1.6 Impurities in coal
Coal rank reflects the maturity of a coal, but another variable is the ratio of combustible organic matter to inorganic impurities found within the coal. As discussed earlier, impurities result mainly from clay minerals washed into the mire prior to its eventual burial. In addition, some impurities are formed from the plant material itself during coalification.
These inorganic impurities are non-combustible and therefore leave an inert residue or ash after coal combustion. High-ash contents increase the volume of particulate matter ejected into the atmosphere following combustion. So coals used as boiler fuels or for coking require less than 10% ash content.
A second important group of impurities are carbonate minerals. During the early stages of coalification, iron carbonate is precipitated either as concretions (hard oval nodules up to tens of centimetres in size) or as infillings of fissures in the coal. Interestingly, it was probably the breakdown of such iron carbonates to molten iron when coal that contained them was burned, which led to the accidental discovery of iron smelting.
Other impurities are nitrogen and sulphur that are chemically reduced during coalification to the gases ammonia (NH4) and hydrogen sulphide (H2S), which become trapped within the coal. However, most sulphur is present as the mineral pyrite (FeS2), which may account for up to a few per cent of the coal volume. Burning coal oxidizes these compounds, releasing oxides of nitrogen (N2O, NO, NO2, etc.) and sulphur dioxide (SO2), notorious contributors to acid rain.
In addition, relatively high concentrations of sodium chloride (NaCl), deposited by seawater floods into coal-forming mires, make the coal virtually unusable in power plants because salt causes severe boiler corrosion. Lastly, trace elements (including germanium, arsenic and uranium) are significantly enriched in coal and are released by burning it, contributing to atmospheric pollution.