q
g
=acceleration due to gravity
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One of the major differences between the single phase
momentum equation and the Eulerian multiphase momentum
equation is the inter-phase momentum exchange coefficient.
For a solid phase ''s,'' the conservation of momentum equation
is:
svm,slift,s
1
sllssllsss
ssssssssss
FFFvvvvKgρ
τppvvρvρt
n
l
slls mm
(5)
Where, ps = the solid pressure of the solid phase ''s''
To describe the conservation of energy equation using the
Eulerian multiphase model, a separate enthalpy equation is
written for each phase:
n
p
qppqq
q
mmS
t
p
1
qppqpqqqq
qqqqqqqq
hhQqv:τ
hvρhρt
(6)
where hq= the specific enthalpy of the phase "q"
= the heat flux of the phase "q"
Sq = a source term that includes sources of enthalpy
Qpq= the rate of heat transfer between the phase "p" and "q"
hpq= the inter-phase enthalpy between the phase "p" and "q"
Devolatilization Model In this study, devolatilization is modeled following a
single rate approach by Badzioch and Hawsley [8] in the
Arrhenius form, . Initially, the primary phase
does not contain any water vapor or volatiles, and the
secondary phase contains liquid water and condensed volatiles
according to the coal composition. As devolatilization (along
with demoisturization) starts and continues, the secondary
phase starts to lose moisture and volatiles, and, in the
meantime, the primary phase starts to gather more water vapor
and volatiles. These two pseudo-chemical reactions are
formulated by Khan and Wang [1] as:
Moisture (H2O)coal phase ↔ Moisture (H2O)gases phase (7)
Vol.(CH2.121O0.586)coal phase ↔ Vol. (CH2.121O0.586)gases phase (8)
The reaction rates of Equations (7) and (8) are shown in
Table 1 for reactions R1 ad R2. It is modeled with two stages.
The first stage occurs in the draft tube with a portion
(estimated to be about 70%) of the volatiles being driven out.
The rest of coal is deflected by the deflector and falls down to
the fluidized bed and continues to absorb energy and release
volatiles matters from the coal matrix.
Chemical Reaction Model The finite rate model is chosen in order to model the
chemical reactions (See Table 1) as homogeneous (gas-gas)
and heterogeneous (gas-solid) reactions together. The Laminar
Finite-Rate Model and Eddy-Dissipation Model for reactions
are calculated and compared. For a homogeneous reaction rate,
the minimum of the two rates is used. For the heterogeneous
reaction, only the finite rate is used.
Table 1 Global gasification reactions model (n=0)
R # Reactions A
(kg / m²-s)
E (J /
kmol)
R1 H2O(l)coal → H2O(g)gases 0.05 1.08104
R2 Volatilescoal →Volatilesgases 0.05 2.6104
R3 C(s) + ½ O2 → CO 0.052 6.1107
R4 C(s) + CO2 → 2CO 0.0732 1.125108
R5 C(s) + H2O → CO + H2 0.0782 1.15108
R6 CO + ½ O2 → CO2 2.21012 1.67108
R7 CO + H2O (g) CO2 + H2 2.751010 8.38107
R8 CH2.121O0.585 → 0.585CO +
0.853H2+0.069C6H6 Eddy Dissipation
R9 C6H6 + 3O2 → 6CO + 3H2 Eddy Dissipation
A total of seven global gasification reactions are included:
three are heterogeneous and four are homogeneous. A two-
stage process is used to model the thermal cracking of
volatiles followed by gasification via benzene.
