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COMPARISON OF INSTANTANEOUS, EQUILIBRIUM, AND FINITE-RATE

GASIFICATION MODELS IN AN ENTRAINED-FLOW COAL GASIFIER

Armin Silaen Ting Wang *

Research Assistant Professor

asilaen@uno.edu twang@uno.edu

Energy Conversion & Conservation Center

University of New Orleans

New Orleans, Louisiana, USA

ABSTRACT

A coal gasification simulation model involves many sub- models and each of the sub-models needs to be investigated and

verified. This paper focuses on comparing three different

gasification reaction models: instantaneous gasification, global

equilibrium, and finite-rate models. The goal is to determine if

the simplified instantaneous gasification model can be used to

quickly capture acceptable approximations of thermal-flow and

reaction behaviors that can be used as a preliminary screening

tool of new design ideas for improving gasifiers’ performance.

The Eulerian-Lagrangian approach is applied to solve the

Navier-Stokes equations and eight species transport equations

with three heterogeneous global reactions and three

homogeneous reactions. The coal particles are tracked with the

Lagrangian method. In the instantaneous gasification model, the

interphase exchange rates of mass, momentum and energy are

assumed to be infinitely fast. Also, the dispersed phase can be

simplified as the gas phase, and the complex two-phase flow is

then treated as a single-phase flow. Two water shift rates are

used. The fast rate is used with the presence of catalyst, while

the slow rate is used without catalyst as in a typical entrained- flow gasifier. The results show that reactions in the

instantaneous gasification model occur fast and finish quickly;

whereas, the reaction in the finite-rate model, which involves

gas-solid reactions, occurs slowly. Varying the coal particle

size of the finite-rate model shows that the syngas heating value

of the smaller particle size is closer to the instantaneous

gasification model. The water shift rate plays a very important

role on affecting the accurate prediction of the syngas

composition. The syngas composition of using fast water shift

rate is very close to that calculated from the global equilibrium

method. The overall result reveals that the instantaneous

gasification approach can provide an overall evaluation of

relative changes of gasifier performance in terms of

temperature, heating value, and gasification efficiency

corresponding to parametric variations, but not adequately

capture the local gasification process predicted by the finite rate

model in most part of the gasifier.

1.0 INTRODUCTION

Gasification is the process of converting various carbon- based feedstocks to clean synthetic gas (syngas), which is

primarily a mixture of hydrogen (H2) and carbon-monoxide

(CO), through an incomplete combustion. Feedstock is

partially combusted with oxygen and steam at high temperature

and pressure with only less than 30% of the required oxygen for

complete combustion being provided. The syngas produced can

be used as a fuel, usually as a fuel for boilers or gas turbines to

generate electricity, or can be used to make a synthetic natural

gas, hydrogen gas or other chemical products. The gasification

technology is applicable to any type of carbon-based feedstock,

such as coal, heavy refinery residues, petroleum coke, biomass,

and municipal wastes.

The ultimate goal of the gasification research team at the

University of New Orleans is to develop a trustworthy

computational tool that can be used to help improve gasifier

designs to achieve better performance, efficiency, and

reliability. It is also desired to reduce the size of gasifiers,

which will lead to reduction of their capital and operational

costs. A good understanding of the gasification process inside a

gasifier is needed to help achieve these goals. The desired

product of a gasifier can mostly be obtained if the gasifier is big

enough so the residence time is sufficiently long to achieve

chemical equilibrium status. However, the corresponding

gasifier will be large and expensive and the product yield will be

low due to the lengthy residence time. To reduce the gasifier

size while augmenting product yield, the authors believe that

performance of a high-efficiency gasifier is closely related to

and affected by the thermal-flow behavior inside the gasifier.

CFD simulation is an economic and effective tool to help

achieve this goal. However, the gasification reaction model is

complicated and requires tremendous time to simulate.

Therefore, it is desired to see if a simplified approach can be

employed to quickly capture acceptable approximations of

thermal-flow and reaction behaviors that can be used as a

preliminary screening tool of new ideas for improving gasifiers’

performance. This study focuses on comparing CFD simulation

Proceedings of the 26th International Pittsburgh Coal Conference, Pittsburgh, USA, September 20-23, 2009

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results of an entrained-flow coal gasifier using the finite-rate

model and the instantaneous gasification model (simplified

approach). The finite-rate model solves gas-solid interactions,

while the instantaneous gasification model assumes a locally- homogeneous flow and solves the flow as a single-phase flow.

The syngas composition of both models are compared with the

that calculated by the global equilibrium method.

