BIOFUELS

Biofuels are biomass (organic matter) or biomass products used for energy production. Energy created from the use of biofuels is often termed bioenergy. Biomass crops grown for the primary purpose of use in biofuels are called energy crops. Biofuels include wood and wood wastes, domestic wastes, agricultural crops and wastes, animal wastes, peat, and aquatic plants. Almost any type of combustible organic matter can potentially be used as an energy source.

Plants store solar energy by photosynthesis. During photosynthesis, carbon dioxide (CO₂) and water (H₂O) in the presence of light are converted into glucose (C₆H₁₂O₆) by the following chemical equation: Further processes in the plant make more complex molecules from the glucose. The exact makeup of biomass varies with type, but in general it has the chemical formula of (CH₂O)_n and on average is about 75 percent carbohydrates or sugars and 25 percent lignin, a polymer that holds plant fibers together.

Biofuels are used to create a wide variety of energy sources. Ever since the harnessing of fire, biomass has been used for heating and cooking. Residential burning of biomass continues to be a primary source of fuel in less industrialized nations, but also has been used as fuel for electricity generation, and converted to liquid transportation fuels.

CURRENT USE OF BIOFUELS

Despite the fact that the world's biomass reserves are declining due to competing land use and deforestation, worldwide there remains more energy stored in biomass than there is in the known reserves of fossil fuels. Trees account for the largest amount of biomass. Currently biomass is the source of about percent of the energy used worldwide, primarily wood and animal dung used for residential heating and cooking. In developing countries, where electricity and motor vehicles are more scarce, use of biofuels is significantly higher (approximately 35 percent on average). At the higher end are countries such as India, where about 55 percent of the energy supply comes from biomass. Geography also is a determining factor; in some industrialized countries that have large sources of natural biomass forests near urban cities, such as Finland, Sweden, and Austria, there is a relatively high utilization of bioenergy (18, 16, and 13 percent, respectively). Municipal waste, which can be incinerated for energy production, also can be a large source of biomass for developed regions. France, Denmark and Switzerland recover 40, 60, and 80 percent of their municipal waste respectively.

At the low end is the United States, where biomass energy accounted for only about 3 percent (2.7 quadrillion Btus) of the total energy consumption in 1997. However, biomass use had been rising over the previous five years at an average rate of about 1 to 2 percent per year, but fell in 1997 due to a warmer-than-average heating season. Bioenergy produced in the United States is primarily from wood and wood waste and municipal solid waste.

These divergent energy production patterns between the developing world and the United States are understandable. Heating and cooking are the major uses of biomass in the developing world because of affordability, availability, and convenience. In the United States, where clean and convenient natural gas, propane, and electricity are widely available and affordable, biomass use has limited potential. Nevertheless, U.S. biomass energy production has been increasing because of technological advances for new and improved biomass applications for electricity generation, gasification, and liquid fuels.

The sources of biofuels and the methods for bioenergy production are too numerous for an exhaustive list to be described in detail here. Instead, electricity production using direct combustion, gasification, pyrolysis, and digester gas, and two transportation biofuels, ethanol and biodiesel, are discussed below.

ELECTRICITY GENERATION

In the United States about 3 percent of all electricity produced comes from renewable sources; of this a little more than half comes from biomass. Most biomass energy generation comes from the lumber and paper industries from their conversion of mill residues to in-house energy. Municipal solid waste also is an important fuel for electricity production; approximately 16 percent of all municipal solid waste is disposed of by combustion. Converting industrial and municipal waste into bioenergy also decreases the necessity for landfill space.

These applications avoid the major obstacles for using biomass for electricity generation: fluctuation in the supply, and the type of biomass available. Seasonal variations and differing quality of feedstock are the biggest barriers to more widespread use. This is especially true for biomass wastes.

COMBUSTION

Combustion is the burning of fuels to produce heat. To produce energy, the heat from the combustion process is used to create steam, which in turn drives turbines to produce electricity.

