Study On The Partitioning Characteristics Of Hg,As,Se And Other Hazardous Elements In Ultra Low Emission Coal Fired Power Plants

With the burgeoning economy,the growing demand for electricity in China relies heavily on coal-fired power plants.The toxic trace metals,such as Hg,As,Se,and Pb,released from coal combustion have received increasing attention due to their high mobility and persistent harm to the health of people.The Ministry of Environmental Protection of the People's Republic of China regulated the emission limit of Hg for power generation for the first time in January 1,2012,suggesting a reinforcement in the assessment and management of the mercury released from coal-fired power plants.In this paper,a comprehensive analysis of the migration,speciation characteristics and mobility of typical trace metals(Hg,As,Cd,Se,Cr,and Pb)in solid and liquid streams in typical ultra-low-emission coal-fired power plants was presented.Firstly,the partitioning of trace metals among air pollution control devices in five ultra-low-emission coal-fired power plants was investigated.Results showed that most of the trace metals were enriched in fly ash.Low-low temperature electrostatic precipitator(LLT-ESP)and electrostatic fabric filter(EFF)greatly increased the relative enrichment factors of Hg,As,and Se in fly ash.In the wet flue gas desulfurization(WFGD)system,the concentrations of trace metals in fine fractions were much higher than those in coarse fractions.The wet electrostatic precipitator(WESP)system removed trace metals mainly by capturing fine particles in the flue gas.The concentrations of Hg in the fine particulates captured by WESP were 16.8 to 60.1 times of those in the fly ash,which could reach up to 17.5 mg/kg.Secondly,the speciation characteristics and mobility of trace elements(As,Se,Cd,Cr and Pb)across ultra-low emission air pollution control devices was studied.Results indicated that most of trace elements partitioned to ash and the mobility of trace elements decreased during this process.37.1%-99.9%of trace elements were bonded in Si-Al-Fe lattice as residual fraction in ash.In wet flue gas desulfurization(WFGD)system,the trace elements in gypsum and waste sludge showed a distinctively higher mobility than in ash.The finer particles captured by WESP had the highest concentrations of trace elements with increased mobility of Se,Cd and Pb.The potential leaching amount per ton of combustion by-products considering acid soluble fraction was in the range of 0-7.56g/t(As),0-5.46g/t(Se),0-1.77g/t(Cd),0.028-82.7g/t(Cr)and 0.012-13.6g/t(Pb),respectively.Thirdly,based on the temperature programmed pyrolysis experiments,the characteristics of pyrolysis curve of mercury in solid products from typical ultra low emission units was investigated.The morphology and distribution proportion of mercury compounds in solid products were obtained.Longitudinal comparison of the composition changes of mercury in different products was presented and the migration mechanism of mercury in the whole process was analyzed.Furthermore,the composition of mercury in the solid products of different units was compared laterally,and the influence of different ultra low emission pollutants control devices on mercury control was obtained.The results showed that mercury in fly ash was mainly in the form of HgCl2 and HgS.HgCl2 tended to be enriched in fine particulate fly ash,and the HgCl2 ratio in WESP slag was up to 90.68%.Gypsum and waste sludge were mainly in the form of HgCl2 and black HgS.The order of thermal stability in solid products is gypsum>waste sludge>fly ash>WESP slag.Finally,the co-benefit of hazardous trace elements such as mercury(Hg),arsenic(As),and selenium(Se)capture in dust removal devices of ultra-low emission coal-fired power plants was investigated.It was found that the low-low temperature electrostatic precipitator(LLT-ESP)and the Electrostatic fabric filter(EFF)had greater abatement capacity for Hg than normal cold-side ESP(CS-ESP).The significance level of the effects on the abatement capacity for Hg,As and Se in ESP were analyzed by Pearson correlation analysis and grey relational analysis.The results indicated that the low inlet temperature of LLT-ESP had significant promotional effect on the simultaneous removal of Hg,As,and Se.The smaller particle size of fly ash can be conducive to the adsorption of hazardous trace elements.The inhibitory effect of sulfur content in coal was significant for the enrichment of Hg and Se in fly ash.