Eddy-Dissipation Model It is assumed that the chemical reaction is faster than the
time scale of the turbulence eddies. Therefore, the reaction
rate is determined by the turbulence mixing of the species. The
reaction is triggered at the same time the reactants meet. The
reaction rate is given by the smaller of the two given
expressions below:
R,wr,R
R
Ri,wr,ir.i
M
YminAMR (9)
N
jj,wr,j
PP
i,wr,ir.i
M
YABMR (10)
where YP is the mass fraction of any product species, P
YR is the mass fraction of a particular reactant, R
A is an empirical constant equal to 4.0
B is an empirical constant equal to 0.5
′i,r is the stoichiometric coefficient for reactant i in reaction r
″j,r is the stoichiometric coefficient for product j in reaction r
The Finite-Rate Model computes the chemical source
terms using the Arrhenius equation and ignores the effects of
turbulence fluctuations. The net source of chemical species i
due to reaction Ri (kg/m3-s) is computed as the sum of the
Arrhenius reaction sources over the NR reactions that the
species participate in, and is given as:
RN
1r
ri,iw,i R̂MR (11)
where Ri,r is the Arrhenius molar rate of
production/consumption of species i in reaction r.
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The r-th reaction can be written in a general form as:
N
1i
i"
ri,
RN
1i
rf,k
rb,ki
'ri, MυMυ
(12)
where kf,r = forward rate constant for reaction, r
kb,r = backward rate constant for reaction, r.
The molar production/consumption of species i as a result
of reaction r, which is ri,R̂ (kmol/m3-s) in Eq. (11), is given as:
rN
1j
"rj,η
rj,
rN
1j
rb,
'rj,η
rj,rf,'
ri,"
ri,ri, CkCkυυΓR̂ (13)
where Cj,r = molar concentration of each reactant and product
species j in reaction r (kmol/m3)
rj ,= forward rate exponent for each reactant and product
species j in reaction, r
rj , = backward rate exponent for each reactant and product
species j in reaction, r.
For Heterogeneous Reactions, the particle reaction, R (kg/m2-
s), is expressed as:
R = D0 (Cg – Cs) = Rc(Cs)N (14)
where D0 = bulk diffusion coefficient (m/s)
Cg = mean reacting gas species concentration in bulk (kg/m³)
Cs = mean reacting gas species conc. at particle surface (kg/m²)
Rc = chemical reaction rate coefficient (units vary)
N = apparent reaction order (dimensionless).
The concentration at the particle surface, Cs, is not known,
so it is replaced by other quantities, and the expression is
recast as follows: N
0gc
D
RCRR
(15)
This equation has to be solved by an iterative procedure,
with the exception of the cases when N = 1 or N = 0. When N
= 1, Eq. (15) can be re-written as
c0
0cg
RD
DRCR
(16)
The reaction stoichiometry of a particle undergoing an
exothermic reaction in a gas phase is given as:
particle species j (s) + gas phase species n products.
Its reaction rate is given as:
rj,jrprj, RYηAR (17)
where Rj,r is defined as:
rN
r0,
rj,nrkin,rj,
D
RpRR
(18)
and where j,r = rate of particle surface species depletion (kg/s)
Ap = particle surface area (m²)
r = effectiveness factor (dimensionless)
Rj,r = rate of particle surface species reaction per unit area
(kg/m²-s)
pn = bulk concentration of gas phase species (kg/m3)
D0,r = diffusion rate coefficient for reaction r
Rkin,r = kinetic rate of reaction r (units vary), and
Nr = apparent order of reaction r.
The effectiveness factor, r, is related to the surface area,
and can be used in each reaction in the case of multiple
reactions.
D0,r is given by :
p
0.75p
r1,r0,d
2TTCD
(19)
Eq. (19) is a modification of the relationship given by Smith
[9] by assuming negligible change in gas density.
The kinetic rate of reaction, r, is defined as:
Rkin,r = Ap Te-(Er/RT) (20)
The rate of particle surface species depletion for reaction
order Nr = 1 is given by:
rkin,r0,
r0,rkin,njrprj,
RD
DRpYηAR
(21)
And for reaction order Nr = 0, the rate of depletion is
given by:
rkin,jrprj, RYηAR (22)
COMPUTATIONAL SCHEME Computational Grid and Grid Sensitivity Study
The geometry is generated and meshed using 64-bit
ANSYS ICEM CFD 14.0. Quadrilateral and triangular surface
meshes are used in the revised 2-D domain. Hybrid meshes,
such as hexahedral, tetrahedral, and pyramidal meshes, are
employed in the 3-D domain. An unstructured mesh is used
for meshing the 2-D geometry as well as the 3-D mild gasifier
(Figs. 6 and 7).
