Mercury Speciation And Isotopic Composition In Flue Gas From Coal-fired Power Plants In Inner Mongolia

This work studied the Hg species in flue gas and the byproducts from three typical coal-fired power plants with different types of coal using C-5000 sampler made in USA ESC according to the United States Environmental Protection Agency(EPA)of the flue gas mercury sampling method(Ontario Hydro Method,OHM),to evaluate actual speciated Hg emission inventories in the largest coal-production province of China,Inner Mongolia.We present the first onsite data for Hg isotope signature in feed coal and the combustion byproducts(bottom ash,fly ash,gypsum,process water,gaseous elemental Hg(GEM)and gaseous oxidized Hg(GOM)at the inlet(i)and outlet(o)of WFGDs)and preliminary discuss mercury isotope fractionation mechanism during coal combustion process from a typical lignite-fired power plant in Inner Mongolia,north of China.We obtain the following conclusions:(1)In normal operation of the current efficient denitrification,particles control and desulphurization devices,Mercury emissions in flue gas from the coal-fired plants are in the dominant form of Hg0(>85%).The Hg2+(<15%)and the Hgp(<2%)emissions are a fraction of Hgtin the flue gas.(2)Mercury content in bitumite and lignite is little difference in Inner Mongolia,which bituminous coal containing mercury is 0.16±0.04 μg/g(1SD),mercury content in lignite is 0.19±0.05 μg/g(1SD).Mercury content in the bottom ash(1.1~11% of the coal in the coal)is lower and mercury is mainly enriched in the fly ash after through precipitators.Mercury enrichment degree of fly ash in fabric dust collector(0.64±0.08μg/g,1SD)is much higher than in electrostatic precipitators(ESPs)(0.22±0.03 μg/g,1SD).The wet limestone flue gas desulphurization device(WFGDs)has much higher removal efficiency for Hg2+ reaching 87~94%.The mercury content in the desulfurization gypsum is almost as same as mercury in coal.(3)The results of mercury mass balance in the three power plants are all good,indicating that the research method is reliable.So we obtained comprehensive Hg discharge factors including direct Hg emission factors to the atmosphere and indirect Hg discharge factors with bottom ash,fly ash and gypsum to the environment,and designated the Hg fates in coal.We estimated the Hg discharges from coal-fired powerplants in Inner Mongolia in 2007-2014 and expanded to China.The maximum of direct Hg emissions from coal-fired power plants of Inner Mongolia was 24.22 t in 2012.The Chinese coal-fired power plants emitted directly about 100 t Hg to the atmosphere every year at the nationwide air control actions.The maximum of Hg discharges with fly ash and gypsum of Inner Mongolia was 80.49 t and 10.56 t in 2012,respectively.The national Hg discharges with fly ash and gypsum every year are respectively about 300 t and 40 t.(4)Fly ash and gypsum as raw material may be reused in building industry.Mercury in fly ash and gypsum during the high temperature combustion section may be reemitted mostly to the atmosphere.Thus,mercury emissions from coal-fired power plants are not the largest single Hg source to the atmosphere in normal operation of the current efficient denitrification,particles control and desulphurization devices.The largest mercury emissions to the atmosphere can or may be the utilization industry of fly ash and gypsum.(5)We present the first onsite Hg isotope data for speciated Hg(GEM and GOM)of flue gas during coal combustion process.Stronger mass dependent fractionation(MDF)takes places during coal combustion and Air Pollution Control Devices(APCDs),but mass independent fractionation(MIF)variation is insignificant.The speciated Hg of flue gas exhibited strong enrichment in heaver Hg isotopes with variation of 0.7‰ and 1.45‰ for GOMi and GEMi relative to the feed coal(δ202Hg)at inlet of WFGDs(or outlet of ESPs)by the combination of SCRs operation and removal of PBM in ESPs,respectively.With the combination of SCRs operation,removal of PBM in ESPs and removal of GOMi/GEMi in WFGDs resulting in gypsum and waste water,the speciated Hg of flue gas emitted to the atmosphere showed the strongest enrichment in heaver Hg isotopes with variation of 1.7‰ and 2.6‰ for GOMo and GEMo relative to the feed coal(δ202Hg),respectively.(6)The statistical analysis(p<0.05)indicates that the δ202Hg values of the feed coals(-2.04±0.50‰,2SD),bottom ash(-0.41±0.06‰,2SD),APCDs byproducts(-2.20‰±0.24‰,2SD)and speciated Hg of flue gas are isotopically distinguishable from each other.(7)All samples in feed coal and its combustion products(bottom ash,fly ash,gypsum,process water,GEM and GOM in flue gases before and after WLFGD)in a lignite-fired power plant are characterized by significant negative Δ199Hg values and the MIF variation is insignificant during coal combustion and Air Pollution Control Devices(APCDs).(8)The combination between significant positive δ202Hg values of GEMo samples(δ202Hg= 0.30~0.85‰)and the significant negative Δ199Hg values may provide direct evidences for tracing and quantifying Hg source from lignite combustion to the atmospheric or the terrestrial ecosystems.(9)Rayleigh fractionation may be one of the main Hg isotopic fractional mechanisms during coal combustion processes.