Article
pubs.acs.org/IECR
Simulation Research on the Fixed-Bed Gasification Process in a TwoStage Combined Gasifier
Yifei Wang,* Weilong Jin, Longchu Zhu, Guangsuo Yu, Zhenghua Dai, and Fuchen Wang
Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, Shanghai Engineering Research
Center of Coal Gasification, East China University of Science and Technology, Shanghai 200237, China
ABSTRACT: The novel two-stage gasification process that is combined entrained-flow for first stage and fixed-bed for second
stage was shown in the study. The benefit of the second stage was to recycle the sensible heat from first stage syngas and enhance
the energy utilization. The fixed-bed gasification reaction of second stage was individually investigated by numerical simulation.
The simulated results were validated well with the experimental data in terms of gas concentration, gas flow rate, and carbon
conversion. The distributions of velocity, temperature, gas concentration, solid mass fraction, and carbon conversion during the
gasification process were analyzed. The influences of coal amount and particle size in the second stage on gasification reactivity
were also discussed in the study. Results showed that the gas temperature was reduced and the effective gas concentration
increased when first stage syngas flowing through the second stage coal layer. The increase of coal amount of second stage
showed a more significant heat recovery, but the heating rate and ultimate carbon conversion would be reduced. The increase of
particle size resulted in the decreasing of effective gas concentration and gas−solid reaction rate, which was attributed to the
change of specific surface area and the increase heat transfer resistance from the particle surface to inside.
1. INTRODUCTION
Coal gasification is an important way to utilize coal resource
efficiently and cleanly, which could convert the primary energy
to a secondary clean energy. The product gases can be used as
the feed stock to treat as fuel, syngas, hydrogen, carbon
monoxide, and so on. Nowadays, coal gasification has been
widely used in synthetic ammonia, methanol synthesis, sponge
iron production, and other fields, and one of the biggest
potential markets is integrated coal gasification combined cycle
(IGCC) power generation.1 Entrained-flow gasification technology has become an advanced coal gasification technology
due to its advantages of high carbon conversion, large singlefurnace production capacity, and good feedstock flexibility.
However, the gasification temperature in the gasifier is
extremely high, resulting in much waste of syngas sensible
heat from the outlet of gasifier. Therefore, a syngas cooling
system is essential for the entrained-flow gasification process.
As to the cooling methods for the existing gasifiers, most of
them still have deficiencies in the technical process. For
example, the Shell gasification process2 uses recycle syngas
quench to recover the syngas heat. The disadvantage is the
increase the circulation compressor and its compression work.
The Texaco gasifier3 featured with water quench process
ignores the sensible heat recovery from high temperature
syngas, while its radiation syngas cooler process increases the
huge investment of the equipment. In view of these situations, a
novel coal-based two-stage combined gasification process with
heat recovery via chemical reaction was proposed by Institute
of Clean Coal Technology, East China University of Science
and Technology, which is aimed at the insufficient heat
recovery of high temperature syngas for the current entrainedflow gasifiers. The novel coal gasification process is an organic
combination of the opposed multiburner (OMB) gasifier at the
first stage and lump coal moving bed gasifier at the second
stage.4,5 Two endothermic reactions of carbon−H2O and
© 2014 American Chemical Society
carbon−CO2 are utilized to recover the sensible heat of high
temperature syngas. The components of CO2 and H2O in the
syngas undergo further reaction with the lump coal in the
second stage, and the cold gas efficiency can be further
improved. The sensible heat of syngas from entrained-flow
gasifier provides the heat that the reactions needed and does
not need additional oxidant for the second stage gasification
reaction. Then, the sensible heat of hot temperature syngas
from entrained flow gasifier is shifted to chemical energy and
stored in the gas products with a simple and compact structure.
It has a dual function for energy recycling efficiently and
reduction of carbon dioxide emission.6
According to the patent named “Coal-Based Two-Stage
Combined Gasification Process”, Huang et al.7 set up a hotmodel experimental device and investigated the influences of
second stage coal amount and gas composition from first stage
on integrated gasification efficiency. Jin et al.8 continued to
perform the experimental research by focusing on the
gasification efficiency influenced by the lump coal type and
particle size. In order to further comprehend the changing
trend of lump coal layer in the second stage and gas
concentration and temperature profile in the furnace, detailed
simulation research should be carried out for the hot-model
two-stage experimental device. Biasi9 adopted the mathematical
method to formulate the biomass gasification process in a
downdraft reactor. The simulation research could predict the
influences of model parameters, kinetic constants, and
operational variables on the produced gas quality well. Yang
et al.10 used a mathematical model to predict the main chemical
Received:
Revised:
Accepted:
Published:
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January 22, 2014
March 14, 2014
April 3, 2014
April 3, 2014
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and physical processes and comparing with the experiment
results in fixed-bed gasifier, assessing the subsequent effect of
model parameters varying on the char gasification characteristics. In recent years, the rapid development of computer
hardware promotes the use of computational fluid dynamics
(CFD) software to simulate and analysis fixed-bed gasification
phenomena. Murgia et al.11 used the Euler−Euler approach to
simulate an air-blown updraft fixed-bed coal gasifier based on
CFD software. The solid phase was considered as continua
according to the kinetic and plastic theory of granular flows.
