《可再生能源》教学资源（阅读资料）04-Tony-Review of fast pyrolysis of biomass and product upgrading

BIOMASS AND BIOENERGY 38 (2012)68-94 Available at www.sciencedirect.com BIOMASS BIOENERGY ScienceDirect ELSEVIER http://www.elsevier.com/locate/biombioe Review of fast pyrolysis of biomass and product upgrading A.V.Bridgwater* Aston University Bioenergy Research Group,Aston Triangle,Birmingham B4 7ET,UK ARTICLE INFO ABSTRACT Article history: This paper provides an updated review on fast pyrolysis of biomass for production of Received 27 August 2010 a liquid usually referred to as bio-oil.The technology of fast pyrolysis is described including Received in revised form the major reaction systems.The primary liquid product is characterised by reference to the 27 January 2011 many properties that impact on its use.These properties have caused increasingly Accepted 28 January 2011 extensive research to be undertaken to address properties that need modification and this Available online 3 March 2011 area is reviewed in terms of physical,catalytic and chemical upgrading.Of particular note is the increasing diversity of methods and catalysts and particularly the complexity and Keywords: sophistication of multi-functional catalyst systems.It is also important to see more Fast pyrolysis companies involved in this technology area and increased take-up of evolving upgrading Biomass processes. Bio-oil 2011 Elsevier Ltd.All rights reserved. Catalyst 1. Introduction technology with applications in most industrialised and developing countries and development is concentrated on Biomass fuels and residues can be converted to more valuable resolving environmental problems [2].Gasification has been energy forms via a number of processes including thermal, practiced for many years and while there are many examples biological,and mechanical or physical processes.While bio- of demonstration and pre-commercial activities [3,4]there are logical processing is usually very selective and produces a small still surprisingly few successful operational units.This review number of discrete products in high yield using biological focuses on the emerging advanced technology of fast pyrol- catalysts,thermal conversion often gives multiple and often ysis both as an integrated process for production of a liquid complex products,in very short reaction times with inorganic fuel that can be used directly and as an intermediate pre- catalysts often used to improve the product quality or spec- treatment step to convert solid biomass into a higher energy trum.Pyrolysis has been applied for thousands of years for content transportable liquid for subsequent processing for charcoal production but it is only on the last 30 years that fast heat,power,biofuels,and chemicals.This technology is pyrolysis at moderate temperatures of around 500C and very widely expected to offer a considerable contribution in the short reaction times of up to 2 s has become of considerable short term in terms of versatility,improved efficiency and interest.This is because the process directly gives high yields of environmental acceptability. liquids of up to 75 wt%which can be used directly in a variety of applications [1]or used as an efficient energy carrier. Fig.1 summarises the markets for the products from the Fast pyrolysis three main thermal processes available for converting bio- mass to a more useful energy form-pyrolysis,gasification and Pyrolysis is thermal decomposition occurring in the absence combustion.Combustion is a well-established commercial of oxygen.Lower process temperatures and longer vapour ·Tel.:+441212043381;fax:+441212043680 E-mail address:a.v.bridgwater@aston.ac.uk. 0961-9534/$-see front matter@2011 Elsevier Ltd.All rights reserved. doi10.1016/j.biombioe.2011.01.048

Review of fast pyrolysis of biomass and product upgrading A.V. Bridgwater* Aston University Bioenergy Research Group, Aston Triangle, Birmingham B4 7ET, UK article info Article history: Received 27 August 2010 Received in revised form 27 January 2011 Accepted 28 January 2011 Available online 3 March 2011 Keywords: Fast pyrolysis Biomass Bio-oil Catalyst abstract This paper provides an updated review on fast pyrolysis of biomass for production of a liquid usually referred to as bio-oil. The technology of fast pyrolysis is described including the major reaction systems. The primary liquid product is characterised by reference to the many properties that impact on its use. These properties have caused increasingly extensive research to be undertaken to address properties that need modification and this area is reviewed in terms of physical, catalytic and chemical upgrading. Of particular note is the increasing diversity of methods and catalysts and particularly the complexity and sophistication of multi-functional catalyst systems. It is also important to see more companies involved in this technology area and increased take-up of evolving upgrading processes. ª 2011 Elsevier Ltd. All rights reserved. 1. Introduction Biomass fuels and residues can be converted to more valuable energy forms via a number of processes including thermal, biological, and mechanical or physical processes. While bio￾logical processing is usually very selective and produces a small number of discrete products in high yield using biological catalysts, thermal conversion often gives multiple and often complex products, in very short reaction times with inorganic catalysts often used to improve the product quality or spec￾trum. Pyrolysis has been applied for thousands of years for charcoal production but it is only on the last 30 years that fast pyrolysis at moderate temperatures of around 500
C and very short reaction times of up to 2 s has become of considerable interest. This is because the process directly gives high yields of liquids of up to 75 wt.% which can be used directly in a variety of applications [1] or used as an efficient energy carrier. Fig. 1 summarises the markets for the products from the three main thermal processes available for converting bio￾mass to a more useful energy form - pyrolysis, gasification and combustion. Combustion is a well-established commercial technology with applications in most industrialised and developing countries and development is concentrated on resolving environmental problems [2]. Gasification has been practiced for many years and while there are many examples of demonstration and pre-commercial activities [3,4] there are still surprisingly few successful operational units. This review focuses on the emerging advanced technology of fast pyrol￾ysis both as an integrated process for production of a liquid fuel that can be used directly and as an intermediate pre￾treatment step to convert solid biomass into a higher energy content transportable liquid for subsequent processing for heat, power, biofuels, and chemicals. This technology is widely expected to offer a considerable contribution in the short term in terms of versatility, improved efficiency and environmental acceptability. 2. Fast pyrolysis Pyrolysis is thermal decomposition occurring in the absence of oxygen. Lower process temperatures and longer vapour * Tel.: þ44 121 204 3381; fax: þ44 121 204 3680. E-mail address: a.v.bridgwater@aston.ac.uk. Available at www.sciencedirect.com http://www.elsevier.com/locate/biombioe biomass and bioenergy 38 (2012) 68 e9 4 0961-9534/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2011.01.048

BIOMASS AND BIOENERGY 38 (2012)68-94 69 Primary Conversion Conversion product Market Char Storage Charcoal Pyrolysis Bio-oil Storage Biofuels chemicals Fuel gas Turbine Gasification Engine Electricity CHP Combustion Heat Boiler Heat Fig.1-Products from thermal biomass conversion. residence times favour the production of charcoal.High Short hot vapour residence times of typically less than 2s to temperatures and longer residence times increase biomass minimise secondary reactions, conversion to gas,and moderate temperatures and short Rapid removal of product char to minimise cracking of vapour residence time are optimum for producing liquids. vapours, Three products are always produced,but the proportions can Rapid cooling of the pyrolysis vapours to give the bio-oil be varied over a wide range by adjustment of the process product parameters.Table 1 and Fig.2 indicate the product distribu- tion obtained from different modes of pyrolysis,showing the As fast pyrolysis for liquids occurs in a few seconds or less, considerable flexibility achievable by changing process heat and mass transfer processes and phase transition conditions.Fast pyrolysis for liquids production is currently of phenomena,as well as chemical reaction kinetics,play particular interest as the liquid can be stored and transported, important roles.The critical issue is to bring the reacting and used for energy,chemicals or as an energy carrier. biomass particles to the optimum process temperature and minimise their exposure to the lower temperatures that 2.1. Principles favour formation of charcoal.One way this objective can be achieved is by using small particles,for example in the flui- In fast pyrolysis,biomass decomposes very quickly to dised bed processes that are described later.Another possi- generate mostly vapours and aerosols and some charcoal and bility is to transfer heat very fast only to the particle surface gas.After cooling and condensation,a dark brown homoge- that contacts the heat source which is used in ablative nous mobile liquid is formed which has a heating value about processes that are described later. half that of conventional fuel oil.A high yield of liquid is The main product,bio-oil,is obtained in yields of up to obtained with most biomass feeds low in ash.The essential 75 wt.%on a dry-feed basis,together with by-product char and features of a fast pyrolysis process for producing liquids are: gas which can be used within the process to provide the process heat requirements so there are no waste streams Very high heating rates and very high heat transfer rates at other than flue gas and ash.Liquid yield depends on biomass the biomass particle reaction interface usually require type,temperature,hot vapour residence time,char separa- a finely ground biomass feed of typically less than 3 mm as tion,and biomass ash content,the last two having a catalytic biomass generally has a low thermal conductivity, effect on vapour cracking. Carefully controlled pyrolysis reaction temperature of A fast pyrolysis process includes drying the feed to typi- around 500C to maximise the liquid yield for most biomass, cally less than 10%water in order to minimise the water in the Table 1-Typical product weight yields(dry wood basis)obtained by different modes of pyrolysis of wood. Mode Conditions Liquid Solid Gas Fast ~500C,short hot vapour residence time ~1s 75% 12%char 13% Intermediate ~500C,hot vapour residence time 10-30s 50%in 2 phases 25%char 25% Carbonisation (slow) ~400C,long vapour residence hours-days 30% 35%char 35% Gasification -750-900°C 5% 10%char 85% Torrefaction(slow) ~290C,solids residence time 10-60 min 0%unless condensed,then up to 5% 80%solid 20%

