Renewable source of energy, unlike fossil fuels, such as petroleum, coal, and natural gas. Some long-exploited biofuels, such as wood, can be used directly as a raw material that is burned to produce heat. The heat, in turn, can be used to run generators in a power plant to produce electricity. A number of existing power facilities burn grass, wood, or other kinds of biomass.
Liquid biofuels are of particular interest because of the vast infra-structure already in place to use them, especially for transportation. The liquid biofuel in greatest production is ethanol (an alcohol), which is made by fermenting starch or sugar. In the United States—the leading producer—ethanol biofuel is made primarily from corn grain, and it typically is blended with gasoline to produce a fuel that is 10% ethanol. In Brazil, which had been the leading producer until 2006, ethanol biofuel is made primarily from sugarcane, and it is commonly used as 100% ethanol fuel or in gasoline blends containing 85% ethanol. The second most common liquid biofuel is biodiesel, which is made primarily from oily plants (such as the soybean or oil palm) and to a lesser extent from other sources (such as cooking waste from restaurants). Biodiesel, which has found greatest acceptance in Europe, is used in diesel engines, usually blended with petroleum diesel in various percentages.
In March the US Department of Energy announced that it would be investing as much as US$385 million in six refineries for cellulosic ethanol.”
Air during combustion will have been removed from the air earlier when the combustible material grew. Such a material is said to be carbon-neutral. In practice, however, the industrial production of agricultural biofuels can result in additional emissions of greenhouse gases that can offset the benefits of using a renewable fuel. These emissions include carbon dioxide from the burning of fossil fuels to produce the biofuel and nitrous oxide from soil.
Other biofuels include methane gas, which can be derived from the decomposition of biomass in the absence of oxygen, and methanol, butanol, and dimethyl ether, which are in development. Much focus is on the development of methods to produce ethanol izer. In this regard, cellulosic biomass is considered to be more beneficial.
Land use is also a major factor in evaluating the benefits of biofuels. Corn and soybeans are important foods, and their use in producing fuel can therefore affect the economics of food price and availability. In 2007 about one-fifth of US corn output was used for biofuel, and one study showed that even if all US corn land was used to produce ethanol, it could replace just 12% of gasoline consumption. Crops grown for biofuel can also compete for the world’s natural habitats. For example, emphasis on ethanol derived from corn is shifting grasslands and brush-lands to corn monocultures, and emphasis on biodiesel is bringing down ancient tropical forests to make way for palm plantations. Loss of natural habitat can change hydrology, increase erosion, and generally reduce biodiversity and wildlife areas. The clearing of land can also result in the sudden release of a large amount of carbon dioxide as the plant matter it contained decays.
Some of the disadvantages apply mainly to low-diversity biofuel sources—corn, soybeans, sugarcane, oil palms—which are traditional agricultural crops. An alternative recently proposed would use high-diversity mixtures of species, with the North American tall-grass prairie as a specific example. Converting degraded agricultural land presently out of production to such high-diversity biofuels could increase wildlife area, reduce erosion, cleanse waterborne pollutants, store carbon dioxide from the air as carbon compounds in the soil, and ultimately restore fertility to degraded lands. Such biofuels could be burned directly to generate electricity or converted to liquid fuels as technologies develop.
The proper way to grow biofuels to serve all needs simultaneously will continue to be a matter of much experimentation and debate, but the fast growth in biofuel production will likely continue. In the European Union, for example, 5.75% of transport fuels are to be biofuels by 2010, with 10% of its vehicles to run exclusively on biofuels by 2020. In December 2007 US Pres. George W. Bush signed into law the Energy Independence and Security Act, which mandated the use of 136 billion liters (36 billion gallons) of biofuels annually by 2020, more than a sixfold increase over 2006 production levels. The legislation required, with certain stipulations, that 79 billion liters (21 billion gallons) of the amount be biofuels other than corn-derived ethanol. In addition the law continued government subsidies and tax incentives for biofuel production. Some observers hoped that the law would encourage the commercialization of technology for producing cellulosic ethanol, for which there were a number of pilot plants in the United States.
The distinctive promise of biofuels not shared by other forms of renewable energy, such as solar power, is that in combination with an emerging technology called carbon capture and storage, biofuels are capable of perpetually removing carbon dioxide from the atmosphere. Under this vision biofuels would remove carbon dioxide from the air as they grew, energy facilities would capture that carbon dioxide when the biofuels were later burned for power, and then the captured carbon dioxide would be sequestered (stored) in long-term repositories such as geologic formations beneath the land, in sediments of the deep ocean, or conceivably as solids such as carbonates. With proper planning, therefore, biofuels have the potential to help create the conditions necessary for a sustainable world.
Clarence Lehman is an Adjunct Professor in the Department of Ecology, Evolution, and Behavior at the University of Minnesota.