Mercury stable isotope fractionation in six utility boilers of two large coal-fired power plants

Abstract

Coal-fired utility boiler (CFUB) emissions of mercury (Hg) represent the largest anthropogenic Hg source to the atmosphere. Hg stable isotope signatures in coal have been shown to vary among coal deposits and coal basins. There is therefore a substantial interest in tracing CFUB Hg emissions at local, regional and global scales. However, CFUB operating conditions, Hg capture technologies and post-emission Hg transformations may potentially alter the original feed coal Hg isotope signatures. Here we investigate Hg isotopic fractionation between feed coal and coal combustion products in six utility boilers of two large power plants in Huainan City, Anhui Province, China. We observe identical trends in all six boilers: relative to feed coal with δ202Hg ranging from −0.67 to −0.18‰, oxidized Hg species in bottom ash and fly ash are enriched in the lighter isotopes with δ202Hg from −1.96 to −0.82‰. Flue gas desulphurization by-product gypsum shows δ202Hg from −0.99 to −0.47‰. No mass independent fractionation was observed during the transport and transformation of Hg inside the boilers. An isotope mass balance suggests that gaseous stack Hg emissions are enriched by up to 0.3‰ in the heavier Hg isotopes relative to feed coal and that the enrichment depends on the Hg capture technology. The observation that oxidized Hg species are enriched in the lighter isotopes suggests that oxidized and reduced forms of Hg in stack emission carry different isotope signatures. This has implications for near-field and far-field Hg emission tracing.