residence times favour the production of charcoal. High temperatures and longer residence times increase biomass conversion to gas, and moderate temperatures and short vapour residence time are optimum for producing liquids. Three products are always produced, but the proportions can be varied over a wide range by adjustment of the process parameters. Table 1 and Fig. 2 indicate the product distribu￾tion obtained from different modes of pyrolysis, showing the considerable flexibility achievable by changing process conditions. Fast pyrolysis for liquids production is currently of particular interest as the liquid can be stored and transported, and used for energy, chemicals or as an energy carrier. 2.1. Principles In fast pyrolysis, biomass decomposes very quickly to generate mostly vapours and aerosols and some charcoal and gas. After cooling and condensation, a dark brown homoge￾nous mobile liquid is formed which has a heating value about half that of conventional fuel oil. A high yield of liquid is obtained with most biomass feeds low in ash. The essential features of a fast pyrolysis process for producing liquids are: Very high heating rates and very high heat transfer rates at the biomass particle reaction interface usually require a finely ground biomass feed of typically less than 3 mm as biomass generally has a low thermal conductivity, Carefully controlled pyrolysis reaction temperature of around 500
C to maximise the liquid yield for most biomass, Short hot vapour residence times of typically less than 2 s to minimise secondary reactions, Rapid removal of product char to minimise cracking of vapours, Rapid cooling of the pyrolysis vapours to give the bio-oil product. As fast pyrolysis for liquids occurs in a few seconds or less, heat and mass transfer processes and phase transition phenomena, as well as chemical reaction kinetics, play important roles. The critical issue is to bring the reacting biomass particles to the optimum process temperature and minimise their exposure to the lower temperatures that favour formation of charcoal. One way this objective can be achieved is by using small particles, for example in the flui￾dised bed processes that are described later. Another possi￾bility is to transfer heat very fast only to the particle surface that contacts the heat source which is used in ablative processes that are described later. The main product, bio-oil, is obtained in yields of up to 75 wt.% on a dry-feed basis, together with by-product char and gas which can be used within the process to provide the process heat requirements so there are no waste streams other than flue gas and ash. Liquid yield depends on biomass type, temperature, hot vapour residence time, char separa￾tion, and biomass ash content, the last two having a catalytic effect on vapour cracking. A fast pyrolysis process includes drying the feed to typi￾cally less than 10% water in order to minimise the water in the P mir yra tcudorp noisrevnoC noisrevnoC tekraM ilP sleufoiB rahC egarotS laocrahC enibruT Ga noitacifis P o y ls r y is sagleuF lio-oiB egarotS sleufoiB & acimehc sl Ga noitacifis oitsubmoC n eH at elE c yticirt C& HP relioB enignE oitsubmoC n eH at relioB taeH Fig. 1 e Products from thermal biomass conversion. Table 1 e Typical product weight yields (dry wood basis) obtained by different modes of pyrolysis of wood. Mode Conditions Liquid Solid Gas Fast w500
C, short hot vapour residence time w 1 s 75% 12% char 13% Intermediate w500
C, hot vapour residence time w 10e30 s 50% in 2 phases 25% char 25% Carbonisation (slow) w400
C, long vapour residence hours / days 30% 35% char 35% Gasification w750e900
C 5% 10% char 85% Torrefaction (slow) w290
C, solids residence time w 10e60 min 0% unless condensed, then up to 5% 80% solid 20% biomass and bioenergy 38 (2012) 68 e9 4 69

70 BIOMASS AND BIOENERGY 38 (2012)68-94 100% ▣Organics 90% 80% Water 70% ▣Char 60% ■Gas 50% 40% 30% 20% 10% 0% Fast Intermediate Carbonisation Gasification Slow Slow-Torrefaction Fig.2-Product spectrum from pyrolysis. product liquid oil,grinding the feed to give sufficiently small product collection,storage and,when relevant,upgrading. particles to ensure rapid reaction,fast pyrolysis,rapid and Several comprehensive reviews of fast pyrolysis processes for efficient separation of solids(char),and rapid quenching and liquids production are available such as [5-9). collection of the liquid product (often referred to as bio-oil). Table 2 lists most of the known recent and current activi- Virtually any form of biomass can be considered for fast ties in fast pyrolysis arranged by reactor type and maximum pyrolysis.While most work has been carried out on wood known throughput.There has been considerable growth and because of its consistency and comparability between tests, expansion of activities over the last few years with more over 100 different biomass types have been tested by many innovation in the types of reactor explored by academic laboratories,ranging from agricultural wastes such as straw, institutions.It is disappointing to see so much re-invention olive pits and nut shells to energy crops such as miscanthus and poor appreciation of the underlying fundamental and sorghum,forestry wastes such as bark and solid wastes requirements of fast pyrolysis as well as a reluctance to carry such as sewage sludge and leather wastes. out basic reviews of past research publications. In all cases,a commercial process comprises three main There are increasing activities on fixed bed and related stages from feed reception to delivery of one or more useful systems that are unlikely to give high liquid yields but are likely products: to give phase separated liquids.Phase separated liquid prod- ucts may be desirable in some applications where fraction- Feed reception,storage,handling,preparation and pre- ation is required,but it would seem preferable to control such treatment; separation rather than rely on poor design and process control. Conversion of solid biomass by fast pyrolysis to a more usable form of energy in liquid form which is known as bio- 2.2.1.Bubbling fluid beds oil; Bubbling fluid beds have the advantages of a well understood Conversion of this primary liquid product by processing, technology that is simple in construction and operation,good refining or clean-up to a marketable end-product such as temperature control and very efficient heat transfer to electricity,heat,biofuels and/or chemicals. biomass particles arising from the high solids density.Fig.3 shows a typical configuration using electrostatic precipita- tors for coalescence and collection of what are referred to as 2.2. Fast pyrolysis reactors aerosols.These are incompletely depolymerised lignin frag- ments which seem to exist as a liquid with a substantial At the heart of a fast pyrolysis process is the reactor.Although molecular weight.Evidence of their liquid basis is found in the it probably represents only about 10-15%of the total capital accumulation of liquid in the ESP which runs down the plates cost of an integrated system,most research and development to accumulate in the bio-oil product.Demisters for agglom- has focused on developing and testing different reactor eration or coalescence of the aerosols have been used but configurations on a variety of feedstocks,although increasing published experience suggest that this is less effective. attention is now being paid to control and improvement of liquid quality and improvement of liquid collection systems 2.2.1.1.Heating.Heating can be achieved in a variety of ways The rest of the fast pyrolysis process consists of biomass and scaling is well understood.However,heat transfer to bed reception,storage and handling,biomass drying and grinding, at large scales of operation has to be considered carefully

product liquid oil, grinding the feed to give sufficiently small particles to ensure rapid reaction, fast pyrolysis, rapid and efficient separation of solids (char), and rapid quenching and collection of the liquid product (often referred to as bio-oil). Virtually any form of biomass can be considered for fast pyrolysis. While most work has been carried out on wood because of its consistency and comparability between tests, over 100 different biomass types have been tested by many laboratories, ranging from agricultural wastes such as straw, olive pits and nut shells to energy crops such as miscanthus and sorghum, forestry wastes such as bark and solid wastes such as sewage sludge and leather wastes. In all cases, a commercial process comprises three main stages from feed reception to delivery of one or more useful products: Feed reception, storage, handling, preparation and pre￾treatment; Conversion of solid biomass by fast pyrolysis to a more usable form of energy in liquid form which is known as bio￾oil; Conversion of this primary liquid product by processing, refining or clean-up to a marketable end-product such as electricity, heat, biofuels and/or chemicals. 2.2. Fast pyrolysis reactors At the heart of a fast pyrolysis process is the reactor. Although it probably represents only about 10e15% of the total capital cost of an integrated system, most research and development has focused on developing and testing different reactor configurations on a variety of feedstocks, although increasing attention is now being paid to control and improvement of liquid quality and improvement of liquid collection systems. The rest of the fast pyrolysis process consists of biomass reception, storage and handling, biomass drying and grinding, product collection, storage and, when relevant, upgrading. Several comprehensive reviews of fast pyrolysis processes for liquids production are available such as [5e9]. Table 2 lists most of the known recent and current activi￾ties in fast pyrolysis arranged by reactor type and maximum known throughput. There has been considerable growth and expansion of activities over the last few years with more innovation in the types of reactor explored by academic institutions. It is disappointing to see so much re-invention and poor appreciation of the underlying fundamental requirements of fast pyrolysis as well as a reluctance to carry out basic reviews of past research publications. There are increasing activities on fixed bed and related systems that are unlikely to give high liquid yields but are likely to give phase separated liquids. Phase separated liquid prod￾ucts may be desirable in some applications where fraction￾ation is required, but it would seem preferable to control such separation rather than rely on poor design and process control. 2.2.1. Bubbling fluid beds Bubbling fluid beds have the advantages of a well understood technology that is simple in construction and operation, good temperature control and very efficient heat transfer to biomass particles arising from the high solids density. Fig. 3 shows a typical configuration using electrostatic precipita￾tors for coalescence and collection of what are referred to as aerosols. These are incompletely depolymerised lignin frag￾ments which seem to exist as a liquid with a substantial molecular weight. Evidence of their liquid basis is found in the accumulation of liquid in the ESP which runs down the plates to accumulate in the bio-oil product. Demisters for agglom￾eration or coalescence of the aerosols have been used but published experience suggest that this is less effective. 2.2.1.1. Heating. Heating can be achieved in a variety of ways and scaling is well understood. However, heat transfer to bed at large scales of operation has to be considered carefully %08 %09 1 %00 scinagrO retaW 06 % %07 %08 rahC saG %03 %04 50% 0% 10% %02 0% Fig. 2 e Product spectrum from pyrolysis. 70 biomass and bioenergy 38 (2012) 68 e9 4

