Pyrite (FeS2) is the most common sulfide mineral in coal and a major source of the sulfur in coal. Pyrite can form in peat while the peat is accumulating, or can form in peats from the introduction of sulfate (SO4) into the peat if the peat was buried by marine waters (such pyrite is called syngenetic or authigenic pyrite). Within the buried peat, the sulfate is reduced to sulfide (S2), which then combines with iron to form pyrite. Pyrite can also be introduced into the peat, after burial (diagenetic or epigenic pyrite), during coalification (Casagrande, 1987).
Pyrite (FeS2) is a mineral of particular importance in several coal markets. Sulfur from pyrite can have a detrimental influence on many chemical processes including coking to make steel. Sulfur from pyrite also contributes to sulfur emissions in industrial boilers and steam coal. During mining, sulfur from pyrite contributes to acid-mine drainage.
Pyrite and acid-mine drainage
Weathering of coal and high-sulfur strata, or abandoned spoil piles with significant pyrite concentrations, can result in acid-mine drainage (Appalachian Regional Commission, 1969; Brady and others, 1998; Greb, 2006). In order to limit the formation of acid-mine drainage, modern mining regulations limit the exposure and storage of high-sulfur coal or roof rocks. Surface and groundwater in the mining area and adjacent to the mining area must also be monitored to ensure that mining is not causing acid-mine drainage. In areas of past mining (before modern regulations were in place), a wide array of mitigation techniques are used to limit acid-mine drainage.
Pyrite and sulfur emissions
Combustion of coal with pyrite results in the emission of sulfur dioxide (SO2) in the flue-gas stream, and can react with water vapor to form sulfurous acid (H2SO3), which oxidizes to sulfuric acid (H2SO4), a major component of acid rain (U.S. Energy Information Administration, 1997; U.S. Environmental Protection Agency, 2016a). In addition, some trace elements that are potentially hazardous air pollutants (antimony, arsenic, cadmium, cobalt, mercury, nickel, and selenium) are commonly associated with pyrite (see, for example, Schweinfurth and Finkelman, 2003).
Many different types of mitigation are employed to remove or reduce sulfurous emissions before and after combustion in power plants. Most coals are “cleaned” at preparation plants, a process called beneficiation or preparation, prior to combustion to lower the sulfur content of coal before it is sent to a power plant. At the power plant, many techniques are also available for limiting sulfurous emissions, including trapping sulfurous gas and sulfur-bearing particles in the flue gas through bag houses (electrostatic precipitators) and scrubbers (flue-gas desulfurization). These methods generally trap most of the sulfur emitted from the combustion of coal.
Major, Minor, and Trace Elements
Coal and the Environment
References for What’s in coal