Most electricity from biofuels is generated by direct combustion. Wood fuels are burned in stoker boilers, and mill waste lignin is combusted in special burners. Plants are generally small, being less than 50 MW in capacity. There is considerable interest in combustion of biomass in a process called cofiring, when biomass is added to traditional fuels for electricity production. Cofiring is usually done by adding biomass to coal, but biomass also can be cofired with oil. There are several biomass cofiring plants in commercial operation in the eastern United States. The U.S. Department of Energy estimates that by 2020 the capacity for biomass cofiring could reach 20 to 30 GW. Cofiring has the advantage of requiring very little capital cost since most boilers can accommodate approximately 5 to 10 percent of biomass without modifications.

Estimates for delivery fuel costs for woody biomass range between $1.25 and $3.90 per million Btus compared to $0.90 to $1.35 per million Btus for coal. The cost associated with biomass electricity depends largely on the proximity of the plant to the biomass source and whether the feed is a waste material. At 10,000 Btu/kWh generation heat rate, each $1 per million Btus translates to 1 cent per kWh electrical cost. Thus biomass electricity costs can range from competitive with coal to several cents per kWh more expensive.

Cofiring biomass has environmental benefits in addition to lowering greenhouse gases. Since biomass has little or no sulfur, sulfur dioxide (SO₂) emissions are less when biomass fuels are used. In the United States, power plants have allowable sulfur dioxide levels for each gigawatt of power produced. If they produce less than the allowable amount of sulfur dioxide, they receive credits with which they can trade on the open market. The price for these sulfur dioxide credits is about $70 to $200 per ton.

Biomass also has lower levels of nitrogen than fossil fuels, leading to lower nitrogen oxide formation. The high water content in biomass also lowers the combustion temperature, decreasing the formation of thermal nitrogen oxides. In some cases this can lead to nonlinear reductions; for example, in one study when 7 percent wood was cofired with coal, nitrogen oxides emissions decreased by 15 percent. However, such reductions are not seen in all cases. Reburning is possible when using most biomass feedstocks and also can lower emissions.

Use of some biomass feedstocks can increase potential environmental risks. Municipal solid waste can contain toxic materials that can produce dioxins and other poisons in the flue gas, and these should not be burned without special emission controls. Demolition wood can contain lead from paint, other heavy metals, creosote, and halides used in preservative treatments. Sewage sludge has a high amount of sulfur, and sulfur dioxide emission can increase if sewage sludge is used as a feedstock.

GASIFICATION

Gasification of biofuels, which is in the early developmental stage, has been the focus of much recent research, since it has the potential of providing high conversion. During gasification, biomass is converted to a combustible gas by heating with a substoichiometric amount of oxygen. The biomass can be heated either directly or with an inert material such as sand. In some cases steam is added. The product gas consists of carbon monoxide, methane and other hydrocarbons, hydrogen, and noncombustible species such as carbon dioxide, nitrogen, and water; the relative amount of each depends on the type of biomass and the operating conditions. Generally the product gas has an energy content about one-half to one-quarter that of natural gas. The gas is cleaned by removing tars, volatile alkali, ash, and other unwanted materials. The gas is then sent to a steam boiler or combustion turbine for electricity production by a Rankine cycle or a combined cycle (IGCC). Use of gasification technology with an IGCC can double the efficiency of average biomass electricity production using advanced turbine technology.

The capital cost of an IGCC plant for biomass or coal is in the range of $1,500 to $2,000 per installed kW. A comparable natural gas fire facility costs about $750 to $1,000. The economics of biomass electricity based on IGCC technology depend on the relative cost of natural gas and biomass fuels. Biomass must be lower in cost than gas to pay back the additional capital cost of gas production and cleaning. A 1999 estimate suggestes that the biomass would have to be $3 per million Btus cheaper than natural gas for biomass to be economical.