BIOMASS AND BIOENERGY 38 (2012)68-94 71 Table 2-Summary of fast pyrolysis reaction systems for liquids,recently and currently operational. Fast pyrolysis Industrial Units Max Research Max built size kg/h size kg/h Fluid bed Agritherm,Canada 200 Adelaide U,Australia Biomass Engineering Ltd,UK 200 Aston U.,UK Dynamotive,Canada 8000 Cirad,France RTI,Canada 20 Curtin U,Australia 2 ECN,NL East China U.Science and nk Technology,Shanghai,China Gent U.,Belgium 03 Guangzou Inst,China 10 Harbin Institute of Technology nk lowa State U.,USA 6 Monash U.Australia NREL,USA 10 PNNL,USA 1 Shandong U.Technology nk Shanghai JiaoTong U, Shenyang U.,China South East U.,China Texas A&M U.,USA 积 TNO,Netherlands 10 U.Basque Country,Spain nk U.Campinas,Brazil 100 U.Maine,USA 0.1 U.Melbourne,Australia 01 U.Naples,Italy 1 U.Science and Technology 650 of China U.Seoul,Korea nk U.Twente,Netherlands U.Western Ontario,Canada U.Zaragoza,Spain k USDA,ARS,ERRC,USA Virginia Tech.U.,USA 0.1 VTT,Finland VTI,Germany 6 Zhejiang U.,China Zhengzhou U.,China 2 Spouted fluid bed Ikerlan,Spain 10 Anhui U.of Science Technology,China U.Basque Country,Spain nk Transported Ensyn,Canada bed CFB Metso/UPM,Finland 81 4000 CPERI,Greece 400 Guangzhou Inst.Energy nk Conversion,China U.Birmingham,UK nk U.Nottingham,UK nk VTT,Finland 20 Rotating cone BTG,Netherlands 4 2000 BTG,Netherlands 10 Integral catalytic BioEcon,Netherlands nk nk Battelle Columbus,USA 1 pyrolysis Kior USA PNNL,USA 1 Technical U.of Munich nk U.Massachusetts-Amhurst,USA nk Virginia Tech.U.,USA 3? Vortex TNO,Netherlands 30 Centrifuge reactor Technical U.Denmark nk Ablative PyTec,Germany 250 Aston U.,UK 20 Institute of Engineering 15 Thermophysics,Ukraine Latvian State Institute,Latvia 0.15 Technical U.Denmark 1.5 Augur or Screw Abritech,Canada 2083 Auburn U.USA 1.0 Lurgi LR,Germany 500 KIT(FZK),Germany 500 Renewable Oil Intl,USA 200 Mississippi State U.,USA 2 Michigan State U.USA 0.5 Texas A&M U.,USA 30 (continued on next page)

Table 2 e Summary of fast pyrolysis reaction systems for liquids, recently and currently operational. Fast pyrolysis Industrial Units built Max size kg/h Research Max size kg/h Fluid bed Agritherm, Canada 2 200 Adelaide U, Australia 1 Biomass Engineering Ltd, UK 1 200 Aston U., UK 5 Dynamotive, Canada 4 8000 Cirad, France 2 RTI, Canada 5 20 Curtin U, Australia 2 ECN, NL 1 East China U. Science and Technology, Shanghai, China nk Gent U., Belgium 0.3 Guangzou Inst, China 10 Harbin Institute of Technology nk Iowa State U., USA 6 Monash U. Australia 1 NREL, USA 10 PNNL, USA 1 Shandong U. Technology nk Shanghai JiaoTong U, 1 Shenyang U., China 1 South East U., China 1 Texas A&M U., USA 42 TNO, Netherlands 10 U. Basque Country, Spain nk U. Campinas, Brazil 100 U. Maine, USA 0.1 U. Melbourne, Australia 0.1 U. Naples, Italy 1 U. Science and Technology of China 650 U. Seoul, Korea nk U. Twente, Netherlands 1 U. Western Ontario, Canada nk U. Zaragoza, Spain nk USDA, ARS, ERRC, USA 1 Virginia Tech. U., USA 0.1 VTT, Finland 1 vTI, Germany 6 Zhejiang U., China 3 Zhengzhou U., China 2 Spouted fluid bed Ikerlan, Spain 1 10 Anhui U. of Science & Technology, China 5 U. Basque Country, Spain nk Transported bed & CFB Ensyn, Canada 8 4000 CPERI, Greece 1 Metso/UPM, Finland 1 400 Guangzhou Inst. Energy Conversion, China nk U. Birmingham, UK nk U. Nottingham, UK nk VTT, Finland 20 Rotating cone BTG, Netherlands 4 2000 BTG, Netherlands 10 Integral catalytic pyrolysis BioEcon, Netherlands þ Kior USA nk nk Battelle Columbus, USA 1 PNNL, USA 1 Technical U. of Munich nk U. MassachusettseAmhurst, USA nk Virginia Tech. U., USA 3? Vortex TNO, Netherlands 30 Centrifuge reactor Technical U. Denmark nk Ablative PyTec, Germany 2 250 Aston U., UK 20 Institute of Engineering Thermophysics, Ukraine 15 Latvian State Institute, Latvia 0.15 Technical U. Denmark 1.5 Augur or Screw Abritech, Canada 4 2083 Auburn U. USA 1.0 Lurgi LR, Germany 1 500 KIT (FZK), Germany 500 Renewable Oil Intl, USA 4 200 Mississippi State U., USA 2 Michigan State U. USA 0.5 Texas A&M U., USA 30 (continued on next page) biomass and bioenergy 38 (2012) 68 e9 4 71

72 BIOMASS AND BIOENERGY 38 (2012)68-94 Table 2 (continued) Fast pyrolysis Industrial Units Max Research Max built size kg/h size kg/h Radiative-Convective CNRS-Nancy U.,France nk Entrained flow, Dalian U.of Technology,China nk Institute for Wood Chemistry,Latvia nk Shandong University of Technology 0.05 Microwave Carbonscape nk nk Chinese Academy of Sciences, nk New Zealand UK Dalian 116023,P.R.China Bioenergy 2020 gmbh, 1 nk National Inst.Advanced Industrial <0.1 Austria Sci.Technol.,Japan Shandong U.China <0.1 Technical U.Vienna,Austria nk U.Malaysia Sarawak <0.1 U.Minnesota,USA 10 U.Mississippi nk U.Nottingham,UK and China nk U.York,UK nk Washington State U.-Tricities,USA <1 Moving bed and Anhui Yineng Bio-energy 3 600 Anadolu University,Turkey nk fixed bed Ltd.,China U.Autonoma de Barcelona,Spain nk U.Science Technology of China -0.5 Ceramic ball Shandong University of 110 downflow Technology,China Unspecified U.Kentucky,USA nk U.Texas,USA nk Technical U.Compiegne,France nk Vacuum Pyrovac,Canada 3500 None known because of the scale-up limitations of different methods of at fast pyrolysis reaction temperatures,rapid and effective heat transfer.Fluid-bed pyrolysers give good and consistent char separation is important.This is usually achieved by performance with high liquid yields of typically 70-75 wt.% ejection and entrainment followed by separation in one or from wood on a dry-feed basis.Small biomass particle sizes of more cyclones so careful design of sand and biomass/char less than 2-3 mm are needed to achieve high biomass heating hydrodynamics is important.The high level of inert gases rates,and the rate of particle heating is usually the rate- arising from the high permanent gas flows required for fluid- limiting step. isation result in very low partial pressures for the condensable vapours and thus care is needed to design and operate efficient 2.2.1.2.Char.Vapour and solid residence time is controlledby heat exchange and liquid collection systems.In addition the the fluidising gas flow rate and is higher for char than for large inert gas flowrates result in relatively large equipment vapours.As char acts as an effective vapour cracking catalyst thus increasing cost. The byproduct char is typically about 15 wt.%of the Prepared Quench GAS products but about 25%of the energy of the biomass feed.It BIOMASS Cyclones cooler export can be used within the process to provide the process heat Dried and sized Gas requirements by combustion or it can be separated and recycle exported,in which case an alternative fuel is required. Depending on the reactor configuration and gas velocities, a large part of the char will be of a comparable size and shape Fluid as the biomass fed.The fresh char is pyrophoric i.e.it spon- bed taneously combusts when exposed to air so careful handling reactor and storage is required.This property deteriorates with time CHAR due to oxidation of active sites on the char surface process heat Electrostatic or export precipitator 2.2.1.3.Background.All the early work on fluid beds was ↓ carried out at the University of Waterloo in Canada,which CHAR BIO-OIL pioneering the science of fast pyrolysis and established a clear lead in this area for many years (e.g.[10-121).Bubbling fluid Recycle gas beds have been selected for further development by several heaterand/or oxidiser companies,including Union Fenosa [13],who built and oper- ated a 200 kg/h pilot unit in Spain based on the University of Fig.3-Bubbling fluid bed reactor with electrostatic Waterloo process which was dismantled some years ago; precipitator. Dynamotive,who operated a 75 kg/h and 400 kg/h pilot unit