The operation of the gasifier was investigated, and the
characteristics of space- and time-dependent behavior were
also addressed. Wu et al.12 used the Euler−Euler approach to
comprehend the biomass gasification in a downdraft fixed-bed
gasifier. The model results exhibited a reasonable agreement
with the experimental data, and the parametric studies of
varying preheating temperature and steam/air ratio were
performed on the basis of the developed model. Their work
is based on a common fixed-bed gasifier with the steady
condition, while no simulation research is about the heat
recovery fixed-bed gasification process in the combined gasifier.
As to this simulation, the high temperature syngas reacting with
the lump coal under the unsteady condition, and a thorough
endothermic reaction is carried out in the second stage fixedbed.
A two-dimensional fixed-bed gasification model is established
to simulate the second stage fixed-bed gasification process in
the proposed two-stage gasifier, which concerned the heat
utilization and gasification efficiency in the syngas atmosphere.
The Euler−Euler approach is adopted to solve gas and solid
phase equations in this model. The accuracy of the model is
verified by the comparison of the simulated results and
experimental data, and the gasification characteristics of fixedbed for the second stage are also analyzed, including the
distributions of velocity, temperature, gas concentration, gas−
solid reaction rate, and carbon conversion. Furthermore, the
influences of coal amount in the second stage and lump coal
particle size on fixed-bed gasification reactivity are also
discussed.
Figure 1. Structure of the combined two-stage gasifier.
Inner Mongolia lignite is used as the feedstock of the second
stage fixed-bed. The proximate and ultimate analyses are shown
in Table 1. The produced gas from second stage outlet is
scrubbed and quenched in a chilling chamber, and the syngas
volume is measured by a V flow-meter. The syngas composition
is analyzed by Aglient7890a GC with the sampling frequency of
10 min after being dried and purified. After the experiment is
finished, the mass of residue in the second stage and its ash are
weighed to calculate the ultimate carbon conversion. The stable
parameters for the first stage syngas are tabulated in Table 2 on
the basis of practical operation conditions.
3. MATHEMATICAL EQUATIONS
The multiphase flow model with Euler−Euler approach is
adopted to simulate the fixed-bed gasification process. In this
model, the high temperature syngas is treated as gas phase and
the lump coal in the fixed-bed as solid phase, in which both of
the phases are dealt with interpenetrating. The conservation
equations of mass, momentum and energy are solved for gas
and solid phases simultaneously. The standard k−ε turbulence
model13 is used to describe the gas phase flowing.
3.1. Continuity Equation. The continuity equations for
the gas and solid phases are defined as14
2. EXPERIMENTAL DEVICE AND FEEDSTOCK
The combined two-stage gasifier is sketched in Figure 1. The
size of alumina tube in the furnace is ϕ160 × 20 mm with the
height of 1010 mm. The first stage is entrained-flow gasification
with diesel and oxygen feed, and the fixed-bed pattern is
adopted as the second stage with coal feed. The purpose of the
study is to address the transient gasification phenomena in the
second stage, so the first stage is adopted to provide the
proposed syngas condition for the second stage inlet. The
experiment process can be divided into two periods: initial
heating process and subsequent gasification reaction process.
The syngas produced from the first stage will be exhausted
from the first stage outlet in order to avoid reacting with coal
during the heating process. When the syngas temperature and
composition reaches the stable in the first stage, the valve of
second stage outlet is turned to open and meanwhile the valve
of first stage outlet is turned to closed. Subsequently, the high
temperature syngas from first stage flows to the lump coal in
the second stage and the produced gas flows out from the
second stage outlet. The latter period of fixed-bed gasification
process in the combined gasifier is just the research process in
this simulation.
∂
(αgρg ) + ∇(αgρg Ug) = Sgs
∂t
(1)
∂
(αsρs ) + ∇(αsρs Us) = Ssg
∂t
(2)
In which α, ρ, U are the volume fraction, density and
instantaneous velocity, respectively. S is the mass source term,
resulting from the heterogeneous reaction between gas and
solid phases, which meets the equation of Ssg = wc∑γcRc = −Sgs,
where wc is the molecular weight, γc is the stoichiometric
coefficient, and Rc is the reaction rate.
