Coal has the approximate empirical formula CH. To convert it to an aliphatic hydrocarbon
mixture requires addition of hydrogen. In the Fischer–Tropsch process, this
is derived ultimately from water. It is, however, possible to hydrogenate coal directly.
This process, called the Bergius process, was operated in Germany in World War II.
Coal, lignite, or coal tar was hydrogenated over an iron catalyst at 450°C and
700 bar, and 4 million tonnes of gasoline were synthesized in this way, largely for
aviation fuel.