A grid sensitivity study has been conducted for 2-D mild
gasifier case using three different grids: 10,115, 21,582, and
46,917 cells for Case 2. Table 2 shows, after 4.4 seconds, the
change of mass-weighted average temperature and CO2 mass
fraction of these three grids are within 1.85% and 8.3%,
respectively. Although they have not reached grid-
independence, these changes are acceptable for the objective
of the present study. In the 3-D domain, 815,666 cells are used.
Table 2 Grid sensitivity study of 2D Case 2
Parameters 10,115
cells
21,582
cells
46,917
cells
difference
of last two
Exit gas
temperature (K) 524.39 413.74 406.22 1.85%
Exit mass
fraction of CO2 0.1067 0.0864 0.0798 8.27%
8
Figure 6 Unstructured mesh (46,917 cells) of the 2-D mild gasifier
Figure 7 Unstructured mesh (816,731 cells) for the 3-D mild gasifier
Boundary and Inlet Conditions The boundary conditions on the surface geometry have
been assigned in Tables 3 and 4 and are summarized below.
a. Velocity inlet: At all the inlet surfaces, the velocity,
temperature, and the mass fractions of all species of the
gas mixture are specified.
Table 3 Selected flow conditions at inlets for fluidized bed and draft tube in 2D domain
Parameters
Inlet position Fluidized bed inlet Draft tube inlet
Feedstock & transporting agent syngas coal & combusted gas
Syngas inlet velocity at horizontal holes, m/s 0.5
Snygas inlet velocity at skew holes, m/s 0.3
Combusted gas velocity at coal inlet, m/s 1.0
Entrained coal velocity at coal inlet, m/s 0.167
Combusted gas velocity at hot gas inlet, m/s 2.0
Temperature for fluidization gas, K 500
Temperature for entrained combusted gas, K 1600
Temperature for entrained coal, K 500
Mass fraction at inlet (%)
CO20.2274(combusted gas)
N20.6738(combusted gas)
Water Vapor0.0976(coal)
0.0987(combusted gas)
CO 0.882
H20.118
Char 0.4533(coal)
Volatiles 0.4491(coal)
Operating pressure (pascal) 101325 101325
Operating temperature (K) 288.16 288.16
Operating density (kg/m3) 1.177 919.56/1.18
Gravitational acceleration (m/s2) 9.81 9.81
Wall temperature, K Adiabatic Adiabatic
Case 1
Table 4 Selected flow conditions at inlets for fluidized bed and draft tube in 3D domain
Parameters
Inlet position Fluidized bed inlet Draft tube inlet
Feedstock & transporting agent syngas coal & combusted gas
Syngas inlet velocity at horizontal holes, m/s 0.5
Snygas inlet velocity at skew holes, m/s 12
Combusted gas velocity at coal inlet, m/s 1.0
Entrained coal velocity at coal inlet, m/s 0.167
Combusted gas velocity at hot gas inlet, m/s 2.0
Temperature for fluidization gas, K 500
Temperature for entrained combusted gas, K 1600
Temperature for entrained coal, K 500
Mass fraction at inlet (%)
CO20.2274(combusted gas)
N20.6738(combusted gas)
Water Vapor0.0976(coal)
0.0987(combusted gas) CO 0.882
H20.118
Char 0.4533(coal)
Volatiles 0.4491(coal)
Operating pressure (pascal) 101325 101325
Operating temperature (K) 288.16 288.16
Operating density (kg/m3) 1.177 919.56/1.18
Gravitational acceleration (m/s2) 9.81 9.81
Wall temperature, K Adiabatic Adiabatic
Case 3
b. Pressure outlet: The outlet surface is assigned as a
constant pressure boundary. In this study, flow separation
occurs at the gas exit due to the sharp 90-degree
connection of the external ducts. For flow separation, the
backflow condition needs to be specified. Typically, a
preliminary study is conducted for the steady-state
condition, and the result of the flow condition and species
composition at the exit are assigned as the backflow's
condition. Several iterations will be taken until the results
converge. However, it is found during the preliminary
9
study that assignment of the syngas composition in the
backflow makes reporting freshly produced syngas
composition unclear because the backflow syngas
contaminates the calculation of the syngas composition at
the exit. To resolve this issue, the backflow is
intentionally assigned without containing any syngas so
the mass flow weighted calculation of the syngas at the
exit consists entirely of the freshly produced syngas. The
steady-state result shows that this simplification does not
affect the result inside the gasifier. However, for a
transient study, it is very difficult to repeat the above
process for each time step. Considering the above two
conditions, the backflow conditions are assigned to be
fresh air from the environment.
