A wide variety of plants produce lipids that can be used as a basis for transport fuels. Main suppliers of lipids are currently plants producing edible oils such as rapeseed or canola, sunflower, oil palm, coconut and soybean. There is also limited use of non-edible lipids such as Jatropha oil and of animal fats (‘yellow grease’) and very limited use of algal lipids. A number of additional potential vegetable sources of lipids have been suggested (Sims et al. 2006; Shao and Chu 2008), and lipids have also been produced microbially from sugars (Zhou et al. 2008). The use of edible oils often has the advantage that co-products may be used in ani­mal feed production. This may be different for non-edible oils. For instance, the oil cake of many of the current Jatropha varieties is not suitable for feeding live­stock because of the presence of toxic compounds such as phorbolester and curcin (Carvalho et al. 2008; Sujatha et al. 2008), but such toxic Jatropha oil cake can be anaerobically converted to methane (Achten et al. 2009) or burned to supply en­ergy.

The lipids used for biofuel production mainly consist of triacylglycerol, in which the acyl groups are fatty acids (Agarwal and Agarwal 2007). In principle, a variety of lipids can be burnt as such in diesel motors, more of them in warm climates. Vegetable oils are used to a limited extent as transport fuel. For instance, there is a significant use of coconut oil in motorcars on the Pacific islands (Cloin 2007), and in Europe there is limited use of rapeseed oil in heavy-duty vehicles. However, for most applications, viscosity is too high. This problem may be solved by dilution, microemulsification and transesterification (Canakci and Sanli 2008). In practice, the solution is mostly transesterification to produce biodiesel, which is compatible with fossil diesel fuel and leads to a more limited increase in maintenance costs. In transesterification, the glycerol OH groups are replaced by the OH groups of either ethanol or, more commonly, methanol.

Transesterification to produce biodiesel from lipids can proceed with the help of an inorganic base catalyst (e. g. NaOH, KOH or NaOCH3). This approach is widely applied in commercial biodiesel production (Canakci and Sanli 2008). Potential al­ternatives are the use of insoluble inorganic catalysts (Shu et al. 2007; Li et al. 2007; Vasudevan and Briggs 2008) and the use of an enzyme: lipase (Harding et al. 2008). These alternatives are under development (Abdullah et al. 2007; Ranganathan et al. 2008). Transesterification by superheated or supercritical alcohols (that are not sen­sitive to free fatty acids and water) has also been studied (Marchetti et al. 2007; Joelianingsih et al. 2008). In the case of lipids that are characterized by the presence of greater than 0.5-1% free fatty acids (that react with base catalysts to soap) — of­ten waste lipids — the use of both homogeneous and heterogeneous acid catalyzed transesterification has been advocated (Abdullah et al. 2007; Vasudevan and Briggs 2008; You et al. 2008; Canakci and Sanli 2008; Park et al. 2008). Alternatively, free fatty acid levels can be reduced to less than 0.5% by the use of ion-exchange resins (Ozbay et al. 2008) or the admixture of virgin lipids.

An alternative option to transesterification is to catalytically remove oxygen from the triacylglycerol, while adding hydrogen. This gives rise to the synthesis of propane and mixtures of hydrocarbons (paraffins) that have diesel-like properties and can also be used in kerosene blends (Holmgren et al. 2007). Such a deoxy­genation process is currently commercially exploited (Rantanen et al. 2005). Still another way of converting virgin or used vegetable oil in transport fuels uses cat­alytic cracking or pyrolysis (heating in the absence of oxygen). In the latter case, this has to be followed by upgrading of the bio-oil that is a product of pyrolysis. In this way, one may produce fuels that are suitable for application in diesel or Otto motors or as a substitute for kerosene that is applicable in air transport (Milne et al. 1990; Knothe 2001; Demirba§ and Kara 2006; Dupain et al. 2007; Ooi and Bhatia 2007; Tamunaidu and Bhatia 2007). It has been noted that in the case of cracking unsatu­rated lipids, the product may contain relatively large amounts of aromatics (Dupain et al. 2007). Also, alkane synthesis from lipids by the bacterium Vibrio furnissii has been reported (Fortman et al. 2008). Finally, there are efforts to produce fatty acid ethylesters and hydrocarbons by re-engineering metabolism in heterotrophic micro­organisms (Wackett 2008). Ultimately, this way of producing biofuels is critically dependent on cheap sources of carbohydrates (Rotman 2008).

In Germany, a major producer of biodiesel, glycerol that is generated by trans­esterification of oils and fats is anaerobically converted into methane (see below). Glycerol can also be gasified to synthesis gas (mainly CO and H2). Synthesis gas (also called ‘syngas’) may be converted into methanol. Methanol, in turn, can be mixed into gasoline up to 15% by volume and applied in Otto motors without major adaptations. Methanol can also be used for the production of biodiesel, as an admix­ture in diesel (Cheng et al. 2008) and to produce methyl tert-butylether (MTBE) to be applied in petrol for use in Otto motors or to produce dimethylether (Arcoumanis et al. 2008). Methanol can, moreover, be reformed on board means of transport in a way that fits the use of H2 in fuel cells (Ferreira-Aparicio et al. 2005).

Converting glycerol into methanol via syngas is now commercially applied. Fur­thermore, synthesis gas derived from glycerol may be turned — via the Fischer — Tropsch reaction — into hydrocarbons that may serve as diesel, petrol or kerosene (Scott et al. 2007; Simonetti et al. 2007; Valliyappan et al. 2008). Also, syngas may be used as a source of H2 (Yazdani and Gonzalez 2007; Valliyappan et al. 2008). There are other options for converting glycerol into transport biofuels, too. Anaer­obic fermentation may convert glycerol into ethanol and/or butanol (Coombs 2007; Yazdani and Gonzalez 2007). And glycerol may be converted into propanol, which can be mixed with conventional gasoline (Coombs 2007; Fernando et al. 2007).