Biofuels are combustible materials directly or indirectly derived from biomass, commonly produced from plants, animals and micro-organisms but also from organic wastes. Biofuels may be solid, liquid or gaseous and include all kinds of biomass and derived products used for energetic purposes. This "bioenergy" is one of the so-called renewable energies. Besides the traditional use of bioenergy, "modern bioenergy"’ comprises biofuels for transport, and processed biomass for heat and electricity production.
Biofuels for transport are commonly addressed according to their current or future availability as first, second or third generation biofuels (OECD/ IEA 2008). Second and third generation biofuels are also called “advanced” biofuels. UNEP (2008) points out that this differentiation is not always straightforward due to overlaps regarding feedstocks and processing technologies, as well as uncertainties regarding environmental impacts. Terms such as "higher generation|" or more "advanced" biofuels suggest superiority; however, superiority in terms of sustainability is not a given and needs to be assessed as critically as for all kinds of biofuels.
"First-generation biofuels" are commercially produced using conventional technology. The basic feedstocks are seeds, grains, or whole plants from crops such as corn, sugar cane, rapeseed, wheat, sunflower seeds or oil palm. These plants were originally selected as food or fodder and most are still mainly used to feed people. The most common first-generation biofuels are bioethanol (currently over 80% of liquid biofuels production by energy content), followed by biodiesel, vegetable oil, and biogas.
Second-generation biofuels can be produced from a variety of non-food sources. These include waste biomass, the stalks of wheat, corn stover, wood, and special energy or biomass crops (e.g. Miscanthus). Second-generation biofuels use biomass to liquid (BtL) technology, by thermochemical conversion (mainly to produce biodiesel) or fermentation (e.g. to produce cellulosic ethanol). Many second-generation biofuels are under development such as biohydrogen, biomethanol, DMF, Bio-DME, Fischer-Tropsch diesel, biohydrogen diesel, and mixed alcohols.
Algae fuel, also called oilgae, is a biofuel from algae and addressed as a third-generation biofuel (OECD/IEA 2008). Algae are feedstocks from aquatic cultivation for production of triglycerides (from algal oil) to produce biodiesel. The processing technology is basically the same as for biodiesel from second-generation feedstocks. Other third -generation biofuels include alcohols like bio-propanol or bio-butanol, which due to lack of production experience are usually not considered to be relevant as fuels on the market before 2050 (OECD/IEA 2008), though increased investment could accelerate their development. The same feedstocks as for first-generation ethanol can be used, but using more sophisticated technology. Propanol can be derived from chemical processing such as dehydration followed by hydrogenation. As a transport fuel, butanol has properties closer to gasoline than bioethanol.