1.1 Global Gasification Chemical Reactions

This study only deals with global chemical reactions of coal

gasification (Smoot and Smith, 1985) that can be generalized in

reactions (R1.1) through (R1.9) below:

Heterogeneous (solid and gas) phase

C(s) + 1⁄2 O2 → CO, ΔH°R = -110.5 MJ/kmol (R1.1)

C(s) + CO2 → 2CO, ΔH°R = +172.0 MJ/kmol (R1.2)

(Gasification, Boudouard reaction)

C(s) + H2O(g) → CO + H2, ΔH°R= +131.4 MJ/kmol (R1.3)

(Gasification)

C + 2H2 → CH4, ΔHo

R = -87.4 MJ/kmol (R1.4)

(Direct methanation)

Homogenous gas phase

CO + 1⁄2 O2 → CO2, ΔH°R = -283.1 MJ/kmol (R1.5)

CO + H2O(g) → CO2 + H2 , ΔH°R = -41.0 MJ/kmol (R1.6)

(Water-shift)

CO + 3H2 → CH4 + H2O, ΔHo

R = -205.7 MJ/kmol (R1.7)

(Methanation)

CH2.121O0.5855 → 0.5855CO + 0.8532H2 + 0.2072C2H2 (R1.8)

(Volatiles cracking)

C2H2 + O2 → 2CO + H2 (R1.9)

(Volatiles gasification via C2H2)

In this study, the methanation reactions are not considered.

Reactions (R1.8) and (R1.9) involve volatiles. The volatiles are

modeled to go through a thermal cracking (R1.8) and

gasification processes (R1.9) via C2H2. Coal used in the study is

sub-bituminous coal from Indonesia. It has a moisture content

of 8.25%. Its moisture-free (MF) proximate and ultimate

analyses compositions are listed in Table 1. The compositions

of volatiles are derived from the values of coal heating value,

proximate analysis, and ultimate analysis.

Table 1 Moisture-free (MF) compositions of Indonesian sub- bituminous 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

1.2 Recent Research

Chen et al. (2000) developed a comprehensive three- dimensional simulation model for entrained coal gasifiers which

applied an extend coal gas mixture fraction model with the

Multi Solids Progress Variables (MSPV) method to simulate the

gasification reaction and reactant mixing process. The model

employed four mixture fractions separately track the variable

coal off-gas from the coal devolatilization, char-O2, char-CO2,

and char-H2O reactions. Chen et al. performed a series of

numerical simulations for a 200 ton per day (tpd) two-stage air

blown entrained flow gasifier developed for an IGCC process

under various operation conditions (heterogeneous reaction rate,

coal type, particle size, and air/coal partitioning to the two

stages).

Bockelie et al. (2002(a)) of Reaction Engineering

International (REI) developed a CFD modeling capability of

entrained flow gasifiers that focuses on two gasifier

configurations: single-stage down fired system and two-stage

with multiple feed inlets. The model was constructed using

GLACIER, an REI in-house comprehensive coal combustion

and gasification tool. The basic combustion flow field was

established by employing full equilibrium chemistry. Gas

properties were determined through local mixing calculations

and are assumed to fluctuate randomly according to a statistical

probability density function (PDF), which is characteristic of the

turbulence. Gas-phase reactions were assumed to be limited by

mixing rates for major species as opposed to chemical kinetic

rates. Gaseous reactions were calculated assuming local

instantaneous equilibrium. The particle reaction processes

include coal devolatization, char oxidation, particle energy,

particle liquid vaporization and gas-particle interchange. The

model also includes a flowing slag sub-model.

U.S. Department of Energy/National Energy Technology

Laboratory (NETL) developed a 3D CFD model of two

commercial-sized coal gasifiers [Guenther and Zitney (2005)].

The commercial FLUENT CFD software is used to model the

first gasifier, which is a two-stage entrained-flow coal slurry-fed

gasifier. The Eulerian-Lagrangian approach is applied. The

second gasifier is a scaled-up design of transport gasifier. The

NETL open source MFIX (Multiphase Flow Interphase

eXchanges) Eulerian-Eulerian model is used for this dense

multiphase transport gasifier. NETL also developed an

Advanced Process Engineering Co-Simulator (APECS) that

combines CFD models and plant-wide simulation. APECS

enables NETL to couple its CFD models with steady-state

process simulator Aspen Plus.

