Fast pyrolysis is a process in which organic materials are rapidly heated to 450 – 650 degrees centigrade in the absence of air. Under these conditions, organic vapors are condensed to bio oil. Up to 80% of the weight of the feed stock is converted into bio crude oil. Fast pyrolysis occurs in a time of few seconds or less, Therefore, not only chemical reaction kinetics but also heat and mass transfer processes, as well as phase transition phenomena, play important roles.

Pyrolysis offers the possibility of de-coupling ( time, place and scale), easy handling of the liquids and a more consistent quality compared to any solid biomass. With fast pyrolysis a clean liquid is produced as an intermediate for a wide variety of applications. This liquid is typically called bio oil.

The critical issue is to bring the reacting biomass particle to the optimum process temperature and minimize its exposure to the intermediate [lower] temperatures that favor formation of charcoal..

The process of pyrolysis is endothermic, requiring a substantial heat input to raise the biomass to reaction temperature, although the heat of reaction is insignificant. Heat transfer in commercial reactors is a significant design feature.

In fast pyrolysis biomass decomposes to generate mostly vapors and aerosols and some charcoal. After cooling and condensation, a dark brown mobile liquid is formed which has a heating value about half that of conventional fuel oil.

The essential features of a fast pyrolysis process for producing liquids are: 

• very high heating and heat transfer rates at the reaction interface, which usually requires
• a finely ground biomass feed
• carefully controlled pyrolysis reaction temperature of around 500ºC and vapor phase
• temperature of 400-450ºC,
• short vapour residence times of typically less than 2 seconds
• rapid cooling of the pyrolysis vapours to give the bio-oil product.

The main product, bio-oil , is obtained in yields of up to  80% wt on dry feed basis, together with by-product char and gas which are used within the process so there are no waste streams other than flue gas and ash.

A fast pyrolysis process includes drying the feed to typically less than 10% water in order to minimize the water in the product liquid oil grinding the feed to give sufficiently small particles to ensure rapid reaction, pyrolysis reaction, separation of solids [char], and collection of the liquid product [bio-oil].

Any form of biomass can be considered for fast pyrolysis.

Char acts as a vapour cracking catalyst so rapid and effective separation from the pyrolysis product vapors is essential to minimize aging and exacerbation of instability.

The gaseous products from fast pyrolysis consist of aerosols, true vapors and noncondensable gases.

These require rapid cooling to minimize secondary reactions and to condense the true vapors, while the aerosols require coalescence or agglomeration.