Speciation of trace elements in Australian coals and combustion residues

Coal fired power stations are recognised as a major anthropogenic source of toxic trace elements to the environment. In Australia, ∼127 MT of coal is used annually for electricity generation. Trace elements are naturally occurring in coal having been incorporated into the coal matrix during the coalification process. During coal combustion these elements are either incorporated into combustion residues, such as bottom and fly ash, or emitted to the atmosphere in vapour or particulate form. The chemical forms and oxidation state of the trace elements may then determine their actual fate and toxicity in the environment. For example, arsenic is more toxic as trivalent As³⁺ than as pentavalent As5+. Hexavalent chromium, Cr⁶⁺ is a carcinogen, while trivalent chromium, Cr³⁺ is an essential nutrient. This study involves determination of the speciation of arsenic, chromium and selenium in coal and combustion products. Samples were collected from five Australian power stations utilizing bituminous ranked coals. Speciation of mercury in flue gas was carried out for one power station. A range of analytical techniques were used to determine the speciation of trace elements. 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 have been used for speciation analysis. Speciation of these trace elements in combustion residues are compared with feed coals to observe trace element transformations under the combustion conditions in large scale power stations.