Silaen and Wang (2005) conducted numerical simulations

of the coal gasification process inside a generic two-stage

entrained-flow gasifier using the commercial CFD solver

FLUENT. They investigated the effects of several parameters

on gasification performance including coal mixture (slurry or

dry powder), oxidant (oxygen-blown or air-blown), wall

cooling, and various coal distributions between the two stages.

The simulation results provide the temperature and species

distributions inside the gasifier. The results indicate that coal- slurry feed is preferred over coal-powder feed to produce

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hydrogen. On the other hand, coal-powder feed is preferred

over coal-slurry feed to produce carbon monoxide. The air- blown operation yields poor fuel conversion efficiency and the

lowest syngas heating value due to air dilution. The effect of

wall cooling has been shown insignificant on the exit gas

composition and heating value. The fuel conversion efficiency

of the case with coal distribution with 75% (first stage) vs. 25%

(second stage) is better than the case with 50% vs. 50% coal

distribution. They stated that a two-stage design has an

advantage of the flexibility to adjust parameters to achieve

desired performance.

In the continuation of that study, Silaen and Wang (2006)

carried out a study that focused on the effect of flow injection

directions on the gasification performance using the same

generic two-stage entrained flow gasifier. Horizontal injection

direction was compared to downward and upward direction.

The results revealed that the horizontal injection direction gave

the best gasifier performance. Changing the direction of the

first-stage injectors downward resulted in a carbon fuel

conversion reduction, but produce more H2. Changing the

direction of the second-stage injectors, however, does little to

affect the overall flow patterns due to the smaller-quantity of

coal injection (25%); therefore the gasifier performance is

essentially insignificantly affected.

Silaen and Wang (2008) conducted a study that investigates

the effects of different parameters on gasification performance

including five turbulence models, four devolatilization models

and three solid coal sizes. The Eulerian-Lagrangian approach

with finite global reaction rates was applied. A two-step

decomposition model was applied to volatiles cracking and

gasification via benzene. The results reveal that the standard k-ε

and the RSM models gave consistent results. High inertia

possessed by large coal particles can propel the particles cross

the gas streamlines and increase particle-gas mixing which

results in enhanced reaction rate. The single rate devolatilization

model and the chemical percolation model produced moderate

and consistent devolatilization rate.

This study is a continuous work of Silaen and Wang (2005,

2006, 2008) and focuses on comparing two different gasification

reaction models – the instantaneous gasification model and the

finite-rate model.

2.0 COMPUTATIONAL MODEL

The models used in the study are the same as used by Silaen

and Wang (2008). The time-averaged steady-state Navier- Stokes equations as well as the mass and energy conservation

equations are solved. Species transport equations are solved for

all gas species involved. The standard k-ε turbulence model is

used to provide closure. Silaen and Wang (2008) reported that

the standard k-ε turbulence model yields reasonable results

without requiring very much computational time when

compared to other turbulence models. Enhanced wall function

and variable material property are used. The P1 model is used

as the radiation model.

Finite-Rate Model -- The flow (continuous phase) is

solved in Eulerian form as a continuum while the particles

(dispersed phase) are solved in Lagrangian form as a discrete

phase. Stochastic model is employed to model the effects of

turbulence on the particles. The continuous phase and discrete

phase are communicated through drag forces, lift forces, heat

transfer, mass transfer, and species transfer. The finite-rate

combustion model is used for the heterogeneous reactions, but

both the finite-rate and eddy-dissipation models are used for the

homogeneous reactions, and the smaller of the two is used as

the reaction rate. The finite-rate model calculates the reaction

rates based on the kinetics, while the eddy-dissipation model

calculates based on the turbulent mixing rate of the flow.

Gasification or combustion of coal particles undergoes the

following global processes: (i) evaporation of moisture, (ii)

devolatilization, (iii) gasification to CO and (iv) combustion of

volatiles, CO, and char. The Chemical Percolation

Devolatilization (CPD) model [Fletcher and Kerstein (1992),

Fletcher et. al (1990), and Grant et. al (1989)] is chosen as the

devolatilization model based on the finding by Silaen and Wang

(2008) that the Kobayashi two-competing rates devolatilization

model [Kobayashi et. al. (1976)] is very slow, while the CPD

model gives a reasonable result.

For solid particles, the rate of depletion of the solid due to

a surface reaction is expressed as a function of kinetic rate, solid

species mass fraction on the surface, and particle surface area.

The reaction rates are all global net rates, i.e., the backward

reaction, calculated by equilibrium constants, are included in the

global rate. Therefore, the finite rate employed in this study

implicitly applies local equilibrium approach. Reaction rate

constants used in this study are summarized in Table 2.

