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Progress in techniques of biomass conversion into syngas
Journal of the Energy Institute xxx (2014) 1e6
Contents lists available at ScienceDirect
Journal of the Energy Institute journal homepage: http://www.journals.elsevier.com/journal-of-the-energyinstitute
Progress in techniques of biomass conversion into syngas Weijuan Lan a, *, Guanyi Chen b, Xinli Zhu b, Xuetao Wang a, Bin Xu a a b
College of Vehicle and Traffic Engineering, Henan University of Science and Technology, Luoyang 471003, China School of Environmental Science & Engineering, Tianjin University, Tianjin 300072, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 August 2013 Accepted 12 May 2014 Available online xxx
Biomass gasification is one of major biomass utilization technologies to get high quality gas. The high quality gas can be subsequently used for gas supply and power generation as well as syngas. Significant efforts have therefore been made in biomass-derived syngas production. The paper reviews the state-ofthe-art biomass-derived syngas production techniques in terms of technical performance. Various kinds of gasification reactor are briefly introduced. Main technologies of syngas production can be divided into four approaches: partial oxidation and steam reforming of biomass pyrolysis oils, co-gasification of biomass and coal, coupled steam hydrogasification of biomass and reforming of methane, gasification of biomass-derived char. Each of these production processes are also analyzed in detail. Among these production technologies, the primary technology for syngas production is steam hydrogasification and reforming. Syngas has a higher H2/CO ratio by using these two technologies: the steam hydrogasification and reforming technology and biomass-derived char technology. © 2014 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Biomass Syngas Gasification reactor Technique
1. Introduction With fossil fuel depleting and increasing serious environmental problems, biomass energy as a clean and renewable resource has been paid more and more attention in the global sourcing strategy [1]. Because of the increasing energy demand and the limitation of the fossil sources, renewable energy sources should be used to the maximum. In order to achieve growth in economic development, it is essential to meet energy needs of all sectors, such as agriculture, industry and transportation [2]. Biomass gasification is one of major biomass utilization technologies to get high quality gas. The high quality gas can be subsequently used for gas supply and power generation as well as syngas. So it is regarded as one of the most attractive options for utilization of biomass. Syngas production from biomass is widely studied, since syngas could be further widely used for many purposes. Many research institutes have involved in biomass gasification for many years. For example, in China, including Guangzhou Institute of Energy Conversion, Tsinghua University, Tianjin University, Huazhong University of Science and Technology, Xi'an Jiao Tong University, East China University of Science and Technology, Shandong Province Academy of Science, and so on. Syngas production technologies can be divided into four approaches: partial oxidation and steam reforming of biomass pyrolysis oils, co-gasification of biomass and coal, coupled steam hydrogasification of biomass and reforming of methane, gasification of biomass-derived char. In the gasification process, the gasification reactor is a critical component as is the main syngas production technology. In this paper, various kinds of gasification reactor are briefly introduced, such as, fixed bed, circulating fluidized bed (CFB), bubbling fluidized bed (BFB). And the main technologies of syngas production are summarized. 2. Gasification reactor type Biomass gasification reactors are classified into two main types: fixed bed and fluidized bed. The sub-categories for the fixed bed type gasifiers are (a) updraft, (b) downdraft. And the sub-categories for the fluidized bed gasifiers are (a) bubbling fluidized bed (BFB) and (b) circulating fluidized bed (CFB).
* Corresponding author. E-mail address: [email protected] (W. Lan). http://dx.doi.org/10.1016/j.joei.2014.05.003 1743-9671/© 2014 Energy Institute. Published by Elsevier Ltd. All rights reserved.
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2.1. Fixed bed gasifier 2.1.1. Updraft gasifier In the updraft gasifier, the material is fed from the top of the reactor and the air is introduced from the bottom of the reactor through a grate. Material and air move in opposite direction in the gasifier [3]. The “combustion” zone is essentially in the lowest part of the gasifier. In this zone, biomass is combusted and char is formed. During the process of combusting, the temperature of the lower part of the gasifier raises to about 750 C. The hot gases pyrolyze the biomass and dry it in further in the upper part of the gasifier. Pyrolysis of biomass results in release of volatiles and formation of a sizeable amount of tar [4]. Some of the tar may be released with the outgoing gases. The humidity of the air used in the gasification plays an important role in controlling the temperature [5]. A schematic of the updraft gasifier is shown in Fig. 1 [5]. 2.1.2. Downdraft gasifier In the downdraft gasifier, the material and air move concurrently from top to bottom of the reactor. Since the exit of the produced gas is close to the combustion zone, tar formed during devolatilization of the biomass is thermally cracked to some extent [5]. Thus, the tar content of the produced gas from the downdraft gasifier is lower than that of the updraft gasifier [6,7]. Downdraft gasifier has been paid more and more attention due to the low tar content in the produced gas. Low tar content gas is always preferred for firing gas engines and turbines [5]. Fig. 2 shows a schematic of the downdraft gasifier [4]. 2.2. Fluidized bed gasifier Fluidized bed gasifier is widely used in recent years [5]. The advantages of these processes are: the temperature distribution in the reactor is uniform, conversion of carbon is high, tar production is low. The fuel type, feed rate, as well as particle size are flexible [8]. In the past two decades, significant experimental and theoretical research has already been carried out in design, development and scale-up of fluidized bed gasifiers [5]. Two kinds of fluidized systems are described.