PRYOLYSIS

Another emerging area in biofuels is pyrolysis, which is the decomposition of biomass into other more usable fuels using a high-temperature anaerobic process. Pyrolysis converts biomass into charcoal and a liquid called biocrude. This liquid has a high energy density and is cheaper to transport and store than the unconverted biomass. Biocrude can be burned in boilers or used in a gas turbine. Biocrude also can be chemical by altered into other fuels or chemicals. Use of pyrolysis may make bioenergy more feasible in regions not near biomass sources. Biocrude is about two to four times more expensive than petroleum crude.

BIOGAS PRODUCTION

Biogas is composed primarily of methane (CH₄) and carbon dioxide. Biogas is a by-product from anaerobic bacteria breaking down organic material. Large amounts of biogas can be released from areas such as livestock waste lagoons, sewage treatment plants, and landfills. Since biogas is primarily methane, it is similar to natural gas and can be used for energy generation, especially electricity using stationary engine-generators. The goals of capturing biogas are often to prevent these greenhouse gases from being released into the atmosphere, to control odor, and to produce fertilizer; energy production is secondary. Methane is a potent greenhouse gas, with twenty-one times the global warming potential of carbon dioxide. However, when methane is burned, it produces less carbon dioxide per Btu than any other hydrocarbon fuel.

Economics for generating electricity from biogas can be favorable. Landfill gas from municipal solid waste can supply about 4 percent of the energy consumed in the United States. In 1997, a total of 90 trillion Btus were generated by landfill gas, about 3 percent of total biomass energy consumption.

TRANSPORTATION FUELS

Although biomass used directly for heating and cooking is the thermodynamically most efficient use, followed by use for electricity generation, the economics are much more favorable to convert to a liquid fuel. Economic considerations outweigh thermodynamics; as an electricity generator, biomass must compete with relatively low-priced coal, but as a liquid fuel the competition is higher-priced oil.

Transportation fuels are the largest consumers of crude oil. Petroleum-based transportation fuels are responsible for 35 percent of greenhouse gas emissions in the United States. Only percent of transportation fuels comes from renewable nonpetroleum-based sources, primarily from the use of corn-based ethanol blended with gasoline to make gasohol. Increased use of biofuels could lower some of the pollution caused by the use of transportation fuels.

ETHANOL

The chemical formula for ethanol is CH₃CH₂OH. Ethanol is less toxic and more biodegradable than gasoline. For its octane boosting capability ethanol can be use as a fuel additive when blended with gasoline.

Demand for gasoline is 125 billion gals (473 billion l) per year according to 1998 estimates. The Clean Air Act Amendment of 1990 mandates the use of oxygenated fuels such as ethanol blends with up to 3.5 percent oxygen by weight in gasoline (E-10 or gasohol). Reformulated gasoline (RFG) is required year-round in areas that are not in compliance for ozone, and oxyfuels are required in the winter in areas that are not in compliance for carbon monoxide. These "program gasolines" total about 40 billion gals (151 billion l) per year.

In 1997 a total 1.3 billion gals of ethanol fuel was produced in the United States. Proposed new low sulfur conventional gasoline standards could greatly increase the demand for ethanol since desulfurization may lower gasoline octane. Almost all fuel ethanol is used as gasohol, but some is used to make E-85 (85% ethanol and 15% gasoline). E-85 can be used in flexible-fuel vehicles (FFVs) which can operate on gasoline or ethanol blends of to 85 percent ethanol.

Eighty-seven percent of the ethanol produced in the United States comes from corn. The remainder comes from milo, wheat, food wastes, and a small amount from wood waste. In Brazil, the largest producer of transportation biofuels, sugar cane is converted into ethanol at the rate of 16 billion l per year. There are 3.6 million cars in Brazil that run on 100 percent ethanol.

