An renewable energy crop, or energy crop is a plant grown for use in agriculture and is produced as a low cost and low maintenance harvest to be used to make biofuels or directly exploited for its energy content. Commercial energy crops in the 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.

Types of energy crops

Biomass

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.

Biogas (Methane) Producers

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.

Biodiesel

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.

Typical oil extraction from 100 kg. of oil seeds

Jatropha is the plant for non-food energy crops. The seed oil can be used to make biodiesel.

Bioethanol

Energy crops for biobutanol are grasses.A non-food crop for the production of cellulosic bioethanol is Switchgrass. There has been a preoccupation with cellulosic bioethanol in America as the agricultural structure supporting biomethane is absent in many regions, with no credits or bonus system in place. Consequently a lot of private money and investor hopes are being pinned on marketable and patentable innovations in enzyme hydrolysis and the like and therefore America is viewed by some technology planners as falling further behind Europe in real bioenenergy gains.

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 set back 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.

Table of Biofuel Crop Yields

The following table shows the yields of common energy crops associated with biofuel production.

- Note: Chinese Tallow (Sapium sebiferum, or Tradica sebifera) is also known as the "Popcorn Tree".

Oils used for biofuel

See also: Vegetable oil used as fuel

A number of the oils listed above are used for biofuel (biodiesel and Straight Vegetable Oil) in addition to having other uses. A number of oils are used only as biofuel.^[72][73]

Although diesel engines were invented, in part, with vegetable oil in mind,^[74] diesel fuel is almost exclusively petroleum-based. Rising oil prices have made biodiesel more attractive. Vegetable oils are evaluated for use as a biofuel based on:

1. Suitability as a fuel, based on flash point, energy content, viscosity, combustion products and other factors
2. Cost, based in part on yield, effort required to grow and harvest, and post-harvest processing cost

Multipurpose oils also used as biofuel

The oils listed immediately below are all (primarily) used for other purposes - all but tung oil are edible - but have been considered for use as biofuel.

• Castor oil, lower cost than many candidates. Kinematic viscosity may be an issue.^[75]
• Coconut oil (copra oil), promising for local use in places that produce coconuts.^[76]
• Corn oil, appealing because of the abundance of maize as a crop.
• Cottonseed oil, shown in one study not to be cost effective when compared with standard diesel.^[77]
• False flax oil, from Camelina sativa, used in Europe in oil lamps until the 18th century.^[47]
• Hemp oil, relatively low in emissions. High flash point. Production is problematic in some countries because of its association with marijuana.^[78]
• Mustard oil, shown to be comparable to Canola oil as a biofuel.^[79]
• Palm oil, very popular for biofuel, but the environmental impact from growing large quantities of oil palms has recently called the use of palm oil into question.^[80]
• Peanut oil, used in one of the first demonstrations of the Diesel engine in 1900.^[74]
• Radish oil. Wild radish contains up to 48% oil, making it appealing as a fuel.^[81]
• Rapeseed oil, the most common base oil used in Europe in biodiesel production.^[73]
• Ramtil oil, used for lighting in India.^[82]
• Rice bran oil, appealing because of lower cost than many other vegetable oils. Widely grown in Asia.^[83]
• Safflower oil, explored recently as a biofuel in Montana.^[84]
• Soybean oil, not economical as a fuel crop, but appealing as a byproduct of soybean crops for other uses.^[73]
• Sunflower oil, suitable as a fuel, but not necessarily cost effective.^[85]

• Tung oil, referenced in several lists of vegetable oils that are suitable for biodiesel.^[86][87]

Inedible oils used only or primarily as biofuel

These oils are extracted from plants that are cultivated solely for producing oil-based biofuel.^[88] These, plus the major oils described above, have received much more attention as fuel oils than other plant oils.

• Algae oil, recently developed by MIT scientist Isaac Berzin. Byproduct of a smokestack emission reduction system.^[89][90]
• Copaiba, an oleoresin tapped from species of genus Copaifera. Used in Brazil as a major source of biodiesel.^[91]
• Honge oil, pioneered as a biofuel by Udipi Shrinivasa in Bangalore, India.^[92]
• Jatropha oil, widely used in India as a fuel oil. Has attracted strong proponents for use as a biofuel.^[93][94]
• Jojoba oil, from the Simmondsia chinensis, a desert shrub.^[95]
• Milk bush, popularized by chemist Melvin Calvin in the 1950s. Researched in the 1980s by Petrobras, the Brazilian national petroleum company.^[96]
• Petroleum nut oil, from the Petroleum nut native to the Philippines. The Philippine government once explored the use of the petroleum nut as a biofuel.^[97]

References

• ENERGY PLANT WEB SITE (Germany)
• Europe's largest Biogas Association
• American Biomass Journal
• BIOCYCLE - America's BioRecycling magazine

See also

• Agricultural byproduct
• Anaerobic digestion
• Biogas
• Short rotation coppice
• Short rotation forestry
• Table of biofuel crop yields
• Vegoil