Coal to liquids (CTL) and gas to liquids (GTL) enable the production of transport fuels from coal and gas respectively. The fuels produced by these processes are called synthetic fuels or synfuels and usually substitute for diesel.
Given Australia's abundant coal and gas reserves there is the potential for large scale synfuel production in Australia and several synfuel projects are under consideration.
CTL
Coal to liquids (CTL) is the production of fuels and petroleum products using coal as feedstock. There are two broad streams of this technology: direct and indirect liquefaction.
In the indirect coal liquefaction process, coal is subjected to intense heat and pressure to create a syngas which is treated to remove impurities and unwanted compounds such as mercury and sulphur. The syngas is then converted to syncrude in the same way as a GTL process.
In direct coal liquefaction, pulverized coal is mixed with oil and hydrogen in a pressurized environment. This directly converts the coal into syncrude without the need for an intermediate stage.
Syncrude produced by CTL processes is essentially identical to that produced through GTL processes and can therefore be further refined or blended in the same way.
Underground coal gasification involves combusting coal in situ which produces a syngas without the need for a specialised plant.
GTL
GTL technology enables the conversion of natural gas into high-value liquid fuels (including methanol, dimethyl ether (DME), and middle distillates), specialty chemicals and waxes.
GTL production involves the initial conversion of natural gas into synthesis gas (syngas) which is a mixture of hydrogen and carbon monoxide. The syngas is then converted into synthetic crude (syncrude). A common method of syngas conversion is the Fischer-Tropsch process which has existed since the 1920s. The resultant syncrude can then be further refined in its own right or used as a refinery blendstock.
Of the various GTL products, the focus of recent Australian proposals has been the production of GTL middle distillate (diesel and jet fuel). Fuels produced by a GTL process are ultra clean with low emissions of particulates, oxides of nitrogen and sulphur, and hydrocarbons. GTL offers a value added pathway to commercialisation of Australia's large, undeveloped gas reserves.