c. Walls: The outside surfaces are defined as a wall
boundary. The walls are stationary with the no-slip
condition imposed (zero velocity) on the surface and are
assumed to be well insulated with zero heat flux (i.e., the
adiabatic condition).
d. Fluidized bed – The bed is initially filled with char (C)
with a depth of 10 inches. The char outlets are designed to
allow controlling of the bed depth by valves. In the CFD
simulation, the char extracting rate is controlled by
changing the char outlets’ pressure values. This is done by
filling the bed with coal (100% solid carbon + 0% H2O
vapor + 0% volatiles) with 40% void.
e. Patching temperature: Akin to using a lighter to ignite
combustion inside a combustor, a high temperature is
needed to start (ignite) the reactions by setting the
temperature of the cells near the injectors to 1600 K.
Numerical Procedure The commercial CFD software package, ANSYS/
FLUENT 14.0, is used in this study. The governing equations
are discretized spatially with second-order accuracy to yield
discrete algebraic equations for each control volume. The
volume fraction of the solid phase is calculated using the
QUICK scheme. The SIMPLE algorithm [10] is used in this
study to couple the pressure and velocity.
The primary phase (gas) enters the computational domain
through the inlets. The iterations are conducted alternatively
between the primary phase and the secondary phase (coal).
The primary phase is updated in the next iteration based on the
secondary phase calculation results, and the process is
repeated. Unsteady flow calculations are performed.
The computations are conducted via an 8-node, dual-core
computer network. For the 2-D case, the iteration time step
size is chosen to be 2×10-4 seconds. Typically, 20,000 time
steps are required to achieve convergence with 200 iterations
in each time step. Thus, a converged run of a 2-D case takes
approximately 48 hours. However, due to the high
computational cost, a fully three-dimensional simulation (with
chemical reactions) requires approximately three weeks to
obtain a converged result with a similar time step size setup as
2-D.
Because there is a lack of experimental data to validate
the numerical result, development of the model starts from
simulating the single-phase turbulent flow and heat transfer to
inspect the thermal-flow behavior, followed by employing the
demoisturization and devolatilization reactions, the seven
global gasification reactions, and the thermal cracking
reactions progressively by adding one equation at a time.
Finally, the particles are introduced. The detailed description
of the development and qualification of the reactive
multiphase CFD model is available from Lu and Wang [11].
The coal used in this study is an Indonesian coal. The
proximate and ultimate analyses are given in Table 5.
Table 5 Proximate and ultimate analyses of an Indonesian coal
Proximate Analysis (MF), wt% Ultimate Analysis (MF), wt%
Volatile 51.29 C 73.32
Fixed Carbon (FC) 47.54 H 4.56
Ash 1.17 O 20.12
100.00 N 0.72
S 0.11
Ash 1.17
100.00
RESULTS AND DISCUSSIONS The study begins with simulations of fluid dynamics and
heat transfer phenomena under different design and operating
considerations. After confirming that the airborne particle
residence time within the gasifier is close to 3 seconds, and the
fluidization velocity of 0.5 m/s is workable, a full, reactive
multiphase model is employed to investigates the effects of
coal particle size, fluidization velocity, coal feed speed, and
residence time on the flow pattern and gasification process.
The impact of selecting different coal transportation agents
(air vs. combusted gases) is also considered. To sustain the
fluidized bed depth to reach steady state, efforts have been
spent to investigate the entrainment control between the
fluidized-bed and the entrained-flow regime in the draft tube,
the effect of draft tube size, and the char chute exit pressure
control. Many cases have been studied and documented in the
report by Lu and Wang [7]. Three representative cases are
analyzed and discussed in this paper.