Fig. 1. The updraft gasifier.
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Fig. 2. The downdraft gasifier.
2.2.1. Bubbling bed gasifier In this kind of system, the bed material rests on a distributor plate. Fluidizing medium is air. Air passes at a velocity about five times faster than that of minimum fluidization velocity through this plate. The temperature in the bed is about 700e900 C [9]. The biomass is pyrolyzed in the hot bed. In this process, char and gases are produced. Then char particles are lifted along with fluidizing air. At the same time gasification reaction occurs in relatively upper part of the bed. Because of contacting with the high temperature bed, the high molecular weight tar compounds are cracked, so that the net tar content of the produced gas is reduced to less than 1e3 g/Nm3 [5]. 2.2.2. Circulating fluidized bed (CFB) Circulating fluidized bed (CFB) is an economic and environmentally acceptable technology for biomass gasification or low-grade burning. In this kind of system, the velocity of the fluidizing air is much higher than the terminal settling velocity of the bed material. The gasifier can be operated at high pressures. The material operated by CFB ranges from 1 to 80 tons [10]. 2.2.3. Entrained flow gasifier In this kind of system, the material (which is pulverized into solid) is fed in the gasifier by pneumatic feeding. The powder is moved by inert gas and injected in a so-called burner in the gasifier. The characterized of this kind of gasifier is that fuel particles dragged along with the gas stream. That is to say: the process needed short residence times, high temperatures and small fuel particles. Furthermore, entrained flow gasifier is often operated under pressure (typically 20e50 bar) and with pure oxygen. Solid fuel and oxygen can be well mixed, Vortex flow patterns are created in the burner. The temperatures in the burner zone can be research 2500 C or even higher. The capacity of the gasifer is hundreds of MW.
3. Syngas production technology Main technologies of syngas production are:
3.1. Syngas production from partial oxidation and steam reforming of biomass pyrolysis oils This method of biomass conversion is called fast pyrolysis, which benefits from years of research and is an industrially technology [11,12]. By rapidly heating biomass in the absence of oxygen, pyrolysis oils (bio-oils) can be formed. Pyrolysis oils contain water and oxygen. They can be pumped or shipped more efficiently and be upgraded at a central facility, thus can be used in an industrial way [13e15]. In addition, pyrolysis represents a chemical step that can be combined with ash removal. In this process, the ash is removed with the solids (char), a byproduct of pyrolysis, thus bio-oils contain much less ash than their original biomass source [16]. Bio-oils vary greatly when different feedstocks and methods were used. They require further upgrading to convert them to usable fuels. Several options can be considered for further upgrading pyrolysis oils to high-quality liquid fuels. Perhaps the most versatile way for producing fuels from pyrolysis oils is gasification to syngas [17]. Typical two-stage biomass gasification process is represented in Fig. 3. Please cite this article in press as: W. Lan, et al., Progress in techniques of biomass conversion into syngas, Journal of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.05.003
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Fig. 3. Biomass two-stage gasification.