3.2. Momentum Equation. 3.2.1. Gas Phase.
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Table 1. Proximate and Ultimate Analyses of Inner Mongolia Lignite
proximate analysis/dry basis
ultimate analysis/dry ash-free basis
FC
VM
ash
C
H
O
N
S
0.5913
0.3556
0.0531
0.8355
0.0480
0.0995
0.0111
0.0059
Table 2. Stable parameters of the first stage syngas
item
temp. (K)
pressure (Pa)
inlet gas velocity (m·s−1)
Gas Components (%)
CO
H2
CO2
H2O
3.3. Energy Equation. 3.3.1. Gas Phase.
∂
(αgρg hg ) + ∇(αgρg Ughg )
∂t
∂p
= −αg
+ tg : ∇Ug − ∇qg⃗ + Sg + Q sg + Ssghsg
∂t
value
1573.15
101 325
0.5153
where hg is the specific enthalpy for gas phase; q⃗g is the heat flux
for the gas phase; Sg is the source term; Qsg is the heat exchange
between gas and solid phases; hsg is the enthalpy between
phases.
3.3.2. Solid Phase.
34.07
27.94
16.52
21.47
∂
(αgρg Ug) + ∇(αgρg UgUg) = −αg∇p + K sg(Us − Ug)
∂t
+ (∇τg) + αgρg g + SgsUs
∂p
∂
(αsρs hs) + ∇(αsρs Uh
+ ts: ∇Us − ∇qs⃗ + Ss
s s) = − αs
∂t
∂t
(3)
+ Q gs + Sgshgs
where Ksg is the momentum exchange coefficient between gas
and solid phases. The Syamlal−O’Brien approach15 is adopted
for drag force model, and the momentum exchange coefficient
is given by the formula of
K sg =
3αsαlρl
4vr2,sds
⎛ Re ⎞
C D⎜⎜ s ⎟⎟|vs⃗ − v l⃗ |
⎝ vr ,s ⎠
⎞2
4.8 ⎟
Res/vr ,s ⎟⎠
∂
(αgρg Yi ) + ∇(αgρg UgYi ) = −∇αgJi + αR g, i + R s, i
∂t
(4)
(5)
vr ,s = 0.5(A − 0.006Res
(0.006Res)2 + 0.12Res(2B − A) + A2 )
ps = αsp*
(6)
The relative Reynolds number is shown as
Res =
where p* is expressed by an empirical power function
ρg ds|vs⃗ − vg⃗ |
μg
p* = 1025(αs − αg*)10
(7)
(8)
3.2.2. Solid Phase.
μs =
∂
(αsρs Us) + ∇(αsρs UU
s s) = − αs∇p − ∇ps + ∇· τs + αsρs g
∂t
+ SsgUs
p sin ϕ
2 I2D
(15)
where ϕ is the angle of internal friction and I2D is the second
invariant of the strain rate tensor.
3.6. Heat Transfer. The interphase heat transfer is
considered as a function of temperature difference between
phases.
(9)
where SsgUs is the momentum transfer from solid particle to gas
phase.
⎛
2 ⎞
τs = αsμs (∇Us − ∇UsT ) + αs⎜λs − μs ⎟∇UI
s
⎝
3 ⎠
(14)
The solid volume fraction for the solid phase is close to the
packing limit due to the fact that the solid phase flow is a dense
flow in the fixed-bed. The generation of shear stress is mainly
due to friction between particles. Therefore, only frictional
viscosity is considered for the shear stress. The shear stress can
be showed as
τ g is the viscous stress tensor and is expressed as follows:
⎛
2 ⎞
τg = αgμg (∇Ug − ∇UgT ) + αg ⎜λg − μg ⎟∇UgI
⎝
3 ⎠
(13)
where Rg,i and Rs,i represent the net rate of production of
component i by homogeneous reaction, and the net rate by
heterogeneous reaction, respectively. Ji is the diffusion flux of
species i, resulting from concentration gradients.
3.5. Solid-Phase Stress. The solid-phase stress is
composed by the solid pressure and shear stress. As for the
fixed-bed gasification process, most of solid phase is in packed
condition. The solid stress arises because of Coulomb friction
between particles in enduring contact. This kind of granular
materials can be treated as plastic flow. The Schaeffer model17
is used for the solid pressure, in which
where vr,s is the correlation for the solid phase terminal velocity
and expressed as follows:
+
(12)
The heat exchange between phases must comply with the local
balance condition Qgs = −Qsg.
3.4. Species Transport Equation. The species transport
equation for the mass fraction Yi of gas phase is given by
The drag coefficient is used by the formula proposed by
Dalla Vallue.16
⎛
C D = ⎜⎜0.63 −
⎝
(11)
Q gs = hgs(Tg − Ts)
(10)
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Table 3. Coefficients of Each Devolatilization Gas
α1
α2
α3
α4
α5
α6
α7
α8
avg. mol wt.
