Bio-syngas production from biomass catalytic gasification

Transcription

1 Energy Conversion and Management 48 (2007) Bio-syngas production from biomass catalytic gasification Pengmei Lv a, *, Zhenhong Yuan a, Chuangzhi Wu a, Longlong Ma a, Yong Chen a, Noritatsu subaki b a Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, No. 2 Nengyuan Road, ianhe, Wushan, Guangzhou , People s Republic of China b School of Engineering, Nagoya University, Gofuku, oyama, Japan Received 26 December 2004; received in revised form 23 April 2006; accepted 29 October 2006 Available online 12 December 2006 Abstract A promising application for biomass is liquid fuel synthesis, such as methanol or dimethyl ether (DME). Previous studies have studied syngas production from biomass-derived char, oil and gas. his study intends to explore the technology of syngas production from direct biomass gasification, which may be more economically viable. he ratio of H 2 /CO is an important factor that affects the performance of this process. In this study, the characteristics of biomass gasification gas, such as H 2 /CO and tar yield, as well as its potential for liquid fuel synthesis is explored. A fluidized bed gasifier and a downstream fixed bed are employed as the reactors. wo kinds of catalysts: dolomite and nickel based catalyst are applied, and they are used in the fluidized bed and fixed bed, respectively. he gasifying agent used is an air-steam mixture. he main variables studied are temperature and weight hourly space velocity in the fixed bed reactor. Over the ranges of operating conditions examined, the maximum H 2 content reaches vol%, while the ratio of H 2 /CO varies between 1.87 and he results indicate that an appropriate temperature (750 C for the current study) and more catalyst are favorable for getting a higher H 2 /CO ratio. Using a simple first order kinetic model for the overall tar removal reaction, the apparent activation energies and pre-exponential factors are obtained for nickel based catalysts. he results indicate that biomass gasification gas has great potential for liquid fuel synthesis after further processing. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Syngas; Biomass; Catalytic gasification 1. Introduction Syngas plays an important role as an intermediate in the production of several industrial products, such as Fischer ropsch liquids, methanol and ammonia. Currently, syngas is produced from fossil fuels, mainly coal, natural gas and naphtha. Syngas from renewable resources, such as biomass, exhibits a promising prospective [1 7]. his is because biomass is a CO 2 neutral resource and is distributed extensively in the world. Several biomass to methanol demonstration projects have been developed recently, such as the Hynol project in the United States, the BioMeet and * Corresponding author. el.: ; fax: address: lvpm@ms.giec.ac.cn (P. Lv). Bio-Fuels projects in Sweden and the BGMSS project in Japan [8 10]. Although more and more interest has been focused on this subject, little study was found to address this topic [11 14]. Classifying the literature involved in this subject, it can be found that three different routes of syngas from biomass were studied. hey were syngas from biomass-derived oil [11], syngas from biomass-derived char [12,13] and syngas from reforming of biomass gasification gas [14]. Panigrahi et al. [11] explored synthesis gas production from steam gasification of biomass-derived oil. Different gasifying agents, mixtures of CO 2 and N 2,H 2,N 2 and steam were used. In their study, syngas (H 2 + CO) ranged from 75 to 80 mol%. Chaudhari et al. [12,13] investigated synthesis gas production from biomass-derived char. heir objective was to provide information on different optimum conditions for producing gases for different applications /$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi: /j.enconman