The two-stage biomass gasification process has advantages, such as: a high pressure pump could be used in stead of the traditional compressor. Pyrolysis oils can be catalytically converted to syngas. Noble metal catalysts could achieve high conversion of bio-oil to H2 and CO. Fast deactivation of catalyst is the disadvantage of this process [18]. 3.2. Syngas production from co-gasification of biomass and coal Syngas production from co-gasification of biomass and coal is considered as a promising technology. The produced syngas is hydrogenrich and contains CH4, which can be used for power plants. This process has several advantages. During the co-gasification process the volatiles readily decompose and form free radicals which react with the organic matter of the coal, thus the conversion rate increases [19]. This process can also reduce CO2, SO2 and NOx emissions. In addition, during the devolatilization of the biomass, the hydrogen-rich molecules form. At the same time, hydrogen may react with the free radicals produced from coal at the moment of their formation and €stro €m reported that the alkali metals in biomass prevent recombination reactions that could produce less reactive secondary tar species. Sjo can act as catalyst for promoting the conversion of the coal char [19]. The disadvantages of this process is that the technical challenges in developing replicated turnkey co-gasificaion plants are considerable, since there is no turnkey supplier of co-gasification technology with associated materials handling [20]. 3.3. Syngas production from coupled steam hydrogasification of biomass and reforming of methane In the process of steam hydrogasification, carbon-containing solid feedstock is converted into methane rich gas in the presence of steam. It is a thermo-chemical process [21]. A lot of research works have demonstrated that the enhanced methane production from steam hydrogasification can be combined with steam reforming to generate syngas with a flexible H2/CO ratio from a number of carbonaceous feedstocks [22e25]. Because the steam methane reformer is a catalytic reactor, contaminants such as chlorine, sulfur and other trace metals in the produced gas stream should be removed. By using a warm-gas cleanup unit, this task can be accomplished. The clean gas stream contained large amount of methane and steam, which is converted into a clean syngas in the steam methane reformer. The syngas from the steam methane reformer can be used for power generation and also used in a fuel synthesis process to produce synthetic hydrocarbon fuels [26]. The H2/CO ratio of the syngas produced from the steam methane reformer is higher than that is required for FischereTropsch synthesis. The excess H2 from the syngas is separated and fed back into the steam hydrogasification reactor. The advantage of this process is that by altering the H2O/C and H2/C ratios of the steam hydrogasification reactor, the final H2/CO ratio of the syngas can be adjusted in a simple way. Fig. 4 shows a schematic diagram of the process [27]. 3.4. Syngas production from gasification of biomass-derived char Biomass pyrolysis can generate fuel products consisting of approximately 70 wt% liquid, 15 wt% char, and 15 wt% gas [28]. In this process, the char is a byproduct and needs to be utilized. A lot of research works have been reported on the gasification of biomass-derived char using several agents such as air, steam and carbon dioxide [29e31]. Gasification using biomass-derived char as feedstock to produce syngas is more preferable than the raw biomass. Syngas obtained from direct gasification of raw biomass is usually rich in tar. As for biomass-derived char gasification, syngas products with lower content of tar can be obtained, since the volatile matter content has been removed during the
Fig. 4. Schematic diagram of the coupled steam hydrogasification and reforming process.
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pyrolysis of biomass [32]. Furthermore, biomass-derived char contains a higher content of fixed carbon, so the productivity can be increased during the process [33]. Gasification of biomass-derived char for syngas production has many advantages and can be widely used [33]. On one hand, the process is rapid and the operating condition is not complicated [34]. On the other hand, the syngas contains a higher content of H2 and CO could be used as fuel gas according to the gas composition [31]. 3.5. Syngas production from pure biomass In the process, gasification can be operated with or without a catalyst in a fixed bed, movable bed or fluidized bed, with the fluidized bed normally yielding better performance [35]. In order to maximize syngas production with minimum tar formation, the reaction temperature should be increased, volatile residence time should be extended [36]. Oxygen or steam as gasifying medium is the most suitable gasifying medium for syngas production [37]. Meanwhile, the advanced technologies including catalytic gasification and thermal plasma gasification can be used for enhancing syngas production [37]. Because impurities from the gasitication process can cause operational problems by eroding and blocking pipelines and may deactivate catalysts [38], so the gas needed to be clean. The first step of gas cleaning is to remove tars. Three methods are usually used to reduce tar content: thermal cracking, catalytic cracking and scrubbing [39]. The method by using catalytic cracking is effect. After the tars are removed, two methods can be applied to cope with other impurities: wet gas cleaning and dry gas cleaning [40]. After the cleaning process, syngas can be collected or further applied. 