0.2040
0.0589
0.0176
0.0687
0.3919
0.2287
0.0096
0.0206
16.852
where hgs = hsg is the heat transfer coefficient between gas and
solid phases; Tg is the gas temperature; Ts is the solid
temperature. The heat transfer coefficient is related to the
Nusselt number for the solid phase and calculated by
hsg =
(17)
where κg is the heat conductivity coefficient of gas phase and
Nus is the solid phase Nusselt number and expressed by the
correlation proposed by Guun18
Nus = (7 − 10αg + 5αg2)(1 + 0.7Res0.2Prg0.33)
(18)
(R5)
4. REACTION MODELS
When the first stage syngas temperature and composition
reaches stable, the second stage fixed-bed gasification reaction
could be started. According to practical experiment condition,
the initial value of lump coal temperature in the second stage is
assumed at 100 °C before the syngas flows to the second stage.
Therefore, the lump coal is regarded as dried lignite, and its
composition is considered to be a mixture of volatile matter,
carbon, and ash. The reaction model can be subdivided into
devolatilization, heterogeneous reaction, and homogeneous
reaction.
4.1. Devolatilization. When the high temperature syngas
from first stage flows to the coal particle in the fixed-bed, the
devolatilization reaction will be carried out immediately. The
nitrogen element in coal is assumed to be converted to N2
entirely during the devolatilization. The sulfur element is
assumed to H2S, and tar is simplified as the benzene.
The composition of the devolatilization gas is calculated by a
mathematical model based on the element balance.19 The
coefficients of each devolatilization gas are listed in Table 3.
The secondary devolatilization of tar is ignored in this study
due to the small amount produced from the devolatilization
process.
(R1)
Coal → Char + VM
Ac =
(1/hM , i) + (1/kj)
(20)
6
y
d p char
(21)
where Ychar is the carbon volume fraction and dp is the particle
diameter.
The chemical reaction rate can be expressed by the Arrhenius
equation
kj = kj ,0T b e−Ea / RT
(22)
The heterogeneous reaction kinetic parameters are listed in
Table 4.23
Table 4. Gas−Solid Reaction Kinetic Parameters
kj (m·s−1·K−b)
Ea (J·kmol−1)
b
19.4
208
2083
2.36 × 10
2.40 × 108
2.30274 × 108
1
1
0
CO2
H2O
CH4
Di,mix is the diffusion coefficient of component i; Sc is the
Schmidt number.
4.3. Homogeneous Reaction. The main gas phase species
includes CO, H2, CO2, and H2O, so the water−gas shift
reaction is only considered for homogeneous reaction.
(R2)
The devolatilization rate is expressed by a single-step global
reaction equation, and its kinetic parameters can be consulted
from ref 20.
k wg
CO + H 2O ←
→ CO2 + H 2
−(E / RT )
ρV
(19)
(R6)
The reaction rate can be expressed by the following equation:24
−1
where the pre-exponential factor A1 = 1.1 × 10 s , and the
activation energy E = 8.86 × 107 kJ·kmol−1.
4.2. Heterogeneous Reaction. In the gasification reaction
region of second stage, three gas−solid heterogeneous reactions
are considered: carbon−CO2, carbon−H2O, and carbon−CH4.
5
8
The gas−solid mass transfer is determined by the formula of
Di ,mix
hM, i =
(2 + 1.1Sc1/3Re 0.6)
dp
(23)
VM → α1CH4 + α2CO + α3CO2 + α4 TAR + α5H 2
+ α6 H 2O + α7 H 2S + α8 N2
Ac ci ,bulk
where Ac is the specific surface area and shown as follows; ci,bulk
is the gas concentration in the bulk; hM,i is the mass transfer
coefficient for solid to gas phase; and kj is the chemical reaction
rate.
where Res is the relative Reynolds number for the solid particle
and Prg is the Prandtl number for gas phase.
C + CO2 → 2CO
C + 2H 2 → CH4
rj =
+ (1.33 − 2.4αg + 1.2αg2)Res0.7Prg0.33
k = A1 e
(R4)
The reaction rate of R5 is significantly slower than R3 and
R4. The overall reaction rate should consider the chemical
reaction rate and mass transfer between gas and solid phases. It
is supposed that the contribution of pores for the gasification
reaction is small,21 and then, it can be considered that the gas−
solid reaction is conducted on the particle surface. The overall
reaction rate can be expressed as follows:22
6κgαsαgNus
ds2
C + H 2O → CO + H 2
(R3)
⎛
CCO2C H2 ⎞
R wg = εk wg ⎜CCOC H2O −
⎟
KE ⎠
⎝
(24)
k wg = 2.78 exp( −1513/Tg)
(25)
KE is the equilibrium constant for water−gas shift reaction.
