Denitrification devices in urban boilers change mercury isotope fractionation signatures of coal combustion products

The installation rate of denitrification devices is accelerating in Chinese urban boilers. Previous studies on pulverized coal-fired boilers without denitrification devices showed that combustion products containing mainly oxidized mercury (Hg) preferably enriched lighter Hg isotopes than feed coals. However, the magnitude of this enrichment becomes less pronounced if denitrification devices are installed. The underlying Hg isotope fractionation mechanisms are still unclear. In this study, three types of urban boilers (two pulverized coal-fired boilers, one circulating fluidized bed boiler and one municipal waste incinerator boiler) all installed with denitrification devices were measured for Hg isotope compositions of their feed fuels and corresponding combustion products. We observed little mass independent fractionation but very significant mass dependent fractionation (MDF) between feed fuels and combustion products. The fly ash and desulfurization products both enriched heavier Hg isotopes than feed coals in three coal-fired boilers, and the enrichment of heavy Hg isotopes increased with sequential removal of combustion products in all boilers. Different from previously suggested kinetic MDF for gaseous Hg⁰(g)→Hgᴵᴵ(g) and gaseous Hgᴵᴵ(g)→particulate Hgᴵᴵ(p) in coal combustion flue gases, we propose an equilibrium MDF for Hg⁰(g)↔Hgᴵᴵ(g) followed by a kinetic MDF for Hgᴵᴵ(g)→Hgᴵᴵ(p). This equilibrium MDF most likely occurs during Hg⁰(g) oxidation in denitrification devices, which enriches heavy Hg isotopes in oxidized products (Hgᴵᴵ(g) and Hgᴵᴵ(p)) that are then sequestrated in fly ash and desulfurization products. The paradigm shift of MDF in boilers with denitrification devices was further verified by parallel Hg isotope measurement in urban atmosphere particulates. Our study clearly demonstrates that modern coal-fired boilers with denitrification devices have a quite different MDF compared to traditional boilers without denitrification devices. This has important implications for estimating isotope signatures of urban boiler Hg emissions, and for isotope tracing of anthropogenic Hg emissions.