4. Overview and conclusions Syngas production is an effective way to utilize biomass resources. Technologies of syngas production from biomass have been discussed in this paper. The primary technology for syngas production is steam hydrogasification and reforming. In this process, methane and steam are catalytically and endothermically converted to syngas. Partial oxidation is an alternative approach, the exothermic, non-catalytic reaction of methane and oxygen to produce syngas. For the technology of co-gasification of biomass and coal, the conversion rate can increase in the process, but there is no turnkey supplier of co-gasification technology with associated materials handling. Syngas production technologies are applicable under different conditions. In principle, syngas can be produced from any hydrocarbon feedstock. The compositions of syngas from different technology are different. In particular, syngas have a higher H2/CO ratio by using these two technologies: the steam hydrogasification and reforming technology and biomass-derived char technology. This represents a distinct advantage for them in hydrogen-production applications. There also have many factors influencing syngas production capacity. With the improvement of gasification technology, the capacity of syngas production from biomass can be significantly improved in the future. Acknowledgments This paper is financially supported by Henan University of Science and Technology Talent Introduction Fund Projects (No. 09001759) and Education Department of Henan Province Science and Technology Key Project (No. 14B470020) and Henan University of Science and Technology Research Fund Project (No. 2014QN004). References [1] Z.Y. Luo, J.S. Zhou, S.R Wang, Biomass utilization as a clean energy in Zhejiang University, in: International Conference on Power Engineering, Hangzhou, China, 23e27 October, 2007, pp. 14e18. [2] G. Fischer, S. Prieler, H.V. Velthuizen, Biomass potentials of miscanthus, willow and poplar: results and policy implications for Eastern Europe, Northern and Central Asia, Biomass Bioenergy 28 (2) (2005) 119e132. [3] M. Kaltschmitt, A.V. Bridgwater, Biomass Gasification and Pyrolysis: State-of-the-art and Future Prospects, CPL Press, Newbury, 1997. [4] V.V.N. Kishore, Renewable Energy Engineering & Technology, a Knowledge Compendium, Energy and Resources Institute, TERI Press, New Delhi, 2008. [5] B. Buragohain, P. Mahanta, V.S. Moholkar, Biomass gasification for decentralized power generation: the Indian perspective, Renew. Sustain. Energy Rev. 14 (1) (2010) 73e92. [6] D. Hebden, H.J.F. Stroud, Coal gasification processes, in: M.A. Elliott (Ed.), Chemistry of Coal Utilization. Second Supplementary Volume, Wiley, New York, 1981 (Chapter 24). [7] P. Liu, X.F. Guo, C.Z. Wu, Gasification characteristics of biomass wastes in fluidized bed gasifier, J. Propuls. Power 16 (2000) 606e608. ~ ez, A. Cabanillas, J.M. Sa nchez, Gasification of leached or ujillo (olive oil waste) in a pilot plant circulating fluidized bed reactor-preliminary results, Biomass [8] P. Garcí-Iban Bioenergy 27 (2) (2004) 83e194. [9] H. Liu, B.M. 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McKibben, Non-catalytic partial oxidation of bio-oil to synthesis gas for distributed hydrogen production, Int. J. Hydrogen Energy 34 (2009) 8519e8534. [16] S. Czernik, A.V. Bridgwater, Overview of applications of biomass fast pyrolysis oil, Energy Fuels 18 (2004) 590e598. [17] D. Rennard, R. French, S. Czernik, et al., Production of synthesis gas by partial oxidation and steam reforming of biomass pyrolysisoils, Int. J. Hydrogen Energy 35 (9) (2010) 4048e4059. [18] R.H. Venderbosch, W. Prins, Entrained Flow Gasification of Bio-oil for Synthesis Gas, BTG Biomass Technology Group b. V. Hallenweg, The Netherlands, 2002. [19] F. Miccio, G. Ruoppolo, S. Kalisz, et al., Combined gasification of coal and biomass in internal circulating fluidized bed, Fuel Process. Technol. 95 (2012) 45e54. [20] N. Koukouzas, A. Katsiadakis, E. Karlopoulos, et al., Co-gasification of solid waste and lignite e a case study for Western Macedonia, Waste Manag. 28 (7) (2008) 1263e1275. [21] J.M. Norbeck, C.S. Park, US Patent Application Number 11/489, 298, 2007. [22] J.M. Norbeck, C.E. Hackett, United States patent number 7,208,530 B2, 2007. [23] S.K. Jeon, C.S. Park, C.E. Hackett, et al., Characteristics of steam hydrogasification of wood using a micro-batch reactor, Fuel 86 (2007) 2817e2823.
