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Biomass is organic material made by plants and animals. Biomass contains stored energy from the sun. At the bottom of the food chain, plants absorb solar radiation through photosynthesis. They then convert it into chemical energy in the form of glucose, or sugar. The chemical energy in plants gets passed on to people and animals that consume them. Animals use glucose as a source of energy.
Burning biomass releases its stored chemical energy as thermal, or heat energy. But, burning solid biomass is not the only way to release its energy. Biomass can be converted to other liquid and gaseous forms. Biofuel is the name given to a flammable liquid fuel produced from biomass.
The two most common types of biofuels are ethanol and biodiesel. Ethanol is also called ethyl alcohol, pure alcohol, or grain alcohol. It is a clear, colorless liquid commonly found in alcoholic beverages produced through fermentation and distillation. Biodiesel is a liquid which varies in color between golden and dark brown, depending on the material from with it is made.
Both ethanol and biodiesel are used primarily for transportation. Motorized vehicles typically have internal combustion engines. These vehicles are powered by the burning of a fuel, mainly gasoline or diesel, in a confined space called a combustion chamber within the engine. Because biofuels are flammable, they can also be used for internal combustion and power motorized vehicles. In this way, biofuels serve as a substitute for fossil fuels that would otherwise be used.
Typically, biofuel is not used on its own, but is blended with a petroleum-based fuel. Any gasoline powered vehicle can use a fuel mixture that contains up to 10% ethanol without causing damage to the engine or fuel system. Cars and light trucks built after 2007 have been modified to handle up to 15% ethanol. Some specially built vehicles, known as flexible fuel vehicles, are able to use fuel mixtures that are up to 85% ethanol. In 2011, almost 14 billion gallons (about 53 billion liters) of ethanol were added to the gasoline consumed in the United States. Almost all of the gasoline now sold in the United States contains some ethanol.
Biodiesel use is relatively small compared to ethanol. In 2011, a little over 800 million gallons (about 3 billion liters) of biodiesel was produced in the United States. Diesel fuel is used by trucks, buses, and tractors. Diesel fuel allows for these vehicles to carry and pull heavy loads better than gasoline. Diesel-based fuels that contain up to 20% biodiesel can be used by most recently built diesel-powered vehicles. Older vehicles may require engine modifications before they can handle biodiesel. The biodiesel industry in the U.S. remains small and biodiesel is available only at some gas stations.
The plant or animal material that is used to make a biofuel is called a feedstock. Almost any plant-based material can be an ethanol feedstock, but some plants are easier to process into ethanol than others. Today, ethanol is produced from three main types of feedstocks: starch-based, sugar-based, or cellulosic.
Starch-based feedstocks are grains, such as corn, sorghum, wheat and barley. These feedstocks contain long complex chains of sugar molecules. Corn is the feedstock for more than 90% of ethanol production in the United States due to its abundance and low price. Most ethanol is produced in the corn-growing states of the Midwest.
Sugar-based feedstocks contain smaller simple sugar molecules. These feedstocks include sugar cane and sugar beets. Sugar-based feedstocks are more commonly used outside of the United States. Brazil is the world's second-largest ethanol producer after the United States. It makes most of its ethanol from sugar cane.
Cellulosic-based feedstocks include trees, grasses, and the unused stalks and leaves of crops. These feedstocks contain cellulose, a rigid molecule used to make the cell walls for the leaves, stems, stalks, and woody portions of plants. Cellulosic ethanol involves a more complicated production process than ethanol made from starch- and sugar-based feedstocks. It is produced on a very small scale at this time. However, much research is being conducted to advance the production of cellulosic ethanol.
Biodiesel is made from vegetable oils and animal fats. Vegetable oil feedstocks include rapeseed and soybean. Soybean oil is the most commonly used raw material for biodiesel in the United States and accounts for about 90 percent of all biodiesel feedstocks. Waste vegetable oil discarded by restaurants can also be used as a biodiesel feedstock. In addition, animal-based biodiesel feedstocks include lard, chicken fat and fish oil.
