With there being a limit to the carbon reductions achievable from petrol, diesel, CNG and LPG, attention has shifted towards fuels that can be used in ICEs and that have substantially lower CO2 emissions. The most significant developments have been in liquid biofuels: renewable fuels that can be produced by the fermentation of energy crops or from vegetable oils or animal fats. Such fuels can reduce the transport sector's dependence on fossil fuels and potentially achieve major cuts in greenhouse gas emissions, since the crops used are part of the natural carbon cycle.
Biofuels can come from a wide number of sources, and include:
- bioethanol, an alcohol that can be produced from virtually any fermentable source of sugar
- methanol, an alcohol that is largely produced from natural gas, but that can also be produced from biomass
- biodiesel, which is most commonly produced from energy crops such as oilseed rape or recycled vegetable oils; the use of waste oil as feedstock is particularly beneficial, as there is little additional CO2 generated in the production of the fuel
- biogas, which is gas produced from biological processes (e.g. from an ); this can be used in vehicles adapted for CNG (Figure 9).
For further information see 'Biofuel types' in Department for Transport, 2012a; U.S. Department of Energy, 2011; 'Methanol' and 'Renewable Natural Gas' in U.S. Department of Energy, 2012; and CPL Press, 2009.