Ethanol is more costly to produce than gasoline. The cost of production of ethanol from corn ranges from about $0.80 per gal ($0.21 per l) for large depreciated wet mills to $1.20 per gal ($0.32 per l) for new dry mills. Better engineering designs, the development of new coproducts, and better uses for existing coproducts will help to lower the production cost. For example, recovering the corn germ in dry mills, which is currently in the development stage could lower ethanol production costs by $0.07 to $0.20 per gal ($0.02 to $0.05 per l). However, ethanol currently used for fuel is not competitive with gasoline without a federal excise tax exemption.

While the corn-to-ethanol industry is mature, conversion of energy crops to ethanol is in the commercial development stage. Engineering studies in 2000 estimate the cost of production per gallon for biomass ethanol at $1.22 per gal ($ 0.32 per l). The U.S. Department of Energy projects that technical advances can lower the cost to $0.60 per gallon. This would make ethanol competitive (without a tax exemption ) on an energy basis with gasoline when petroleum is $25 per barrel.

To use biomass material, ethanol needs to be produced from the cellulose portion of the biomass, not just from the starch or sugars. Cellulose is more resistant to breakdown than starch or sugars, so different production methods are required. Acid-catalyzed reactions can be used for the breakdown of cellulose into products that can be converted into alcohol. This process, however, is expensive, and there are problems with the environmental disposal of dilute acid streams. Research for the development of an enzyme to break down cellulose began after World War II. It was discovered that a specific microbe, Trichoderma reesei, was responsible for the decomposition of canvas (cellulose) tents in tropical areas. Research on this microbe and others is being conducted. Using genetic engineering, new enzymes are being produced with the primary goal to increase efficiency of alcohol production from cellulose.

BIODIESEL

Biodiesel is diesel fuel produced from vegetable oils and other renewable resources. Many different types of oils can be used, including animal fats, used cooking oils, and soybean oil. Biodiesel is miscible with petroleum diesels and can be used in biodiesel-diesel blends. Most often blends are 20 percent biodiesel and 80 percent traditional diesel. Soy diesel can be used neat (100%), but many other types of biodiesel are too viscous, especially in winter, and must be used in blends to remain fluid. The properties of the fuel will vary depending on the raw material used. Typical values for biodiesel are shown in Table 1.

Biodiesel does not present any special safety concerns. Pure biodiesel or biodiesel and petroleum diesel blends have a higher flash point than conventional diesel, making them safer to store and handle. Problems can occur with biodiesels in cold weather due to their high viscosity. Biodiesel has a higher degree of unsaturation in the fuel, which can make it vulnerable to oxidation during storage.

To produce biodiesel, the oil is transformed using

a process of transesterification; agricultural oil reacts with methanol in the presence of a catalyst to form esters and glycerol. These monoalkyl esters, otherwise known as biodiesel, can operate in traditional diesel combustion-ignition engines. Glycerol from the transesterification process can be sold as a coproduct. Low petroleum prices continue to make petroleum-based diesel a more economical choice for use in diesel engines.

Current consumption of transportation diesel fuel in the United States is 25 billion gal (94.6 billion l) per year. The total production of all agricultural oils in the United States is about 2 billion gal (7.6 billion l) per year of which 75 percent is from soybeans. Total commodity waste oils total about 1 billion gal (3.8 billion l) per year. The amount of other truly waste greases cannot be quantified. Sewage trap greases consist of primarily free fatty acids and are disposed of for a fee. Trap greases might amount to 300 million gal (1.1 billion l) per year of biodiesel feedstock. The production of biodiesel esters in the United States in 1998 was about 30 million gal (114 million l). The most common oil used is soybean oil, accounting for 75 percent of oil production used for most biodiesel work. Rapeseed oil is the most common starting oil for biodiesel in Europe.