Almost all ethanol produced in the United States comes from starch-based feedstocks, primarily corn. There are two types of facilities that produce ethanol from starch-based feedstocks: dry mill and wet mill. Dry mill is the most common type of production method in the U.S., due to the lower costs involved in setting up a dry mill facility. The main difference between dry mill and wet mill is in the initial treatment of the grain.
In a dry-mill facility, the corn or other grain is first ground into a fine powder. Next, the powder is mixed with water. Enzymes are then added that convert the starch in the mixture into simple sugars. The mixture is then heated to kill off any bacteria. After the mixture has cooled, yeast is added. During this step, the yeast eats the sugar and in the process produces heat, ethanol, and carbon dioxide. This conversion takes between 40 and 50 hours. After this, the mixture is pumped through a series of columns and sieves that separate the ethanol. Finally, the ethanol is pumped into storage tanks. It is then ready for shipment to gasoline terminals or retailers.
In a wet-mill facility, the first step is to soak the grain in hot water for 24 to 48 hours. The wet grain is then passed through a series of grinders, centrifuges and screens that separate the grain into its component parts, including starch. The separated starch is then treated with enzymes, heat, and yeast, in a manner similar to a dry-mill facility, in order to produce ethanol.
The process of producing ethanol from sugar-based feedstocks is simpler than converting starch-based feedstocks. Sugar-based feedstocks do not need to be heated or the addition of enzymes. Instead, they need only to be treated with yeast, which eats the simple sugars contained in the feedstocks to produce ethanol.
Producing ethanol from cellulosic feedstocks is more complicated because it is difficult to break down cellulosic materials into usable sugars that can be treated with yeast. Cellulose is composed of long chains of sugar molecules. Current methods for breaking these chains into simpler sugars include treating the feedstocks with an acid, such as sulfuric acid. Other methods include heat treatment and exposure to expensive, specially selected enzymes.
Biodiesel is primarily produced by chemically reacting alcohol (commonly methanol) and the fats within biodiesel feedstocks. Substances are added to the mixture to speed up the chemical reaction. This reaction creates organic, chemical compounds called esters which are the main components of biodiesel. Another by-product is glycerin, which is removed from the mixture because it can damage engines.
Biofuels offer a number of benefits over fossil fuels. They can extend fuel supplies and reduce dependency on imported fossil fuels. Biofuels are considered a renewable energy source because they are made from crops that can be replanted. Fossils fuels, on the other hand, are considered nonrenewable because they exist in limited quantities. Burning biofuels releases carbon dioxide into the atmosphere, much like fossil fuels do. However, the plants from which biofuels are made also take carbon dioxide out of the atmosphere during photosynthesis. As a result, biofuels do not contribute to greenhouse gases, unlike fossil fuels. Biofuels are also biodegradable and safer to handle than fossil fuels, making spills less hazardous and easier to clean up. As the technology used to produce biofuels becomes more widely available, their costs will decrease. It is possible that in the future, they will become less expensive than fossil fuels which will only become more expensive as reserves are depleted.
There are also a number of disadvantages to biofuels. One concern is that production of biofuels could lead to rising food costs. For example, the most common feedstock used to produce ethanol is corn. Corn is used in many types of manufactured foods. As demand for food crops such as corn grows to produce biofuel, it could also raise the prices of foods that contain these crops. In addition, farmland has to be used to grow profitable biofuel feedstocks. There is concern that growing these feedstocks will take the place of growing food, which could lead to a food shortage. Another worry is that more land will have to be cleared in order to grow more crops for an increasing biofuel demand. This could lead to the destruction of important ecosystems and cause soil erosion. One more disadvantage is that currently, biofuels are not widely available and most vehicles are not equipped to run on fuels that contain a high percentage of biofuel. This limited availability makes biofuels a less attractive energy source than fossil fuels. Another problem is that biofuel feedstocks require large quantities of water for irrigation, which strains local and regional water resources. Such methods might not be suitable in dryer climates where water is scarce. Finally, biofuel production can produce strong and noxious smells which are undesirable to nearby communities.