Pyrolysis processes can be classified on the basis of the rate of heating as: Slow pyrolysis, fast pyrolysis, and intermediate pyrolysis. All these types of pyrolysis processes are carried out in the absence of oxygen. Depending on the medium in which it is carried out, it is classified as either hydrous pyrolysis (carried out in presence of water) or hydropyrolysis (carried out in presence of hydrogen).

Slow pyrolysis

The process of slow pyrolysis is used mainly for production of char and involves slow heating of the biomass over long periods of time (ranging from minutes to days).

Torrefaction is a slow pyrolysis process carried out at low temperatures (230-300°C) in the absence of oxygen. It is a form of pretreatment of biomass to improve its energy density, reduce the oxygen/carbon and hydrogen/carbon ratio, and reduce its hygroscopicity. This pretreatment makes the biomass more suitable for other biomass conversion processes. For example, the high oxygen content of biomass increases thermodynamic losses during the gasification process. Reduc­tion of the oxygen/carbon ratio as a result of torrefaction reduces the thermody­namic losses during the gasification process. The microfibrils in biomass, comprising cellulose, are supported or bound together by hemicellulose. The process of torrefaction depolymerizes this hemicellulose, causing a consequent reduction in binding of cellulose fibrils. This causes the structure to become friable and brittle, reducing the energy requirement for size reduction process which precedes most bioconversion processes. Torrefaction is also accompanied with a color change in most cases. The process of roasting of coffee beans by heating the green beans to 200-300°C over a long period of time is the most popular example of the torrefaction process. During the process of torrefaction, there is some reduction in the energy content of the biomass due to the partial devolatilization occurring during the process. However, this reduction in energy content is com­pensated by the increase in energy density of the biomass during the process. Basu [11] gives details of the changes in terms of energy density, heating value, etc. in bagasse after torrefaction.