Commercial energy crops are typically densely planted, high yielding crop species. Where the energy crops will be burnt to generate power woody crops such as Miscanthus, Salix or Populus are widely utilised. If carbohydrate content is desired for the production of biogas whole-crops such as maize, sudangrass, millet, white sweet-clover and many others, can be made into silage and the converted into biogas.
Through genetic modification and application of biotechnology plants can be manipulated to create greater yields, reduce associated costs and require less water. However, high energy yield can be realized with existing crops, especially maize.
Note: The terms biofuel, biomass, and so on, are often used interchangeably.
Energy generated by burning plants grown for the purpose, often after the dry matter is pelletized. Energy crops are used for firing power plants, either alone or co-fired with other fuels. Alternatively they may be used for heat or combined heat and power (CHP) production.
Anaerobic digesters or biogas plants can be directly supplemented with energy crops once they have been ensiled into silage. The fastest growing sector of German biofarming has been in the area of "Renewable Energy Crops" (Nachwachsender Rohstoff = "NaWaRo") on nearly 500,000 ha land (2006). Energy crops can also be grown to used boost gas yields where feedstocks have a low energy content, such as manures and spoiled grain. It is estimated that the energy yield presently of bioenergy crops converted via silage to methane is about 20,000 kwh per hectare. Small mixed cropping enterprises with animals can use a portion of their acreage to grow and convert energy crops and sustain the entire farms energy requirements with about 1/5 the acreage. In Europe and especially Germany, however, this rapid growth has occurred only with substantial government support, as in the German bonus system for renewable energy (ranging from €0.02-€0.16/kwh made from renewable sources). Similar developments of integrating crop farming and bioenergy production via silage-methane have been almost entirely overlooked in N. America, where political and structural issues and a huge continued push to centralize energy production has overshadowed positive developments.
European production of biodiesel from energy crops has grown steadily in the last decade, principally focused on rapeseed used for oil and energy. In North America rapeseed was renamed "Canada Oil = Canola". Production of oil/biodiesel from rape covers more than 1.2 million-hectares in Germany alone, and has doubled in the past 15 years. Typical yield of oil as pure biodiesel may be is 1,000 liters per hectare or more, making biodiesel crops economically attractive, provided sustainable crop rotations exist that are nutrient-balanced and preventative of the spread of disease such as clubroot. Biodiesel yield of soybeans is significantly lower than that of rape.
Bioethanol also refers to the technology of using animal and human grains, principally corn (maize seed) to make ethanol directly through fermentation, a process that is widely reputed to consume as much energy as it produces, therefore being non-sustainable. New developments in converting grain stillage (referred to as distillers grain stillage or DGS) into biogas energy looks promising as a means to improve the poor energy ratio of this type of bioethanol process. 2007 saw a setback in the economics of building grain refineries in the USA while the shipment of grains and ethanol by rail car has prompted the train industries largest growth phase since 50 years.
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