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

Jingjing Yuan; Ruoyu Sun; Ruwei Wang; Biao Fu; Mei Meng; Wang Zheng; Jiubin Chen

doi:10.1016/j.envpol.2020.115753

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

Environ Pollut. 2021 Jan 1;268(Pt B):115753. doi: 10.1016/j.envpol.2020.115753. Epub 2020 Oct 5.

Authors

Jingjing Yuan¹, Ruoyu Sun², Ruwei Wang³, Biao Fu⁴, Mei Meng¹, Wang Zheng¹, Jiubin Chen¹

Affiliations

¹ Institute of Surface-Earth System Science, School of Earth System Science, Tianjin University, Tianjin, 300072, China.
² Institute of Surface-Earth System Science, School of Earth System Science, Tianjin University, Tianjin, 300072, China. Electronic address: ruoyu.sun@tju.edu.cn.
³ Guangdong Key Laboratory of Environmental Pollution and Health, School of Environment, Jinan University, Guangzhou, 511443, China.
⁴ State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China.

PMID: 33045583
DOI: 10.1016/j.envpol.2020.115753

Abstract

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)→Hg^II(g) and gaseous Hg^II(g)→particulate Hg^II(p) in coal combustion flue gases, we propose an equilibrium MDF for Hg⁰(g)↔Hg^II(g) followed by a kinetic MDF for Hg^II(g)→Hg^II(p). This equilibrium MDF most likely occurs during Hg⁰(g) oxidation in denitrification devices, which enriches heavy Hg isotopes in oxidized products (Hg^II(g) and Hg^II(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.

Keywords: China; Coal-fired boilers; Denitrification devices; Mass dependent fractionation; Mercury isotopes.

MeSH terms

Air Pollutants* / analysis
Atmosphere
Coal / analysis
Coal Ash
Denitrification
Mercury Isotopes / analysis
Mercury* / analysis
Power Plants

Substances

Air Pollutants
Coal
Coal Ash
Mercury Isotopes
Mercury