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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

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.

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. Table 10 gives the biofuel yields for different feedstocks and countries.

See also: Alcohols, Algae Fuel, Bioalcohol, Bioethanol, Biodiesel.