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[24] C.S. Park, S.P. Singh, J.M. Norbeck, Steam hydrogasification of carbonaceous matter to liquid fuels, in: Proceedings of the 24th Annual International Pittsburgh Coal Conference, Johannesburg, South Africa, 2007. [25] A.S.K. Raju, C.S. Park, J.M. Norbeck, Production of synthesis gas for liquid fuel synthesis using a steam hydrogasification and reforming process, in: Fifteenth European Biomass Conference, Berlin, Germany, 2007. [26] A. Aguirre, C.S. Park, J.M. Norbeck, Method for high energy density biomass-water slurry, US Patent Application Number 11/488, 299, 2007. [27] A.S.K. Raju, C.S. Park, J.M. Norbeck, Synthesis gas production using steam hydrogasification and steam reforming, Fuel Process. Technol. 90 (2009) 330e336. [28] S.T. Chaudhari, A.K. Dalai, N.N. Bakhshi, Production of hydrogen and/or syngas (H2 þ CO) via steam gasification of biomass-derived chars, Energy Fuels 17 (4) (2003) 1062e1067. €stro €m, Reactivity of char from pyrolysis of birch wood, J. Anal. Appl. Pyrolysis 40 (1997) 491e499. [29] G.X. Chen, Q.Z. Yu, K. Sjo [30] M. Sakaguchi, A. Paul-Watkinson, N. Ellis, Steam gasification reactivity of char from rapid pyrolysis of bio-oil/char slurry, Fuel 89 (2010) 3078e3084. [31] K. Matsumoto, K. Takeno, T. Ichinose, et al., Gasification reaction kinetics on biomass char obtained as a by-product of gasification in an entrained-flow gasifier with steam and oxygen at 900e1000 C, Fuel 88 (2009) 519e527. [32] H. Haykiri-Acma, S. Yaman, S. Kucukbayrak, Gasification of biomass chars in steam-nitrogen mixture, Energy Convers. Manag. 47 (2006) 1004e1013. [33] P.W. He, S.Y. Luo, G. Cheng, et al., Gasification of biomass char with air-steam in a cyclone furnace, Renew. Energy 37 (1) (2012) 398e402. [34] F. Mermoud, S. Salvador, L. Van de Steene, et al., Influence of the pyrolysis heating rate on the steam gasification rate of large wood char particles, Fuel 85 (2006) 1473e1482. [35] J.D. Holladay, J. Hu, D.L. King, Y. Wang, An overview of hydrogen production technologies, Catal. Today 139 (2009) 244e260. [36] J.F. Wang, J. Chang, X.L. Yin, T.J. Wang, C.Z. Wu, Synthesis of clean fuel dimethyl ether by indirect liquefaction of biomass, Acta Energiae Solaris Sin. 26 (2005) 414e418. [37] X.L. Yin, J. Chang, J.F. Wang, Y. Fu, C.Z. Wu, D.Y.C. Leung, Exploration of possibility and technical route for methane synthesis via biomass gasification in China, Coal Convers. 27 (2004) 17e22. [38] C. Li, D. Hirabayashi, K. Suzuki, Development of new nickel based catalyst for biomass tar steam reforming producing H2-rich syngas, Fuel Process. Technol. 90 (2009) 790e796. [39] C.N. Hamelinck, A.P.C. Faaij, H. den Uil, H. Boerrigter, Production of FT transportation fuels from biomass; technical options, process analysis and optimization, and development potentials, Energy 29 (2004) 1743e1771. [40] X.L. Guo, Y.C. Zhao, H. Huang, Methanol production via biomass gasification, Energy Eng. 28 (2004) 28e31.