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KE = 0.0265exp(3966/Tg)
Article
The concentration of CO shows a significant decrease during
the initial stage, followed by a maximum value, and then turns
to be decreased gradually. The concentration of H2 first
increases and then decreases throughout the gasification
process, while the concentration of CO2 shows an opposed
variation trend of H2. The concentration of CH4 increases
dramatically during the initial stage, and becomes to close to nil
after 40 min, showing that in the first 40 min the main reaction
is dominated by devolatilization. At the end of the gasification
reaction, the concentrations of CO, H2, and CO2 are close to
the initial values.
The comparison of gas flow rate between simulation and
experiment is showed in Figure 2b. It also turns to be a
reasonable agreement between two values, which can further
validate the accuracy of the gasification model. The flow rates of
H2 and CH4 rapidly increase in the initial stage, while the flow
rate of CO increases smoothly. This is mainly due to the
devolatilization process in the initial stage, in which a large
amount of H2 and CH4 is emitted and dilutes the concentration
of CO. Therefore, this can reasonably explain the drop of CO
concentration in the initial stage in Figure 2a. The curve of
H2O flow rate shows the amount of H2O increases due to
devolatilization in the beginning and reaches the maximum
value after 10 min. Then, it decreases gradually and reaches the
minimum value after 50 min, showing that the gasification
reaction starts to play the main role in the time period. The
slight decreasing of CO2 flow rate also shows the CO2 is
reacted with coal char simultaneously. Finally, the flow rates of
H2O and CO2 increase to the near initial values.
5.2. Analysis of Simulation Results. 5.2.1. Velocity and
Temperature Distributions. Figure 3a shows the distributions
of gas velocity at the moment of 60, 180, and 300 min. It can be
found that the gas velocity rapidly increases, when hot
temperature syngas flows through the fixed-bed layer. The
superficial gas velocity increases because of the porous region of
second stage, and also, the total gas volume flow rate increases
due to gas generated from gasification reactions. As the
gasification reaction proceeds, the influence of gas velocity from
the fixed-bed layer becomes weaker as a result of the mass
decreasing of lump coal in the second stage. Furthermore, the
syngas is more inclined to flow from the center during the later
reaction process, which is attributes to that the velocity
distribution in the inlet syngas is decreasing from the center to
the tube wall,25 resulting a smaller pressure drop resistance at
center in the coal layer ascribed to a more enhanced gasification
reaction process than that nearby wall surface.
Figure 3b shows the distributions of gas temperature at the
moment of 60, 180, and 300 min. After the syngas flowing
through the coal layer, a significant temperature drop has taken
on comparing with the initial gas temperature. The outlet gas
temperatures are 1372.1, 1422.3, and 1507.4 K at 60, 180, and
300 min, respectively. This shows that the heat energy from the
inlet syngas used in the gasification is decreased due to the
decreasing of coal mass during the gasification process. It also
can be found that the interface of the temperature drop
between the coal layer and inlet syngas also shows the surface
of coal char in lump coal, and the depth area for the high
temperature in the fixed-bed layer increases with gasification
reaction as a result of the absence of carbon in the upper layer.
In the later stage of reaction, the gas temperature distribution in
the coal layer shows a more obvious characteristic of deep at
center and shallow nearby wall surface.
(26)
4.4. Solution Method and Boundary Condition. The
unsteady two-dimensional model is used to simulate the second
stage fixed-bed gasification process, and the total number of the
grids is chosen as 24 800 after the grid independence test. The
convective terms in transport equations are discretized with
second order upwind scheme. A phase coupled SIMPLE
scheme is adopted for the velocity−pressure coupling
calculation. The adiabatic wall is chosen for the wall heating
condition. The gas conditions from first stage based on
experiment results is used as the inlet condition, and pressure
outlet is set as outlet. The calculated step time is controlled as 1
s, and the total physical time is 300 min.
5. RESULTS AND DISCUSSION
5.1. Comparison between Simulated and Experimental Results. The settings in the simulation model are based on
experimental operating conditions. The mass of coal in the
second stage is 1400 g, and the particle size range is 17.5 mm
instead of the 15−20 mm in experiment for the near-spherical
shape of raw coal particle. The true density of Inner Mongolia
lignite is 1200 kg·m−3 and 707 kg·m−1 for the bulk density. The
comparison of gas concentration at outlet between simulation
and experiment is showed in Figure 2a based on H2O free,
which exhibits a good agreement with the experimental values7
and an identical variation trend during the gasification process.