2 P. Lv et al. / Energy Conversion and Management 48 (2007) Nomenclature k kinetic constant for tar elimination (m 3 ( b,wet )/ kg h) A pre-exponential factor for k (m 3 ( b,wet )/kg h) R gas constant (kj/mol K) E apparent activation energy for overall tar removal (kj/mol) Q gas flow rate [m 3 ( b,wet )/h] = b temperature measured in center of fixed bed ( C) X W tar conversion, dimensionless weight of catalyst in fixed bed (kg) Greek Symbols o tar concentration in flue gas (kg/m 3 ) s space time, defined as W/Q [kg h/m 3 ( b,wet )] Wang et al. [14] studied syngas from reforming of biomass gasification gas by adding of biogas. From the above discussion, it was easily discovered that few studies referred to syngas from direct gasification of biomass. Hence, there is a need for a systematic study in this direction. herefore, the purpose of this study is to test the feasibility of syngas from direct biomass gasification, which is a more compact process because pure biomass air gasification produces a gas with a low H 2 /CO ratio, which is not suitable for further synthesis of liquid fuel. herefore, gasifying agents air and steam were used in this study and common catalysts were applied to improve the H 2 /CO ratio as well as to decrease tar yield. 2. Experimental section 2.1. Feed materials and catalysts Pine sawdust obtained from a timber mill in Guangzhou City, China, was used as the feedstock for experimental runs. he particle size of this pine sawdust is between 0.3 and 0.45 mm. Its proximate and ultimate analyses are reported in able 1. Calcined dolomite and nickel based catalysts were used in the experiments, which were proved to be quite active for tar elimination [15 17]. he dolomite was first crushed and sieved to obtain a fraction with a particle sized mm and then calcined in air at 900 C for 4 h. Its chemical composition is presented in able 2. Nickel based able 1 Proximate and ultimate analysis of pine sawdust Moisture content (wt% wet basis) 8 Higher heating value (MJ/kg) 20.2 Proximate analysis (wt% dry basis) Volatile matter 81.0 Fixed carbon 18.5 Ash 0.5 Ultimate analysis (wt% dry basis) C H 5.54 O N 0.18 S 0.23 catalysts of Z409R were used in the catalytic reactor, which were produced in Qilu PetroChemical Company, Shandong Province, China. Z409R is annular with a size of / 16 / mm and a composition of NiO P 22 wt%, K 2 O of 6.5 ± 0.3 wt% Apparatus he tests were performed in an atmospheric pressure, indirectly heated, fluidized bed gasification system, which is shown schematically in Fig. 1. Its major components are: a fluidized bed gasifier, a biomass feeder, a steam generator, an air compressor, a cyclone and a catalytic fixed bed reactor. he reactor is made of 1Cr18Ni9i stainless steel pipe and is externally heated by two electric furnaces. he total height of the reactor is 1400 mm, with a bed diameter of 40 mm and a freeboard diameter of 60 mm. Along the total height of the reactor, there are 5 temperature and pressure taps for temperature and pressure detection. Below the reactor, one air distributor is installed for better air distribution. he distributor is 3 mm in thickness and 25 holes (i.d. 1 mm) are perforated uniformly in it. he biomass is fed into the reactor through one screw feeder driven by a variable speed metering motor. Air is used as the fluidizing agent and comes from the air compressor. Before the air enters the reactor, it is preheated to 65 C for better performance. he steam of 154 C is produced in a steam generator (Model SZ , Guangzhou Zhongli Boilers Auxiliary Machine Co., Ltd., Guangdong, China). Before the steam flows into the reactor above the biomass feeding point, it is metered by a steam flow meter. he produced gas flow exits the reactor, then passes through a cyclone, which is heated to 200 C to prevent the tar contained in the gas from condensing in it. he fixed bed reactor is externally heated by an electric furnace. Its length is 400 mm with an inner diameter of 38.5 mm. able 2 Chemical composition of uncalcined dolomite (wt%) Sample CaO MgO SiO 2 Al 2 O 3 Fe 2 O 3 Loss otal Dolomite