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Contents lists available at ScienceDirect
Journal of the Energy Institute journal homepage: http://www.journals.elsevier.com/journal-of-the-energyinstitute
Progress in techniques of biomass conversion into syngas Weijuan Lan a, *, Guanyi Chen b, Xinli Zhu b, Xuetao Wang a, Bin Xu a a b
College of Vehicle and Traffic Engineering, Henan University of Science and Technology, Luoyang 471003, China School of Environmental Science & Engineering, Tianjin University, Tianjin 300072, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 August 2013 Accepted 12 May 2014 Available online xxx
Biomass gasification is one of major biomass utilization technologies to get high quality gas. The high quality gas can be subsequently used for gas supply and power generation as well as syngas. Significant efforts have therefore been made in biomass-derived syngas production. The paper reviews the state-ofthe-art biomass-derived syngas production techniques in terms of technical performance. Various kinds of gasification reactor are briefly introduced. Main technologies of syngas production can be divided into four approaches: partial oxidation and steam reforming of biomass pyrolysis oils, co-gasification of biomass and coal, coupled steam hydrogasification of biomass and reforming of methane, gasification of biomass-derived char. Each of these production processes are also analyzed in detail. Among these production technologies, the primary technology for syngas production is steam hydrogasification and reforming. Syngas has a higher H2/CO ratio by using these two technologies: the steam hydrogasification and reforming technology and biomass-derived char technology. © 2014 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Biomass Syngas Gasification reactor Technique
1. Introduction With fossil fuel depleting and increasing serious environmental problems, biomass energy as a clean and renewable resource has been paid more and more attention in the global sourcing strategy [1]. Because of the increasing energy demand and the limitation of the fossil sources, renewable energy sources should be used to the maximum. In order to achieve growth in economic development, it is essential to meet energy needs of all sectors, such as agriculture, industry and transportation [2]. Biomass gasification is one of major biomass utilization technologies to get high quality gas. The high quality gas can be subsequently used for gas supply and power generation as well as syngas. So it is regarded as one of the most attractive options for utilization of biomass. Syngas production from biomass is widely studied, since syngas could be further widely used for many purposes. Many research institutes have involved in biomass gasification for many years. For example, in China, including Guangzhou Institute of Energy Conversion, Tsinghua University, Tianjin University, Huazhong University of Science and Technology, Xi'an Jiao Tong University, East China University of Science and Technology, Shandong Province Academy of Science, and so on. Syngas production technologies can be divided into four approaches: partial oxidation and steam reforming of biomass pyrolysis oils, co-gasification of biomass and coal, coupled steam hydrogasification of biomass and reforming of methane, gasification of biomass-derived char. In the gasification process, the gasification reactor is a critical component as is the main syngas production technology. In this paper, various kinds of gasification reactor are briefly introduced, such as, fixed bed, circulating fluidized bed (CFB), bubbling fluidized bed (BFB). And the main technologies of syngas production are summarized. 2. Gasification reactor type Biomass gasification reactors are classified into two main types: fixed bed and fluidized bed. The sub-categories for the fixed bed type gasifiers are (a) updraft, (b) downdraft. And the sub-categories for the fluidized bed gasifiers are (a) bubbling fluidized bed (BFB) and (b) circulating fluidized bed (CFB).
* Corresponding author. E-mail address: [email protected] (W. Lan). http://dx.doi.org/10.1016/j.joei.2014.05.003 1743-9671/© 2014 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: W. Lan, et al., Progress in techniques of biomass conversion into syngas, Journal of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.05.003
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2.1. Fixed bed gasifier 2.1.1. Updraft gasifier In the updraft gasifier, the material is fed from the top of the reactor and the air is introduced from the bottom of the reactor through a grate. Material and air move in opposite direction in the gasifier [3]. The “combustion” zone is essentially in the lowest part of the gasifier. In this zone, biomass is combusted and char is formed. During the process of combusting, the temperature of the lower part of the gasifier raises to about 750 C. The hot gases pyrolyze the biomass and dry it in further in the upper part of the gasifier. Pyrolysis of biomass results in release of volatiles and formation of a sizeable amount of tar [4]. Some of the tar may be released with the outgoing gases. The humidity of the air used in the gasification plays an important role in controlling the temperature [5]. A schematic of the updraft gasifier is shown in Fig. 1 [5]. 2.1.2. Downdraft gasifier In the downdraft gasifier, the material and air move concurrently from top to bottom of the reactor. Since the exit of the produced gas is close to the combustion zone, tar formed during devolatilization of the biomass is thermally cracked to some extent [5]. Thus, the tar content of the produced gas from the downdraft gasifier is lower than that of the updraft gasifier [6,7]. Downdraft gasifier has been paid more and more attention due to the low tar content in the produced gas. Low tar content gas is always preferred for firing gas engines and turbines [5]. Fig. 2 shows a schematic of the downdraft gasifier [4]. 2.2. Fluidized bed gasifier Fluidized bed gasifier is widely used in recent years [5]. The advantages of these processes are: the temperature distribution in the reactor is uniform, conversion of carbon is high, tar production is low. The fuel type, feed rate, as well as particle size are flexible [8]. In the past two decades, significant experimental and theoretical research has already been carried out in design, development and scale-up of fluidized bed gasifiers [5]. Two kinds of fluidized systems are described.
Fig. 1. The updraft gasifier.
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Fig. 2. The downdraft gasifier.
