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Biofuel  

A biofuel is a fuel that is produced over a short time span from biomass rather than by the extremely slow natural processes that create fossil fuels. This term is usually used only for liquid or gaseous fuels that are used for transportation and does not encompass solid biomass that can be used directly as a fuel. Biofuel can be produced from purposely grown plants and from agricultural, household, and industrial waste. Its greenhouse gas mitigation potential varies considerably, ranging from emission levels higher than or comparable to fossil fuels to negative emissions. The two most common type of biofuels are bioethanol and biodiesel, with the U.S. the largest producer of the former and the EU the largest producer of the latter.

Biofuels are often classified in terms of generations. First generation biofuels are fuels made from food crops grown on arable land and whose sugar, starch, or oil content is converted into biodiesel or ethanol using yeast fermentation. Second generation biofuels are made from woody biomass or agricultural residues and waste that are byproducts of the main crop or are grown on marginal land. Third generation biofuels, which are produced from algae, offer several important advantages including that the algae can be genetically modified to produce various kinds of fuel directly, that the fuels can be easily refined, and that the yields per acre are far higher than for other biofuels. However, they have the still unsolved disadvantage of requiring very large amounts of water, nitrogen and phosphorus for growing.