Article
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Simulation of Syngas Production from Municipal Solid Waste
Gasification in a Bubbling Fluidized Bed Using Aspen Plus
Miaomiao Niu, Yaji Huang,* Baosheng Jin, and Xinye Wang
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Key Laboratory of Energy Thermal Conversion and Control, Ministry of Education, School of Energy and Environment, Southeast
University, Nanjing 210096, People’s Republic of China
ABSTRACT: A comprehensive process model is developed to simulate municipal solid waste (MSW) gasification in a bubbling
fluidized bed using an Aspen Plus simulator. The model is based on a combination of modules that the Aspen Plus simulator
provides, representing the three stages of gasification (devolatilization, partial oxidation, and steam reforming). The restricted
equilibrium method is used to correct the deviation caused by uncompleted equilibrium of gasification system. Effects of
operating parameters, including gasification temperature, equivalence ratio (ER), oxygen percentage in the enriched air (OP),
MSW moisture content, and steam/MSW ratio (S/M), on the syngas composition and gasifier efficiency are analyzed. Higher
temperature favors the production of H2 and CO and leads to higher gasification efficiency. Increasing ER improves the CO yield
and the carbon conversion of MSW at lower ERs. The optimal value of ER for air gasification of MSW in this study is found to be
0.35. The use of enriched air elevates syngas heating value and gasification efficiency by increasing the concentration of
combustible components, but shows little improvement at temperatures higher than 900 °C. Higher moisture content degrades
the syngas quality and cold gas efficiency. Steam injection results in higher H2/CO ratio and gasification efficiency. Optimal S/M
value shifts from 0.5 to 1.0 with an increase in OP from 21% to 100%.
1. INTRODUCTION
In China, MSW generation has been increasing dramatically
with rapid economic growth and massive urbanization, reaching
a point at which changes must be made to implement
municipal solid waste (MSW) minimization.1 Discovering
environmentally benign and economically feasible methods
for the disposal of MSW has become an urgent issue for
China.2 Within the alternatives for MSW treatment, landfilling
is most widely used, because of the lower cost involved, but it
occupies large amounts of land and gives rise to serious
environmental problems. In fact, pressure against dumping rises
so fast that there are scarce landfill sites left in many highly
populated developing cities.3 Incineration has been used
extensively in developed countries, because of its heavy
reduction amount and sound resource recovery. However,
because of the high emissions of carcinogens such as dioxins
and furans, the spread of this technology has encountered
increasingly hostile resistance from the general public in
China.2 Besides, the serious corrosion and low incineration
temperature of the incineration system have led to a relatively
low economic and energy efficiency.4,5 Diminishing landfill
volume and public opposition to new incinerators strongly
increase the interest on the study of MSW gasification. As a
novel waste-to-energy technology, gasification has several
potential benefits over traditional incineration, mainly related
to lower emissions and more flexible and efficient utilization of
MSW energy.6,7 Because of the low levels of oxygen present in
gasification processes, the formation of dioxins, furans, and
NOx is effectively inhibited and most of the heavy and alkali
metals (except mercury and cadmium) are retained in the
bottom ash.8 The product gases mainly contain CO, H2, CO2,
CH4, and other hydrocarbons (such as C2H2, C2H4, C2H6,
C3H8, etc.). They can be used to power gas engines or gas
turbines, or can be converted to higher-value commercial
© 2013 American Chemical Society
products such as transportation fuels, chemicals, fertilizers, and
substitute natural gas.
In recent years, several studies on MSW gasification have
been published in the scientific literature. The effect of
operating parameters on the main performance parameters,
such as gas composition and calorific value, has been the focus
of most research. He et al.8 built a laboratory-scale continuously
feeding fixed-bed reactor and found a strong potential for
producing hydrogen-rich gas from MSW using a simple steam
gasification process with dolomite as a catalyst. Ahmed et al.9
conducted a comparison between gasification and pyrolysis of
rubber waste at temperatures of 800 and 900 °C. They proved
that gasification was more beneficial than pyrolysis, since
gasification resulted in a 500% increase in hydrogen yield,
compared to pyrolysis at 800 °C. Xiao et al.10 studied the effect
of ER, bed height, and fluidization velocity on the fluidized-bed
gasification of polypropylene plastic waste. They observed that
ER is more important than the effect of bed height and
fluidization velocity. However, to date, there is still a lack of
research and reporting on MSW gasification, since current
studies are mainly focused on the gasification of single
components such as wood, rubber, and plastics in MSW.
