Speciation and modelling of toxic trace elements in Australian coal fired power stations

Coal fired power stations are a source of toxic trace elements to the environment. In Australia, ~85% of electricity is generated from coal fired power stations. Use of large quantities of coal makes it a major anthropogenic source of toxic trace element emission in the environment. Trace elements are naturally occurring in coal having been incorporated into the coal matrix during the coalification process. During coal combustion these elements partition to the combustion ash residues, such as bottom ash and fly ash, or are released to the atmosphere in gaseous or particulate form. It is widely known that the distribution, mobility and biological availability of these species in the environment not only depends on their total concentrations but, critically on their various chemical forms and oxidation states (speciation). For example, arsenic is more toxic as trivalent As(III) than as pentavalent As(V). Hexavalent chromium, Cr(VI) is a carcinogen, while trivalent chromium, Cr(III) is an essential nutrient. The study involves speciation analysis of arsenic, chromium, mercury and selenium in coal and combustion residues generated from five bituminous coal utilizing Australian power stations. Ion chromatography coupled with inductively coupled plasma mass spectrometry (IC-ICPMS), X-ray absorption near edge structure spectrometry (XANES) and the Ontario Hydro method of stack sampling for mercury was used for speciation analyses. The transformation of trace element species during combustion and post combustion has also been examined using calculations of thermodynamic equilibrium and kinetic modelling. Results are also discussed and compared with respect to operating conditions of the power stations.