Case 1: 2-D, hot air at 500K is blown into both the draft tube
and the fluidized bed.
Case2: 2-D, syngas at 500K is blown to the fluidized bed and
1600K combusted gas is blown to the draft tube.
Case3: 3-D, syngas at 500K is blown to the fluidized bed and
1600K combusted gas is blown to the draft tube.
Case 1 In this case, hot air, preheated to 500K, is blown into the
fluidized bed, and the coal transported by air is also preheated
to 500K. The coal enters at 0.167 m/s and the transporting air
enters at 1 m/s. (Note, it is understood that coal particles
should not be transported by hot air at 500K because it can
cause combustion during transportation. In this simulation, the
coal/air mixture is assigned an inlet boundary condition of
500K for the convenience of simulation.) The major portion of
10
the air enters the draft tube from the outer annular passage at 2
m/s with a temperature of 1600K to simulate the hot gas that
will be used to provide energy for devolatilization and mild
gasification.
In the fluidized bed, 0.25 mm diameter carbon solid
particles are packed at a volume fraction of 0.6. The air
consisting of 21% O2 and 78% N2 by volume (O2 + 3.76 N2)
enters the horizontal perforated plates at 0.5 m/s and enters the
inclined perforated plate at 0.3 m/s and 500K. There are a total
of 28 perforated openings in the 2-D geometry.
The draft tube is designed to prevent the fluidized bed
from contacting the oxygen in the draft tube air while still
transferring heat to the fluidized bed through the draft tube
wall. Above the draft tube, a deflector is installed to block the
particles from being entrained out of the gasifier. There are
four outlets: two for char at middle portion and two for the
produced syngas at the top portion of the gasifier.
Figure 10 shows the transient distribution of the solid
carbon mass fraction. Due to the large difference in solid
carbon’s mass fraction between that in the draft tube and in the
fluidized bed, two separated color maps are used in Fig. 10
respectively. The first row in Fig. 10 emphasizes the
information inside the draft tube and second row emphasizes
the same information inside the fluidized bed. With this
composite presentation in Fig. 10, it can be seen that the coal
particles are successfully blocked by the deflector and most of
the coal particles fall off to the fluidized bed. For those that
escape from the deflector and rise to the freeboard, some of
them fall off to the top of the deflector and accumulate there
as time increases.
t = 0.2 sec t = 0.4 sec t = 0.8 sec
t = 0.2 sec t = 0.4 sec t = 0.8 sec
Figure 10 2-D transient distribution of volume fraction of carbon solid with an emphasis on draft tube (top row) and fluidized bed (bottom row) respectively for Case 1
The separate velocity vector plots of both the coal particle
phase and the gas phase colored by corresponding phase
temperature at time t = 0.58 seconds are shown in Fig. 11. The
particle velocity field in Fig. 11(a) clearly shows the
circulation in the fluidized bed, which is different from the gas
circulation shown in Fig. 11(b) in strength and pattern. A large
portion of the particles moves downward in the outer region of
the fluidized bed with a relatively higher velocity than the
slower-moving gas in the same region. Based on the particle
vector plot in Fig. 11(a), not many particles can be seen in the
top of the gasifier.
(a) (b)
Figure 11 Velocity vector plots for (a) particles and (b) gas with corresponding particle and gas temperature contours (K) at 0.58 seconds for Case 1
O2 N2
H2 CO
CO2
C6H6
Figure 12 2-D transient distribution of mass fractions of
various species at time t = 1.94 seconds for Case 1
11
The transient distribution of various species mass fraction
in the fluidized bed mild gasifier is shown in Fig. 12. It looks
like the volatile matters have been partially thermally cracked,
and some minor mild gasification also occurs in the freeboard
in Fig. 12, with traces of benzene, carbon monoxide, and
hydrogen. The existence of CO2 in this case implies that some
minor combustion occurs, since air (and hence oxygen) is
blown in this case. The presence of a small amount of these
species (C6H6, CO, and H2) only in the upper part (i.e.
freeboard region) of the gasifier implies that thermal cracking
and mild gasification are not very active inside the fluidized
bed. The notably reduced temperature in Fig. 11 also provides
further evidence that endothermic reactions such as thermal
cracking and gasification occur in the freeboard region. Please
note that due to the transient nature of the calculation, the
instantaneous snapshot of the figures usually capture the
dynamic motion of the multiphase such as periodic swaying of
the flow and vapor bubble dynamics, so they are not always
symmetric as is typically demonstrated by and expected from
a steady-state calculation.
