Synthetic fuels are gaseous and liquid fuels produced synthetically, primarily from coal and oil shale. They are commonly referred to "synfuels."

The general principle behind coal-based synfuels is that coal can be converted to gaseous or liquid forms that are more easily transported and that burn more cleanly than coal itself. In either case, hydrogen is added to coal, converting some of the carbon it contains into ozone-depleting hydrocarbons. Coal liquefaction involves the conversion of solid coal to a petroleum-like liquid. The technology needed for this process is well known and has been used in special circumstances since the 1920s. During World War II, the Germans used this technology to produce gasoline from coal. In the Bergius process used in Germany, coal is reacted with hydrogen gas at high temperatures (about 890°F/475°C) and pressures (200 atmospheres). The product of this reaction is a mixture of liquid...