Simulation of Syngas Production from Municipal Solid Waste

Sep 6, 2013 - Five types of unit operation blocks have been used to simulate the gasification process (see Table 1). Since the real-world MSW contains...
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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 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: Revised: Accepted: Published: 14768

January 4, 2013 August 25, 2013 September 6, 2013 September 6, 2013 dx.doi.org/10.1021/ie400026b | Ind. Eng. Chem. Res. 2013, 52, 14768−14775

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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 14769

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Figure 2. Aspen Plus simulation flowchart.

Table 1. Description of Aspen Plus Unit Operation Blocks Aspen Plus block type

Table 2. Gasification Reactions reaction number

block ID

RStoic

DRIER

RYield

DECOMP COMBUST

RGibbs

GASIF

Flash2

GAS-SEP

Sep

ASH-SEP

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 name

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

(R7) (R8)

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

heat of reactiona (MJ/kmol)

(R1)

(R6)

a

reaction

+165

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 14770

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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. 14771

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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 14772

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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. 14773

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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. 14774

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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.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-25-83794744. Fax: +86-25-83795508. E-mail address: [email protected]. Notes

The authors declare no competing financial interest.

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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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dx.doi.org/10.1021/ie400026b | Ind. Eng. Chem. Res. 2013, 52, 14768−14775