The microbial desulfurization of coal.

The chemical structure of coal macerals is usually characterized by the presence of inorganic and organic sulfur. Inorganic sulfur consists mostly of iron sulfides, the so-called "pyritic sulfur," whereas organic sulfur is covalently bound to the carbon atoms of the coal macromolecule. Comminution of coal to sizes that liberate the iron sulfide grains makes their removal with mineral beneficiation processes theoretically possible, but practically profitless. Microbial removal of pyritic sulfur has been extensively investigated over the last 50 years and the very promising results obtained have encouraged the design and construction of a semi-commercial pilot plant in the framework of Project JOULE 0039 funded by the European Commission. The results of the 1-year operation of this plant are reported here, the most significant being the 90 % pyrite removal achieved in five stirred tank bioreactors operating with a 40 % solids suspension and the pyritic iron solubilization rate of 36 mg dm(-3) h(-1). Taking into account the very high price of the kWh in Italy, a rough estimate of the overall costs is in the range from 25 to 30 <euro> per tonne of dry coal. So far the development of a microbial process for organic sulfur removal has shown to be much more difficult and less successful, although significant progress in laboratory research is reported.