Simulation Analysis of Biomass Gasification in an Autothermal Gasifier Using Aspen Plus

Transcription

1 Simulation Analysis of Biomass Gasification in an Autothermal Gasifier Using Aspen Plus Zhongbin Fu, Yaning Zhang, Hui Liu, Bo Zhang, and Bingxi Li Abstract Based on simulation, biomass gasification in an autothermal gasifier is analyzed, the effects of the equivalence ratio (ER), reactor temperature and gasification pressure on the composition and the higher heating values (HHV) of the product gas are also covered. The results indicate that the temperature in the gasifier increases when the ER increases, while the HHV of the product gas decreases. In an autothermal gasifier, the temperature which is controlled by varying ER, has the same influence on the composition and HHV of the product gas as the ER does. Higher gasification pressure slightly increases the temperature in the gasifier and the HHV of the product gas. Keywords Biomass gasification Autothermal gasifier Simulation analysis Aspen Plus 1 Introduction Application of biomass energy can weaken the fossil energy crisis and improve the environmental air quality with much less SO 2 and NO x emissions due to less content of N and S in the biomass, respectively [1]. It is important to develop effective technologies for biomass utilization that can be economically accepted and extensively commercialized [2]. Biomass gasification is one of the most promising conversion technologies [3]. In autothermal gasifiers, fuel reacts with gasification agent and produces heat to sustain high temperature atmosphere for the other reactions. Being simple and practicable, biomass gasification with air in autothermal gasifiers is widely studied and used [4 6]. Under the situation that experiments have limits in adjusting to the varied aims and conditions, and usually cost too much, simulation is a useful and important tool. This paper aims to study the effects of the ER, reactor temperature and gasification pressure on the composition Z. Fu Y. Zhang H. Liu B. Zhang B. Li (*) School of Energy Science and Engineering, Harbin Institute of Technology, Harbin , China libx@hit.edu.cn and the higher heating values of the product gas from biomass gasification in an autothermal gasifier by simulation using Aspen Plus software. 2 Simulation Procedures The simulation analysis in this paper is based on Aspen Plus 11.1, which is widely applied in simulation of distillation [7, 8], plant [9, 10], combustion [11, 12] and gasification [13, 14]. 2.1 Gasification Principle Biomass gasification process can be separated to three linked processes: pyrolysis (or devolatilisation, thermal decomposition or carbonization), gasification and combustion. If gasification and combustion are combined, gasification can be described as two stages: solid pyrolysis and char conversion (gasification and combustion) [15]. Partial combustion is necessary because it supplies the heat required for the endothermic gasification reactions. H. Qi and B. Zhao (eds.), Cleaner Combustion and Sustainable World, DOI / _66, # Springer-Verlag Berlin Heidelberg and Tsinghua University Press

2 480 Z. Fu et al. The main reactions in biomass gasification process are as follows [14]: Char partial combustion: 2C þ O 2 ¼ 2CO 222 kj mol 1 (1) Boudouard: C þ CO 2 $ 2CO þ 172 kj mol 1 (2) Water-gas: C þ H 2 O $ CO þ H 2 þ 131 kj mol 1 (3) Methanation: C þ 2H 2 $ CH 4 75 kj mol 1 (4) CO partial combustion: 2CO þ O 2 ¼ 2CO kj mol 1 H 2 partial combustion: 2H 2 þ O 2 ¼ 2H 2 O 484 kj mol 1 (5) (6) CO shift: CO þ H 2 O $ CO 2 þ H 2 42 kj mol 1 (7) Steam-methane reforming: CH 4 þ H 2 O $ CO þ 3H 2 þ 206 (8) H 2 S formation: H 2 þ S ¼ H 2 S (9) NH 3 formation: N 2 þ 3H 2 $ 2NH 3 (10) 2.2 Flowsheet and Description According to the gasification process which is splitted into three linked processes, a simulation flowsheet can be displayed in Fig. 1. Table 1 displays a brief description of the unit operation blocks which are shown in Fig. 1. In the simulation flowsheet which is shown in Fig. 1, the stream BIOMASS represents biomass. The ultimate and proximate analyses of biomass (rice husk) are shown in Table 2. BIOMASS is specified as a non-conventional stream, the ultimate and proximate analyses are input. The thermodynamic condition and the mass flow rate are also specified and entered. The yields from block DECOPM are set by a calculator block to produce two streams: the energy stream Q-DECOPM which supplies COMBUST with heat, and the mass stream which includes ash, sulphur, nitrogen, oxygen, hydrogen and carbon. The carbon in the TO-C combusts with AIR in the DECOPM block, which transfers the major heat and products to react in the GASIFY with the gases from the SEP block, while the minor heat Q-LOSS is set negative to indicate the lost energy. The product GASES from the GASIFY is separated by the SEPH2O to form DRYGAS, which is product gas with higher heating value. Fig. 1 Aspen Plus flowsheet of biomass gasification Table 1 Description of Aspen Plus flowsheet unit operation blocks presented in Fig.1 Name Name Description RYIELD DECOPM Converts the non-conventional stream BIOMASS into conventional components SEP2 SEP Separates the inert ash and some un-reacted carbon from the gas to allow removal from the system SEPH2O Separates steam from the GASES RGIBBS COMBUST Simulates partial oxidation GASIFY Restricts chemical equilibrium of the specified reactions to set the gases composition

