Biomass gasification is a highly endothermic process requiring large quantities of heat .The heat can be supplied to the gasifier via an independent and external source, so-called 'direct' heating or by circulating an inert bed material between a gasification and a combustion zone, 'indirect' heating. Although the first arrangement has the advantage of physically separating the gasifier from the source of heat and utilising spare energy from processes that might otherwise be wasted, the inevitable drawback is that it is attached to other heat generating processes and requires large surface areas for the heat transfer. Indirect heating produces a gas with consistent heating values regardless of the feedstock's moisture content, however, care must be taken in order to minimise product gas contamination.

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