| Coal-fired power plants are the primary coal consumers.During the combustion process of coal,many trace elements are discharged with flue gas.With the improvement of China’s environmental protection requirements,the emissions of trace elements from coal-fired power plants have received extensive attention.The limestone-gypsum wet flue gas desulfurization(WFGD)system can not only efficiently absorb SO2,but also affect the migration and distribution of trace elements.Desulfurization gypsum and sludge are the main by-products.In the subsequent treatment and reuse process,trace elements may cause negative environmental impacts.This paper selected the WFGD systems of three ultra-low emission coal-fired units in Anhui Province as the research objects,and systematically collected environmental samples such as inlet/outlet flue gas,limestone,industrial water,sludge,gypsum,and desulfurization wastewater.The distribution and migration characteristics of trace elements such as Cr,As,Cd,Pb,and Hg were studied,and the forms and leaching characteristics of trace elements in desulfurization by-products were analyzed to evaluate the potential environmental impact of trace elements in process of desulfurization by-products reuse.The main results and conclusions are as follows:(1)Regarding the distribution and migration of trace elements in WFGD system,the removal rate of trace elements in dust was over 95%.The concentrations of Cr,As,Cd,Pb,and Hg in inlet dust were 55.2～127.8,22.1～62.5,0.5～2.2,13.1～26.0,and 2.5～9.2 μg/m3,respectively,which were reduced to below 0.1 μg/m3 after desulfurization.The contents of Cr,As,and Hg in sludge were 29.98～52.26,12.52～36.71,and 2.21～6.46 mg/kg,respectively,about 7～9 times the contents of Cr,As,and Hg in gypsum.Elemental mass balance analysis showed that trace elements mainly came from dust and limestone,and were mainly trapped to gypsum(0%～89%)and sludge(3%～100%).Gypsum was enriched in As and Cd,while sludge was enriched in Cr and Pb,and only a small amount of Cd(7%～10%)was enriched in wastewater.(2)Regarding the elemental forms of trace elements in desulfurization by-products,the sequential chemical extraction method was used to analyze the forms of trace elements Cr,As,Cd,and Pb.Cr and As in gypsum were mainly in residual form(accounting for 82%),Cd in gypsum of unit 1#was mainly in residual form,and Cd in gypsum of units 2#and 3#were mainly in oxidizable form(accounting for over 70%).The effective form of Pb in gypsum accounted for more than 55%,and the acetic acid extractable form of Cr and Pb in sludge only accounted for 5%,As in sludge was mainly in effective form,and the acetic acid extractable form of Cd was relatively high(about 30%),with a certain degree of mobility.The migration ability of Pb and Cd in gypsum was stronger than that in sludge.The transferability of trace elements in gypsum and sludge was in the order of Pb>Cd>Cr>As,As>Cd>Cr>Pb.Most trace elements existed in easily transferable components,which were easily leached under certain environmental conditions and may cause environmental pollution.(3)In terms of the leaching characteristics of trace elements in desulfurization byproducts,Toxicity Characteristic Leaching Procedure(TCLP)was used to simulate the leaching process of trace elements in acidic environments.Overall,the leaching rate of trace elements increased with the increase of liquid-solid ratio(LSR)and decreased with the increase of pH.The relationship between leaching rate of Cd and pH of the leaching solution was not significant.The concentrations of trace elements in gypsum and sludge leaching solution of three units were far below the leaching toxicity limits of the "Identification Standards for Hazardous Wastes".The average leaching rates of Cr,As,Cd,and Pb in gypsum were 0.39%～4.16%,0.29%～1.41%,0.69%～6.14%,and 1.50%～38.95%,and in sludge were 0.24%～0.76%,0.87%～0.90%,1.97%～2.89%,and 2.716～5.26%,respectively.Generally,the leaching rate of trace elements in gypsum was higher than that in sludge,and the leaching ability of trace elements was in order of Pb>Cd>As>Cr.In treatment and disposal,corresponding management and disposal methods needed to be developed based on the characteristics and risk level.(4)In terms of environmental risks of desulfurization by-products,the results of single factor contaminant index,geo-accumulation index,and risk assessment code indicated that the pollution risk of sludge was higher,with varying degrees of pollution risk present in As,Cd,Pb,and Hg,except for Cr.The leaching risk of trace elements in by-products under natural landfill conditions was simulated through the concentration model of leaching substances,indicating that the probability of Cd and Pb concentrations in sludge exceeding the limit value of Class Ⅲ groundwater was high.Therefore,attention should be paid to the leaching risks of Cd and Pb in disposal. |
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