Case 2
In this 2-D case, the hot air is replaced with more realistic
combusted gases in the draft tube inlet, and a small portion of
the raw syngas exiting the gasifier is extracted to fluidize the
char in the fluidized-bed. The combusted gases consist of
carbon dioxide, water vapor, and nitrogen, which are produced
from an external combustor located below the draft tube
entrance. The oxygen is assumed to be completely consumed
in combustion, so no oxygen is included in the combusted
gases. By making this change, an oxygen-free situation is
created for mild gasification. Part of the mild gasification
processes (C + CO2 2CO and C + H2O CO + H2) could
occur with CO2 from the combusted gases and H2O (water
vapor) from the coal and combusted gases. The lighter volatile
products (modeled as benzene) from thermal-cracking of the
heavy volatiles will not have sufficient oxygen to be broken
down (modeled as C6H6 + 3O2 → 6CO + 3H2 in this study) .
The combusted gases can come from burning the raw
syngas or chars extracted from the syngas exits or char chutes
of this gasifier. Burning char is more involved than burning
raw syngas in the current system arrangement. Therefore, in
this case, a portion of the raw syngas is extracted and
combusted in the external combustor. In consideration of mild
gasification, the raw syngas consists mainly of volatiles, so the
combusted gases are assumed to be the products of complete
combustion of the volatiles following reactions under
stoichiometric conditions:
CH2.121O0.5855 + 1.2378 O2 + 4.714288 N2 → CO2 + 1.061
H2O (g) + 4.714288 N2 (23)
The coal (transported by nitrogen) enters the inner section
of the draft tube at 0.167 m/s and 500K. The remaining
portion of the combusted gases is used to entrain the coal from
the annular duct surrounding the inner coal-fed tube. It enters
at 1 m/s and 1600K.
Regarding the fluidization fluid in Case 1, hot air was
used. However, in Case 2, the raw syngas consisting of 88%
carbon monoxide and 12% hydrogen by weight is used instead.
O2 N2
H2 CO
CO2
C6H6
Figure 13 2-D transient distribution of mass fractions of various species at time t = 2 seconds for Case 2
t = 0.1 sec t = 0.5 sec t = 2.0 sec
t = 0.1 sec t = 0.5 sec t = 2.0 sec
Figure 14 2-D transient distributions of mass fractions of volatiles in coal phase (top row) vs. volatiles in gas phase (bottom row) from 0.1 s to 2.0 s for Case 2 with an emphasis on the draft tube
Although this syngas may not have the same composition
as that expelled from the gasifier outlet in this study,
considering that the composition of syngas changes with time
and different operating parameters, this fixed composition (88%
CO and 12% H2 by mass fraction) is used for convenience. The
12
raw syngas is assigned a temperature of 500K with an inlet
velocity of 0.5 m/s at the horizontal perforated plate and 0.3
m/s at the inclined perforated openings into the fluidized bed.
The distributions of the mass fractions of various species in
the mild gasifier are shown in Fig. 13, 14, and 15. Clearly, a
coal devolatilization model has been successfully carried out
in the 2-D computational domain. Again, due to the very dense
char packed within the fluidized bed region, the mass fractions
of volatiles and water vapor are too low to be seen in Figs. 14
and 15. The mass weighted average temperatures for the gas
phase and solid phase are 849.92K and 818.47K, respectively,
at the outlet. The syngas composition at the exit is given in
Table 6.
