Biomass pyrolysis is the thermal breakdown of biomass using high temperature in the absence of oxygen. Pyrolysis, similar to other thermochemical conversion technologies, results in three products: solid (biochar), liquid (bio-oil), and gas (syngas/producer gas). For pyrolysis, the target product is usually either bio-oil (using fast pyrolysis) or biochar (using slow pyrolysis). Slow pyrolysis has been used for centuries to produce solid, cleaner burning fuels. Only recently (1980s) has fast pyrolysis been recognized as an alternative to produce liquid fuel (Meier and Faix 1999). The main differences between the fast and slow pyrolysis are summarized in Table 4.

Slow pyrolysis was discovered and used many centuries ago, when charcoal and coal-tar were produced using slow pyrolysis of wood and coal. Charcoal was used as a fuel to create a smokeless flame and increase the combustion temperature. The coking process is also used in manufacture of steel. Recent interest in liquid fuels has changed the focus to fast pyrolysis, which results in much higher liquid yield.