Energy crops are plants grown specifically for fuel, divided into herbaceous (grasses) and woody (trees and shrubs) categories. Switchgrass is a popular choice due to high yields, low production costs, and resistance to pests and drought. Energy crops are used for electricity generation and biofuel production, with butanol, ethanol, and biodiesel being common products.
Energy crops are plants grown and harvested with the specific intent of using them as fuel. These plants are typically divided into two categories: herbaceous, such as grasses, and woody, such as trees and shrubs. The energy stored in the plants is accessible by direct combustion, gasification and conversion into liquid fuels. Energy crops are generally chosen for their energy content, the ease with which they can be grown and harvested, as well as the related processing costs of the final product. The inedible parts of food plants used for the same purpose are considered agricultural byproducts, not energy crops.
Several types of grasses show potential as an energy source, with switchgrass generating the most interest; yields are generally higher and production costs lower than for other herbaceous plants. Switchgrass can reach full productive capacity within three years and grows from a permanent root system, requiring no replanting for up to 15 years. Only a quarter of the amount of water and fertilizer is required for most food crops, and the grass is remarkably resistant to pests and drought. Standard farm equipment can be used to harvest branch grass, which is baled just like hay.
Trees and shrubs used for energy crops are generally not grown to maturity, as lumber is not the final product. Short rotation woody crops (SRWC) are grown with an expected harvest within four to ten years, while fast-growing hardwoods, such as poplar and willow, can grow up to ten feet per year under ideal conditions. Cultivated woodlands also produce up to ten times more wood per acre than natural forests.
The two main uses of energy crops are for the generation of electricity and as a feedstock for the production of biofuels. A power plant could use crops in a steam boiler, burning them directly or together with coal, a practice called co-firing. Through the gasification process, biomass can be transformed into syngas, a mixture of carbon monoxide and hydrogen, or methane. It can be used to ignite steam turbines or as an energy source for manufacturing.
Butanol, a long-chain hydrocarbon similar to gasoline, can be produced by herbaceous plants such as grass creepers, miscanthus and elephant grass. Ethanol, an alcoholic fuel, is made from wheat, corn, sugar cane, or any plant that can be used to make an alcoholic beverage, while biodiesel fuel can be made from vegetable oils produced from energy crops such as soybeans, canola, and hemp. The lipids, or fats, in the oil are reacted with the alcohol to produce biodiesel. While some biofuels can power vehicles directly, most are used in blends with traditional fuels.
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