Table 2 Summary of reaction rate constants used in this

study

Reaction Rate Constant Parameters

C(s) + 1⁄2O2 → CO k = ATn exp(-E/RT) n = 0

(Combustion) A = 0.052 kg/m2

.Pa-0.5

E = 6.1x107 J/kmol

C(s) + CO2 → 2CO k = ATn exp(-E/RT) n = 0

(Gasification, Boudouard reaction) A = 0.0732 kg/m2

.Pa-0.5

E = 1.125x108

J/kmol

C(s) + H2O(g) → CO + H2 k = ATn exp(-E/RT) n = 0

(Gasification) A = 0.0782 kg/m2

.Pa-0.5

E = 1.15x108

J/kmol

CO + 1⁄2 O2 → CO2 k = ATn exp(-E/RT) n = 0

A = 2.2x1012

E = 1.67x108

J/kmol

CO + H2O(g) → CO2 + H2 k = ATn exp(-E/RT) n = 0

(Watershift) A = 2.75x102 *

E = 8.38x107

J/kmol

CH2.121O0.5855 → 0.5855CO + 0.8532H2 + 0.2072C2H2 Eddy-dissipation only

C2H2 + O2 → 2CO + H2

* This rate is reduced from the original value of Jones and Lindstedt (1988)

Eddy-dissipation only

Gas phase homogeneous reactions:

Solid-gas heterogeneous reactions:

The reaction rate of the water-shift, adopted from Jones and

Lindstedt (1988), is found to be too fast in this study because the

rate is obtained with the presence of catalyst. Considering no

catalyst is added in a typical gasifier, the water shift reaction

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rate is purposely slowed down to make the syngas composition

consistent with that in the actual production of a commercial

entrained-flow gasifier with coal-slurry feed from bottom.

For liquid droplets, water evaporates from the particle’s

surface when temperature is higher than the saturation

temperature (based on local water vapor concentration). The

evaporation is controlled by the water vapor partial pressure

until 100% relative humidity is achieved. When the boiling

temperature (determined by the air-water mixture pressure) is

reached, water continues to evaporate even though the relative

humidity reaches 100%. After the moisture is evaporated due to

either high temperature or low moisture partial pressure, the

vapor diffuses into the main flow and is transported away.

Please refer to Silaen and Wang (2008) for the detailed

devolatilization and gasification models.

Instantaneous Gasification Model -- The interphase

exchange rates of mass, momentum and energy are assumed to

be infinitely fast. Carbon particles are made to gasify

instantaneously, thus the solid-gas reaction process can be

modeled as homogeneous combustion reactions. This approach

is based on the locally-homogeneous flow (LHF) model

proposed by Faeth (1987), implying infinitely-fast interphase

transport rates. The instantaneous gasification model can

effectively reveal the overall combustion process and results

without dealing with the details of the otherwise complicated

heterogeneous particle surface reactions, heat transfer, species

transport, and particle tracking in turbulent reacting flow. The

eddy-dissipation model is used to model the chemical reactions.

The eddy-dissipation model assumes the chemical reactions are

faster than the turbulence eddy transport, so the reaction rate is

controlled by the flow motions.

Since the water-shift rate plays an important role on the

formation of the final syngas composition, two water shift rates

are used. The fast rate is used with the presence of catalyst,

while the slow rate is used without catalyst as in a typical

entrained-flow gasifier. The fast rate from Jones and Lindstedt

(1988) was first used and it was discovered that the results are

similar to using the eddy-dissipation rate. Considering using the

eddy-dissipation rate is convenient, the fast water shift rate is

hence replaced by the eddy-dissipation rate in this study. The

slow water shift rate is the same as that used in the previous

finite rate simulation as shown in Table 2.

The instantaneous gasification model can significantly

reduce the computational time but can only provide a qualitative

trend of gasification process. Although the instantaneous

gasification model is crude, it catches the effect of thermal-fluid

field (including turbulence structure) on chemical reactions,

which are not readily available from the equilibrium method.

Chemical Equilibrium Method – In the chemical

equilibrium method, CFD scheme is not employed. The C-H2O

gasification process (R1.3) is assumed to consume the steam

first before the water-shift takes place to use up the remaining

steam. This assumption is based on the fact that water shift is

slow without catalyst in a typical gasifier.

