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Sulfur in Coal

Dividing Line

Geologists believe that these general distribution trends from low to high sulfur content are attributable to changes in climate through geologic time (wetter and less seasonal, to drier and more seasonal). The ancient climates affected the geometry of the peat-forming swamps, which in turn affected the amount of sulfur entering the peat before it became coal. Coal swamps under a drier climate were low-lying, flat expanses, easily penetrated by waters containing sulfur and iron which combined to form pyrite within the peat. Pyrite (FeS2) is the major sulfur mineral in coal. Coal swamps under a wetter climate developed raised water tables because of the excessive rainfall, enabling the peat to accrue into large domed swamps which were impenetrable to surface water, limiting or eliminating pyrite formation.

Other factors are associated with the depositional setting. For example, coal beds overlain by rocks deposited in marine waters (i.e., the sea) are often relatively higher in sulfur than coals stratigraphically close by without overlying marine roof rocks.

When coal is mined, fresh sulfur-bearing minerals in the coal and rocks are exposed to air and water. The resulting chemical reactions produce sulfuric acid and precipitates. The acid water flowing from coal mines, if not treated, can damage life forms in the receiving streams. The iron and sulfate precipitates often discolor stream beds with yellow and orange stains. In a similar manner, when burned, sulfur escaping in the flue gases can combine with water in the atmosphere to produce acidic precipitation ("acid rain"). For the same reasons, burning high-sulfur coal can be corrosive to the metal equipment used in a power plant.

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