3 1134 P. Lv et al. / Energy Conversion and Management 48 (2007) ,P Gasifier Gas and tar sampling Cyclone Fixed-bed reactor Gas and tar sampling Valve o exhaust burner Heators Steam flowmeter Hopper P Steam generator,p Air distributor Air compressor Gas flowmeter Fig. 1. Schematic diagram of biomass catalytic gasification for syngas production. Prior to each test, an amount of 120 g/(kg h 1 ) biomass calcined dolomite mixed with 30 g silica sand ( mm) were put in the gasifier. Since calcined dolomite is soft, it erodes during the test and is eluted out of the bed with the flue exit gas. herefore, some calcined dolomite was mixed with the pine sawdust carefully by hand and continuously fed into the gasifier to attain a steady state. he feeding rate of calcined dolomite was determined by preliminary test. At the end of each test, calcined dolomite left in the gasifier is separated and measured. hus, the weight percent of calcined dolomite in the gasifier bed during operation can be known, and it is about wt% over the ranges of experimental conditions examined. he measurement accuracy for the gas flow meter, temperature sensor, manometer, steam flow meter is ±0.06 Nm 3 /h, ±10 C, ±0.25 kpa, ±0.05 kg/h, respectively. o ensure the reliability of the test data, each experiment was repeated 2 times and the results had good agreement. he data reported in this paper is average values of the two times Gas and tar analysis he cool, dry, clean gas was sampled using gas bags and analyzed on a gas chromatograph (Model GC-2010, Shimadzu, Japan), which is fitted with a GS carbon plot column (30 m mm 3.00 lm), flame ionization Electric heating Quartz fiber filter Steel tube Volume flow meter Silica gel Ice bath Cool bath Gas washing bottles content CH2C 2 Fig. 2. Schematic diagram of tar sampling system.

4 P. Lv et al. / Energy Conversion and Management 48 (2007) detector (FID) and thermal conductivity detector (CD), and standard gas mixtures are used for quantitative calibration. he tar sampling line is shown in Fig. 2. Dichloromethane cooled to approximately 10 C is used to condense and collect the tar. A gas chromatograph HP-4890 is used to analyze the tar sample. he operating conditions are: 30 m 0.25 mm 0.25 lm, HP-5 capillary column; carrier gas, N 2 ; temperature program: 75 C (hold 5 min) to 285 C at 3 C/min (hold 40 min); injector and detector temperature, 280 C. first increases and then decreases with temperature and reaches the maximum value of 4.45 at the temperature of 750 C, as shown in Fig. 3. Fig. 3 indicates that the value of H 2 /CO ranges between 3.11 and 4.45, a quite high ratio. Fig. 3 also indicates that temperature is an important factor for controlling H 2 /CO ratio CO þ H 2 O ¼ CO 2 þ H 2 þ 41 kj=mol ð1þ CH 4 þ H 2 O ¼ CO þ 3H kj=mol ð2þ C þ H 2 O ¼ CO þ H kj=mol ð3þ C þ CO 2 ¼ 2CO 172 kj=mol ð4þ 3. Results and discussion 3.1. Effects of different fixed bed reactor temperature As listed in able 3, run no. 1, the operating conditions in the fluidized bed were kept constant, while the temperature in the catalytic reactor was varied to perform a series of tests. In able 3, equivalence ratio is defined as the actual oxygen to fuel ratio divided by the stoichiometric oxygen to fuel ratio needed for complete combustion. he experimental results are presented in able 4. able 4 shows that the content of H 2 exhibits an increasing trend with temperature. his is an expected result because most H 2 production reactions are endothermic. he content of CH 4 decreases with temperature because a higher temperature strengthens the steam reforming reaction of CH 4. he content of CO first decreases and then increases with temperature, which indicates reactions (1) (4) happen simultaneously in the process. As a result of the variation trend of H 2 and CO, the ratio of H 2 /CO also Value of H 2 /CO emperature ( ) Fig. 3. Effect of temperature on the value of H 2 /CO. able 3 Operating conditions in the fluidized bed reactor Run no. 1 2 Run no. 1 2 Biomass feed rate (kg/h) Steam-to-biomass ratio Air (N m 3 /h) Calcined dolomite feeding rate (g/h) Steam (kg/h) Calcined dolomite in the gasifier (g) Equivalence ratio Gasifier bed temperature ( C) Gasifier bed outlet gas composition(dry, inert-free, vol%) Run no. H 2 CH 4 CO CO 2 C 2 Gas yield (N m 3 /kg biomass, wet basis) ar yield (g/kg biomass) able 4 Experimental results of different fixed bed temperature at the condition of run no. 1 Reactor temperature ( C) Dry, inert-free, gas composition (vol%) H CH CO CO C Gas yield (N m 3 /kg biomass) (wet basis) ar yield (g/kg biomass) (wet basis)

