Encyclopedia of Renewable Energy. James G. Speight

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СКАЧАТЬ significant production of plant-based chemicals will only be economically viable in highly integrated and efficient production complexes producing a diverse range of chemical products. Also contrasting with fossil fuels, biofuels do not contribute to the stock of total carbon dioxide in the atmosphere, since the sources of many biofuels are plants which normally remove carbon dioxide from the atmosphere, and then give off the same amount when burned and are, therefore, considered to be carbon dioxide neutral, although some observers would doubt this balance as being the result of mathematical manipulation without taking into consideration other important and influential factors.

In any given location, the type of biofuel used will depend on, among other minor factors, the available natural resources and local energy needed for processing requirements. Some of the main reasons for the shifting of interest from fossil fuels to biofuels are the rising prices of oil and increasing emissions of greenhouse gases, low barriers to entry of biofuels, and increasing government support. There is also a requirement for the utilization of biofuels which is the reduction or total elimination of noxious emissions in an economical manner.

      Biofuels can be classified into (i) first generation biofuels, (ii) second generation biofuels, and (iii) third generation biofuels, otherwise known as advanced biofuels, and the composition varies according to the feedstock and conversion techniques used. First-generation biofuels are derived from simple materials, such as mono and disaccharides (simple sugars), amylose and amylopectin (starch), esters of glycerol and fatty acids, free fatty acids and mono and diglycerides (fats and vegetable oils for biodiesel), methanol, ethanol, propanol and butanol (bioalcohols), methyl-tertiary-butyl-ether (or MTBE) and ethyl-tertiary-butyl-ether (or ethyl-t-butyl-ether, ETBE) – also known as bioether derivatives – methane, carbon-dioxide, nitrogen, hydrogen, hydrogen sulfide, oxygen, carbon monoxide and hydrogen (synthesis gas, often referred to as syngas) and cellulose, hemicellulose derivatives and lignin (wood for solid biofuels). Second generation biofuels are typically produced by the biomass to liquid (BTL) technology and are fuels such as ethanol from lignocellulose, biohydrogen, and wood diesel. The third generation type of biofuel is a biofuel from algae otherwise known as oilgae. Botryococcus braunii and Chlorella vulgaris, as well as macroalgae (seaweed), are typically used to produce such biofuels.

      In fact, the free fatty acid from non-edible oils can be mixed with crude oil-derived diesel fuel (up to 30% w/w of the non-edible oil with diesel) may be usable without having too much of an adverse effect on the properties and efficiency of the diesel fuel. However, it must be remembered that olefins in fuel (such as gasoline or diesel fuel) can often result in the form of gums that have an adverse effect on fuel flow and efficiency. This tendency can be mitigated if the fatty acids are modified to produce more suitable fuels (i.e., fuels that do not for gum products) that can act as a diesel substitute. One method involves cracking at high temperatures on certain metal oxide catalysts (decarboxylation), which yields hydrocarbon products that can be hydrogenated to the saturated hydrocarbon derivative (n-heptadecane, C₁₇H₃₆). Using linoleic acid as the example, the pyrolysis product is heptadecadiene which hydrogenates to heptadecane:

      More generally, many food sources and plant crops are the main feedstock for biofuels, and as such, one must take into consideration the deleterious effects of biofuel production on the natural resources of any particular region. These feedstocks may enter into animal as well as human food chains, and as biofuel production has increased, there has been criticism for diverting food away from human consumption, which can lead to food shortages and increased food prices. While the demand for biofuels is directly correlated to the price of oil, other issues also arise such as deforestation, soil erosion, water usage, carbon emissions, and biofuel prices which complicate the food-vsfuel debate with regard to energy balance and efficiency. Henceforth, while discussing the benefits of biofuels, the pitfalls must be kept apparent, before sustainable biofuel production can be realized.

      In conclusion, biofuels are classed according to source and type. They are derived from forest, agricultural, or fishery products or municipal wastes, as well as from agro-industry, food industry, and food service by-products and wastes. They may be solid, such as fuelwood, charcoal and wood-pellets; liquid, such as ethanol, biodiesel and pyrolysis oils; or gaseous, such as biogas.

      See also: Geothermal Energy, Hydrogen, Hydroelectric Energy, Nuclear Energy, Ocean Energy, Solar Energy, Tidal Energy, Waste, Wind Energy.

      Biofuels - Classification

Put simply, a biofuel is a fuel that is produced through contemporary processes from a biological source (biomass) rather than a fuel produced by the geological processes involved in the formation of fossil fuels. In addition, the word biofuel is usually reserved for liquid or gaseous fuels, used for transportation. Biofuels can be produced from plants (such as energy crops), or from agricultural, commercial, domestic, and/or industrial wastes (if the waste has a biological origin).

      Renewable biofuel sources generally involve carbon fixation, such as those that occur in plants or in micro-algae through the process of photosynthesis. Furthermore, some observers argue that biofuel can be carbon-neutral because all biomass crops sequester carbon to a certain extent because all crops move carbon dioxide from aboveground circulation to below-ground storage in the roots and the surrounding soil. However, the simple proposal that biofuel is carbon-neutral requires that the total carbon sequestered by the root system of the energy crop must compensate for all the above-ground emissions which must include any emissions caused by direct or indirect land-use change. Many first generation biofuel projects are not carbon-neutral given these demands – in fact, some may even have higher total emissions of greenhouse gases than some fossil-based alternatives.

Generally, the classification of biofuels is related to the method of preparation which, in turn, is related to the starting material of the agent by which the biomass if converted to biofuels (Table B-10). For convenience, biofuels are generally classified as being (i) first generation biofuels, (ii) second generation biofuels, and (iii) third generation biofuels. Examples of solid biofuels include wood, sawdust, grass cuttings, domestic refuse, charcoal, agricultural waste, non-food energy crops, and dried manure.

Table B-10 General classification of biofuels.

1st level	2nd level	Brief definition
Woodfuels	Direct Woodfuels	Wood from forests, shrubs, and other trees
	Indirect Woodfuels	Solid biofuels produced from wood processing
	Recovered Woodfuels	Wood used directly or indirectly as fuel
	Wood-based fuels	Liquid and gaseous biofuels from woody biomass
Agrofuels	Fuel crops	Growing plants for the production of biofuels
	Agricultural by-products	By-products from crop
	Animal by-products	Primarily excreta from farm animals
	Agro-industrial by-products	Biomass as bagasse and rice husks

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