Migration And Transformation Of As,Se And Pb In Coal-fired Power Plants

Coal combustion is still the dominant source for power generation in the energy mix in the foreseeable future.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.Coal combustion for power generation is considered as one of the major emission sources of hazardous trace elements.In this work,the migration and transformation of typical trace elements(As,Se and Pb)in flue gas of four power plants obtained and the effects of several factors,in terms of coal types,combustion manners and pollutants control devices are verified.Firstly,the transform mechanism of As,Se and Pb has been comprehensively presented during coal combustion.The method of chemical equilibrium is analyzed.HSC,as a typical software of chemical thermodynamics,is used to predict the speciation of three kinds of trace elements(As,Se and Pb)in the flue gas during coal combustion.The results show that temperature and additive limestone are main factors to affect the distributions of trace elements.The volatilization characteristics of trace elements are markedly different in different temperature ranges.For the pulverized coal boiler(PCB),As and Se mainly exist in the form of gaseous oxides,and Pb also gradually changes from solid oxides to gaseous oxides at highest temperature.For the circulating fluidized combustion boiler(CFB),As mainly exists in the form of solid calcium arsenate,and SeO is the main form.Secondly,a new method based on microwave digestion and atomic fluorescence spectrometer is established to investigate the partitioning of trace elements among air pollution control devices in coal-fired power plants.This method can realize fast digestion and accurate measurement,and the loss of As,Se and Pb can be reduced as far as possible in the whole experimental process.Finally,the distribution of trace elements,including As,Se and Pb from coal,fly ash and bottom ash from four power plants are explored.The results show that most of As and Se are enriched in fly ash,and the enrichment trend of lead is related to the combustion temperature of power plants unit.Pb is more enriched in the fly ash for PCB,whilst in the bottom ash for CFB.The above findings correspond to the results from chemical thermodynamic calculation.