Coal occurs in immense quantities in many countries. The estimated lifetime of coal reserves substantially exceeds those of natural gas and petroleum. Prospects for commercializing biofuels increase continuously, but it remains a question whether it will be possible to grow enough biomass to meet liquid fuel market demands, particularly without impacting food production. At the same time, an immense investment has been made worldwide in vehicles, airplanes, ships, and stationary combustion sources that use liquid fuels. Replacing this infrastructure with alternatives using solid or gaseous fuels or electricity would require many decades. A need for liquid fuels will continue throughout the foreseeable future. For these reasons, technologies for producing liquid fuels from coal merit serious attention.

Gasification followed by Fischer–Tropsch synthesis represents the currently dominant coal-to-liquids technology. However, many other processes are available to produce useful liquid fuels from coal. In its broadest sense, the term liquefaction refers to a conversion of something (usually a solid) into a liquid. Aside from the indirect liquefaction discussed in Chapter 21, liquids can be produced from coal by thermal breakdown of the coal structure (pyrolysis), by dissolution of coal components (solvent extraction), or by reaction of coal with hydrogen or with solvents capable of donating hydrogen (hydroliquefaction). All four approaches, i.e. including F–T, constitute the field of coal-to-liquids (CTL) technology. Although the word liquefaction could embrace any of these, as customarily used it refers to F–T synthesis or hydroliquefaction.