5 1136 P. Lv et al. / Energy Conversion and Management 48 (2007) able 5 Experimental results of different reactor WHSV (h 1 ) at the condition of run no. 2, b = 700 C WHSV (h 1 ) Dry, inert-free, gas composition (vol%) H CH CO CO C Gas yield (N m 3 /kg biomass) (wet basis) Experimental results of different Weight Hourly Space Velocity (WHSV) Value of H 2 /CO b =700 b =800 Weight hourly space velocity (WHSV) is defined as the mass flow rate of biomass fed to the gasifier divided by the mass of catalyst in the catalytic reactor. Keeping the operating conditions listed in able 3, run no. 2 constant, the experimental results of different WHSVs in the catalytic reactor is presented in able 5. As shown in able 5, the content of H 2 shows an upward trend with increasing residence time, while the content of CH 4 and CO decreases with increasing residence time. he content of CO 2 has little change. he above phenomena indicate that applying more catalysts is favorable for H 2 production. he value of H 2 /CO for two different temperatures in the fixed bed reactor is shown in Fig. 4. For the temperatures of 700 C and 800 C, the values of H 2 /CO vary between 2.11 and 3.32, 1.87 and 2.78, respectively. Fig. 4 indicates that the ratio of H 2 /CO increases with residence time. his is an obvious result, which can be seen from able 5. Another phenomena found from Fig. 4 is that the ratio of H 2 /CO at 700 C is higher than that at 800 C. his result is in accordance with the conclusion drawn from Fig. 3. hat is, there exists a maximum value of H 2 /CO for the different temperatures. his is possibly caused by the water gas shift reaction (1) being exothermic; so a higher temperature is not favorable for CO consumption, which lowers the CO transformation rate, which results in a smaller value of H 2 /CO. Observing Fig. 4, we can also find that there appears an extraordinary decrease in the H 2 /CO ratio at WHSV just higher than 4 h 1. his can be explained by the residence time being shorter than the minimum time needed for some secondary reactions of biomass gasification gas to proceed when WHSV is higher than 4 h 1. his results in a slight increase of H 2 content and, thus, makes a quite small increment of H 2 /CO ratio when WHSV is higher than 4 h Weight Hourly Space Velocity (h -1 ) Fig. 4. Effect of WHSV on the value of H 2 /CO Comparison of different routes for bio-syngas production able 6 lists a comparison of the different routes for biosyngas production and process D represents this study. As able 6 indicates, process A yields the least hydrogen, able 6 Comparison of different routes for bio-syngas production Process A [11] B [13] C [14] D Feedstock Biomass-derived oil Biomass-derived char Biomass-derived gas Biomass Gasifying agent H 2 and N 2 Steam Air-steam Air-steam Reforming method Biogas or CH 4 reforming Use of catalyst No No Yes Yes ypical gas composition (dry, inert-free, mol%) H CO CO CH Others Range of H 2 /CO

6 P. Lv et al. / Energy Conversion and Management 48 (2007) while process B produces a maximum H 2 content and thereby the highest H 2 /CO ratio. Processes C and D produce comparable H 2 quantities. From able 6, we can know that the H 2 /CO ratio is quite high from direct biomass catalytic gasification. Process D has the highest content of CO 2, which is caused by the catalytic activity of nickel catalysts on the water gas shift reaction (1). his can be settled by controlling operation conditions and modifying the nickel catalyst to make it have low selectivity on the WGSR. he advantage of process D is that it produces syngas in one step, while the other processes need two steps. his means it has more compact equipment and may be more economically viable. Based on the above analysis, it can be concluded that biomass catalytic gasification has great potential for syngas production and has a good economic outlook. Fig. 5. Gas chromatogram of tar sample in the inlet of fixed bed reactor. Fig. 6. Gas chromatogram of tar sample in the exit of fixed bed reactor.

