2.Fast pyrolysis
Very rapid feedstock heating leads to a much greater proportion of bio-oil and less biochar
(Table 1). It was with the objective of achieving this high yield of liquid fuel that fast pyrolysis
technology was developed.
The time taken to reach peak temperature of the endothermic process (the ‘resistance time’)
is approximately one or two seconds, rather than minutes or hours as is the case with slow
pyrolysis. The lower operating temperature also enhances the overall conversion efficiency
of the process relative to slow pyrolysis.
Maintaining a low feedstock moisture content of around 10% and using a fine particle size of
<2mm permits rapid transference of energy despite relatively moderate peak temperatures of
around 450°C (and in the range 350 to 500°C). In many systems the transfer is further
increased by mechanically enhancing feedstock contact with the heat source or maximising
heat source surface area. Various technologies have been used and proposed or tested
including: fixed beds, augers, ablative methods, rotating cones, fluidized beds and circulating
fluidized beds. Surface charring must be continuously removed during reaction to prevent
pyrolysis of particle interiors being inhibited by its insulating effect.
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