Conventional pyrolysis (carbonization, destructive distillation, dry distillation, retorting) consists of the slow, irreversible, thermal degradation of the organic components in biomass, most of which are lignocellulosic polymers, in the absence of oxygen. Slow pyrolysis has traditionally been used for the production of charcoal. Detailed studies of biomass pyrolysis beginning in the 1970s have led to methods of controlling the selectivities and yields of the gaseous, liquid, and solid products by controlling the pyrolysis temperature and heating rate (cf. Stevens, 1994). Short-residence-time pyrolysis (flash, rapid, ultra pyrolysis) of biomass at moderate temperatures can afford up to 70 wt % yields of liquid products (cf. Bridgwater and Bridge, 1991). Pyrolysis conditions can be used that provide high yields of gas or liquid products and char yields of less than 5%. One configuration of an advanced biomass pyrolysis system, for example, involves an ablative, vortex reactor for pyrolysis at biomass residence times of fractions of a second coupled to a downstream vapor cracker (Diebold and

Scahill, 1988). The products can be varied to yield up to about 56% liquids (dry) or 90% gases; the char yields are about 10-15% in each case. Advanced pyrolysis processes are discussed in more detail in Section III.