To increase the amount of liquid product a technique known as fast pyrolysis (more properly defined as thermolysis since it is the thermal degradation of a chemical compound into a range of product chemicals) was developed in the early to mid-1980s.57-61 The yield of oil or liquid can be as high as 80% of the feedstock biomass but is normally around 65-75%. The corresponding amounts of char and gas are typically 10-25 and 10-20%, respectively. Fast pyrolysis processes occur in the time frame of a few seconds or less and the product range obtained is highly dependent on chemical kinetics as well as heat and mass transfer in the reaction chamber as discussed by Bridgwater.62 The residence time of solid in fast pyrolysis is of the order of a few seconds or less with a heating rate of tens or hundreds of °C/s and the temperature range used is normally above 650-1000°C. Bridgwater and Peacocke have reviewed various forms of fast pyrolysis reactors.63 It is clear, that to achieve such heating rates that these reactors will be considerably more complicated than for conventional pyrolysis. There are some obvious requirements for equipment used in fast pyrolysis and these are:

1 High heating rate to allow reactants to react in short periods of time and minimise char formation.

2 Very rapid heat transfer to allow feedstock to reach optimum temperature during their short residence time in the reactor.

3 Rapid cooling (condensation) of the pyrolysis products on exit from the reactor which prevents further reaction to char and tar-like materials.

It is difficult experimentally, as well as being expensive, to achieve the high heating rates required in static (batch) reactors. This is particularly true for the large scale reactors that would be required commercially. Whilst the heating rates may be achieved in small volume fixed bed reactors (fixed bed — the solid does not move) they can be more readily achieved if the feedstock is fed in and out of the reactor held at fixed temperature. This is achieved using a gas flow to suspend the feedstock solid. Probably the best design is a fluidised bed reactor (FBR).53,63 Here, a particulate solid, such as fine sand, together with a powder of the feedstock is supported on a high velocity gas flow forming a fluid that can be passed trough the reactor.64 The support solid is normally re-circulated through the reactor after separation of the products and this is known as the circulating FBR. The need to suspend the feedstock in the fluid requires that it is in the form of a fine particulate. This is also a necessity for rapid heat transport from the environment of the reactor to the solid. For these reasons the solid feedstock is normally ground or milled into particulates of the order of 1mm diameter or less. The FBR also facilitates the rapid cooling required to collect the bio-oil. Separation of char is usually achieved using cyclone technologies. Despite the apparent technical complexity, FBRs are well-established technology64 and probably represent the most cost-efficient form of pyrolysis.65