MICROBIAL DESULFURIZATION OF COAL

ABSTRACT

The sulfur found in coal is generally divided into three forms: pyritic, sulfate, and organic sulfur. Originating from irt-situ oxidation of metal sulfides, sulfates axe present in coal at low concentrations. Being soluble in water, they are relatively easy to be leached from the coal. Similarly, microbial depyritization promotes the oxidative conversion of inorganic sulfur compounds to water-soluble products. The pyrite removal results from the combined effects of direct bacterial attack and indirect chemical solubilization. In the former, pyrite (FeS₂) is oxidized by bacteria into Fe₂(SO4) ₃; in the latter, ferric iron is the actual oxidizing agent and microorganisms serve to regenerate the ferric iron from ferrous iron.

Organic sulfur is chemically bound to the carbon skeleton of coal. It is believed to be present mainly in the forms of organic sulfides, disulfides, thiols and thiophenes. Two pathways have been proposed for microbial desulfurization of heterocycles. One emphasized the oxidation of carbon ring structures to polar teachable derivatives. The other stressed the sulfur-specific metabolism without degradation of the carbon skeleton. Both pathways have been demonstrated with model organosulfur compounds, especially dibenzothiophene (DBT). Reports on organic sulfur removal from coal and water-soluble coal-derived products by microorganisms are also available. Unfortunately, due to the difficulty in accurate analysis.of organic sulfur content, the technical feasibility of microbial organic sulfur removal from coal is still in dispute.

The costs of independent microbial depyritization and organic:sulfur removal were estimated to be $10–14 and $14 per ton of coal, respectively. A cost analysis for a tow-step process, which was designed to achieve complete removal of inorganic sulfur and a 40% reduction of organic sulfur, yieded $11 per ton of coal. These are lower than the costs of other techniques, e.g. $20–40 per ton of coal for flue-gas desulfurization.