Institute of Surface-Earth System Science, School of Earth System Science, Tianjin University, Tianjin, 300072, China.
Institute of Surface-Earth System Science, School of Earth System Science, Tianjin University, Tianjin, 300072, China.
Environ Pollut. 2021 Jan 1;268(Pt B):115753. doi: 10.1016/j.envpol.2020.115753. Epub 2020 Oct 5.
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 Hg(g)→Hg(g) and gaseous Hg(g)→particulate Hg(p) in coal combustion flue gases, we propose an equilibrium MDF for Hg(g)↔Hg(g) followed by a kinetic MDF for Hg(g)→Hg(p). This equilibrium MDF most likely occurs during Hg(g) oxidation in denitrification devices, which enriches heavy Hg isotopes in oxidized products (Hg(g) and Hg(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.
中国城市锅炉的脱硝装置安装率正在加速。以前对没有脱硝装置的煤粉锅炉的研究表明，主要含有氧化汞 (Hg) 的燃烧产物比进料煤更优先富集较轻的 Hg 同位素。然而，如果安装了脱硝装置，这种富集的幅度变得不那么明显。Hg 同位素分馏的潜在机制仍不清楚。在这项研究中，对三种类型的城市锅炉（两台煤粉锅炉、一台循环流化床锅炉和一台城市垃圾焚烧锅炉）都安装了脱硝装置，测量了其进料燃料和相应燃烧产物的 Hg 同位素组成。我们观察到进料燃料和燃烧产物之间几乎没有质量独立分馏，但存在非常显著的质量依赖分馏 (MDF)。在三台煤粉炉中，飞灰和脱硫产物都比进料煤富集较重的 Hg 同位素，而且在所有锅炉中，随着燃烧产物的顺序去除，重 Hg 同位素的富集程度增加。与先前在煤燃烧烟道气中提出的气态 Hg(g)→Hg(g) 和气态 Hg(g)→颗粒态 Hg(p)的动力学 MDF 不同，我们提出了 Hg(g)↔Hg(g) 的平衡 MDF，随后是 Hg(g)→Hg(p)的动力学 MDF。这种平衡 MDF 很可能发生在脱硝装置中 Hg(g)氧化过程中，导致氧化产物（Hg(g) 和 Hg(p)）中重 Hg 同位素富集，然后被封存在飞灰和脱硫产物中。在装有脱硝装置的锅炉中 MDF 的范式转变通过城市大气颗粒物中平行的 Hg 同位素测量得到了进一步验证。我们的研究清楚地表明，装有脱硝装置的现代煤粉锅炉与没有脱硝装置的传统锅炉相比，具有相当不同的 MDF。这对估计城市锅炉 Hg 排放的同位素特征以及人为 Hg 排放的同位素示踪具有重要意义。