because of the scale-up limitations of different methods of heat transfer. Fluid-bed pyrolysers give good and consistent performance with high liquid yields of typically 70e75 wt.% from wood on a dry-feed basis. Small biomass particle sizes of less than 2e3 mm are needed to achieve high biomass heating rates, and the rate of particle heating is usually the rate￾limiting step. 2.2.1.2. Char. Vapour and solid residence time is controlled by the fluidising gas flow rate and is higher for char than for vapours. As char acts as an effective vapour cracking catalyst at fast pyrolysis reaction temperatures, rapid and effective char separation is important. This is usually achieved by ejection and entrainment followed by separation in one or more cyclones so careful design of sand and biomass/char hydrodynamics is important. The high level of inert gases arising from the high permanent gas flows required for fluid￾isation result in very low partial pressures for the condensable vapours and thus care is needed to design and operate efficient heat exchange and liquid collection systems. In addition the large inert gas flowrates result in relatively large equipment thus increasing cost. The byproduct char is typically about 15 wt.% of the products but about 25% of the energy of the biomass feed. It can be used within the process to provide the process heat requirements by combustion or it can be separated and exported, in which case an alternative fuel is required. Depending on the reactor configuration and gas velocities, a large part of the char will be of a comparable size and shape as the biomass fed. The fresh char is pyrophoric i.e. it spon￾taneously combusts when exposed to air so careful handling and storage is required. This property deteriorates with time due to oxidation of active sites on the char surface. 2.2.1.3. Background. All the early work on fluid beds was carried out at the University of Waterloo in Canada, which pioneering the science of fast pyrolysis and established a clear lead in this area for many years (e.g. [10e12]). Bubbling fluid beds have been selected for further development by several companies, including Union Fenosa [13], who built and oper￾ated a 200 kg/h pilot unit in Spain based on the University of Waterloo process which was dismantled some years ago; Dynamotive, who operated a 75 kg/h and 400 kg/h pilot unit Table 2 (continued) Fast pyrolysis Industrial Units built Max size kg/h Research Max size kg/h Radiative-Convective CNRS e Nancy U., France nk Entrained flow, Dalian U. of Technology, China nk Institute for Wood Chemistry, Latvia nk Shandong University of Technology 0.05 Microwave Carbonscape New Zealand & UK nk nk Chinese Academy of Sciences, Dalian 116023, P. R. China nk Bioenergy 2020 þ gmbh, Austria 1 nk National Inst. Advanced Industrial Sci. & Technol., Japan <0.1 Shandong U. China <0.1 Technical U. Vienna, Austria nk U. Malaysia Sarawak <0.1 U. Minnesota, USA 10 U. Mississippi nk U. Nottingham, UK and China nk U. York, UK nk Washington State U.-Tricities, USA <1 Moving bed and fixed bed Anhui Yineng Bio-energy Ltd., China 3 600 Anadolu University, Turkey nk U. Auto`noma de Barcelona, Spain nk U. Science & Technology of China w0.5 Ceramic ball downflow Shandong University of Technology, China 110 Unspecified U. Kentucky, USA nk U. Texas, USA nk Technical U. Compiegne, France nk Vacuum Pyrovac, Canada 1 3500 None known Fig. 3 e Bubbling fluid bed reactor with electrostatic precipitator. 72 biomass and bioenergy 38 (2012) 68 e9 4

BIOMASS AND BIOENERGY 38 (2012)68-94 73 [14]in Canada based on an RTI design and have subsequently solids flow match the process and feed requirements.Heat built a 100 t/d and a 200 t/d plant in Canada;Wellman,who transfer is a mixture of conduction and convection in the riser. built a 250 kg/h unit [15]in the UK which has not operated; One of the unproven areas is scale up and heat transfer at high Biomass Engineering Ltd in the UK who are finalising throughputs. construction of a 250 kg/h pilot unit and Fortum who built and extensively tested a 500 kg/h plant in Finland which has now 2.2.2.2.Char.All the char is burned in the secondary reactor been dismantled [16].More recent activities include Ikerlan to re-heat the circulating sand,so there is no char available for who are developing a spouted fluid bed in Spain [17].Metso export unless an alternative heating source is used.If sepa who are working with UPM and VTT in Finland who have rated the char would be a fine powder. constructed and are operating a 4 MWth unit in Tampere Finland [18]and Anhui University of Science and Technology 2.2.2.3.Background.Larger scale examples include the in China who are overseeing the construction of three 650kg/h ENEL plant in Italy built by Ensyn [20,21]which has not demonstration plants in China up to 600 kg/h [19].Many operated for some years,several Ensyn units in the USA at Red research units have also been built at universities and Arrow in Wisconsin for production of food flavourings up to research institutions around the world,as they are relatively 1700 kg/h,and the Ensyn units at their R&D centre in Renfrew easy to construct and operate and give good results,and many Canadaup to 2000kg/h with plans for unitsup to 1000td-1[22]. are listed in Table 2. 2.2.3.Rotating cone 2.2.2.Circulating fluid beds and transported beds The rotating cone reactor,invented at the University of Circulating fluid bed (CFB)and transported bed reactor Twente [23]and developed by BTG [241,is a relatively recent systems have many of the features of bubbling beds described development and effectively operates as a transported bed above,except that the residence time of the char is almost the reactor,but with transport effected by centrifugal forces in same as for vapours and gas,and the char is more attrited due a rotating cone rather than gas.A 250 kg/h unit is now oper- to the higher gas velocities.This can lead to higher char ational,and a scaled up version of 50t/d was commissioned in contents in the collected bio-oil unless more extensive char Malaysia in mid 2005.A 120 t/d plant is at an advanced plan- removal is included.A typical layout is shown in Fig.4.An ning stage [25].Fig.5 shows an early prototype on the left and added advantage is that CFBs are potentially suitable for larger its role in an integrated fast pyrolysis process on the right.The throughputs even though the hydrodynamics are more key features are: complex as this technology is widely used at very high throughputs in the petroleum and petrochemical industry. centrifugation(at~10 Hz)drives hot sand and biomass up a rotating heated cone; 2.2.2.1.Heating.Heat supply is usually from recirculation of vapours are collected and processed conventionally; heated sand from a secondary char combustor,which can be char and sand drop into a fluid bed surrounding the cone, either a bubbling or circulating fluid bed.In this respect the process is similar to a twin fluid-bed gasifier except that the whence they are lifted to a separate fluid bed combustor where char is burned to heat the sand,which is then drop reactor(pyrolyser)temperature is much lower and the closely ped back into the rotating cone; integrated char combustion in a second reactor requires char is burned in a secondary bubbling fluid bed combustor. careful control to ensure that the temperature,heat flux and The hot sand is recirculated to the pyrolyser; carrier gas requirements in the pyrolysis reactor are much Cyclones GAS less than for fluid bed and transported bed systems; Forexport however,gas is needed for char burn-off and sand transport; Quench Pyrolyser a more complex integrated operation of three subsystems is cooler required:rotating cone pyrolyser,riser for sand recycling, ESP and bubbling bed char combustor; Prepared Flue BIOMASS gas liquid yields of 60-70%on dry feed are typically obtained. Dried and sized As with CFB and transported beds all the char is burned so is Sand+ not a byproduct,although the char could in principle be sepa- Char rated and recovered if an alterative heatingsource is provided. 2.2.4.Ablative pyrolysis 77 Hot BIO-OIL Ablative pyrolysis is substantially different in concept sand compared with other methods of fast pyrolysis.In all the other methods,the rate of reaction is limited by the rate of heat Combustor transfer through the biomass particles,which is why small AIr particles are required.The mode of reaction in ablative Ash pyrolysis is like melting butter in a frying pan-the rate of Gas recycle melting can be significantly enhanced by pressing the butter down and moving it over the heated pan surface.In ablative Fig.4-Circulating fluid bed reactor. pyrolysis,heat is transferred from the hot reactor wall to