3 Simulation Analysis of Biomass Gasification in an Autothermal Gasifier Using Aspen Plus Assumptions The main assumptions for the simulation of biomass gasification using Aspen Plus software are: 1. Steady state conditions. 2. Zero-dimensional model. 3. Isothermal (uniform bed temperature). 4. Drying and pyrolysis are instantaneous in the gasifier [16]. 5. Char is 100% carbon (graphite). 6. All of the sulphur reacts to form H 2 S[17]. 7. Only NH 3 formed, no nitrogen oxides considered [17]. 8. Separation efficiency is 100%. 9. 2% carbon loss in ash [18]. 10. Heat loss equals 3% of the total input heat [19]. 2.4 Simulation Operation The ER is defined as [20]: Air Flow rate Biomass Consumption Rate ER ¼ Air Flow rate Biomass Consumption Rate Stoichiometric (11) The higher heating value of dry gas at the standard state of kpa and 298 K can be estimated from the components of the product gas [18]: HHV ¼ 12:75H 2þ12:63CO þ 39:82CH 4 þ63:46c 2 H 4 þ 100 (12) where H 2, CO, CH 4 and C 2 H 4 represent the molar or volume fractions of H 2, CO, CH 4 and C 2 H 4 in the product gas in %, respectively. Table 2 Ultimate and proximate analyses of rice husk Ultimate analysis (dry basis) Carbon wt.% Hydrogen wt.% 5.1 Oxygen wt.% Nitrogen wt.% 2.17 Sulphur wt.% 0.12 Chlorine wt.% 0 Ash wt.% 15.8 Proximate analysis (dry basis) Volatile matter wt.% 69.3 Fixed carbon wt.% 10.2 Ash wt.% 15.8 Moisture wt.% 4.7 HHV(dry basis) MJ kg In the simulation process, the ER is changed by inputting different air flow rates while keeping the rice husk flow rate fixed. In this autothermal gasifier, the gasification temperature is also controlled by varying air flow rate while keeping the rice husk flow rate constant, being the same as changing the ER. The different gasification pressures are directly input into the COMBUST and GASIFY blocks. 3 Results and Discussion 3.1 Effect of ER Effect of ER on the Temperature in the Gasifier The effect of ER on the temperature in the gasifier is illustrated in Fig. 2. When ER increases from 0.20 to 0.40, the temperature increases from to C. This obvious influence can be explained by the fact that biomass will combust or gasify more completely to generate more energy which heightens the temperature in the chamber when there are more air supplied Effect of ER on the Composition of Product Gas Figure 3 shows the effect of ER on the composition of product gas. In the process of increasing ER from 0.20 to 0.40, N 2 increases from 38.9 to 54.7%, CO 2 and CH 4 respectively decrease from 15.4 and 7.8% to 10.5 and 0.04%, while H 2 and CO increase first and then decline with the critical ERs are The molar fractions of H 2 and CO are in ranges of % and %, respectively. The probable mechanism can be displayed as follows: (1) Before the critical ER value of 0.3: CH 4 +CO 2! CO + 2H 2,CH 4! C+2 H 2 ; (2) After the critical ER value of 0.3: CO + H 2! H 2 O+ C, CO 2 +2H 2! C+2H 2 O, H 2 +CO 2! H 2 O + CO Effect of ER on the HHV of Product Gas The effect of ER on the HHV of product gas is also displayed in Fig. 2. When ER increases from 0.2 to 0.4, the HHV of Fig. 2 Effect of ER on the HHV of product gas

4 482 Z. Fu et al. Fig. 3 Effect of ER on the composition of product gas Fig. 5 Effect of pressure on the temperature and the HHV of product gas Fig. 4 Effect of temperature on the composition and HHV of product gas product gas declines from to MJ m 3, this is mainly caused by the dilution [21]ofN 2 which has zero high heating value and high molar fractions. 3.2 Effect of Temperature on the Composition and HHV of Product Gas Figure 4 shows the effect of temperature on the composition and HHV of the product gas. The trends of the composition and HHV are the same as it was shown in the Sect. 3.1, this is because temperature is controlled by varying the air flow rate while keeping the rice husk flow rate constant, being the same as changing the ER. 3.3 Effect of Gasification Pressure Effect of Gasification Pressure on the Temperature The effect of pressure on the temperature in the gasifier is illustrated in Fig. 5. The temperature increases from to C when the pressure increases from 1 to 30 atm, this can be explained that the higher pressure supplies molecules with more chance to meet with each other, resulting in more complete gasification. Fig. 6 Effect of pressure on the composition of product gas Effect of Gasification Pressure on the Composition of Product Gas Figure 6 shows the effect of gasification pressure on the composition of product gas. In the process of increasing pressure from 1 to 30 atm, CO and CO 2 vary slightly, N 2 increases slightly from 44.7 to 47.9%, H 2 declines from 21.7 to 16.1%, while CH 4 increases from 0.5 to 2.9%. This phenomenon can be explained by the probable mechanism: C+2H 2! CH Effect of Gasification Pressure on the HHV of Product Gas The effect of gasification pressure on the HHV of product gas is displayed in Fig. 5. When the pressure increases from 1 to 30 atm, the HHV of product gas increases slightly from to MJ m 3, this is mainly caused by the increase of CH 4 which has higher HHV. 4 Conclusions Based on the simulation of biomass gasification in an autothermal gasifier, some main conclusions can be obtained: 1. Higher ER increases the temperature in the gasifier while decreases the HHV of the product gas.

