Sulfur in Coal
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
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.
Page last revised: November 11, 2005
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