7 1138 P. Lv et al. / Energy Conversion and Management 48 (2007) ln(k) /(1/K) Fig. 7. Arrhenius plot for tar catalytic cracking. catalytic gasification, which is favorable for methanol or DME synthesis. However, besides H 2 and CO, there is also a lot of CO 2 and CH 4 in the gases, which needs to be decreased through modifying the catalysts and controlling operating conditions. he maximum H 2 content reaches vol%. An appropriate temperature (750 C for the current study) and more catalyst are more favorable for getting a higher H 2 /CO ratio. From the comparison of this technology with others, it can be seen that great potential exists for syngas production from direct biomass gasification. A single one lump model is perfect for tar destruction analysis. Applying this model, E and A can be determined as 51 kj/mol and (m 3 ( b,wet )/kg h), respectively. Acknowledgements 3.4. ar yield analysis and kinetic model construction for tar cracking Figs. 5 and 6 show the gas chromatogram of tar samples taken from the inlet and outlet of the fixed bed reactor, respectively. It is obvious that both the species and the quantity of tar are reduced greatly in the presence of nickel based catalysts. Although there are hundreds of species in the tar sample, in order to simplify the analysis, all the species are treated as a single one lump. his approach has been accepted by many institutions working worldwide in catalytic hot gas cleaning [18]. It uses the following single first order kinetic equation: d@ dt ¼ k@ ð5þ which can be used in integrated form, working under piston or plug flow conditions, as k ¼½ lnð1 X ÞŠ=s ð6þ Also, for k, it abides by the Arrhenius equation as k ¼ A expð E=R Þ ð7þ Eq. (7) can be further transformed as lnðkþ ¼lnðAÞþ E 1 ð8þ R Eq. (8) is linear. By applying k (determined by Eq. (6)) and to it; the values of E and A can be determined. A calculation result for the condition listed in able 4 is presented in Fig. 7, which shows good linearity. he E and A values determined by Fig. 7 are 51 kj/mol and (m 3 ( b,wet )/ kg h), respectively. his value of E is in near agreement with Aznar et al. s experimental data, 58 kj/mol [18]. 4. Conclusion he results show that quite a high ratio of H 2 /CO, ranging between 1.87 and 4.45, can be obtained from biomass he financial support received from the National Natural Science Foundation of China (Project No ), Guangdong Province Key Laboratory Open Foundation (Project No ) and Guangdong Province Natural Science Foundation (Project No ) is gratefully appreciated. References [1] Bridgwater AV, Double JM. Production costs of liquid fuel from biomass. Fuel 1991;70(10): [2] Dong Y, Steinberg M. Hynol an economic process for methanol production from biomass and natural gas with reduced CO 2 emission. Int J Hydrogen Energy 1997;22(10 11): [3] Hamelinck CN, Faaij APC. Future prospects for production of methanol and hydrogen from biomass. J Power Sources 2002;111(1):1 22. [4] Chmielniak, Sciazko M. Co-gasification of biomass and coal for methanol synthesis. Appl Energy 2003;74(3 4): [5] ijmensen MJA, Faaij APC, Hamelinck CN, van Hardeveld MRM. Exploration of the possibilities for production of Fischer ropsch liquids and power via biomass gasification. Biomass Bioenergy 2002;23(2): [6] Faaij A, Hamelinck C, ijmensen M. Long term perspectives for production of fuels from biomass; integrated assessment and R&D priorities preliminary results. In: Kyritsis S et al., editors. In: Proceedings of the first world conference on biomass for energy and industry. London, UK: James & James Ltd., vol. 1/2, p [7] Larson ED, Jin H. Biomass conversion to Fischer ropsch liquids: preliminary energy balances. In: Overend R, Chornet E, editors. Proceedings of the fourth biomass conference of the Americas. UK: Elsevier Science Kidlington; p. 43. [8] Norbeck JM, Johnson K. pdf, [9] Brandberg ÅRL, Ekbom. [10] Mitsubishi Heavy Industries. Biomass gasification methanol synthesis system. April, [11] Panigrahi S, Dalai AK, Chaudhari S, Bakhshi NN. Synthesis gas production from steam gasification of biomass-derived oil. Energy Fuels 2003;17(3): [12] Chaudhari S, Bej SK, Bakhshi NN, Dalai AK. Steam gasification of biomass-derived char for the production of carbon monoxide-rich synthesis gas. Energy Fuels 2001;15(3):

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