James G. Speight - Encyclopedia of Renewable Energy

Thus, bioenergy crops include fast-growing trees such as hybrid poplar, black locust, willow, and silver maple in addition to annual crops such as corn, sweet sorghum, and perennial grasses such as switch grass. The first-generation bioenergy crops include corn, sorghum, rapeseed, and sugarcane, whereas the second-generation bioenergy crops are comprised of switchgrass, miscanthus, alfalfa, reed canary grass, Napier grass, and other plants.

Briefly, switch grass is a thin-stemmed, warm season, perennial grass that has shown high potential as a high yielding crop that can be readily grown in areas that are also suitable for crop production. In fact, there are many perennial crops (grass and tree species) that show high potential for production of cost-competitive cellulosic biomass. Switch grass can be viewed as a surrogate for many perennial energy crops when estimating biomass supply and availability.

Bioenergy crops increase soil carbon and fix atmospheric carbon. In addition, bioenergy crops (miscanthus, sorghum, and poplar) could also be used for the phytoremediation of heavy metal-contaminated soils. The bioenergy crops include specific plants that are grown and maintained at lower costs for biofuel production. Many other crops are possible, and the optimal crop will vary with growing season and other environmental factors. Most fast-growing woody and annual crops are high in hemicellulose sugars such as xylose.

See also: Bioenergy, Biofuel, Biomass.

A bioenergy system is an energy system which is comprised of a source which generates energy and modulates it in some manner such that it conveys energy. There is also a mechanism connecting the bioenergy source to a transfer medium and a transfer medium through which the bioenergy flows. There is a coupling mechanism connecting the transfer medium bioenergy sink and a terminal sink which includes a mechanism for the storage and use of the energy. The input and output coupling depend on properties of the source and the transfer medium, likewise for the sink.

To evaluate the performance of a bioenergy system, the entire chain from biomass production up to the end-use should be considered. A major criterion for comparing total bioenergy systems is the net energy yield per hectare. If this net yield is low, the amount of land needed for the net production of a certain amount of energy is high and vice versa. Since land is a scarce commodity, high net energy yields per hectare are favored. Also, the environmental impacts of a specific energy crop are a criterion for selection. Logically, another major criterion is the cost of the biomass per GJ produced (or per GJ of fossil energy replaced).

Biomass for energy conversion is usually considered as a local resource. With appropriate logistic systems, access to biomass can be improved over a large geographical area. In this study, life cycle inventory has been used as a method to investigate the environmental load of selected bioenergy transport chains. As a case study, chains starting in Sweden and ending in Holland have been investigated. Biomass originates from tree sections or forest residues, the latter upgraded to bales or pellets. The study is concentrated on production of electricity; hot cooling water is considered as a loss.

See also: Bioenergy, Bioenergy Crops, Biomass.

Bioethanol is ethanol produced from biomass feedstocks, which includes ethanol produced from the fermentation of crops, such as corn, as well as cellulosic ethanol produced from woody plants or grasses. The resource base is gradually widening to cellulosic crops, and even wood. Such low-cost feedstock would result in a global increased production potential for low-cost ethanol.

Unlike crude oil, bioethanol is a form of renewable energy that can be produced from agricultural feedstocks. It can be made from various crops such as sugar cane, potato, and maize.

The long-term prospects of bioethanol, or any other biofuels such as methanol and biodiesel, depend on biomass availability. Concerns related to its production and use relate to the large amount of arable land required for crops, as well as the energy and pollution balance of the whole cycle of ethanol production. The availability of bioethanol depends on future food demand and food patterns, other types of land use, and agricultural productivity.

See also: Alcohols, Bioethanol Production, Ethanol.

Bioethanol can be produced from a variety of crops, although the traditional feedstocks for the production of ethanol are still starch crops like corn, wheat and cassava and from sugar crops like sugar cane and sugar beet.

The development of lignocellulosic technology has meant that not only high energy content starch and sugar crops can be used but also woody biomass or waste residues from forestry. This process requires an additional pretreatment process of pulping and enzymes to break down the organic compounds.

Additional steps that may be taken in a biorefinery include the drying and pelletization of bagasse (the waste component of sugar cane after juice extraction) to produce a secondary product along with ethanol, which makes the plant more economically feasible. In addition to the production of fuel pellets, one can also produce feed for animals, which could be used locally thereby reducing carbon dioxide emissions from associated transport energy spent.

See also: Alcohols, Bioethanol.

Ethers are a class of carbon compounds that contain an ether group (C-O-C) in which either carbon may be attached to other carbon atoms as well (such as the commonly-used diethyl ether, CH 3CH 2OCH 2CH 3). The most commonly used fuel additive ethers are methyl-tertiary-butyl-ether (MTBE) and ethyl-tertiary-butyl-ether (ETBE).

Bioethers ( Table B-8) are produced by the reaction of reactive iso-olefin derivatives, such as iso-butylene, with bioethanol, which is ethanol produced from bio-sources.

Table B-8Examples of bioether derivatives used in fuels.

When added to gasoline, a bioether can make the gasoline burn cleanly and completely and enhance engine performance, while reducing engine wear and toxic exhaust emissions, as well as the amount of ground-level ozone.

Traditional gas cleaning and air pollution control technologies for pollutant gases, such as adsorption, absorption, and combustion, were developed to treat high concentration waste gas streams associated with process emissions from stationary point sources. Although these technologies rely on established physico-chemical principles to achieve effective control of gaseous pollutants, in many cases, the control technique yields products which require further treatment before disposal or recycling of treatment materials. In the case of treatment of dilute waste gas streams, however, these traditional methods are relatively less effective, more expensive, and wasteful in terms of energy consumption and identification of alternative control measures is warranted. A suitable alternate air pollution control technology is biofiltration, which utilizes naturally occurring microorganisms supported on a stationary bed (filter) to continuously treat contaminants in a flowing waste gas stream.