Flash pyrolysis is an extension to fast pyrolysis where heating rates reach around 1000°C/s. Flash pyrolysis has been reviewed several times.66-68 The residence time of the solid is less than a second and depending on the type of reactor the temperatures can be as low as 500 or as high as 1200°C. The very high rate of heating requires very rapid heat transfer from the reactor environment to the feedstock and, because of this, particle sizes of less than 0.5 mm but usually less than 100 um are required. The small particle sizes of feedstock also result in small particles of char and this is a major disadvantage of the technique.65 Great care must be taken to remove particles of char from the as-produced bio-oil because it can catalyse polymerisation of some of the products and increased viscosity of the bio-oil.65 The major advantage of flash pyrolysis is the improved energy efficiency of the process which can be in excess of 70%.69 Although we have not discussed the efficiency of pyrolysis processes here, it should be noted that pyrolysis is a strongly endothermic process and energy must be supplied to affect the heating in the reaction. The source of energy for heating is the feedstock itself, either before or after pyrolysis. The challenge for engineers is to configure reactor technologies to minimise losses through heat recovery and other methods to allow efficient heating of the feedstock during pyrolysis. There are various reactor technologies used in flash pyrolysis and are briefly introduced below.

1 Fluidised bed and circulating FBRs: These are probably closest to integration into large scale commercial use, and large scale pilot plants have been demonstrated.70

2 Entrained flow reactor: This reactor has been scaled to allow pyrolysis of 500 kg/h of feedstock.71 In this reactor a carrier gas and a combustion gas (to produce the pyrolysis temperature by combustion) are fed into a reactor tube and powdered feedstock is fed into the high flow gas stream. Whilst the design is relatively simple the use of carrier gas (usually nitrogen) is a disadvantage.

3 Vacuum pyrolysis: This is a relatively new technique where the sample is heated under vacuum; the vacuum removes pyrolysis generated volatiles which are then condensed to the bio-oil.72 This technique results in low residence times and also allows for rapid separation of the oil and char. Its major advantage is that it can operate at relatively low temperatures of 500°C.

4 Rotating cone reactor: This type of reactor was developed by scientists in The Netherlands.73,74 In the rotating cone reactor, biomass particles are fed to the bottom of a rotating cone with inert heat carrier particles and are pyrolysed whilst being transported spirally upwards along the cone wall. The advantage is the absence of a carrier gas and high oil yields.

5 Ablative pyrolysis: There are several versions of this methodology which varies considerably from the other techniques discussed. It consists of solid particles being exposed directly to heat via contact with a heated surface or radiatively.75 The action of pressing the particles against the hot surface reduces heat transfer requirements.

The final type of pyrolysis often differentiated in the literature is catalytic pyrolysis, the main subject of this article. In reality this is not a completely different form of pyrolysis and can be used with the same type of reactors, etc. outlined above. Catalytic pyrolysis is outlined in depth below.