t = 0.1 sec t = 0.5 sec t = 2.0 sec
t = 0.1 sec t = 0.5 sec t = 2.0 sec
Figure 15 2-D transient distributions of mass fractions of water vapor in coal phase (top row) vs. water vapor in gas phase (bottom row) from 0.1 s to 2.0 s for Case 2 with an emphasis on the draft tube Table 6 Composition of product syngas at gasifier exit at t = 2.0 s for Case 2
Temp
(K) Components
Mass
(%)
Vol
(%)
Gas phase
99.4%
volume)
849.9
2
O2 8.62 6.89
N2 58.58 51.82
Volatiles 0.77 0.04
Moisture 5.12 6.82
CO 8.44 7.27
CO2 14.68 8.76
H2 1.48 17.69
C6H6 2.31 0.71
Coal
phase
(0.6%
volume)
818.4
7
Char 51.03
Volatiles 0
Moisture 0
Case 3 In Case 3, the entire 3-D mild gasification process has
been simulated. The fluidization gas velocity was selected to
be 0.167 m/s in the previous 2-D cases. However, in order to
keep the same percentage ratio of the fluidization mass flow
rate versus the volume of fluidized medium as in the 2-D cases,
the fluidization gas inlet velocity of the 3-D case has to be
increased to 12 m/s for the perforated holes, located on the
inclined plate, but the other portion of the fluidization velocity
through the horizontal slots remain unchanged.
Figure 16 shows the 3-D transient distribution of the
volume fraction of carbon solid in the whole domain up to 2
seconds into the simulation. Figure 16 also shows that the
arched deflector successfully removes char accumulation on
its roof, which was previously shown on a flat-top deflector in
Khan and Wang's study [1]. Cutaway of two perpendicular
cross planes in Figure 17 show that carbon solid undergoes a
vivid fluidization movement in the fluidized bed, as displayed
on the two perpendicular planes.
Fig. 18 displays the velocity vector field for both the gas
phase and the solid phase. In the fluidized bed, the particles
appear to be moving more rigorously in large circulations than
the gases. In the previous 2-D cases, recall that the pressure at
the char chutes was purposely assigned a higher value in order
to simulate the control valve action as well as to adjust the
ratio of the char chute cross-sectional area over the gasifier
cross-sectional area in order to sustain the height of the
fluidized bed. In this 3-D case, no such manipulation is needed.
The fluidized height is sustained well with the given input
fluidization velocities.
Figure 16 3-D transient distribution of the volume fraction of carbon solid from t = 0.2-2.0 seconds for Case 3
\
t = 0.2sec t = 0.6 sec t = 0.8 sec
t = 1.4 sec t = 1.6 sec t = 2.0 sec
13
Figure 17 3-D transient distribution of the volume fraction of carbon solid on two crossed mid-planes for Case 3
Figure 18 A 3-D snapshot of (a) velocity vectors of the coal phase and (b) velocity vectors of the gas phase at t = 0.3s with the volume fraction of carbon solid being displaced in color for Case 3
Figures 19 and 20 indicate that the devolatilization
process that has been developed in the 2-D model has been
successfully incorporated into the 3-D model. Volatiles and
water vapor, which are inherent in the coal, have been driven
out by the combusted hot gases. Figure 21 presents the volume
fraction of various species in the gas phase. It can be seen that
the production of lighter volatiles, which are modeled as
benzene (C6H6) in this study, mainly occurs in the later part of
the draft tube and immediately outside the draft tube's exit.
This means that the coal feeding speed with the hot gas
entrainment is adequately assigned in this current design for
successfully achieving mild-gasification. However, some
thermal cracking and gasification seems to occur in the
freeboard area, so the benzene concentration decreases, while
those of CO and H2 increase. The lower volume fraction of
C6H6 in the freeboard area is also caused by the dilution of the
gases coming from the fluidized bed. Since not much volatile
matter has been detected at the syngas exits in the 3-D case, it
will be just a matter of trial-and-error to obtain the different
degrees of mild gasification for practical applications. On the
other hand, the production of CO can be seen primarily
occurring in the fluidized bed. The composition of syngas at
the exit is listed in Table 7. Based on these results, the
objectives have been successfully achieved in this project.
