A biomass fuel is an energy source derived from living organisms. Most commonly it is plant residue, harvested, dried and burned, or further processed into solid, liquid, or gaseous fuels. The most familiar and widely used biomass fuel is wood. Agricultural waste, including materials such as the cereal straw, seed hulls, corn stalks and cobs, is also a significant source. Native shrubs and herbaceous plants are potential sources. Animal waste, although much less abundant overall, is a bountiful source in some areas.

Wood accounted for 25% of all energy used in the United States at the beginning of this century. With increased use of fossil fuels, its significance rapidly declined. By 1976, only 1–2% of United States energy was supplied by wood, and burning of tree wastes by the forest products industry accounted for most of it. Although the same trend has been evident in all industrialized countries, the...