2.2.1. Bubbling bed gasifier In this kind of system, the bed material rests on a distributor plate. Fluidizing medium is air. Air passes at a velocity about five times faster than that of minimum fluidization velocity through this plate. The temperature in the bed is about 700e900 C [9]. The biomass is pyrolyzed in the hot bed. In this process, char and gases are produced. Then char particles are lifted along with fluidizing air. At the same time gasification reaction occurs in relatively upper part of the bed. Because of contacting with the high temperature bed, the high molecular weight tar compounds are cracked, so that the net tar content of the produced gas is reduced to less than 1e3 g/Nm3 [5]. 2.2.2. Circulating fluidized bed (CFB) Circulating fluidized bed (CFB) is an economic and environmentally acceptable technology for biomass gasification or low-grade burning. In this kind of system, the velocity of the fluidizing air is much higher than the terminal settling velocity of the bed material. The gasifier can be operated at high pressures. The material operated by CFB ranges from 1 to 80 tons [10]. 2.2.3. Entrained flow gasifier In this kind of system, the material (which is pulverized into solid) is fed in the gasifier by pneumatic feeding. The powder is moved by inert gas and injected in a so-called burner in the gasifier. The characterized of this kind of gasifier is that fuel particles dragged along with the gas stream. That is to say: the process needed short residence times, high temperatures and small fuel particles. Furthermore, entrained flow gasifier is often operated under pressure (typically 20e50 bar) and with pure oxygen. Solid fuel and oxygen can be well mixed, Vortex flow patterns are created in the burner. The temperatures in the burner zone can be research 2500 C or even higher. The capacity of the gasifer is hundreds of MW.
3. Syngas production technology Main technologies of syngas production are:
3.1. Syngas production from partial oxidation and steam reforming of biomass pyrolysis oils This method of biomass conversion is called fast pyrolysis, which benefits from years of research and is an industrially technology [11,12]. By rapidly heating biomass in the absence of oxygen, pyrolysis oils (bio-oils) can be formed. Pyrolysis oils contain water and oxygen. They can be pumped or shipped more efficiently and be upgraded at a central facility, thus can be used in an industrial way [13e15]. In addition, pyrolysis represents a chemical step that can be combined with ash removal. In this process, the ash is removed with the solids (char), a byproduct of pyrolysis, thus bio-oils contain much less ash than their original biomass source [16]. Bio-oils vary greatly when different feedstocks and methods were used. They require further upgrading to convert them to usable fuels. Several options can be considered for further upgrading pyrolysis oils to high-quality liquid fuels. Perhaps the most versatile way for producing fuels from pyrolysis oils is gasification to syngas [17]. Typical two-stage biomass gasification process is represented in Fig. 3. Please cite this article in press as: W. Lan, et al., Progress in techniques of biomass conversion into syngas, Journal of the Energy Institute (2014), http://dx.doi.org/10.1016/j.joei.2014.05.003
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Fig. 3. Biomass two-stage gasification.
The two-stage biomass gasification process has advantages, such as: a high pressure pump could be used in stead of the traditional compressor. Pyrolysis oils can be catalytically converted to syngas. Noble metal catalysts could achieve high conversion of bio-oil to H2 and CO. Fast deactivation of catalyst is the disadvantage of this process [18]. 3.2. Syngas production from co-gasification of biomass and coal Syngas production from co-gasification of biomass and coal is considered as a promising technology. The produced syngas is hydrogenrich and contains CH4, which can be used for power plants. This process has several advantages. During the co-gasification process the volatiles readily decompose and form free radicals which react with the organic matter of the coal, thus the conversion rate increases [19]. This process can also reduce CO2, SO2 and NOx emissions. In addition, during the devolatilization of the biomass, the hydrogen-rich molecules form. At the same time, hydrogen may react with the free radicals produced from coal at the moment of their formation and €stro €m reported that the alkali metals in biomass prevent recombination reactions that could produce less reactive secondary tar species. Sjo can act as catalyst for promoting the conversion of the coal char [19]. The disadvantages of this process is that the technical challenges in developing replicated turnkey co-gasificaion plants are considerable, since there is no turnkey supplier of co-gasification technology with associated materials handling [20]. 3.3. Syngas production from coupled steam hydrogasification of biomass and reforming of methane In the process of steam hydrogasification, carbon-containing solid feedstock is converted into methane rich gas in the presence of steam. It is a thermo-chemical process [21]. A lot of research works have demonstrated that the enhanced methane production from steam hydrogasification can be combined with steam reforming to generate syngas with a flexible H2/CO ratio from a number of carbonaceous feedstocks [22e25]. Because the steam methane reformer is a catalytic reactor, contaminants such as chlorine, sulfur and other trace metals in the produced gas stream should be removed. By using a warm-gas cleanup unit, this task can be accomplished. The clean gas stream contained large amount of methane and steam, which is converted into a clean syngas in the steam methane reformer. The syngas from the steam methane reformer can be used for power generation and also used in a fuel synthesis process to produce synthetic hydrocarbon fuels [26]. The H2/CO ratio of the syngas produced from the steam methane reformer is higher than that is required for FischereTropsch synthesis. The excess H2 from the syngas is separated and fed back into the steam hydrogasification reactor. The advantage of this process is that by altering the H2O/C and H2/C ratios of the steam hydrogasification reactor, the final H2/CO ratio of the syngas can be adjusted in a simple way. Fig. 4 shows a schematic diagram of the process [27]. 3.4. Syngas production from gasification of biomass-derived char Biomass pyrolysis can generate fuel products consisting of approximately 70 wt% liquid, 15 wt% char, and 15 wt% gas [28]. In this process, the char is a byproduct and needs to be utilized. A lot of research works have been reported on the gasification of biomass-derived char using several agents such as air, steam and carbon dioxide [29e31]. Gasification using biomass-derived char as feedstock to produce syngas is more preferable than the raw biomass. Syngas obtained from direct gasification of raw biomass is usually rich in tar. As for biomass-derived char gasification, syngas products with lower content of tar can be obtained, since the volatile matter content has been removed during the
Fig. 4. Schematic diagram of the coupled steam hydrogasification and reforming process.
