Biomass Fuels

Biomass fuel is fuel produced from organic material of recent biological origin. Biomass suitable for the production of fuels is classified as either bio-mass waste or energy crops.

Biomass waste is any material of recent biological origin that has been discarded because it has no apparent value. Examples of biomass waste include residues from agricultural crops, municipal solid waste, and even sewage. Because this waste originates from biomass that recycles to the environment on a nearly annual basis, it is a sustainable energy resource; that is, the resource will be available for future generations.

Energy crops are defined as plants grown specifically as an energy resource. An energy crop is planted and harvested periodically. The cycle of planting and harvesting over a relatively short period assures that the resource is used in a sustainable fashion. Energy crops include woody crops harvested on a rotation of five to seven years and herbaceous energy crops harvested on an annual basis. Examples of woody crops include hybrid poplars and willows. Examples of herbaceous crops include switchgrass and sweet sorghum. Peat moss is also harvested and used as a fuel.

Solid fuels such as biomass are at an enormous disadvantage when compared to petroleum and natural gas because they are more difficult to transport and handle. The goal of many biomass conversion processes is to convert solid fuel into more useful gaseous or liquid forms. Gaseous fuels can substitute for natural gas in machinery to produce heat and power. Liquid fuels can substitute for gasoline or diesel fuel in automobiles and trucks. Both gaseous and liquid fuels can also be used to produce chemicals and materials currently made from petroleum resources.

Either anaerobic digestion or thermal gasification can produce gaseous biomass fuels. Anaerobic digestion is the decomposition of organic solids to gaseous fuel by bacteria in an oxygen-free environment. The product is a mixture of methane, carbon dioxide, and some trace gases. Thermal gasification is the conversion of solid fuels into flammable gas mixtures consisting of carbon monoxide, hydrogen, methane, nitrogen, carbon dioxide, and smaller quantities of higher hydrocarbons.

Liquid biomass fuels can be produced from solid biomass by three processes: fermentation to ethanol, processing of vegetable oils to biodiesel, or thermal processing to pyrolysis oils. Pyrolysis is the high-temperature decomposition of organic compounds in the absence of oxygen to produce liquids. The mixture of oxygenated hydrocarbons is similar to that found in fuel oil.

Fermentation is the decomposition of complex organic molecules into simpler compounds by the action of microorganisms. A variety of carbohydrates (such as sugars, starches, hemicellulose, and cellulose) can serve as feed-stock in ethanol fermentation as long as they can be broken down to sugars that are susceptible to microbial biological action. Cellulose and hemicellulose found in the plant cell walls of woody and herbaceous energy crops can also be converted to fermentable sugars, but the process is relatively difficult.

Vegetable oils from soybeans, peanuts, and other grains and seeds can be used as fuel in diesel engines after chemical modification to improve their combustion properties.

see also Agriculture, Modern; Forestry; Oils, Plant-Derived; Peat Bogs.

Robert C. Brown

Bibliography

Brown, Robert C. "Capturing Solar Energy Through Biomass." In Principles of Solar Engineering, 2nd ed. D. Y. Goswami, F. Kreider, and F. Kreith, eds. Washington, DC: Taylor & Francis, 1999.

Needham, Cynthia, Mahlon Hoagland, Kenneth McPherson, and Bert Dodson. Intimate Strangers: Unseen Life on Earth. Washington, DC: ASM Press, 2000.

Wayman, Morris, and Sarad R. Parekh. Biotechnology of Biomass Conversion. Milton Keynes, England: Open University Press, 1990.