The computation is carried out using the finite-volume- based commercial CFD software FLUENT (Version 6.3.26)

from Ansys, Inc. The simulation is steady-state and uses the

pressure-based solver, which employs an implicit pressure- correction scheme and decouples the momentum and energy

equations. SIMPLE algorithm is used to couple the pressure

and velocity. Second order upwind scheme is selected for

spatial discretization of the convective terms. For the finite rate

model where the Eulerian-Lagrangian approach is used, the

iterations are conducted alternatively between the continuous

and the dispersed phases. After twenty continuous phase

iterations, one dispersed phase iteration is performed. The drag,

particle surface reaction, and mass transfer between the

dispersed and the continuous phases are calculated. The

continuous phase is updated in the next iteration based on the

dispersed phase calculation results, and the process is repeated.

Converged results are obtained when the residuals satisfy mass

residual of 10-3, energy residual of 10-5, and momentum and

turbulence kinetic energy residuals of 10-4. These residuals are

the summation of the imbalance in each cell, scaled by a

representative for the flow rate. The computation was carried

out in parallel processing on two dual-core Pentium clusters

with 12 nodes each.

2.1 Physical Characteristics of the Model and Assumptions

This paper studies a one-stage entrained flow coal gasifier.

Fundamental investigation is first conducted on a simplified 2-D

geometry (Fig. 1) to perform a parametric study of the effect of

coal particle sizes on gasification performance and two different

approaches of modeling coal slurry.

Inlet Inlet

Outlet

9m

1.5m

Inlet Inlet

(a) Schematic (b) Meshed computational domain (86k

elements)

Fig. 1 Schematics of a simplified 2-D one-stage entrained

flow gasifier configuration studied and its meshed

computational domain.

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Diffuser

Reductor

Combustor

1.5 m

0.75 m

Top view of second stage

injectors

Top view of first stage

injectors

2nd

Stage

Inlets

1

st Stage

Inlets

Coal Slurry

& Oxygen

Coal Slurry

Raw Syngas

9.5 m

0.5 m

0.5 m

3.75 m

0.5 m

(a) Schematic of 3-D gasifier

(b) Meshed computational domain (969k elements)

Fig. 2 Schematics of 3-D one-stage entrained flow gasifier

configuration (adopted from Bockelie et al. 2002a) and its

meshed computational domain. The second stage is not

used in this study.

From the 2-D results, a fixed coal particle size and one coal

slurry model are selected to conduct 3-D simulation. The

geometry of the 3-D one-stage gasifier is adopted from Bockelie

el al. (2002a) and is shown in Fig. 2. Two opposing injectors are

located near the bottom of the gasifier. In the simulations, the

buoyancy force is considered, varying fluid properties are

calculated for each species and the gas mixture, and the walls

are impermeable. The following general assumptions are made:

the flow is steady and no-slip condition (zero velocity) is

imposed on wall surfaces.

3.0 BOUNDARY AND INLET CONDITIONS

Indonesian sub-bituminous coal is used as feedstock in this

study. Its composition is given in Table 1 and the feed rates

used are given in Table 3. The 2-D feed rate is prorated lower

from the 3D feed rate to the extent that the injection velocity is

comparable to the 3-D case. The coal/water weight ratio of the

coal slurry is 60%-40%. The oxidant used is 95% O2 and 5%

N2. Oxidant/coal slurry feed rate used in Table 3 gives O2/coal

equivalence ratio of 0.3.

Table 3 Feed rates used in the study

2D gasifier 3D gasifier

Coal slurry 18.15 21.39

Oxidant 6.04 7.12

Feed rate (kg/s)

In the finite-rate model, the oxidant is considered as a

continuous flow and coal slurry is considered as a discrete flow.

The discrete phase only includes the fixed carbon and water

from the moisture content of coal and water added to make the

slurry. Two approaches are adopted to model the coal slurry

injection. The first approach injects the slurry coal with each

particle containing both coal and liquid water. The second

approach injects coal (as a solid particle) and liquid water (as

droplets) separately. Other components of the coal, such as N,

H, S, O, and ash, are injected as gas, together with the oxidant in

the continuous flow. N is treated as N2, H as H2, and O as O2. S

and ash are lumped into N2. The coal slurry size is uniformly

given as 50 μm respectively in the baseline case. In the

instantaneous gasification model, all species are injected as gas.

The walls are assigned as adiabatic with internal emissivity

of 0.8. The boundary condition of the discrete phase at walls is

assigned as “reflect”, which means the discrete phase elastically

rebound off once reaching the wall. At the outlet, the discrete

phase simply escapes/exits the computational domain. The

gasifier is operating at 24 atm.

