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