Assessment of elements, speciation of As, Cr, Ni and emitted Hg for a Canadian power plant burning bituminous coal

Abstract

Abstract A detailed assessment of elements was carried out at a power plant rated at 150 MW burning western Canadian medium volatile bituminous coal with an ash content of 34 wt.%. The distributions of elements of environmental concern (As, Cd, Cr, Hg, Ni and Pb) in feed coals, ashes, and stack-emitted materials were determined using NAA, ICPES and ICP-MS, GFAA for Pb, and CVAA for Hg. The speciation of As, Cr, and Ni was examined using XANES spectroscopy. The results show that the elements in the feed coal are within the same range for As, Hg and Pb and higher for Cd, Cr and Ni compared to other Canadian feed coals and within range for world coals. The combination of a Mechanical Cyclone Separator (MCS) and Fabric Filter (FF) removes a significant portion of the elements of environmental concern, as indicated by their relative enrichment (RE) ratios greater than 0.7. The fly ash from within the FF has a higher content of elements, such as Hg (1.58 mg/kg) than the MSC (0.13 mg/kg) due its lower temperature (130 °C) and the ability for finer particles to be captured by the baghouse. Arsenic in the feed coal is dominated by arsenical pyrite and less toxic As + 5 in arsenate forms. Arsenic is very low in the bottom ash, while in the fly ash it is largely (> 90%) present as As + 5 . Chromium in the milled coal is present as Cr + 3 in association primarily with illite. This occurrence gives rise to an aluminosilicate association in ash materials, with the chromium oxidation state remaining as Cr + 3 . Nickel in both the feed coal and ashes occurs as Ni + 2 predominantly in coordination with oxygen. No evidence for carcinogenic Ni sub-sulfides was observed. Mercury is emitted from the plant at a rate of 1.8 g/h and is mostly in the form of reactive gaseous mercury, followed by elemental mercury and particulate mercury. The rates of input of elements of environmental concern, As, Cd, Cr, Hg, Ni and Pb, for this station were 10.45, 1.13, 123.3, 0.29, 36.3 and 23.1 kg/day, respectively, of which only 0.08, 0.01, 0.71, 0.04, 0.44, and 0.17 kg/day were emitted from the stack. Indicating that most of these elements (> 99%) were captured by the particulate removing devices. The concentrations of elements in the air and in the vicinity of this power plant are low or within the range of published data for ambient air in urban and rural areas.