Direct coal liquefaction was developed as a commercial process in Germany based on research pioneered by Friedrich Bergius. Seven direct liquefaction plants were in operation in Germany just before WWII. Five additional plants were constructed during the war and produced more than 3 million t of oil per annum. These plants produced about 90% of the available aviation fuel for the German war effort.
Most of the direct processes developed in the 1980s were modifications or extensions of Bergius's original concept. The coal is ground so that it can be mixed into a coal derived recycle solvent to form a coal-oil slurry feed. The slurry containing 30-50% coal is then heated to about 450°C in a hydrogen atmosphere between 13 900-20 900 kPa pressure for about one hour.
A variety of catalysts are used to improve the rates of conversion to liquid products. One tonne of coal yields about one-half tonne of liquids. Processes have been developed to use coals from low rank lignites to high volatile bituminous coals. Higher-rank coals are less reactive and anthracites are essentially non-reactive.
The liquids produced have molecular structures similar to those found in aromatic compounds and need further upgrading to produce specification fuels such as gasoline and fuel oil.