[14] in Canada based on an RTI design and have subsequently built a 100 t/d and a 200 t/d plant in Canada; Wellman, who built a 250 kg/h unit [15] in the UK which has not operated; Biomass Engineering Ltd in the UK who are finalising construction of a 250 kg/h pilot unit and Fortum who built and extensively tested a 500 kg/h plant in Finland which has now been dismantled [16]. More recent activities include Ikerlan who are developing a spouted fluid bed in Spain [17], Metso who are working with UPM and VTT in Finland who have constructed and are operating a 4 MWth unit in Tampere Finland [18] and Anhui University of Science and Technology in China who are overseeing the construction of three demonstration plants in China up to 600 kg/h [19]. Many research units have also been built at universities and research institutions around the world, as they are relatively easy to construct and operate and give good results, and many are listed in Table 2. 2.2.2. Circulating fluid beds and transported beds Circulating fluid bed (CFB) and transported bed reactor systems have many of the features of bubbling beds described above, except that the residence time of the char is almost the same as for vapours and gas, and the char is more attrited due to the higher gas velocities. This can lead to higher char contents in the collected bio-oil unless more extensive char removal is included. A typical layout is shown in Fig. 4. An added advantage is that CFBs are potentially suitable for larger throughputs even though the hydrodynamics are more complex as this technology is widely used at very high throughputs in the petroleum and petrochemical industry. 2.2.2.1. Heating. Heat supply is usually from recirculation of heated sand from a secondary char combustor, which can be either a bubbling or circulating fluid bed. In this respect the process is similar to a twin fluid-bed gasifier except that the reactor (pyrolyser) temperature is much lower and the closely integrated char combustion in a second reactor requires careful control to ensure that the temperature, heat flux and solids flow match the process and feed requirements. Heat transfer is a mixture of conduction and convection in the riser. One of the unproven areas is scale up and heat transfer at high throughputs. 2.2.2.2. Char. All the char is burned in the secondary reactor to re-heat the circulating sand, so there is no char available for export unless an alternative heating source is used. If sepa￾rated the char would be a fine powder. 2.2.2.3. Background. Larger scale examples include the 650 kg/h ENEL plant in Italy built by Ensyn [20,21] which has not operated for some years, several Ensyn units in the USA at Red Arrow in Wisconsin for production of food flavourings up to 1700 kg/h, and the Ensyn units at their R&D centre in Renfrew Canada up to 2000 kg/h with plans for units up to 1000 t d1 [22]. 2.2.3. Rotating cone The rotating cone reactor, invented at the University of Twente [23] and developed by BTG [24], is a relatively recent development and effectively operates as a transported bed reactor, but with transport effected by centrifugal forces in a rotating cone rather than gas. A 250 kg/h unit is now oper￾ational, and a scaled up version of 50 t/d was commissioned in Malaysia in mid 2005. A 120 t/d plant is at an advanced plan￾ning stage [25]. Fig. 5 shows an early prototype on the left and its role in an integrated fast pyrolysis process on the right. The key features are: centrifugation (at w10 Hz) drives hot sand and biomass up a rotating heated cone; vapours are collected and processed conventionally; char and sand drop into a fluid bed surrounding the cone, whence they are lifted to a separate fluid bed combustor where char is burned to heat the sand, which is then drop￾ped back into the rotating cone; char is burned in a secondary bubbling fluid bed combustor. The hot sand is recirculated to the pyrolyser; carrier gas requirements in the pyrolysis reactor are much less than for fluid bed and transported bed systems; however, gas is needed for char burn-off and sand transport; a more complex integrated operation of three subsystems is required: rotating cone pyrolyser, riser for sand recycling, and bubbling bed char combustor; liquid yields of 60e70% on dry feed are typically obtained. As with CFB and transported beds all the char is burned so is not a byproduct, although the char could in principle be sepa￾rated and recovered if an alternative heating source is provided. 2.2.4. Ablative pyrolysis Ablative pyrolysis is substantially different in concept compared with other methods of fast pyrolysis. In all the other methods, the rate of reaction is limited by the rate of heat transfer through the biomass particles, which is why small particles are required. The mode of reaction in ablative pyrolysis is like melting butter in a frying pandthe rate of melting can be significantly enhanced by pressing the butter down and moving it over the heated pan surface. In ablative Fig. 4 e Circulating fluid bed reactor. pyrolysis, heat is transferred from the hot reactor wall to biomass and bioenergy 38 (2012) 68 e9 4 73

74 BIOMASS AND BIOENERGY 38 (2012)68-94 Char combustor →Flue gas Pyrolysis gases and vapours Sawdust feed Biomass Sand Ash Hot sand Condenser →Gas Rotating Vapours Cone Sand char Reactor A Bio-oil storage Fig.5-Rotating cone pyrolysis reactor and integrated process. "melt"wood that is in contact with it under pressure.As the 2.2.4.2.Background.Much of the pioneering fundamental wood is moved away,the molten layer then vaporises to work on ablative pyrolysis reactors was performed by the a product very similar to that derived from fluid bed systems. CNRS laboratories in Nancy,France where extensive basic The pyrolysis front thus moves unidirectionally through research has been carried out onto the relationships between the biomass particle.As the wood is mechanically moved pressure,motion and temperature [27].The National Renew- away,the residual oil film both provides lubrication for able Energy Laboratory (NREL)in Boulder,Colorado developed successive biomass particles and also rapidly evaporates to the ablative vortex reactor,in which the biomass was accel- give pyrolysis vapours for collection in the same way as other erated to supersonic velocities to derive high tangential processes.There is an element of cracking on the hot surface pressures inside a heated cylinder [26].Unreacted particles from the char that is also deposited.The rate of reaction is were recycled and the vapours and char fines left the reactor strongly influenced by pressure of the wood onto the heated axially for collection.Liquid yields of 60-65 wt.%on dry-feed surface;the relative velocity of the wood and the heat basis were typically obtained. exchange surface;and the reactor surface temperature.The Aston University has developed an ablative plate reactor key features of ablative pyrolysis are therefore as follows: [28]in which pressure and motion is derived mechanically obviating the need for a carrier gas.Liquid yields of High pressure of particle on hot reactor wall,achieved by 70-75 wt.%on dry-feed basis are typically obtained.A second- centrifugal force in the NREL,USA,concept,(no longer generation reactor has recently been built and commissioned operational [26))or mechanically at Aston University,UK and has been patented [29](Fig.6). which is described below and at PyTec in Germany; Another configuration is the mechanically driven PyTec High relative motion between particle and reactor wall; process in Germany [30].The company has built and tested Reactor wall temperature less than 600C. a laboratory unit based on hydraulically feeding wood rods onto a rotating electrically heated cone.The liquid collection As reaction rates are not limited by heat transfer through system is analogous to the other systems described above [30]. the biomass particles,larger particles can be used and in A 6 t/d unit has been built in north Germany in 2006 which is principle there is no upper limit to the size that can be pro- undergoing testing and designs are in progress for a 50 t/d unit. cessed.The process,in fact,is limited by the rate of heat supply The liquid is used in an engine for power generation. to the reactor rather than the rate of heat absorption by the pyrolysing biomass,as in other reactors.There is no require- 2.2.5.Other reaction systems ment for inert gas,so the processing equipment is smaller and the reaction system is thus more intensive.In addition the 2.2.5.1.Entrained flow.Entrained flow fast pyrolysis is,in absence of fluidising gas substantially increases the partial principle,a simple technology,but most developments have pressure of the condensable vapours leading to more efficient not been so successful because of the poor heat transfer collection and smaller equipment.However,the process is between a hot gas and a solid particle.High gas flows are surface-area-controlled so scaling is less effective and the required to effect sufficient heat transfer,which requires large reactor is mechanically driven,and is thus more complex. plant sizes and entails difficult liquid collection from the low vapour partial pressure.Liquid yields have usually been lower 2.2.4.1.Char.The char is a fine powder which can be sepa- than fluid bed and CFB systems at 50-55 wt%as in Georgia rated by cyclones and hot vapour filters as for fluid bed reac- Tech Research Institute [31]and Egemin [32]but neither is now tion systems. operational.There is some basic research in this area in China