5 Simulation Analysis of Biomass Gasification in an Autothermal Gasifier Using Aspen Plus Temperature which is controlled by varying ER in an autothermal gasifier, has the same influence on the composition and HHV of the product gas as the ER does. 3. Higher gasification pressure increases the temperature in the gasifier and the HHV of the product gas, whereas the effect is slight. Acknowledgment The authors gratefully acknowledge the support provided by the Science and Technology Planning Projects of Shenyang City (Grant No ). References 1. Tan LL, Li CZ. Formation of NO x and SO x precursors during the pyrolysis of coal and biomass. Part III: further discussion on the formation of HCN and NH 3 during pyrolysis. Fuel. 2000;79 (15): Devi L, Ptasinski KJ, Janssen FJJG. A review of the primary measures for tar elimination in biomass gasification process. Biomass Bioenergy. 2003;24(2): Ptasinski KJ, Prins MJ, Pierik A. Exergetic evaluation of biomass gasification. Energy. 2007;32(4): Kaewluan S, Pipatmanomai S. Potential of synthesis gas production from rubber wood chip gasification in a bubbling fluidised bed gasifier. Energy Convers Manage. 2011;52(1): Narváez I, Orío A, Aznar MP, Corella J. Biomass gasification with air in an atmospheric bubbling fluidized bed: effect of six operational variables on the quality of the produced raw gas. Ind Eng Chem Res. 1996;35(7): Mansaray KG, Ghaly AE, Al-Taweel AM, Hamdullahpur F, Ugursal VI. Air gasification of rice husk in a dual distributor type fluidized bed gasifier. Biomass Bioenergy. 1999;17(4): Yang B, Wu J, Zhao G, Wang H, Lu S. Multiplicity analysis in reactive distillation column using ASPEN PLUS. Chin J Chem Eng. 2006;14(3): More RK, Bulasara VK, Uppaluri R, Banjara VR. Optimization of crude distillation system using aspen plus: effect of binary feed selection on grass-root design. Chem Eng Res Des. 2010;88 (2): Ongiro AO, Ugursal VI, Al Taweel AM, Blamire DK. Simulation of combined cycle power plants using the ASPEN PLUS shell. Heat Recovery Syst CHP. 1995;15(2): Zheng L, Furimsky E. ASPEN simulation of cogeneration plants. Energy Convers Manage. 2003;44(11): Cimini S, Prisciandaro M, Barba D. Simulation of a waste incineration process with flue-gas cleaning and heat recovery sections using Aspen Plus. Waste Manage. 2005;25(2): Oexmann J, Hensel C, Kather A. Post-combustion CO 2 -capture from coal-fired power plants: preliminary evaluation of an integrated chemical absorption process with piperazine-promoted potassium carbonate. Int J Greenh Gas Control. 2008;2(4): Nikoo MB, Mahinpey N. Simulation of biomass gasification in fluidized bed reactor using ASPEN PLUS. Biomass Bioenergy. 2008;32(12): Doherty W, Reynolds A, Kennedy D. The effect of air preheating in a biomass CFB gasifier using ASPEN Plus simulation. Biomass Bioenergy. 2009;33(9): Di Blasi C, Signorelli G, Di Russo C, Rea G. Product distribution from pyrolysis of wood and agricultural residues. Ind Eng Chem Res. 1999;38(6): Zanzi R, Sjostrom K, Bjornbom E. Rapid pyrolysis of agricultural residues at high temperature. Biomass Bioenergy. 2002;23 (5): Schuster G, Loffler G, Weigl K, Hofbauer H. Biomass steam gasification an extensive parametric modeling study. Bioresour Technol. 2001;77(1): Li XT, Grace JR, Lim CJ, Watkinson AP, Chen HP, Kim JR. Biomass gasification in a circulating fluidized bed. Biomass Bioenergy. 2004;26(2): Donolo G, Simon GD, Fermeglia M. Steady state simulation of energy production from biomass by molten carbonate fuel cells. J Power Sources. 2006;158(2): Reed TB, Das A. Handbook of biomass downdraft gasifier engine systems. Golden: The Biomass Energy Foundation Press; Mahishi MR, Goswami DY. Thermodynamic optimization of biomass gasifier for hydrogen production. Int J Hydrog Energy. 2007;32(16):