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pyrolysis of biomass [32]. Furthermore, biomass-derived char contains a higher content of fixed carbon, so the productivity can be increased during the process [33]. Gasification of biomass-derived char for syngas production has many advantages and can be widely used [33]. On one hand, the process is rapid and the operating condition is not complicated [34]. On the other hand, the syngas contains a higher content of H2 and CO could be used as fuel gas according to the gas composition [31]. 3.5. Syngas production from pure biomass In the process, gasification can be operated with or without a catalyst in a fixed bed, movable bed or fluidized bed, with the fluidized bed normally yielding better performance [35]. In order to maximize syngas production with minimum tar formation, the reaction temperature should be increased, volatile residence time should be extended [36]. Oxygen or steam as gasifying medium is the most suitable gasifying medium for syngas production [37]. Meanwhile, the advanced technologies including catalytic gasification and thermal plasma gasification can be used for enhancing syngas production [37]. Because impurities from the gasitication process can cause operational problems by eroding and blocking pipelines and may deactivate catalysts [38], so the gas needed to be clean. The first step of gas cleaning is to remove tars. Three methods are usually used to reduce tar content: thermal cracking, catalytic cracking and scrubbing [39]. The method by using catalytic cracking is effect. After the tars are removed, two methods can be applied to cope with other impurities: wet gas cleaning and dry gas cleaning [40]. After the cleaning process, syngas can be collected or further applied. 4. Overview and conclusions Syngas production is an effective way to utilize biomass resources. Technologies of syngas production from biomass have been discussed in this paper. The primary technology for syngas production is steam hydrogasification and reforming. In this process, methane and steam are catalytically and endothermically converted to syngas. Partial oxidation is an alternative approach, the exothermic, non-catalytic reaction of methane and oxygen to produce syngas. For the technology of co-gasification of biomass and coal, the conversion rate can increase in the process, but there is no turnkey supplier of co-gasification technology with associated materials handling. Syngas production technologies are applicable under different conditions. In principle, syngas can be produced from any hydrocarbon feedstock. The compositions of syngas from different technology are different. In particular, syngas have a higher H2/CO ratio by using these two technologies: the steam hydrogasification and reforming technology and biomass-derived char technology. This represents a distinct advantage for them in hydrogen-production applications. There also have many factors influencing syngas production capacity. With the improvement of gasification technology, the capacity of syngas production from biomass can be significantly improved in the future. Acknowledgments This paper is financially supported by Henan University of Science and Technology Talent Introduction Fund Projects (No. 09001759) and Education Department of Henan Province Science and Technology Key Project (No. 14B470020) and Henan University of Science and Technology Research Fund Project (No. 2014QN004). References [1] Z.Y. Luo, J.S. Zhou, S.R Wang, Biomass utilization as a clean energy in Zhejiang University, in: International Conference on Power Engineering, Hangzhou, China, 23e27 October, 2007, pp. 14e18. [2] G. Fischer, S. Prieler, H.V. Velthuizen, Biomass potentials of miscanthus, willow and poplar: results and policy implications for Eastern Europe, Northern and Central Asia, Biomass Bioenergy 28 (2) (2005) 119e132. [3] M. Kaltschmitt, A.V. 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