The earliest work was done on microbial decomposition of German hard coals.
Reactors have been designed to use a variety of bacteria and fungi to break down the large molecular stucture into smaller units that may be useful as intermediates(solubilization) or as liquid and gaseous fuels (conversion).
Efforts have focused on lower rank coals,lignite or brown coal and subbituminous coal.
Because of greater reactivity.
The conversion processes frequently introduce chemically combined oxygen through hydrolysis or related reactions to make the solid soluble in the reaction mixture as an initial step.
Further reaction involves biological degradation of the resulting material to form gases or liquids.