Experiments of the real-world MSW, especially at large scale,
are often expensive and complicated to be done, making it
difficult for studying the characteristics of gasification. Modeling
can not only save time and money but also support preparation
and optimization of experiments to be undertaken in a real
system. In order to design the gasification processes quickly
Received: January 4, 2013
Revised: August 25, 2013
Accepted: September 6, 2013
Published: September 6, 2013
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with minimal financial costs, it is indispensable to develop a
practical model for systematically predicting gasification
characteristics.
As a commercial simulator, Aspen Plus can break complex
chemical processes down into small sections and test them as
separate modules before they are integrated. It has a large
property databank containing various stream properties
required to describe the material streams in a process involving
solids in addition to vapor and liquid streams.11 Thus, it is
considered to be an excellent design tool for modeling
gasification system. So far, many investigators have used
Aspen Plus to simulate coal and biomass power generation
systems.12,13 However, relatively few studies have been done on
MSW or combustible waste gasification. Mitta et al.14 modeled
a fluidized-bed tire gasification plant with air and steam using
Aspen Plus, and they validated the simulation results with a
gasification pilot plant. Chen et al.15 compared MSW
gasification characteristics in two types of fixed-bed reactors,
based on Aspen Plus, and found that the introduction of flue
gas improves the syngas quality.
In this work, MSW gasification process with enriched air and
steam in a bubbling fluidized bed was proposed. The primary
objective was to develop a computer simulation model for
predicating thermodynamic performance of MSW gasification
system under various operating conditions. The model is based
on Gibbs free-energy minimization. The restricted equilibrium
method is employed by using a specifying temperature
approach for implemented reactions. The influence of the
gasification temperature, equivalence ratio (ER), oxygen
percentage in the enriched air (OP), MSW moisture content,
and steam/MSW ratio (S/M) on the syngas composition,
lower heating value (LHV) of syngas, and process efficiency
was investigated. The research provides reference data for the
further study on syngas production from MSW gasification.
Figure 1. Schematic diagram of a bubbling fluidized bed municipal
solid waste (MSW) gasifier.
simulation is developed under the following assumptions: the
process is steady state; both the combustion block and the
gasification block are isothermal; all considered reactions are at
equilibrium; and the drying and devolatilization of MSW are
instantaneous. Char, which contains more than 90% carbon,
with the results of ultimate analysis reported by Antal et al.17 as
reference, is assumed to be 100% carbon (graphite). Ash is
assumed to be inert and does not participate in chemical
reactions, in view of its physicochemical properties. Also, the
presence of tars is not considered to simplify the model.
Considering the reducing atmosphere, all sulfur is assumed to
go to H2S while all chlorine goes to HCl. Only NH3 forms as
the N-containing product and no nitrogen oxides are produced.
The contents of sulfur, chlorine and nitrogen are so low that
negligible inaccuracies in the simulation results will be caused
by these simplifications.
Figure 2 depicts the Aspen Plus simulation flowsheet for an
indirect bubbling fluidized bed gasifier (heat is supplied by an
external source via a heat exchanger or an indirect process).
Five types of unit operation blocks have been used to simulate
the gasification process (see Table 1). Since the real-world
MSW contains large amounts of moisture, RStoic is introduced
to model the predrying process, where the moisture content of
MSW is reduced by converting a portion of the fed MSW to
form water. When the moisture content of the dried MSW is
specified, the corresponding conversion of MSW to water could
be calculated by writing a FORTRAN statement in the
calculator block and then using it in the RStoic block. Since
the RStoic block has a single outlet stream, a Flash2 block is
2. TECHNICAL AND MODELING APPROACH
2.1. Uncoupling the Gasification Process. The
schematic of a bubbling fluidized bed is shown in Figure 1.
The gasifying agent is blown upward through a distributor plate
to keep the bed particles in a state of suspension.11 MSW
introduced at the bottom of the reactor is quickly mixed with
the bed material, followed by an intense exchange of heat and
mass. The gasification process can be divided into three linked
stages: initially, drying and devolatilization; subsequently,
partial oxidation of volatiles and char; and, finally, char
gasification and steam reforming reactions.16 Drying occurs at
∼100−200 °C with the evaporation of moisture in MSW.