Production costs for biodiesel from soybean oil exceeds $2.00 per gal ($0.53 per l), compared to $0.55 to $0.65 per gal ($0.15 to $0.17 per l) for conventional diesel. The main cost in biodiesel is in the raw material. It takes about 7.7 lb (3.5 kg) of soybean oil valued at about $0.25 per lb (0.36 per kg) to make 1 gal (3.8l) of biodiesel. Waste oils, valued at $1 per gal ($3.79 per l) or less, have the potential to provide low feedstock cost. However, much "waste oil" is currently collected, reprocessed as yellow and white greases, and used for industrial purposes and as an animal feed supplement. Production of biodiesel from less expensive feedstocks such as commodity waste oil still costs more than petroleum diesel. Research has been done to develop fast-growing high-lipid microalgae plants for use in biodiesel production. These microalgae plants require high amounts of solar radiation and could be grown in the southwestern United States.

In addition to greenhouse benefits, biodiesels offer environmental advantages over conventional diesel. Biodiesels produce similar NO_x emissions to conventional diesel, fuel but less particulate matter. Biodiesel is more biodegradable that conventional diesel making any spills less damaging in sensitive areas. In general biodiesel provides more lubrication to the fuel system than low-sulfur diesel.

ENERGY INPUT-ENERGY OUTPUT OF BIOFUELS

Since the Sun, through photosynthesis, provides most of the energy in biomass production, energy recovered from biofuels can be substantially larger than the nonsolar energy used for the harvest and production. Estimates on conversion efficiency (energy out to non-solar energy in) of ethanol can be controversial and vary widely depending on the assumptions for type of crop grown and farming and production methods used. Net energy gain estimates for converting corn to ethanol vary between 21 and 38 percent. Conversion efficiencies can be increased if corn stover (leaves and stocks) is also used and converted to ethanol. Research is being conducted on converting other crops into ethanol. Switchgrass, a perennial, is one of the most promising alternatives. It has a net energy gain as high as 330 percent since it only has to be replanted about every ten years and because there are low chemical and fertilizer requirements. Net energy gains for the production of biodiesel are also high, with estimates ranging between 320 and 370 percent.

FUTURE USE OF BIOFUELS

One of the main benefits from future use of biofuels would be the reduction of greenhouse gases compared to the use of fossil fuels. Carbon dioxide, a greenhouse gas that contributes to global warming, is released into the air from combustion. Twenty-four percent of worldwide energy-related carbon emissions in 1997 were from the United States. Carbon and due to rising energy consumption, are expected to increase 1.3 percent per year through 2015.

When plants grow, they adsorb carbon dioxide from the atmosphere. If these plants are used for biofuels, the carbon dioxide released into the atmosphere during combustion is that which was adsorbed from the atmosphere while they were growing. Therefore the net balance of carbon dioxide from the use of biofuels is near zero. Since some fossil fuel use is required in both the planting and the production of bioenergy, there are some net carbon dioxide and other greenhouse gases released into the atmosphere. In determining the net carbon dioxide balance, important variables include growth rates, type of biomass, efficiency of biomass conversion, and the type of fossil fuel used for production. The amount of carbon accumulated in the soil and the amount of fertilizers used also have a large effect on the carbon balance. In particular, nitrous oxide (N₂O), a powerful greenhouse gas, can be released as a result of fertilizer application. Estimates for the amount of greenhouse emissions recycled using biomass for energy production range from a low of 20 to a high of 95 percent. Wood and perennial crops have higher greenhouse gas reduction potential than annual crops. Using biomass to replace energy intensive materials also can increase the carbon balance in favor of energy crops. It is estimated that the nation's annual carbon dioxide emissions could be reduced by 6 percent if 34.6 million acres were used to grow energy crops.

There is some greenhouse gas benefit from planting forests or other biomass and leaving the carbon stored in the plants by not harvesting. However, over the long term, increased carbon dioxide benefits are realized by using land that is not currently forested for growing some energy crops such as fast-growing poplar. The added benefits come from the displacing fossil fuels by the use of biofuels, since energy crops can be repeatedly harvested over the same land.

