Indirect liquefaction processes require first gasifying the solid feedstocks into a syngas. Therefore, while direct coal liquefaction (DCL) takes coal directly into a liquid phase, indirect coal liquefaction (ICL) consists of two major steps: (a) gasification to produce a synthesis gas (syngas); and (b) conversion of the carbon monoxide (CO) and hydrogen (H2) in the syngas to a range of hydrocarbon fuels/products such as gasoline, diesel, methanol and chemicals). Most frequently, Fischer-Tropsch (FT) synthesis followed by subsequent liquids product refining is used to convert syngas to fuels; alternately, methanol formed from syngas can be converted to gasoline via ExxonMobil’s MTG process.
Direct coal liquefaction requires an external source of hydrogen, which may have to be provided by gasifying additional coal feed and/or the heavy residue produced from the DCL reactor. Many argue that indirect liquefaction with the current state-of-the-art technologies is more competitive than direct liquefaction. ICL has been demonstrated commercially by Sasol since the 1950s, and the ICL process is more amenable to carbon dioxide (CO2) capture.