Pyrolysis involving a series of complex physical and chemical
processes starts at ∼230 °C, generating char, tars, H2, CO, CO2,
CH4, H2O, and other hydrocarbons (such as C2H2, C2H4,
C2H6, C3H8, etc.) as major species. These products are then
either burnt or gasified. The char, tar as well as some H2 and
CO are partially reacted with oxidant and steam in the dense
phase zone, releasing large amount of heat that could be used to
sustain the endothermic gasification reactions. Further gasification and steam reforming reactions involving the
Boudouard reaction, the water-gas reaction, and the water-gas
shift reaction mainly occur in the freeboard region, where
oxygen is insufficient.
2.2. Model Description. A gasification model has been
developed according to principles of mass, energy, and chemical
balance using Aspen Plus. Since the simulation is based on the
minimization of the total Gibbs free energy at equilibrium, the
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Figure 2. Aspen Plus simulation flowchart.
Table 1. Description of Aspen Plus Unit Operation Blocks
Aspen
Plus
block
type
block ID
RStoic
DRIER
RYield
DECOMP
COMBUST
RGibbs
GASIF
Flash2
GAS-SEP
Sep
ASH-SEP
Table 2. Gasification Reactions
description
stoichiometric reactor − reduces the moisture
content of MSW
yield reactor − converts nonconventional MSW
into conventional components based on mass
balance
Gibbs free-energy reactor − simulates the partial
oxidation
Gibbs free-energy reactor − restricts chemical
equilibrium of specified reactions to simulate
the gasification
separator − separates the dried MSW from the
moist gas by flashing
separator − separates gases from ash by
specifying split fractions
reaction
number
reaction
heat of reactiona
(MJ/kmol)
(R1)
C + 0.5O2 → CO
−111
(R2)
CO + 0.5O2 → CO2
−283
(R3)
H2 + 0.5O2 → H2O
−242
(R4)
(R5)
C + CO2 ↔ 2CO
C + H2O ↔ CO +
H2
CO + H2O ↔ CO2
+ H2
CH4 + H2O ↔ CO
+ 3H2
CH4 + 2H2O ↔
CO2 + 4H2
+172
+131
char partial
combustion
CO partial
combustion
H2 partial
combustion
Boudouard
water-gas
−41
water-gas shift
+206
steam-methane
reforming
steam-methane
reforming
(R6)
(R7)
(R8)
a
used to model the removal of the gas stream containing water
vapor from the dried MSW. When MSW is fed into gasifier,
RYield is introduced to simulate the drying and devolatilization
section. The total yield of volatiles is assumed to equal the
volatile content of the proximate analysis. MSW is converted to
its constituting components, including carbon, hydrogen,
oxygen, sulfur, nitrogen, and ash, by specifying the yield
distribution according to the MSW ultimate analysis (calculated
using the FORTRAN statement). Although several experimental studies have been conducted on gasification of MSW
model compounds and real MSW, by now, there is no accepted
mechanistic reaction path with kinetics constants for the
reaction taking place in a gasifier. When the reaction kinetics is
not known, a rigorous reactor and multiphase equilibrium
based on the minimization of the total Gibbs free energy of the
product mixture (an RGibbs block) is preferred to predict the
equilibrium composition of the produced syngas.11 Thus, both
partial oxidation section and gasification section based on the
principle of minimization of Gibbs free energy can be modeled
in the RGibbs block. The main reactions of MSW gasification
considered in the gasifier are given in Table 2.
There are two ways of supplying heat to the gasifier in the
indirect gasification: by external heat or by internal combustion
of gas and char.16 In the model, the heat flow among pyrolysis,
combustion and gasification reactions is modeled by two heat
streams (Q-DEC and Q-COM) into the oxidation and
gasification section, respectively. Heat flow between the gasifier
+165
reaction name
Heats of reactions at standard temperature (25 °C).
and external environment is carried out by a heat stream (QGASFY) out of the gasification section. Besides, a Sep block is
used to model the inert ash removal from the product gas.
