Lignites and subbituminous coals combustion in Polish power plants as a source of anthropogenic mercury emission

Abstract

Coal combustion is one of the main anthropogenic sources of mercury emission to the atmosphere. A total of 121 subbituminous coal samples and 29 lignite samples representative for coals burned in Polish power and heat and power plants were acquired and examined. The average mercury content in subbituminous coal samples was 104μgkg−1 (4.591gHgTJ−1), varying from 18 to 518μgkg−1. The average chlorine and bromine content were 2200ppm and 12ppm, respectively. For lignite samples the average mercury content was 197μgkg−1 (21.380gHgTJ−1), varying from 60 to 665μgkg−1. Lignite samples contained significantly less chlorine (40ppm on average) and bromine (4ppm) than coal samples. The Cl·Hg−1 ratio for subbituminous coal samples varied between 103 and 105. For lignite samples this ratio was around 10. Mercury mass balances were performed for two subbituminous coal-fired plants and one lignite-fired plant. The chemical composition of each type of coal had a significant influence on mercury concentration and speciation in the flue gas. The proportion of oxidized mercury (Hg2+) correlated with chlorine content. The relatively large proportion of particulate-bound mercury (Hgp) and Hg2+ measured favored mercury removal by the electrostatic precipitator (65% on average), and by the wet flue gas desulfurization units (32%). Results of mercury emissions from the stack to the atmosphere were 2.0–5.0 and 15.1μgm−3, for plants burning subbituminous coal and lignite, respectively. The mercury leaving the stack from three plants was over 90% elemental mercury (Hg0). Calculated mercury emission factors for plants burning subbituminous coal and lignite were: 0.71–1.80gHgTJ−1 (subbituminous coal) and 6.09gHgTJ−1 (lignite).