Energy crops are plants grown and harvested with the specific intent to use them as fuel. These plants are typically divided into two categories: herbaceous, such as grasses, and woody, like trees and shrubs. Stored energy in the plants may be accessed 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 associated end-product processing costs. The non-edible 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 typically higher and production costs lower than for other herbaceous plants. Switchgrass can reach full yield capacity within three years and grows from a permanent root system, not requiring replanting for up to 15 years. Only one-quarter the amount of water and fertilizer necessary for most food crops is required and the grass is remarkably pest and drought resistant. Standard farm equipment can be used to harvest switchgrass, which is baled just like hay.
Trees and shrubs used for energy crops are not generally grown to maturity, as lumber is not the end product. Short rotation woody crops (SRWC) are cultivated 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 stands of trees also produce up to ten times more wood per acre than natural forests.
The two main uses for energy crops are to generate electricity and as a raw material for biofuel production. A power plant might use the crops in a steam boiler, burning them directly or along with coal, a practice called cofiring. By the process of gasification, biomass can be processed into syngas, a mixture of carbon monoxide and hydrogen, or methane. Either can be used to fire steam turbines or as an energy source for manufacturing.
Butanol, a long-chain hydrocarbon similar to gasoline, can be produced from herbaceous plants such as switchgrass, miscanthus and elephant grass. Ethanol, an alcohol fuel, is made from wheat, corn, sugarcane 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 soy, rapeseed and hemp. Lipids, or fats, in the oil are reacted with alcohol to produce the biodiesel. Although some biofuels can directly power vehicles, most are used in mixture with traditional fuels.