Considering that the system may not strictly reach complete
equilibrium, a restricted equilibrium method is employed in the
RGibbs block by specifying the temperature approach for
individual reactions. The temperature approach is the number
of degrees above the reactor temperature at which chemical
equilibrium is determined. The equilibrium constant could be
calculated depending on the temperature approach. Moreover,
the gas composition could be adjusted to match the published
literature reference data in this way. The correction was
introduced by Gumz18 and has been well-established. Kunze et
al.19 also used restricted equilibrium in a entrained flow gasifier
model. In this way, the deviation from the equilibrium can be
reflected in the simulation results and the practical gasification
characteristic can be reproduced.
2.3. Model Operating Conditions. In these simulations,
the ambient temperature is 25 °C and system pressure is set at
atmosphere pressure. Since the fluidized-bed gasifier is usually
operated below the ash sintering temperature, to prevent the
formation of excessive sintered agglomerates, the temperature
of gasification section ranges from 500 °C to 1000 °C, while
that of the combustion section is kept at 900 °C. The
equivalence ratio (ER) is defined as the ratio between the actual
air−fuel ratio and the stoichiometric air−fuel ratio required for
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combustion.20 The value of ER varies from 0.2 to 0.8 while the
oxygen percentage of enriched air is set at the range of 21%−
100%. The steam/MSW ratio (S/M) is defined as the flow rate
of steam fed to the gasifier divided by the MSW flow rate (dry
and ash-free).4 S/M varies from 0 to 1 by adjusting steam
injection amount. Saturated steam (500 kPa) and enriched air
are both injected into the gasifier at 150 °C. The heat duty is 0
kJ/kg in the predrying section. The dried MSW temperature is
controlled by the inlet temperature of the drying gas and the
degree of predrying, displaying an increase of ∼20−40 °C after
drying. The MSW used in the study was collected from
Huizhou, Guangdong Province, China, with a feed rate of 1.0
kg/h. The characteristic of the feeding MSW are given in Table
3.
Table 3. Characteristics of MSWa
parameter
moisture
Proximate Analysis (dry basis)
volatile matter
fixed carbon
ash
high heating value
average particle size
Ultimate Analysis (dry basis)
C
H
O
N
S
Cl
a
value
Figure 3. Effect of gasification temperature on syngas composition.
51.87
57 wt %
12.19 wt %
30.81 wt %
15044.65 kJ/kg dry basis
0.1−1 mm
accordance with Le Chatelier’s principle. The increase of H2
concentration could be ascribed to endothermic reactions R5,
R7, and R8, while enhanced reactions R4 and R5 at higher
temperature are responsible for the increase in CO.
Simultaneously, by reverse water-gas shift reaction (reaction
R6), H2 is converted to CO and a faster growth rate is observed
in CO than H2. Although endothermic reaction R8 could
release CO2, reactions R4 and R5 would be more favored22 and
lead to the increase in CO and decrease in CO2. Besides, the
strengthened endothermic steam-methane reactions (reactions
R7 and R8) result in a decrease in CH4.
To study the performance of gasification process, dry
product gas lower heating value (LHV), carbon conversion
efficiency (CCE), and cold gas efficiency (CGE) are
respectively defined as follows:4
40.44 wt %
4.75 wt %
21.13 wt %
0.94 wt %
1.72 wt %
0.21 wt %
Data taken from ref 21.
3. RESULTS AND DISCUSSION
The model is used to perform sensitivity analysis on five
parameters, namely, temperature, ER, OP, moisture content,
and S/M. The baseline case with all input parameters listed is as
follows: temperature at 800 °C, ER at 0.2, OP at 21%, moisture
content at 5%, and S/M at 0.0. One parameter will be changed
within the range presented in section 2.3, while the other
parameters are kept constant. Simulation results for cold gas
efficiency (CGE) versus OP at different temperatures and CGE
versus S/M at different OPs are also included for a full-scale
understanding.