“melt” wood that is in contact with it under pressure. As the wood is moved away, the molten layer then vaporises to a product very similar to that derived from fluid bed systems. The pyrolysis front thus moves unidirectionally through the biomass particle. As the wood is mechanically moved away, the residual oil film both provides lubrication for successive biomass particles and also rapidly evaporates to give pyrolysis vapours for collection in the same way as other processes. There is an element of cracking on the hot surface from the char that is also deposited. The rate of reaction is strongly influenced by pressure of the wood onto the heated surface; the relative velocity of the wood and the heat exchange surface; and the reactor surface temperature. The key features of ablative pyrolysis are therefore as follows: High pressure of particle on hot reactor wall, achieved by centrifugal force in the NREL, USA, concept, (no longer operational [26]) or mechanically at Aston University, UK which is described below and at PyTec in Germany; High relative motion between particle and reactor wall; Reactor wall temperature less than 600
C. As reaction rates are not limited by heat transfer through the biomass particles, larger particles can be used and in principle there is no upper limit to the size that can be pro￾cessed. The process, in fact, is limited by the rate of heat supply to the reactor rather than the rate of heat absorption by the pyrolysing biomass, as in other reactors. There is no require￾ment for inert gas, so the processing equipment is smaller and the reaction system is thus more intensive. In addition the absence of fluidising gas substantially increases the partial pressure of the condensable vapours leading to more efficient collection and smaller equipment. However, the process is surface-area-controlled so scaling is less effective and the reactor is mechanically driven, and is thus more complex. 2.2.4.1. Char. The char is a fine powder which can be sepa￾rated by cyclones and hot vapour filters as for fluid bed reac￾tion systems. 2.2.4.2. Background. Much of the pioneering fundamental work on ablative pyrolysis reactors was performed by the CNRS laboratories in Nancy, France where extensive basic research has been carried out onto the relationships between pressure, motion and temperature [27]. The National Renew￾able Energy Laboratory (NREL) in Boulder, Colorado developed the ablative vortex reactor, in which the biomass was accel￾erated to supersonic velocities to derive high tangential pressures inside a heated cylinder [26]. Unreacted particles were recycled and the vapours and char fines left the reactor axially for collection. Liquid yields of 60e65 wt.% on dry-feed basis were typically obtained. Aston University has developed an ablative plate reactor [28] in which pressure and motion is derived mechanically, obviating the need for a carrier gas. Liquid yields of 70e75 wt.% on dry-feed basis are typically obtained. A second￾generation reactor has recently been built and commissioned and has been patented [29] (Fig. 6). Another configuration is the mechanically driven PyTec process in Germany [30]. The company has built and tested a laboratory unit based on hydraulically feeding wood rods onto a rotating electrically heated cone. The liquid collection system is analogous to the other systems described above [30]. A 6 t/d unit has been built in north Germany in 2006 which is undergoing testing and designs are in progress for a 50 t/d unit. The liquid is used in an engine for power generation. 2.2.5. Other reaction systems 2.2.5.1. Entrained flow. Entrained flow fast pyrolysis is, in principle, a simple technology, but most developments have not been so successful because of the poor heat transfer between a hot gas and a solid particle. High gas flows are required to effect sufficient heat transfer, which requires large plant sizes and entails difficult liquid collection from the low vapour partial pressure. Liquid yields have usually been lower than fluid bed and CFB systems at 50e55 wt.% as in Georgia Tech Research Institute [31] and Egemin [32] but neither is now operational. There is some basic research in this area in China. Fig. 5 e Rotating cone pyrolysis reactor and integrated process. 74 biomass and bioenergy 38 (2012) 68 e9 4

BIOMASS AND BIOENERGY 38 (2012)68-94 75 Biomass 方 ea Heat Char out Heat Heat Vapours out Fig.6-Aston University Mark 2 ablative fast pyrolysis reactor. 2.2.5.2.Vacuum pyrolysis.Vacuum pyrolysis,as developed in higher.KIT has promoted and tested the concept of producing Canada by the University of Laval and Pyrovac,is arguably not a slurry of the char with the liquid to maximise liquid yield in a true fast pyrolysis as the heat transfer rate to and through the terms of energy efficiency [35],but this would requires an solid biomass is much slower than in the previously described altemative energy source to provide heat for the process. reactors although the vapour residence time is comparable. The basic technology was developed at the University of Laval 2.2.5.4.Fixed bed fast pyrolysis.There have been claims of using a multiple hearth furace but was upscaled to a purpose- fast pyrolysis in fixed beds but it is difficult to envisage a fixed designed heated horizontal moving bed [33].The process bed pyrolysis process that satisfies the basic requirements of operated at 450C and 100 kPa.Liquid yields of 35-50%on dry fast pyrolysis which can be constructed at anything above feed were typically obtained with higher char yields than fast laboratory or bench scale pyrolysis systems.The process was complex and costly because the high vacuum necessitates the use of very large 2.2.5.5.Microwave pyrolysis.Some basic research has been vessels and piping.The advantages of the process are that it carried out on microwave driven pyrolysis.Microwave heating can process larger particles than most fast pyrolysis reactors, is fundamentally difference from all other pyrolysis tech- there is less char in the liquid product because of the lower gas niques as the biomass particles are heated from within and velocities,and no carrier gas is needed.The process has not not by external heat transfer from a high temperature heat operated for some years and no activities are currently known source.Microwave heating requires a material with a high using vacuum pyrolysis. dielectric constant or loss factor,of which water is a good example.So in microwave pyrolysis,water is rapidly driven 2.2.5.3.Screw and augur kilns.There have been a number of off then the particle heats up to start forming char.It is not developments that mechanically move biomass through a hot clear that this can be considered fast pyrolysis.This is elec- reactor rather than using fluids.These include screw and trically conductive and eddy currents are created that provide augur reactors.Heating can be with recycled hot sand as at the very rapid heating.Therefore control of a microwave system Biolig plant at KIT(FZK until 2009)[34],with heat carriers such is quite challenging.A further problem to be considered is that as steel or ceramic balls,or external heating.The nature of penetration of microwaves is limited to typically 1-2 cm,so mechanically driven reactors is that very short residence the design of a microwave reactor presents interesting scale times comparable to fluid and circulating fluid beds are diffi- up challenges.Activities are included in Table 2 cult to achieve,and hot vapour residence times can range One of the potentially valuable aspects of microwave from 5 to 30 s depending on the design and size of reactor. pyrolysis is that due to the absence of thermal gradients,an Examples include screw reactors and more recently the Lurgi environment is created for studying some of the fundamen- LR reactor at Karlsruhe Institute of Technology(KIT)[35]and tals of fast pyrolysis.This offers possibilities to examine the the Bio-oil International reactors which have been studied at effect of the thermal gradient in a pyrolysing particle and the Mississippi State University [36].Screw and augur reactors secondary reactions that occur both within and without have also been developed as intermediate pyrolysis systems the biomass particle. such as Haloclean also at KIT (e.g.[37))and also as slow pyrolysis systems which are not included in this review. 2.2.5.6.Hydropyrolysis.In an effort to reduce the oxygen Screw reactors are particularly suitable for feed materials content of the bio-oil product within a single step process, that are difficult to handle or feed,or are heterogeneous.The some attention has returned to the concept of integrating liquid product yield tends to be somewhat lower than fluid pyrolysis and hydrocracking in which hydrogen is added to beds and is often phase separated due to the longer residence the pyrolysis reactor.GTIis starting a new hydropyrolysis and times and contact with byproduct char.Also the char yields are hydroconversion programme to make gasoline and diesel in

2.2.5.2. Vacuum pyrolysis. Vacuum pyrolysis, as developed in Canada by the University of Laval and Pyrovac, is arguably not a true fast pyrolysis as the heat transfer rate to and through the solid biomass is much slower than in the previously described reactors although the vapour residence time is comparable. The basic technology was developed at the University of Laval using a multiple hearth furnace but was upscaled to a purpose￾designed heated horizontal moving bed [33]. The process operated at 450
C and 100 kPa. Liquid yields of 35e50% on dry feed were typically obtained with higher char yields than fast pyrolysis systems. The process was complex and costly because the high vacuum necessitates the use of very large vessels and piping. The advantages of the process are that it can process larger particles than most fast pyrolysis reactors, there is less char in the liquid product because of the lower gas velocities, and no carrier gas is needed. The process has not operated for some years and no activities are currently known using vacuum pyrolysis. 2.2.5.3. Screw and augur kilns. There have been a number of developments that mechanically move biomass through a hot reactor rather than using fluids. These include screw and augur reactors. Heating can be with recycled hot sand as at the Bioliq plant at KIT (FZK until 2009) [34], with heat carriers such as steel or ceramic balls, or external heating. The nature of mechanically driven reactors is that very short residence times comparable to fluid and circulating fluid beds are diffi- cult to achieve, and hot vapour residence times can range from 5 to 30 s depending on the design and size of reactor. Examples include screw reactors and more recently the Lurgi LR reactor at Karlsruhe Institute of Technology (KIT) [35] and the Bio-oil International reactors which have been studied at Mississippi State University [36]. Screw and augur reactors have also been developed as intermediate pyrolysis systems such as Haloclean also at KIT (e.g. [37]) and also as slow pyrolysis systems which are not included in this review. Screw reactors are particularly suitable for feed materials that are difficult to handle or feed, or are heterogeneous. The liquid product yield tends to be somewhat lower than fluid beds and is often phase separated due to the longer residence times and contact with byproduct char. Also the char yields are higher. KIT has promoted and tested the concept of producing a slurry of the char with the liquid to maximise liquid yield in terms of energy efficiency [35], but this would requires an alternative energy source to provide heat for the process. 2.2.5.4. Fixed bed fast pyrolysis. There have been claims of fast pyrolysis in fixed beds but it is difficult to envisage a fixed bed pyrolysis process that satisfies the basic requirements of fast pyrolysis which can be constructed at anything above laboratory or bench scale. 2.2.5.5. Microwave pyrolysis. Some basic research has been carried out on microwave driven pyrolysis. Microwave heating is fundamentally difference from all other pyrolysis tech￾niques as the biomass particles are heated from within and not by external heat transfer from a high temperature heat source. Microwave heating requires a material with a high dielectric constant or loss factor, of which water is a good example. So in microwave pyrolysis, water is rapidly driven off then the particle heats up to start forming char. It is not clear that this can be considered fast pyrolysis. This is elec￾trically conductive and eddy currents are created that provide very rapid heating. Therefore control of a microwave system is quite challenging. A further problem to be considered is that penetration of microwaves is limited to typically 1e2 cm, so the design of a microwave reactor presents interesting scale up challenges. Activities are included in Table 2. One of the potentially valuable aspects of microwave pyrolysis is that due to the absence of thermal gradients, an environment is created for studying some of the fundamen￾tals of fast pyrolysis. This offers possibilities to examine the effect of the thermal gradient in a pyrolysing particle and the secondary reactions that occur both within and without the biomass particle. 2.2.5.6. Hydropyrolysis. In an effort to reduce the oxygen content of the bio-oil product within a single step process, some attention has returned to the concept of integrating pyrolysis and hydrocracking in which hydrogen is added to the pyrolysis reactor. GTI is starting a new hydropyrolysis and hydroconversion programme to make gasoline and diesel in Fig. 6 e Aston University Mark 2 ablative fast pyrolysis reactor. biomass and bioenergy 38 (2012) 68 e9 4 75

