Indirect liquefaction processes were developed in Germany at the same time as direct processes. In the early 1920s, Franz Fischer and Hans Tropsch patented a process to produce a mixture of alcohols, aldehydes, fatty acids and hydrocarbons known as synthol, from a synthesis gas of hydrogen and carbon monoxide.
The Fischer-Tropsch (F-T) process forms the basis for indirect liquefaction of coal. The process is indirect since the coal structure is completely broken down into synthesis gas by gasification with steam and oxygen. Then the synthesis gas is reacted over an appropriate F-T catalyst to form predominantly paraffinic liquid hydrocarbons having wide molecular weight.
This method was used to produce motor fuel during WWII and South Africa has used it to produce motor fuels and petrochemical feedstocks since the 1960s. The indirect route yields a large number of byproducts and overall has a lower thermal efficiency.