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 Biofuel is any fuel that is derived from biomass - recently living organisms or their metabolic byproducts, such as manure from cows. It is a renewable energy source, unlike other natural resources such as petroleum, coal and nuclear fuels. One definition of biofuel is any fuel with an 80% minimum content by volume of materials derived from living organisms harvested within the ten years preceding its manufacture.
Like coal and petroleum, biomass is a form of stored solar energy. The energy of the sun is "captured" through the process of photosynthesis in growing plants. One advantage of biofuel in comparison to most other fuel types is it is biodegradable, and thus relatively harmless to the environment if spilled.
Agricultural products specifically grown for use as biofuels include corn and soybeans, primarily in the United States; as well as flaxseed and rapeseed, primarily in Europe; sugar cane in Brazil and palm oil in South-East Asia. Biodegradable outputs from industry, agriculture, forestry, and households can also be used to produce bioenergy; examples include straw, timber, manure, rice husks, sewage, biodegradable waste and food leftovers. These feedstocks are converted into biogas through anaerobic digestion. Biomass used as fuel often consists of underutilized types, like chaff and animal waste.
History
 Biofuel was used since the early days of the car industry. Nikolaus August Otto, the German inventor of the combustion engine, conceived his invention to run on ethanol. While Rudolf Diesel, the German inventor of the Diesel engine, conceived it to run on peanut oil. The Ford Model T, a car produced between 1903 and 1926 used ethanol.
However, when crude oil began being cheaply extracted from deeper in the soil (thanks to drilling starting in the middle of the 19th century), cars began using fuels from oil. Nevertheless, before World War II, biofuels were seen as providing an alternative to imported oil in countries such as Germany, which sold a blend of gasoline with alcohol fermented from potatoes under the name Reichskraftsprit. In Britain, grain alcohol was blended with petrol by the Distillers Company Ltd under the name Discol and marketed through Esso's affiliate Cleveland.
Biologically produced alcohols
 Biologically produced alcohols, most commonly ethanol and methanol, and less commonly propanol and butanol are produced by the action of bacteria:
Methanol, which is currently produced from natural gas, can also be produced from biomass - although this is not economically viable at present. The methanol economy is an interesting alternative to the hydrogen economy.
Biomass to liquid, synthetic fuels produced from syngas. Syngas in turn, is produced from biomass by gasification.
Ethanol fuel produced from sugar cane is being used as automotive fuel in Brazil. Ethanol produced from corn is being used mostly as a gasoline additive (oxygenator) in the United States, but direct use as fuel is growing. Cellulosic ethanol is being manufactured from straw (an agricultural waste product). ETBE containing 47% Ethanol is currently the biggest biofuel contributor in Europe.
Biologically produced gases
 Biogas is produced by the process of anaerobic digestion of organic material by anaerobes. Biogas can be produced either from biodegradable waste materials or by the use of energy crops fed into anaerobic digesters to supplement gas yields. The solid output, digestate, can also be used as a biofuel.
Biogas contains methane and can be recovered in industrial anaerobic digesters and mechanical biological treatment systems. Landfill gas is a less clean form of biogas which is produced in landfills through naturally occurring anaerobic digestion. Paradoxically if this gas is allowed to escape into the atmosphere it is a potent greenhouse gas.
Biologically produced oils and gases can be produced from various wastes:
Thermal depolymerization of waste can extract methane and other oils similar to petroleum.
Pyrolysis oil may be produced out of biomass, wood waste etc. using heat only in the flash pyrolysis process. The oil has to be treated before using in conventional fuel systems or internal combustion engines (water + pH).
One company, GreenFuel Technologies Corporation, has developed a patented bioreactor system that utilizes nontoxic photosynthetic algae to take in smokestacks flue gases and produce biofuels such as biodiesel, biogas and a dry fuel comparable to coal.
Applications of biofuels
 One widespread use of biofuels is in home cooking and heating. Typical fuels for this are wood, charcoal or dried dung. The biofuel may be burned on an open fireplace or in a special stove. The efficiency of this process may vary widely, from 10% for a well made fire (even less if the fire is not made carefully) up to 40% for a custom designed charcoal stove. Inefficient use of fuel may be a minor cause of deforestation (though this is negligible compared to deliberate destruction to clear land for agricultural use) but more importantly it means that more work has to be put into gathering fuel, thus the quality of cooking stoves has a direct influence on the viability of biofuels.
Direct biofuel
 Direct biofuels are biofuels that can be used in existing unmodified petroleum engines. Because engine technology changes all the time, exactly what a direct biofuel is can be hard to define; a fuel that works without problem in one unmodified engine may not work in another engine. In general, newer engines are more sensitive to fuel than older engines, but new engines are also likely to be designed with some amount of biofuel in mind.
Straight vegetable oil can be used in some (older) diesel engines. Only in the warmest climates can it be used without engine modifications, so it is of limited use in colder climates. Most commonly it is turned into biodiesel. No engine manufacturer explicitly allows any use of vegetable oil in their engines.
Biodiesel can be a direct biofuel. However, no current manufacturer covers their engine under warranty for 100% biodiesel some have allowed 100% in the past, and it appears that changes in emission standards are the only reason they don't today, but no official statement exists. Many people have driven thousands of miles on biodiesel without problem, and many studies have been made on 100% biodiesel.
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