|Liquid petroleum fuels (gasoline, diesel, kerosene, jet fuel, bunker fuel)||Conventional oil fields (ground and shore-based). Non-conventional sources (tar sands)||Refining|
|Liquid synthetic fuels||Natural gas, coal||Gasification|
|Biodiesel||Oil seed crops||Esterification, hydrogenation|
|Ethanol||Grain crops||Saccharification and distillation|
|Sugar crops (cane)||Distillation|
|Advanced biodiesel||Biomass from crops or waste products||Gasification|
|Compressed natural gas (CNG)||Natural gas||Gasification|
|Electricity||Coal, gas, petroleum, nuclear, renewables (hydro, wind)||Electric generator (source dependent)|
|Hydrogen||Natural gas||Reforming, compression|
|Direct production using other sources||High temperature process|
Source: adapted from IEA/OECD (2009) Transport, Energy and CO2: Moving toward sustainability. Paris: International Energy Agency.
Each fuel has specific sources and processes related to its transformation. The more marginal the sources and the more complex the processes, the higher the price and scarcity. A major challenge concerns the sustainability of the energy sources as well as of the processes leading to the creation of fuels that can be used by transportation.