3.1. Effect of Gasification Temperature. The gasification
temperature affects all the chemical reactions involved in the
gasification process. Figure 3 shows the variation of syngas
composition as a function of temperature. All gas components
are plotted on a dry basis and the N2 concentration is not
displayed. The H2 concentration increased sharply with the
increasing temperature while the CO2 concentration revealed
an opposite trend. The CO concentration increased remarkably
as the temperature increased and exceeded that of H2 at ∼800
°C. CH4 concentration decreased steadily within the entire
temperature range. At relatively low temperatures (500−600
°C), endothermic char gasification and steam reforming
reactions (reactions R4, R5, R7, and R8) are limited by the
lack of energy, making the pyrolysis of MSW play a more
dominant role. The CH4 obtained in the syngas is mainly
present as a product of pyrolysis.16 As the temperature
increases, the endothermic reactions are strengthened, in
LHV (MJ/Nm 3)
(CO × 126.36 + H 2 × 107.98 + CH4 × 358.18)
=
1000
where CO, H2, and CH4 are the mole percentages of the syngas
components;
CCE (%) =
carbon content in the syngas × syngas flow rate
× 100
carbon content in the fed MSW × MSW flow rate
CGE (%) =
LHV of the syngas × syngas flow rate
LHV of the fed MSW × MSW flow rate
× 100
The effect of gasification temperature on the gasification
performance is shown in Figure 4. It is indicated that increasing
the temperature enhanced the syngas LHV, CCE, and CGE
values effectively and led to better gasification performance.
This could be explained by the continuous increase of the CO
and H2 content. In particular, there was a substantial increase in
the gasification quality between the temperature range of 650−
900 °C, with the CCE increasing from 39.9% to 78.7% and the
CGE increasing from 32.6% to 87.6%. At low temperatures
(500−650 °C), the system efficiency showed a relatively small
growth with rising temperature, because of the sustained
decrease in CH4 content.
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Figure 6. Effect of ER on syngas LHV, CCE, and CGE.
Figure 4. Effect of gasification temperature on syngas LHV, CCE, and
CGE.
quality and affected the CGE, the CGE would first rise due to
the increasing amount of syngas yield and then decrease with
increasing ER. The optimum ER was found to be 0.35, where
the CGE reached a maximum of 86.8%. Recent experimental
studies on ER show similar variation trend with the simulation.
Caballero et al.24 investigated the effect of bed height and air
ER on the air gasification of dried sewage sludge in bubbling
fluidized bed. They found that, with increasing ER, the
concentrations of H2, CO, and CH4 decreased while that of
CO2 increased. Arena et al.25 studied fluidized bed gasification
of five alternative waste-derived fuels obtained from MSW and
also found similar changes in gas composition with increasing
ER.
3.3. Effect of Oxygen Percentage in the Enriched Air.
The use of enriched air is effective for reducing the nitrogen
dilution effect and achieving medium heating value gas.24
Influence of oxygen percentage in the enriched air (OP) on the
syngas composition is given in Figure 7. The H2 concentration
3.2. Effect of Equivalence Ratio (ER). The equivalence
ratio (ER) is one of the most important operating parameter
for syngas production.4 Figure 5 presents the variation of
Figure 5. Effect of ER on syngas composition.
syngas composition as a function of ER. The increase of ER
means that the feeding air or oxygen increases. With increasing
ER, more active oxidization reactions (reactions R1−R3) will
occur, normally increasing the CO2 and H2O concentrations at
the expense of CO and H2. For the entire range of ER, the H2
concentration in the obtained gas decreased from 22.7% to
7.9%, while CH4 concentration was very low (max. 3.0%) and
continued to decrease. The concentration of CO initially
increased slightly from 22.3% to 24.8% with ER, because of
increased conversion of MSW, but decreased after a certain
value (∼0.35), because of complete oxidization of the feed.
Meanwhile, CO2 first showed a modest decrease from 6.8% to
5.7% and then increased to 15.4%.
The influence of ER on gasification performance is illustrated
in Figure 6. The LHV of dry gas diminished with increasing ER
because of the enlarging dilution effect of nitrogen.23
Meanwhile, conversion of carbon present in the MSW
increased since the amount of oxygen supplied to the gasifier
increased. As shown in Figure 6, the CCE increased and kept
constant when it reached the maximum value as the ER
increased. Although the decline of LHV degraded the syngas
Figure 7. Effect of OP on syngas composition.
increased with rising OP from 22.7% to 37.1%, while CO
concentration showed a greater increment from 22.3% to
48.4%. CO2 concentration first revealed a slight increase from
6.8% to 9.0% and then decreased at higher OPs while CH4
concentration showed a slight increase trend. With increasing
OP, the amount of nitrogen sent into the gasifier decreases
gradually, causing changes in the initial concentration of
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Particularly, it should be noticed that OP has a significant effect
on CGE only up to 900 °C, after which point little effect has
been found for CGE increasing. The slight decrease of CGE at
1000 °C results from the synthetical effect of decreasing syngas
yield and increasing LHV with OP. For lower temperatures, the
increase of OP could achieve an evident improvement in
combustible component, leading to a greater increase in CGE.
