Ethanol can be used in petrol engines up to a concentration of about 24%, any more and either engine modifications are required or a flexible fuel engine is required. However, with modifications, a petrol engine can run on 100% ethanol, although it contains less energy than petrol. Biobutanol is a possible replacement for ethanol, as it has a higher energy density but there is little information about its use in engines. FT petrol is essentially the same as petrol and should cause no problems in supply and use. Methanol has been used in petrol engines, but it is not used as a fuel at the moment but could be a very useful fuel in the supply of hydrogen to fuel cells.

Diesel replacements

Biodiesel can be used up to 100% in a conventional diesel engine and jet engine with­out modification, but bio-oil requires considerable processing before it can be used because of its high viscosity and acidic nature. FT diesel is the same as diesel and can be used without modification. The microalgal biodiesel appears to have similar prop­erties to biodiesel, but so far there is no information on its use in diesel engines.

Gas turbine engines are used in aircraft, marine propulsion and electricity gener­ation and can function on a variety of fuels, paraffin, natural gas and propane, but the formulation for aircraft engines is regulated by international specifications DEF

169 I

STAN 91-91 and ASTM D1655. Jet A and Jet A-1 are known as aviation kerosene, and Jet B is a wide cut fuel. Biodiesel has been used at concentrations of 2, 20 and 30% in jet fuels to power aviation turbine engines with no apparent ill effects (Wardle, 2003). However, the biodiesel results from diesel engines cannot be transferred directly to aircraft engines as the combustion system responds differently, influenced by density more than chemistry (Ebbinghaus and Weisen, 2001). The problems with the supply of first-generation biofuels may preclude the use of biodiesel in aircraft but it is another option.

Thus, biofuels are capable of supplementing or replacing fossil fuels for transport and electricity generation and could be integrated into the present fuel infrastructure. The only exception is hydrogen which may require a completely new infrastructure.