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 separated 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 operational, 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 planning 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 dropped 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 separated 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 processed. 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 requirement 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 separated by cyclones and hot vapour filters as for fluid bed reaction 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 Renewable Energy Laboratory (NREL) in Boulder, Colorado developed the ablative vortex reactor, in which the biomass was accelerated 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 secondgeneration 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 purposedesigned 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 techniques 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 electrically 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 fundamentals 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 development 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 insignificant. 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 combustion 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 satisfactory 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 coalescence 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