For example, at 500 °C, the CGE increased from 30.4% to
65.3% with OP. With regard to temperatures of >900 °C, as
shown in Figure 9, the quality of syngas was already very high
when OP was 21%, leaving little room for improvement.
3.4. Effect of MSW Moisture Content. Figure 10 presents
the dependence of the syngas composition on the varying
reactants. The partial combustions (reactions R1−R3) are
improved and more CO, CO2, and H2O are produced in the
combustion section. A higher concentration of CO2 and H2O
favors the forward Boudouard reaction (reaction R4), the
water-gas reaction (reaction R5), and steam-methane reforming
reaction (reaction R7), eventually leading to the increase of H2
and CO concentration. The change of CO2 concentration
could be ascribed to the comprehensive effect of combustion
and reforming reactions (reactions R2, R4, R6, and R8). The
increasing trend of CH4 is mainly related to the decrease of
nitrogen in the produced syngas. As shown in Figure 8, the
Figure 8. Effect of OP on syngas LHV and CCE.
LHV value of syngas and the CCE value both increased greatly
with increasing OP and reached the maximum values (12.2 MJ/
kg and 81.2%, respectively) when OP = 100%. Yoon et al.26
compared the syngas produced from gasification of biodiesel
byproduct with air and oxygen. They also discovered that the
syngas LHV and carbon conversion using the oxygen agent was
higher than using air. Huynh et al. investigated the characteristics of a biomass gasification system using oxygen-enriched air
and steam. The results indicated that oxygen-enriched air
gasification favors the production of combustible gas
components, including H2, CO, and CH4.27
Figure 9 shows that, consistent with the changes in gas
composition and LHV, the CGE value increased with OP.
Figure 10. Effect of MSW moisture content on syngas composition.
MSW moisture content. The moisture level was varied over a
realistic range for MSW (5%−50%). It can be seen that rising
moisture content increased the H2 concentration slightly, from
22.4% to 26.4%, but decreased the CO concentration sharply,
from 22.3% to 7.8%. The CO2 concentration increased
remarkably, from 6.8% to 14.8%, while the CH4 concentration
decreased slowly at a low level (max. 2.1%). Higher moisture
content favors the reactions involving H2O (reactions R5−R8),
especially the water-gas shift reaction.28 CO and CH4 is shifted
and reformed with H2O producing CO2 and H2, causing a
decrease in CO and an increase in CO2. Although CH4 could
be reformed with H2O producing CO in reaction R7, the CH4
concentration is too low to change the decreasing trend of CO.
The effect of MSW moisture content on gasification performance is shown in Figure 11. The LHV value of syngas decreased
significantly, from 6.0 MJ/m3 to 4.0 MJ/m3, because of the
continuous decline of CO and CH4. The CCE and CGE also
decreased from 61.5% to 41.9% and from 64.2% to 39.5%,
respectively, as the MSW moisture content increased from 5%
to 50%. Meanwhile, more heat input to the gasifier would be
needed for turning liquid water into superheated steam. Since
increasing the moisture content degrades gasifier performance
and causes greater energy consumption, the input MSW should
be predried for optimal use.
3.5. Effect of Steam/MSW Ratio. Influence of S/M on
syngas composition is shown in Figure 12. Compared with the
results in Figure 10, it can be seen that steam injection caused a
greater increase in H2 yield than adding an equal amount of
bound moisture into MSW directly. Given that H2O enters the
Figure 9. Effect of OP on CGE for the complete temperature range.
