This process promotes dissolution of the coal from large, complex molecules to smaller molecules with more hydrogen. The portion of coal that remains is mostly inorganic ash-forming material, which does not dissolve. Part of the liquid is removed as a product to be refined further into a hydrogen fuel, and the remainder is fed back into the system to act as the solvent and assist in dissolving more coal. Two of Bergius's contemporaries, the German chemists Franz Fischer and Hans Tropsch, developed the first indirect coal-liquefaction process. In indirect liquefaction today, coal is heated in the presence of steam and oxygen to produce a mixture of carbon monoxide and hydrogen, or synthesis gas.
Extraction is a special kind of direct liquefaction in which a solvent is used without hydrogen.
Depending on the solvent, high pressures and temperatures are not always necessary in extraction. The solvent is usually a material that is not coal derived and can be recovered and recycled after the extraction process is completed. Fossil Fuel Liquefaction Science has demonstrated that waste materials such as plastics, rubber tires, and paper can be converted into high quality oil using direct coal liquefaction technology. This could be a revolutionary solution to waste disposal problems that will lead to a major new industry providing a new domestic source of oil. Both produced promising results.