Emission and species distribution of mercury during thermal treatment of coal fly ash

Abstract

The thermal treatment of coal fly ash (FA) utilized by industries will inevitably lead to the emission of mercury, potentially causing atmospheric pollution. The present study is aimed at understanding the emission amount and species distribution of mercury during FA thermal treatment, and attempts were made to further provide basic data of mercury emission for environmental utilization of the FA by industries as raw materials. The physio-chemical properties of FA were analyzed and FA samples were heated at 200–1200°C in a tubular furnace in an oxidizing atmosphere. Thermogravimetric analysis (TGA) was carried out to determine the content of residual carbon in FA. The mercury species distribution was determined using United States EPA method 3200. The results indicated that semi-mobile mercury was the main mercury species in FA. Owing to the adsorption of residual carbon on Hg0, more residual carbon which lead to higher proportion of semi-mobile obtained in coarser size grades. 20% of total mercury emitted at 206°C, followed by a rapid growth of the emission rate of mercury at above 206°C, and the emission of total mercury achieved to be 50% and 90% corresponding to 291°C and 515°C, respectively. A distinct transformation from semi-mobile mercury to extractable mercury occurred at 200°C due to catalytic oxidation of Hg0 and enhance the mobility and potential toxicity of mercury. Both extractable and semi-mobile mercury decreased sharply at 300–400°C. Finally, the non-mobile mercury was emitted with combustion of residual carbon in FA at 600–800°C. It was thus concluded that measures would be taken to control emission of mercury for environmentally friendly utilization of coal FA by industries when the FA was heated higher than 300°C.