Encyclopedia of Renewable Energy - James G. Speight

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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:

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      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

      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.

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

      When raw biomass is already in a suitable form (such as firewood), it can burn directly in a stove or furnace to provide heat or raise steam. When raw biomass is in an inconvenient form (such as sawdust, wood chips, grass, urban waste wood, and agricultural residues), the typical process is to densify the biomass. This process includes grinding the raw biomass to an appropriate particulate size, which depending on the densification type can be from 1 to 3 cm, which is then concentrated into a fuel product. The current types of processes are pellet, cube, or puck. The pellet process is most common in Europe and is typically a pure wood product. The other types of densification are larger in size compared to a pellet and are compatible with a broad range of input feedstocks. The resulting densified fuel is easier to transport and feed into thermal generation systems such as boilers.

      Raw biomass emits considerable amounts of pollutants such as particulate matter and polyaromatic hydrocarbon derivatives, (PAHs also known as polynuclear aromatic hydrocarbon derivative, PNAs, PAHs) during combustion. Nevertheless, biomass fuels appear to have significantly less impact on the environment than fuels based on fossil sources.

      A derivative of solid biofuel is biochar, which is produced by biomass pyrolysis. Biochar made from agricultural waste can substitute for wood charcoal. As wood stock becomes scarce, this alternative fuel is gaining popularity.

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