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LHV and CCE. The LHV of syngas first increased slightly from
6.0 MJ/m3 to 6.3 MJ/m3 and then decreased with S/M. The
CCE kept increasing until reaching a maximum value of 71.4%
for S/M = 0.5. Steam injection could adjust the H2/CO ratio of
syngas by the combination of the water-gas reaction (reaction
R5) and water-gas shift reaction (reaction R6). Increasing the
amount of steam shifts reactions R5 and R6 toward the right
and results in an increase of H2 and CO2 concentration and a
decrease in CO concentration. The presence of peak values for
LHV and CCE could be attributed to the changes in syngas
composition and syngas yield.
The variation of CGE as a function of S/M with different
OPs is shown in Figure 14. For a specified OP, the CGE value
Figure 11. Effect of MSW moisture content on syngas LHV, CCE, and
CGE.
Figure 14. Effect of S/M on CGE for complete OP range.
would first increase to reach a maximum and then slowly
decrease with increasing amounts of steam. Consistent with the
trend shown in Figure 9, higher OP tends to cause higher CGE
for a specified S/M, which is probably related to the removal of
nitrogen. The optimization of steam injection in air gasification
leads to a maximum CGE value of 74.7% for S/M ≈ 0.5.
Notably, steam injection had a significant improvement on
CGE at lower S/M values and the increasing trend of CGE
tends to decrease for higher OPs. Besides, the threshold of S/M
corresponding to the maximum CGE became larger at higher
OPs, increasing from 0.5 to 1.0 when the value of OP increased
from 21% to 100%. Campoy et al.29 studied the effect of steam
injection by enriched air-steam biomass gasification tests in a
bubbling fluidized bed gasification plant. They found that the
addition of steam using enriched air of 40% OP leads to a
maximum efficiency of 70% for steam-to-biomass ratio of ∼0.3,
which conformed well to the simulation results in this study.
Figure 12. Effect of S/M on syngas composition.
gasifier in the form of steam, the heat input for gasification with
steam injection will be less than that for wet MSW gasification.
With increasing S/M, H2 and CO2 concentrations increased
remarkably while the CO concentration smoothly decreased.
The CH4 concentration increased slightly and was approximately constant. Figure 13 shows the effect of S/M on syngas
4. CONCLUSIONS
Simulation of municipal solid waste (MSW) gasification in a
bubbling fluidized bed is performed using Aspen Plus. The
effect of gasification temperature, equivalence ratio (ER),
oxygen percentage in the enriched air (OP), moisture content,
and steam/MSW ratio (S/M) on the composition of syngas,
lower heating value (LHV), carbon conversion efficiency
(CCE), and cold gas efficiency (CGE) has been discussed.
The gasification temperature is found to have a strong influence
on the syngas composition. Increasing the temperature
improves gasifier performance, enhancing the production of
Figure 13. Effect of S/M on syngas LHV and CCE.
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Industrial & Engineering Chemistry Research
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H2 and CO in syngas, which ultimately leads to higher LHV,
CCE, and CGE. ER influences the syngas production by
determining carbon conversion and the oxidation of part of
syngas. Increasing ER strengthens carbon conversion and
increases CGE at lower ER values, but causes evident decreases
in syngas LHV and CGE at higher ER values. The optimal
value of ER for air gasification of MSW at 800 °C is found to be
0.35. The use of enriched air reduces the nitrogen dilution
effect, thus increasing the syngas LHV. Higher OP could lead to
a significantly increase of CGE at lower temperatures but show
little improvement for gasification at temperatures higher than
900 °C. Higher moisture content in the MSW increases the
heat input to gasifier, decreases the concentration of the
combustible components, and results in lower gasification
efficiency. Steam injection should be employed if a H2-rich gas
is desired. With increasing S/M, the CGE shows greater
increase for lower OPs than for higher OPs. Optimal S/M value
shifts from 0.5 to 1.0 with OP increasing from 21% to 100%.
This study provided some insight to the characteristics of
MSW gasification in search of efficient and clean utilization of
MSW energy. The simulation results can serve as a guideline
for further process optimization studies.
■
AUTHOR INFORMATION
Corresponding Author
*Tel.: +86-25-83794744. Fax: +86-25-83795508. E-mail
address: heyyj@seu.edu.cn.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work was supported by the National Basic Research
Program of China (973 Program, No. 2011CB201505) and the
National Nature Science Foundation of China (No. 51006023).
■
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