76 BIOMASS AND BIOENERGY 38 (2012)68-94 early 2010[38]and a new patent has been applied for that 2.4. Char removal includes hydrogen in the pyrolysis reactor with claims of producing hydrocarbons,alcohols and other oxygenates [39. Char acts as a vapour cracking catalyst so rapid and effective The concept has some contradictory requirements-high separation from the pyrolysis product vapours is essential pressure in pyrolysis increases char yields e.g.Antal [40]and Cyclones are the usual method of char removal,however reduces liquid yields while high pressures are required to some fines always pass through the cyclones and collect in the provide effective hydrogenation. liquid product where they accelerate aging and exacerbate the instability problem which is described below. 2.3. Heat transfer in fast pyrolysis Some success has been achieved with hot vapour filtration which is analogous to hot gas cleaning in gasification systems There are a number of technical challenges facing the devel- e.g.[41-44].Problems arise with the sticky nature of fine char opment of fast pyrolysis,of which the most significant is heat and disengagement of the filter cake from the filter. transfer to the reactor.Pyrolysis is an endothermic process, Pressure filtration of the liquid for substantial removal of requiring a substantial heat input to raise the biomass to particulates(down to <5 um)is very difficult because of the reaction temperature,although the heat of reaction is insig- complex interaction of the char and pyrolytic lignin,which nificant.Heat transfer in commercial reactors is a significant appears to form a gel-like phase that rapidly blocks the filter. design feature and the energy in the by-product charcoal Modification of the liquid microstructure by addition of would typically be used in a commercial process by combus- solvents such as methanol or ethanol that solubilise the less tion of the char in air.The char typically contains about 25%of soluble constituents can improve this problem and contribute the energy of the feedstock,and about 75%of this energy is to improvements in liquid stability,as described below. typically required to drive the process.The by-product gas only contains about 5%of the energy in the feed and this is not 2.5. Liquids collection sufficient for pyrolysis.The main methods of providing the necessary heat are listed below: The gaseous products from fast pyrolysis consist of aerosols, true vapours and non-condensable gases.These require rapid through heat transfer surfaces located in suitable positions cooling to minimise secondary reactions and condense the in the reactor; true vapours,while the aerosols require additional coales- by heating the fluidisation gas in the case of a fluid bed or cence or agglomeration.Simple indirect heat exchange can circulating fluid bed reactor,although excessive gas cause preferential deposition of lignin-derived components temperatures may be needed to input the necessary heat leading to liquid fractionation and eventually blockage in resulting in local overheating and reduced liquid yield,or pipelines and heat exchanges.Quenching in product bio-oil or alternatively very high gas flows are needed resulting in in an immiscible hydrocarbon solvent is widely practised. unstable hydrodynamics.Partial heating is usually satis- Orthodox aerosol capture devices such as demisters and factory and desirable to optimise energy efficiency. other commonly used impingement devices are not reported by removing and re-heating the bed material in a separate to be as effective as electrostatic precipitation,which is reactor as used in most CFB and transported bed reactors; currently the preferred method at both laboratory and by the addition of some air,although this can create hot commercial scale units.The vapour product from fluidbed and spots and increase cracking of the liquids to tars transported bed reactors has a low partial pressure of condensable products due to the large volumes of fluidising There are a variety of ways of providing the process heat gas,and this is an important design consideration in liquid from byproduct char or gas;or from fresh biomass.This facet collection.This disadvantage is reduced in the rotating cone of pyrolysis reactor design and optimisation is most important and ablative reaction systems,both of which exclude inert gas for commercial units and will attract increasing attention as which leads to more compact equipment and lower costs [45] plants become bigger.Examples of options include: 2.6 By-products combustion of byproduct char,all or part combustion ofbyproduct gas which requires supplementation, Char and gas are by-products,typically containing about 25 combustion of fresh biomass instead of char,particularly and 5%of the energy in the feed material respectively.The where there is a lucrative market for the char pyrolysis process itself requires about 15%of the energy in the gasification of the byproduct char and combustion of the feed,and of the byproducts,only the char has sufficient resultant producer gas to provide greater temperature energy to provide this heat.The heat can be derived by control and avoid alkali metal problems such as slagging in burning char in orthodox reaction system design,which the char combustor. makes the process energy self sufficient.More advanced use of byproduct gas with similar advantages as above, configurations could gasify the char to a LHV gas and then although there is unlikely to be sufficient energy available in burn the resultant gas more effectively to provide process heat this gas without some supplementation, with the advantage that the alkali metals in the char can be use of bio-oil product, much better controlled. use of fossil fuels where these are available at low cost,do The waste heat from char combustion and any heat from not affect any interventions allowable on the process or surplus gas or by-product gas can be used for feed drying and in product,and the by-products have a sufficiently high value. large installations could be used for export or power generation

