Ethanol, derived from plant material, is a renewable energy source because the production and combustion of ethanol completes a cycle. The amounts of carbon dioxide and water that plants use are returned back to the atmosphere when ethanol, made from plant starch or cellulose, is combusted. Ethanol is considered a "green" fuel because it is nontoxic and reduces air pollution. The 13.3 billion gallons of ethanol that were produced in the United States in 2013 reduced greenhouse gas emissions by 38 million metric tons, equivalent to removing 8 million vehicles from the road, according to the Renewable Fuels Association.
Ethanol from Plants
Ethanol is generated from glucose that is derived from plant material. Glucose can be obtained from plant starch or cellulose. In the United States, most of the ethanol is derived from the starch in corn kernels. Other countries may use other plants. For example, Brazil uses sugarcane to make ethanol. Although it is relatively easy to produce ethanol from corn starch, growing large amounts of corn for energy production may cause shortages in corn food production. Nonfood plants, such as wheat straw or switch grass, contain cellulose, which is a polymer of glucose molecules. Ethanol obtained from cellulose requires pretreatment of plant material to soften it and make the cellulose more susceptible to hydrolysis, which is the breakdown of the cellulose polymer.
The production of ethanol from plant material and ethanol's subsequent combustion form a complete cycle. Glucose within plants is synthesized from carbon dioxide and water by photosynthesis, a process that uses solar energy. Yeast fermentation generates ethanol from glucose. When ethanol is used as an energy source and combusted, it produces carbon dioxide and water. Plants convert the carbon dioxide and water back to glucose, completing the cycle.
Energy Use in Ethanol Production
Although ethanol is a renewable source of energy, energy is needed to collect and transport plant material to the production facility. More energy is consumed in the fermentation process. Ethanol production has a positive energy balance, however, because the solar energy, captured during photosynthesis by plants and stored as glucose, produces more energy upon combustion of ethanol than was used in the production of ethanol. For example, one unit of energy, used in the production of ethanol from corn, produces 2.3 units of usable energy from ethanol, according to U.S. Department of Agriculture research.
Ethanol as Fuel
In the early days of motor vehicles, Henry Ford and others thought that ethanol would become the primary fuel. Today, about 95 percent of U.S. gasoline contains low amounts of ethanol, usually 10 percent. Flexible fuel vehicles, more common in Brazil, can run on gasoline or high-ethanol blends. High-ethanol blends contain 85 percent or more of ethanol. Ethanol has a higher octane rating than gasoline and helps to prevent engine knocking. Even low-level ethanol blends have a higher octane rating than gasoline. Compared to gasoline, ethanol contains less energy per gallon, but the higher octane rating increases vehicle performance.
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