4.0 RESULTS AND DISCUSSIONS

4.1 Comparison of Finite-Rate, Chemical Equilibrium , and

Instantaneous Gasification Cases

Table 4 presents the exit syngas temperature and

compositions for the finite-rate and instantaneous gasification

cases. Carbon conversion is defined as the amount of unburned

char contained in the exit gas divided by the total char injected

through the inlets. Carbon conversion for the finite-rate case is

91% while the carbon conversion for the instantaneous

gasification is 100%. As mentioned earlier, carbon particles are

made to gasify instantaneously in the instantaneous gasification

model. Thus, the solid-gas reaction process can be modeled as

homogeneous combustion reactions. The homogenous reactions

of char in the instantaneous gasification model are much faster

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than the heterogeneous reactions of char in the finite-rate model.

In the instantaneous gasification, char is injected as gas which

means that it can immediately react as soon as it leaves the

inlets. Coal particles in the finite-rate model have to undergo

evaporation and devolatilization before the char can be burned.

The existence of unburned chars in the exit gas is consistent

with the operating of current gasifiers reported in available

literatures.

Table 4 Exit syngas temperature and compositions for 2D

cases with 50 μm particles .

Finite

rate

Instantaneous

gasification,

slow water shift

Instantaneous

gasification,

fast water shift

Equilibrium

1181 1179 1204 N/A

91.0% 100.0% 100.0% 100.0%

78% 84% 84% 85%

Mole fraction:

CO 28.4% 28.9% 4.3% 6.1%

H2 29.7% 36.4% 59.6% 59.4%

CO2 8.5% 10.5% 34.7% 33.4%

CH2.121O0.5855 0.0% 0.0% 0.0% 0.0%

H2O 31.2% 23.1% 0.0% 0.0%

C2H2 1.0% 0.0% 0.3% 0.0%

N2 1.0% 1.1% 1.1% 1.1%

O2 0.0% 0.0% 0.0% 0

9.7 10.4 10.4 10.5

2D

HHV at 25C (MJ/kg)

Temperature (K)

Cold gasification

efficiency

Carbon conversion

Syngas temperature for both finite-rate and instantaneous

gasification cases with a slow water shift rate, listed in Table 4,

are almost identical. The exit syngas compositions are also close

except more hydrogen is produced in the instantaneous

gasification model at the expense of steam. However, Figs. 4

and 5 show very different local temperature and mass-weighted

species distributions between these two cases. The reactions

happen and finish very quickly as expected, but the CFD results

provide a quantitative measure of the reaction time. On the

other hand, the finite-rate model shows a hot region (around

2200 K) below the injection points and a gradual increase above

the injection points. The heating value of the finite rate is a bit

low as expected.

Above is the comparison between the finite rate and the

instantaneous gasification using the same slow water shift rate

as shown in Table 2. . If the slow water shift rate is replaced

with the fast rate using the eddy-dissipation approach, the exit

syngas composition changes drastically with a large reduction of

CO and steam and a significant increase of H2 and CO2 – an

unmistaken result due to the water shift reaction (R1.6).

Unsurprisingly, the exit syngas composition is almost the same

as the equilibrium results. The little bit higher exit temperature

is resulted from the heat released from the exothermic water

shift process.

(a) Finite-rate (b) Instantaneous gasification

Fig. 4 Temperature distribution for 2D finite-rate case and

instantaneous gasification case with a slow water shift rate.

0

1

2

3

4

5

6

7

8

9

400 800 1200 1600 2000

Gas temperature (K)

Gasifier height (m)

Finite

Instant

0

1

2

3

4

5

6

7

8

9

0.0 0.1 0.2 0.3 0.4

Mole fraction

Gasifier height (m)

CO - finite

H2 - finite

CO2 - finite

C2H2 - finite

CO - instant

H2 - instant

CO2 - instant

C2H2 - instant

Fig. 5 Mass weighted average of gas temperature and

species mole fraction for 2D finite-rate and instantaneous

gasification cases with a slow water shift rate.

4.2 Effects of Different Coal Particle Sizes

As mentioned earlier, the reaction rates in the instantaneous

gasification are much faster than those in the finite-rate model.

Several cases with different coal particle diameters are studied

to examine if perhaps the results would be similar if particle size

is reduced because smaller particle sizes have larger surface

areas, which can help expedite the reaction. Three other

different coal slurry particle diameters used are 1μm, 10μm, and

100μm. The results, along with the results for the 50μm- diameter coal slurry particle case above, are compared in Table

5 with the instantaneous gasification case using the slow water

shift rate.