In the calculation of greenhouse gas benefits of planting energy crops, many assumptions are made. Among them is that the land will be well managed, appropriate crops for the region will be used, there will be careful use of fertilizers and other resources, and efficient production methods will be employed to get the maximum amount of energy from the biomass. Most importantly, it is assumed that biomass is grown in a sustainable manner. Harvested biomass that is not replanted increases greenhouse gas emissions in two ways: Carbon dioxide that had been previously stored in trees is released in the atmosphere, and future carbon fixation is stopped.

To comply with carbon reduction goals, some countries impose taxes on carbon dioxide emissions. Since biofuels have lower full-cycle carbon dioxide emissions than fossil fuels, biofuels are more cost-competitive with fossil fuels in regions where these taxes are imposed.

Another advantage to using biomass as an energy source is a possible increase in energy security for countries that import fossil fuels. More than two-thirds of the oil reserves are in the Middle East. More than half of the oil consumed in the United States is imported and oil accounts for approximately 40 percent of the trade deficit of the United Sates. A substantial biofuels program could help to the increase energy independence of importing nations and lessen the impact of an energy crisis.

There are some disadvantages with the use of biofuels as well. Some of the high-yield energy crops also have significant removal rates of nutrients from the soil. Each year the cultivation of row crops causes a loss of 2.7 million metric tons of soil organic matter in the United States. However, there are exceptions: Through the use of good farming practices, Brazilian sugarcane fields have had minimal deterioration from the repeated planting of sugarcane. Moreover, using switchgrass and other grasses increases soil organic matter and thus can help in reducing the soil erosion caused by the cultivation of rowcrops. Research is being conducted into improving sustainable crop yield with a minimal of fertilizer application. Possible solutions include coplanting energy crops with nitrogen-fixing crops to maintain nitrogen levels in the soil.

It is estimated that biomass is cultivated at a rate of 220 billion dry tons per year worldwide. This is about ten times worldwide energy consumption. Advocates suggest that by 2050, better use of cultivated biomass could lead to biomass providing 38 percent of the world's direct fuel and 17 percent of electricity generation. However, a large increase in bioenergy seems unlikely. When the U.S. Energy Information Administration (EIA) does not include any new greenhouse gas legislation into its energy utilization projections, only limited growth for renewable energy is predicted. The EIA estimates an average increase of 0.8 percent per year for fuels through 2020 and an average increase of 0.5 percent for renewable electrical generation without new legislation. Most of the increase comes from wind, municipal solid waste, and other biomass. The reason for low expected growth in biofuels is that natural gas and petroleum prices are expected to remain relatively low over the next few decades; in 2020 the average crude oil price is projected to be $22.73 a barrel (in 1997 dollars). The average wellhead price for natural gas is projected to increase from $2.23 per thousand cu ft ($2.17 per million Btus to $2.68 per thousand cu ft ($2.61 per million Btus in 2020 (prices in 1997 dollars). Low fossil fuel prices make it difficult for alternative fuels to compete. Projections for the amount of biomass energy use do rise, however, if it is assumed that the Kyoto protocols limiting greenhouse gases will be adopted, since biofuels contribute fewer greenhouse emissions than do fossil fuels. In the case where greenhouse gas emissions are kept to 1990 levels, renewable energy could account for as much as 22 percent of electricity generation in 2020. Even under this scenario, the biggest change in greenhouse gas emissions comes from a decrease in coal use and an increase in natural gas use.

While considerable amounts of biomass exist as wastes, the costs of collection, storage, transportation, and preparation are high. The largest obstacle for the wider us of biofuels is economics, but niche opportunities exist. Strategies to improve economics include extracting high-valued coproducts from the cellulosic matrix, offsetting disposal costs and mitigating environmental problems by using the waste.