Figure 2. Comparisons of outlet gas concentration and flow rate
between simulation and experiment.
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Figure 3. Distributions of gas velocity and temperature at the moment
of 60, 180, and 300 min.
5.2.2. Gas Concentration Distribution. Figure 4 shows the
variation of average gas concentrations along the axial position
at the moment of 60, 180, and 300 min. It can be seen that the
gas concentration outside the fixed-bed layer basically remains
unchanged. The concentrations of CO and H2 increase, while
the concentrations of CO2 and H2O show downward trend
when the syngas flows through the fixed-bed layer.
Furthermore, the declining rate of H2O is much faster than
CO2, revealing the conversion rate of H2O is higher than that of
CO2 in the fixed-bed gasification reaction. This is because that
the chemical reaction rate of C + H2O is faster than that of C +
CO2, and the H2O concentration in syngas is higher than that
of CO2, resulting a higher partial pressure of H2O than that of
CO2. As the gasification reaction proceeds, the changes of gas
concentration above and below the fixed-bed layer become
smaller, coupling with a smaller temperature drop, which
indicates a weaker endothermic gasification reaction. At the
reaction time of 300 min, there is no significant change of gas
concentration in the upper lump coal layer due to much
consuming of carbon and remaining by ash layer.
5.2.3. Solid Components Distribution. Figure 5 shows the
solid mass and mass fraction of each component in coal layer
Figure 4. Average gas concentrations along the axial position at the
moment of 60, 180, and 300 min.
with reaction time. It can be found that volatile gas has been
released out within 40 min, which reveals that the
devolatilization process has already been performed in this
time period. The mass of char is almost unchanged within 20
min, indicating a weak gasification reaction during the initial
stage, and then is attenuated significantly and eventually
approaches nil. As inert matter, the ash mass amount remains
unchanged in the coal layer during the whole gasification
process, while its mass fraction gradually increases as a result of
the consumption of the other two matters.
Figure 6 shows the mass fractions of each component in coal
layer along the axial position at the moment of 60, 180, and 300
min. It can be seen that the mass fraction of char in the upper
layer is lower than that at bottom, which indicates that char in
the coal layer is consumed from the top toward downward
during the fixed-bed gasification process. As a result, ash will
become the main material in the upper layer, when the char is
reacted gradually. As the gasification reaction proceeds, the
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Figure 5. Solid mass and mass fraction of each component in coal
layer.
content of char in the upper layer decreases significantly,
resulting a moving-down process of gasification reaction zone,
which can also be observed from the declining rate of solid
temperature.
5.2.5. Carbon Conversion and Conversion Rate. Figure 7
shows the variation of carbon conversion and conversion rate in
the fixed-bed during the gasification process. The carbon
conversion gradually increases as the reaction proceeds, while
the conversion rate shows a bimodal distribution. According to
the variation of conversion rate curve, the gasification reaction
process can be divided into two zones, namely Zone 1 and
Zone 2. In Zone 1, the first peak of conversion rate is formed in
50 min with a faster conversion rate, indicating that the
carbonaceous matter in volatiles is released to the gas block
rapidly. In Zone 2, a secondary peak for conversion rate is
formed after 50 min. The conversion rate increases gradually
due to a more obvious gasification reaction between coal char
and gasifying agent. As the gasification reaction proceeds, the
conversion rate of carbon decreases, resulting from the mass
decrease of effective carbon in coal layer during gasification
reaction.26 It can also be found that the ultimate carbon
conversion of simulation is close to the experimental data,
which is 97.9%.
5.3. Effects of Feedstock Parameters. 5.3.1. Various
Coal Amounts. The effect of various coal amounts for the
second stage on gasification efficiency is also investigated in the
proposed model. The added coal amount is 1000 g, 1200 g,
1400 g, and 1600 g, respectively. Figure 8a shows the gas
Figure 6. Mass fractions of each component in coal layer along the
axial position at the moment of 60, 180, and 300 min.
Figure 7. Carbon conversion and conversion rate in the fixed-bed.
temperature at the outlet as the reaction with various coal
amounts proceeds. The outlet gas temperature shows a
temperature fluctuation in the initial stage, which is caused by
a distinct heat recovery through devolatilization reaction. A
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Figure 9. Effective gas concentration at outlet with various coal
amounts.
be the main factor for gasification reaction in the condition of
less temperature difference. In the case of 1600 g, the height of
the coal layer and the carbon content are higher than the other
cases, which will increase the surface area for gasification
reaction and lengthen the flow path in coal layer. This results in
the highest effective gas concentration in Zone 2 of the 1600 g
coal amount.
