The high heat transfer rate that is necessary to heat the particles sufficiently quickly imposes a major design requirement on achieving the high heat fluxes required to match the high heating rates and endothermic pyrolysis reactions. Reed et al. originally suggested that to achieve true fast pyrolysis conditions, heat fluxes of 50 W/cm2 would be required but to achieve this in a commercial process is not practicable or necessary (6).

Each mode of heat transfer imposes certain limitations on the reactor operation and may increase its complexity. The two dominant modes of heat transfer in fast pyrolysis technologies are conductive and convective. Each one can be maximised or a contribution can be made from both depending on the reactor configuration. The penalties and interactions are summarised in Table 3.2 below with some speculations on heat transfer modes.

For ablative pyrolysis in a vortex reactor, a furnace arrangement equivalent to an ethylene cracking furnace has been proposed by the IEA Bioenergy Agreement pyrolysis and liquefaction group (7, 8). Other possibilities to achieve the pyrolysis temperatures and heat transfer rates necessary have included vapour condensation such as sodium, induction heating of the reactor wall and the use of contact electrical heaters. In a circulating fluid bed, the majority of the heat transfer wilt be from the hot circulating sand which therefore requires an efficient sand re­heating system. In a conventional fluid bed the sand requires an external heat source.

A commercial system would be expected to utilise the by-product char and gas for the process heat requirements as an integrated system as proposed for the NR EL wood to gasoline process evaluated by the IEA (7, 8).

Reactor type Suggested mode Advantages/disadvantaqes/features of heat transfer