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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, ylv2@uno.edu and 504-280-7183, twang@uno.edu

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"

qq

= 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

Fluidized Bed Mild Gasifier," International Journal of

Clean Coal and Energy, 2013, vol. 2, pp. 35-53,

http://dx.doi.org/10.4236/ijcce.2013.23005, Published

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