Figure 10 shows the gas−solid reaction rates of C + CO2 and
C + H2O in the reaction with various coal amounts. It can be
Figure 8. Average temperatures of outlet gas and coal layer with
various coal amounts.
larger coal amount shows a more obvious heat recovery from
high-temperature syngas. As the gasification reaction proceeds,
the gas temperature at the outlet approaches to the initial value
(1573.15 K), indicating a gradual weaker effect of endothermic
reaction at the later gasification process. Figure 8b shows the
average temperature of coal layer with various coal amounts. It
can be found that the temperature of 1000 g case is always
higher than other three cases, especially in the first 50 min,
indicating that a smaller coal amount results in a faster heating
rate. After the reaction time of 50 min, the increase of
temperature becomes slowly and the temperature difference
among the four cases becomes smaller.
The effective gas concentration is defined as the concentration of CO + H2 on dry basis. Figure 9 shows the effective
gas concentration at the outlet with various coal amounts. It can
be seen that for all the cases the effective gas concentration
shows a significant decrease in the initial stage due to the
dilution by a large amount of CH4 released from devolatilization process. After about 60 min, the effective gas concentration
gradually decreases as the gasification reaction proceeds, due to
the reducing of carbon content in the coal layer. With various
coal amounts, the effective gas concentration of 1000 g case is a
little higher than that of larger coal amounts in the first 50 min,
while effective gas concentration for 1600 g case takes on the
highest value after 50 min. This is because a faster heating rate
of the 1000 g case in the initial stage will take the lead in the
gasification reaction process. Subsequently, the contact surface
area between high temperature syngas and lump coal turns to
Figure 10. Gas−solid reaction rates of C + CO2 and C + H2O with
various coal amounts.
seen that the gas−solid reaction rates exist an obvious trend of
increasing first and then decreasing. The gas−solid reaction rate
of the 1000 g case is much higher than other three cases in the
initial gasification process. As the reaction proceeds, the smaller
coal amount shows a much more rapid decreasing trend on the
gas−solid reaction rate due to the consuming of carbon
content. Furthermore, it is also confirmed that the reaction rate
of C + H2O is much higher than that of C + CO2, revealing that
the main gasification reaction in the gasifier is C + H2O. Figure
11 shows the carbon conversion and conversion rate with
various coal amounts. The carbon conversion of the 1000 g
case is always the highest during the gasification process, and
the ultimate value is 99.6%. The carbon conversion of 1600 g
case turns to be lowest (96.9%) after 300 min reaction time.
Therefore, the carbon conversion will be reduced within the
limited reaction time for a large coal amount in the second
stage. In the viewpoint of conversion rate, a smaller coal
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Figure 11. Carbon conversion and conversion rate with various coal
amounts.
amount shows a more rapid devolatilization and gasification
reacting rate during the reaction process.
5.3.2. Various Coal Particle Sizes. The effect of various coal
particle sizes on gasification efficiency is studied with the coal
mass keeping at 1400 g. The coal particle size for the second
stage is ranged from 5−10 mm, 10−15 mm, 15−20 mm, and
20−25 mm, replaced by the intermediate uniform diameter in
the simulation. Figure 12 shows the temperature of the outlet
gas and the coal layer as the with various coal particle sizes
proceeds. It can be seen that the change of particle sizes has
slight effect on temperature comparing with that caused by coal
amounts. The outlet gas temperature of 5−10 mm case is a
little lower than other three cases in the first half period and
shows a distinct increase and exceeds other three cases in the
later period, which reveals that a more significant heat recovery
efficiency for smaller particle size in initial stage and slight
endothermic effect at later stage. In Figure 12b the temperature
difference of coal layer with various coal particle sizes is small,
so it can be obtained that the reaction rate difference is not
consisted in the chemical reaction rate.
Figure 13 shows the effective gas concentration at outlet with
various coal particle sizes. It can be found that the effective gas
concentration of 5−10 mm case increases immediately after the
initial drop, which shows that the gasification reaction of 5−10
mm case has already been conducted significantly during the
devolatilization process. The effective gas concentration of 5−
10 mm case is the highest one among these four cases during
the gasification process, in which the maximum value increases
by 3.9 percentage point to 82.9% compared with the initial gas
concentration.
Figure 14 shows the gas−solid reaction rates of C + CO2 and
C + H2O with various coal particle sizes. It can be found that
the gas−solid reaction rate for 5−10 mm case increases
immediately after the reaction time of 5 min, while after 25 min
for 20−25 mm case. Therefore, with the increase of coal
particle size the time for gasification reaction starting will be
prolonged and the gas−solid reaction rate will also be
decreased. This is because larger particle size contains a larger
heat transfer resistance from the particle surface to inside.