Figure 19 3-D transient distribution of the mass fraction of volatiles within the gas phase in two crossed mid-planes for Case 3
Figure 20 3-D transient distribution of the mass fraction of water vapor within the gas phase in two crossed mid-planes for Case 3
Table 7 Species composition of exit syngas at t = 1.56 s for Case 3
Temp(K) Species Mass (%) Vol (%)
Gas
phase
(99.9%
volume)
1587
O2 0.03 0
N2 43 27.03
Volatiles 0 0
Moisture 4.93 4.62
CO 37.74 23.19
CO2 7.64 2.98
H2 4.89 41.79
C6H6 1.76 0.39
Coal
phase
(0.1%
volume)
1587
Char 10.58
Volatiles 0
Moisture 0
t = 0.3 sec t = 0.5 sec t = 0.9 sec
(a) (b)
t = 0.1 sec t = 0.2 sec t = 0.3 sec
t = 0.1 sec t = 0.2 sec t = 0.3 sec
14
CO C6H6
CO2 H2
Figure 21 A 3-D snapshot of the transient distribution of the volume fraction of various species on two crossed mid-planes at t = 0.9 s for Case 3
CONCLUSIONS In this study, a mild gasification process was simulated in
a conceptual mild gasifier. The Eulerian-Eulerian multiphase
model is employed to simulate both the primary phase (air)
and the secondary phase (coal particles). The transient
multiphase Navier-Stokes equations and species transport
equations are solved with heterogeneous (gas-solid) and
homogeneous (gas-gas) global gasification reactions and a
two-step volatile cracking reaction. Multiphase constitutive
equations derived from kinetic theory are used to calculate the
effective shear viscosities, bulk viscosities, and other
interaction coefficients between the primary and secondary
phases.
The study begins with simulations of fluid dynamics and
heat transfer phenomena under different design and operating
considerations. After confirming that the airborne particle
residence time within the gasifier is close to 3 seconds, and the
fluidization velocity of 0.5 m/s is workable, a full, reactive
multiphase model is employed to investigate the effects of
coal particle size, fluidization velocity, coal feed speed, and
residence time on the flow pattern and gasification process.
The impact of selecting different coal transportation agents
(air vs. combusted gases) is also considered. To sustain the
fluidized-bed depth to reach steady state, efforts have been
spent to investigate the entrainment control between the
fluidized-bed and the entrained-flow regime in the draft tube,
the effect of draft tube size, and the char chute exit pressure
control. The results are summarized below:
1. A series of mild gasifier design modifications have been
considered to achieve effective mild gasification by
controlling the particle's residence time in the draft tube
less than one second and keeping the gases and air-borne
particles in the mild-gasifier for about 3 seconds.
2. In the 2-D cases, the appropriate velocity needed to
sustain fluidization with effective and vigorous mixing
but without depleting the fluidized bed contents has been
identified to be in the range of 0.2 – 0.5 m/s.
3. The goal of sustaining the fluidized bed in the 2-D
configuration has been achieved by closing the
entrainment slot and pressurizing the char chute at the exit
or by reducing the char chute so that the ratio of char
chute diameter over the gasifier perimeter remains the
same as that of the 3-D geometry.
4. The devolatilization and demoisturization processes have
been successfully implemented in the 3-D case. Using the
results obtained in the 2-D cases, the fluidized bed can be
successfully operated and sustained in the 3-D case at a
desired height.
5. The heavy volatiles have been shown to be fully cracked
in the freeboard region, but the modeled lighter product
"benzene" exists in the produced syngas at the exit. This
means that a certain degredd of mild gasification has been
successfully achieved. More sophisticated models for
volatile cracking and mild gasification will be
implemented in the future.
ACKNOWLEDGMENTS This study was partially supported by the Louisiana
Governor's Energy Initiative via the Clean Power and Energy
Research Consortium (CPERC) under the auspices of
Louisiana Board of Regents and partially supported by the U.S.
Department of Energy.
REFERENCES
1. Khan, J.R. and Wang, T., "Implementation of a
Demoisturization and Devolatilization Model in Multi-
Phase Simulation for a Hybrid Entrained-Flow and
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