Agricultural wastes such as corn stover (stalks, leaves, etc.) have been proposed as bioenergy sources. The annual planted corn acreage is near 80 million acres, and up to 1.5 tons of stover per acre could be collected. In many farm locations stover has a competitive use as animal feed, but in areas where higher-valued uses do not exist, it may be collected and used as an industrial feedstock. In California, rice straw presents a disposal problem, since burning has been disallowed, and the rice straw could be used for ethanol production. Alfalfa growers in Minnesota are developing a technology to separate stems from the protein-containing leaves. Since protein sources are economically valued on a ton-of-protein basis, the stems are available at essentially no cost for electricity generation. Diversion of demolition wood collected in urban areas from landfills also could yield low-cost fuels. However, if biomass is to become a large component of U.S. energy use, it will have to be grown commercially as an energy crop.

Because the energy density of biomass is much lower than that of fossil fuels, most cost analyses suggest that in order for conversion of biomass to fuels to be economical, the biomass source needs to be close to the processing facility, usually within fifty miles. Lower energy density also means that storage costs can be higher than with fossil fuels, and unlike fossil fuels, it is wholly important that storage time is minimized because weather and bacteria can lower the energy quality of the biomass.

The U.S. Department of Agriculture reports that in 1997 there were 432 million acres of cropland in the United States, of which 353 million acres were planted. Idled cropland accounted for 79 million acres, of which 33 million acres were in the Conservation Reserve Program (CRP). Some planted cropland as well as some or all the idled cropland may be available for energy crops depending on the ability of energy crops to compete economically with traditional crops and on public policy related to the use of CRP land. A 1999 study from University of Tennessee's Agricultural Policy Analysis Center and Oak Ridge National Laboratory used the POLYSYS (Policy Analysis System) model to estimate the amount of land that might be used for energy crops in 2008 based on two different scenarios. Under both scenarios it is assumed that producers are allowed to keep 75 percent of the rental rate paid by the U.S. government for CRP acreage. In both cases, switchgrass was the energy crop with the most economic potential. In the first scenario, it is assumed that the price for energy crops is $30 per dry ton ($2 per million Btus) and there are strict management practices in the CRP; in this case it is estimated that switchgrass would be competitive on 7.4 million acres. In the second scenario, it is assumed that the price for energy crops is $40 per dry ton ($2.70/per million Btus) and that there are lenient management practices in the CRP; under this scenario it is estimated that switchgrass would be competitive on 41.9 million acres. This would result in an increased annual ethanol production on the order of 4 billion to 21 billion gal (15 billion to 79 billion l) compared to the current corn ethanol production of about 1.5 billion gal (5.7 billion l) per year, or sufficient fuel for 6,000 to 36,000 MW of electrical generating capacity. Such a program could provide additional benefit to farmers by reducing the supply of commodity crops and in turn raising crop prices.

With dedicated feedstock supply systems, energy crops are grown with the primary purpose of energy generation. This means that fuel processors and growers will need to enter into long-term fuel supply contracts that provide early incentives to growers to tie up land. Woody species require four to seven years from planting to harvest. Switchgrass crops require approximately two years from planting to first harvest. High-growth species of poplar, sycamore, eucalyptus, silver maple, and willow are all being tested as energy crops. Hybrid species are being developed for pest and disease resistance. Willows have the advantage that common farm equipment can be modified for harvesting. Selection of biomass depends on many factors including climate, soil, and water availability.

Research is being done in the United States and worldwide to lower some of the barriers to biofuels. Researchers hope to develop high-yield, fast-growing feedstocks for reliable biomass fuel supplies. Research is also being done to improve the efficiency of energy conversion technologies so that more of the biomass is utilized.

Deborah L. Mowery

See also: Agriculture; Biological Energy Use, Cellular Processes of; Biological Energy Use, Ecosystem Functioning of; Diesel Fuel; Environmental Economics; Environmental Problems and Energy Use; Fossil Fuels; Gasoline and Additives; Geography and Energy Use; Green Energy; Hydrogen; Methane; Nitrogen Cycle; Renewable Energy; Reserves and Resources; Residual Fuels; Waste-to-Energy Technology.

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