Supercritical Water Processing of Coal to Liquid Fuels

Abstract

Coal, especially low-rank coals are a major domestic resource and account for about 48% of the demonstrated US coal reserves. Characterized by their high moisture content, high oxygen content, and lower calorific value than bituminous coals, low-rank coals are more difficult to convert to liquid fuels through conventional gasification than bituminous coals. Sodium hydroxide could function as a “once through” or disposable catalyst because of its low cost. Indirect coal liquefaction methods, such as the Fischer–Tropsch process, are common practice for liquid fuels production. Coal liquefaction by supercritical treatment can tolerate high pollutants in the syngas, while the Fischer–Tropsch process is very sensitive to catalyst poison in the syngas feed. Lignite coal is high in sodium and alkali metals, carboxyl groups i.e., oxygen, and moisture. The moisture is “bound” or incorporated into the coal matrix and is difficult to remove by drying.