Biomass is here defined as biological material derived from living (or recently living) plants that is used as a source of energy; the term also encompasses peat. It is assumed that any biomass used is harvested sustainably without endangering food security, habitats, or soil conservation. Biomass is a complex fuel. Its primary components are cellulose, hemicellulose and lignin,

which are the main constituents of the plant cell wall. These species also account for the bulk of its energy content (calorific value). The non-uniformity and varied sources of biofuels make it necessary to convert them to standardized solid, liquid, or gaseous fuels.