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Fuel produced from renewable resources, especially plant biomass, vegetable oils, and treated municipal and industrial wastes. Biofuels are considered neutral with respect to the emission of carbon dioxide because the carbon dioxide given off by burning them is balanced by the carbon dioxide absorbed by the plants that are grown to produce them. The use of biofuels as an additive to petroleum-based fuels can also result in cleaner burning with less emission of carbon monoxide and particulates.

◆ Ethanol produced by fermenting the sugars in biomass materials such as corn and agricultural residues is known as bioethanol. Bioethanol is used in internal-combustion engines either in pure form or more often as a gasoline additive.

◆ Biodiesel is made by processing vegetable oils and other fats and is also used either in pure form or as an additive to petroleum-based diesel fuel.

◆ Biogas is a mixture of methane and carbon dioxide produced by the anaerobic decomposition of organic matter such as sewage and municipal wastes by bacteria. It is used especially in the generation of hot water and electricity.