early 2010 [38] and a new patent has been applied for that includes hydrogen in the pyrolysis reactor with claims of producing hydrocarbons, alcohols and other oxygenates [39]. The concept has some contradictory requirements e high pressure in pyrolysis increases char yields e.g. Antal [40] and reduces liquid yields while high pressures are required to provide effective hydrogenation. 2.3. Heat transfer in fast pyrolysis There are a number of technical challenges facing the devel￾opment of fast pyrolysis, of which the most significant is heat transfer to the reactor. Pyrolysis is an endothermic process, requiring a substantial heat input to raise the biomass to reaction temperature, although the heat of reaction is insig￾nificant. Heat transfer in commercial reactors is a significant design feature and the energy in the by-product charcoal would typically be used in a commercial process by combus￾tion of the char in air. The char typically contains about 25% of the energy of the feedstock, and about 75% of this energy is typically required to drive the process. The by-product gas only contains about 5% of the energy in the feed and this is not sufficient for pyrolysis. The main methods of providing the necessary heat are listed below: through heat transfer surfaces located in suitable positions in the reactor; by heating the fluidisation gas in the case of a fluid bed or circulating fluid bed reactor, although excessive gas temperatures may be needed to input the necessary heat resulting in local overheating and reduced liquid yield, or alternatively very high gas flows are needed resulting in unstable hydrodynamics. Partial heating is usually satis￾factory and desirable to optimise energy efficiency. by removing and re-heating the bed material in a separate reactor as used in most CFB and transported bed reactors; by the addition of some air, although this can create hot spots and increase cracking of the liquids to tars. There are a variety of ways of providing the process heat from byproduct char or gas; or from fresh biomass. This facet of pyrolysis reactor design and optimisation is most important for commercial units and will attract increasing attention as plants become bigger. Examples of options include: combustion of byproduct char, all or part combustion of byproduct gaswhich requires supplementation, combustion of fresh biomass instead of char, particularly where there is a lucrative market for the char, gasification of the byproduct char and combustion of the resultant producer gas to provide greater temperature control and avoid alkali metal problems such as slagging in the char combustor, use of byproduct gas with similar advantages as above, although there is unlikely to be sufficient energy available in this gas without some supplementation, use of bio-oil product, use of fossil fuels where these are available at low cost, do not affect any interventions allowable on the process or product, and the by-products have a sufficiently high value. 2.4. Char removal Char acts as a vapour cracking catalyst so rapid and effective separation from the pyrolysis product vapours is essential. Cyclones are the usual method of char removal, however some fines always pass through the cyclones and collect in the liquid product where they accelerate aging and exacerbate the instability problem which is described below. Some success has been achieved with hot vapour filtration which is analogous to hot gas cleaning in gasification systems e.g. [41e44]. Problems arise with the sticky nature of fine char and disengagement of the filter cake from the filter. Pressure filtration of the liquid for substantial removal of particulates (down to <5 mm) is very difficult because of the complex interaction of the char and pyrolytic lignin, which appears to form a gel-like phase that rapidly blocks the filter. Modification of the liquid microstructure by addition of solvents such as methanol or ethanol that solubilise the less soluble constituents can improve this problem and contribute to improvements in liquid stability, as described below. 2.5. Liquids collection The gaseous products from fast pyrolysis consist of aerosols, true vapours and non-condensable gases. These require rapid cooling to minimise secondary reactions and condense the true vapours, while the aerosols require additional coales￾cence or agglomeration. Simple indirect heat exchange can cause preferential deposition of lignin-derived components leading to liquid fractionation and eventually blockage in pipelines and heat exchanges. Quenching in product bio-oil or in an immiscible hydrocarbon solvent is widely practised. Orthodox aerosol capture devices such as demisters and other commonly used impingement devices are not reported to be as effective as electrostatic precipitation, which is currently the preferred method at both laboratory and commercial scale units. The vapour product from fluid bed and transported bed reactors has a low partial pressure of condensable products due to the large volumes of fluidising gas, and this is an important design consideration in liquid collection. This disadvantage is reduced in the rotating cone and ablative reaction systems, both of which exclude inert gas which leads to more compact equipment and lower costs [45]. 2.6. By-products Char and gas are by-products, typically containing about 25 and 5% of the energy in the feed material respectively. The pyrolysis process itself requires about 15% of the energy in the feed, and of the byproducts, only the char has sufficient energy to provide this heat. The heat can be derived by burning char in orthodox reaction system design, which makes the process energy self sufficient. More advanced configurations could gasify the char to a LHV gas and then burn the resultant gas more effectively to provide process heat with the advantage that the alkali metals in the char can be much better controlled. The waste heat from char combustion and any heat from surplus gas or by-product gas can be used for feed drying and in large installations could be used for export or power generation. 76 biomass and bioenergy 38 (2012) 68 e9 4

BIOMASS AND BIOENERGY 38 (2012)68-94 77 An important principle of fast pyrolysis is that a well- Organics yield,wt.%of dry feed designed and well-run process should not produce any 80% emissions other than clean flue gas i.e.COz and water, Maple although they will have to meet local emissions standards and 70% requirements. Poplar Aspen 60% 50% Pyrolysis liquid-bio-oil Cellulose Bagasse 40% Crude pyrolysis liquid or bio-oil is dark brown and approxi- mates to biomass in elemental composition.It is composed of 30% a very complex mixture of oxygenated hydrocarbons with an Pine Bark appreciable proportion of water from both the original mois- 20% ture and reaction product.Solid char may also be present. 10% Typical organics yields from different feedstocks and their variation with temperature is shown in Figs.7 and 8 shows the 0% temperature dependence of the four main products from 400 450 500 550 600 650 a variety of feedstocks [46].Similar results are obtained for most biomass feedstocks,although the maximum yield can Reaction temperature,C occur between 480 and 520C depending on feedstock. Fig.8-Organics yield from different feedstocks [46]. Grasses,for example,tend to give maximum liquid yields of around 55-60 wt.%on a dry-feed basis at the lower end of this temperature range,dependingon the ash content of the grass. The liquid is formed by rapidly quenching and thus bio-oil',it will not mix with any hydrocarbon liquids.It is freezing'the intermediate products of flash degradation of composed of a complex mixture of oxygenated compounds hemicellulose,cellulose and lignin.The liquid thus contains that provide both the potential and challenge for utilisation. many reactive species,which contribute to its unusual attri- There are some important properties of this liquid that are butes.Bio-oil can be considered a micro-emulsion in which summarised in Table 3. the continuous phase is an aqueous solution of holocellulose Pyrolysis oil typically is a dark brown,free-flowing liquid decomposition products,that stabilises the discontinuous Depending on the initial feedstock and the mode of fast phase of pyrolytic lignin macro-molecules through mecha- pyrolysis,the colour can be almost black through dark red- nisms such as hydrogen bonding.Aging or instability is brown to dark green,being influenced by the presence of believed to result from a breakdown in this emulsion.In some micro-carbon in the liquid and chemical composition.Hot ways it can be considered to be analogous to asphaltenes vapour filtration gives a more translucent red-brown appear- found in petroleum. ance owing to the absence of char.High nitrogen content can impart a dark green tinge to the liquid. 3.1. Bio-oil characteristics There are many particular characteristics of bio-oil that require consideration for any application.These are sum- marised in Table 4 with causes,effects and solutions.Oasmaa Fast pyrolysis liquid has a higher heating value of about 17 MJ/kg as produced with about 25 wt.%water that cannot and Peacocke have reviewed physical property character- readily be separated.While the liquid is widely referred to as isation and methods [47].The more significant characteristics are discussed below. Yield,wt.%of dry feed 80% 70% Table 3-Typical properties of wood-derived crude bio- oil. 60% Organics Physical property Typical value 50% Moisture content 25% 40% pH 2.5 Gas Specific gravity 1.20 30% Char Elemental analysis 20% C 56% 10% H 6% 0 38% Reaction water 0% N 0-0.1% 400 450 500 550 600 650 HHV as produced 17 MJ/kg Reaction temperature,C Viscosity (40C and 25%water) 40-100mpas Solids(char) 0.1% Fig.7-Variation of products from Aspen Poplar with Vacuum distillation residue up to 50% temperature【46

An important principle of fast pyrolysis is that a well￾designed and well-run process should not produce any emissions other than clean flue gas i.e. CO2 and water, although they will have to meet local emissions standards and requirements. 3. Pyrolysis liquid e bio-oil Crude pyrolysis liquid or bio-oil is dark brown and approxi￾mates to biomass in elemental composition. It is composed of a very complex mixture of oxygenated hydrocarbons with an appreciable proportion of water from both the original mois￾ture and reaction product. Solid char may also be present. Typical organics yields from different feedstocks and their variation with temperature is shown in Figs. 7 and 8 shows the temperature dependence of the four main products from a variety of feedstocks [46]. Similar results are obtained for most biomass feedstocks, although the maximum yield can occur between 480 and 520
C depending on feedstock. Grasses, for example, tend to give maximum liquid yields of around 55e60 wt.% on a dry-feed basis at the lower end of this temperature range, depending on the ash content of the grass. The liquid is formed by rapidly quenching and thus ‘freezing’ the intermediate products of flash degradation of hemicellulose, cellulose and lignin. The liquid thus contains many reactive species, which contribute to its unusual attri￾butes. Bio-oil can be considered a micro-emulsion in which the continuous phase is an aqueous solution of holocellulose decomposition products, that stabilises the discontinuous phase of pyrolytic lignin macro-molecules through mecha￾nisms such as hydrogen bonding. Aging or instability is believed to result from a breakdown in this emulsion. In some ways it can be considered to be analogous to asphaltenes found in petroleum. 3.1. Bio-oil characteristics Fast pyrolysis liquid has a higher heating value of about 17 MJ/kg as produced with about 25 wt.% water that cannot readily be separated. While the liquid is widely referred to as ‘bio-oil’, it will not mix with any hydrocarbon liquids. It is composed of a complex mixture of oxygenated compounds that provide both the potential and challenge for utilisation. There are some important properties of this liquid that are summarised in Table 3. Pyrolysis oil typically is a dark brown, free-flowing liquid. Depending on the initial feedstock and the mode of fast pyrolysis, the colour can be almost black through dark red￾brown to dark green, being influenced by the presence of micro-carbon in the liquid and chemical composition. Hot vapour filtration gives a more translucent red-brown appear￾ance owing to the absence of char. High nitrogen content can impart a dark green tinge to the liquid. There are many particular characteristics of bio-oil that require consideration for any application. These are sum￾marised in Table 4 with causes, effects and solutions. Oasmaa and Peacocke have reviewed physical property character￾isation and methods [47]. The more significant characteristics are discussed below. Fig. 7 e Variation of products from Aspen Poplar with temperature [46]. Fig. 8 e Organics yield from different feedstocks [46]. Table 3 e Typical properties of wood-derived crude bio￾oil. Physical property Typical value Moisture content 25% pH 2.5 Specific gravity 1.20 Elemental analysis C 56% H 6% O 38% N 0e0.1% HHV as produced 17 MJ/kg Viscosity (40
C and 25% water) 40e100 mpa s Solids (char) 0.1% Vacuum distillation residue up to 50% biomass and bioenergy 38 (2012) 68 e9 4 77