Furthermore, the specific surface area of bed layer decreases
with the increase of coal particle diameter,27 resulting in a lower
reaction rate between high temperature syngas and lump coal as
the increasing of coal particle size. Figure 15 shows the carbon
conversion and conversion rate with various coal particle sizes.
It can be seen that the changing of particle size has a significant
Figure 12. Average temperatures of outlet gas and coal layer with
various coal particle sizes.
Figure 13. Effective gas concentration at outlet with various coal
particle sizes.
effect on carbon conversion and conversion rate. The carbon
conversion declines obviously with the increase of coal particle
size. The carbon conversion of 5−10 mm case has already been
87.7% after 120 min reaction time, while 58.7% for 20−25 mm
case. As the aspect of conversion rate, the lump coal of 5−10
mm diameter shows a much more rapid devolatilization rate
than others, followed by taking the lead of the gasification
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a higher coal layer and more carbon content. The largest
coal amount (1600 g) leads to the lowest carbon
conversion, which is 96.9% within 300 min gasification
reaction time.
(3) A significant heat recovery is observed for 5−10 mm
diameter from the outlet gas temperature during the first
half period. An increase of particle size results in the
reducing of effective gas concentration and gas−solid
reaction rate during the gasification process, which is
caused by a smaller specific surface area and relative
lower heat transfer from particle surface to inside. The
response time for gasification reaction starting is
proposed to be prolonged with the increase of particle
size, and the carbon conversion is also reduced. A smaller
coal particle size is propitious to the gasification reaction
in the fixed-bed gasification process.
Figure 14. Gas−solid reaction rates of C + CO2 and C + H2O with
various coal particle sizes.
■
AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-021-64252522. Fax: +86-021-64251312 Email: [email protected].
Notes
The authors declare no competing financial interest.
■
■
ACKNOWLEDGMENTS
This work is supported by foundation of Shanghai outstanding
academic leader (No. 08XD1401306).
Figure 15. Carbon conversion and conversion rate with various coal
particle sizes.
reaction process. Thus, a smaller coal particle size is propitious
to the gasification reaction in the second stage.
6. CONCLUSIONS
The second stage fixed-bed gasification reaction in the
combined two-stage gasifier, which recycles the sensible heat
from first stage syngas to improve the energy utilization, is
investigated by numerical simulation in this study. Results can
be summarized as follows:
(1) The simulated results show a reasonable agreement with
the experimental data in terms of gas concentration, gas
flow rate, and carbon conversion. The gas velocity
increases and gas temperature descends when flowing
through the fixed-bed layer, and the concentrations of
CO and H2 in syngas increase by gasification reaction. As
the gasification reaction proceeds, the temperature
difference and changes of gas concentration become
weaker comparing with initial values. The volatile matter
in coal has been released out within 40 min, and the coal
char in coal layer is consumed from the top toward the
bottom gradually.
(2) An increase of coal amount leads to the decline of outlet
gas temperature and coal layer temperature in the second
stage. A smaller coal amount turns to be a faster heating
rate during the initial stage, resulting a little higher gas−
solid reaction rate and effective gas concentration.
During the later gasification stage, a larger coal amount
shows a relative higher effective gas concentration due to
NOMENCLATURE
A1 = pre-exponential factor
Ac = specific surface area (m−1)
ci,bulk = gas concentration in the bulk
CD = drag coefficient
dp = particle diameter (m)
Di,m x = diffusion coefficient of component i (m2·s−1)
E = activation energy (kJ·kmol−1)
hg = specific enthalpy for gas phase (J·kg−1)
hM,i = mass transfer coefficient (m·s−1)
hsg = heat transfer coefficient between gas and solid phases
(W m−2·K−1)
I2D = second invariant of the strain rate tensor
k = turbulent kinetic energy
kj = chemical reaction rate
KE = equilibrium constant of water shift reaction
Ksg = momentum exchange coefficient between gas and solid
phases (kg m−3·s−1)
Nus = Nusselt number of the solid phase
ps = solid pressure
Prg = Prandtl number of gas phase
qg⃗ = heat flux of the gas phase (W·m−2)
Qsg = heat exchange between gas and solid phases (W·m−3)
Res = Relative Reynolds number
S = mass source term
Sc = Schmidt number
Tg = gas temperature (K)
Ts = solid temperature (K)
U = instantaneous velocity (m·s−1)
vr,s = terminal velocity of solid phase (m·s−1)
Ychar = carbon volume fraction
Greek Symbols
α = volume fraction
αi = coefficients of each devolatilization gas
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ρ = density (kg·m−3)
τ g = viscous stress tensor (Pa)
μs = shear stress (Pa·s)
ϕ = angle of internal friction
κ = thermal conductivity (W·m−1·K−1)
ε = turbulent dissipation rate (m2·s−3)
■
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