Experimental Study On Mercury Removal Based On SCR And WFGD System

Mercury and its compounds are highly biotoxic,and have a threat to both human health and environment,which attracts considerable attention in worldwide.Coal-fired flue gas has been targeted as the major anthropogenic source of mercury emissions,and it is an urgent task to control mercury emissions from coal-fired power plants.The selective catalytic reduction(SCR)denitration catalyst for NO removal can oxidize elemental mercury(Hg0)into oxidized mercury(Hg2+),and the Hg2+ can be captured by the subsequent wet flue gas desulfurization(WFGD)system,which achieves the mercury control in coal-fired flue gas.However,the Hg0 oxidation activity of the existing commercial SCR catalyst is relatively low since the flue gas components ald the reaction conditions have a significant influence on Hg0 oxidation,which limits the mercury removal efficiency.Enhancing the Hg0 oxidation activity of the commercial SCR catalyst and improving the mercury removal effect of desulfurization slurry can effectively increase the mercury removal activity of the existing air pollution control devices,which can achieve the simultaneous control of multipollutant.This meets the economic and technical requirements of mercury removal,promoting the green and sustainable development of coal-fired power plants.In this thesis,CuCl2 modified commercial SCR catalysts(CuCl2-SCR)were prepared by the improved impregnation method for catalytic oxidation of Hg0.The effects of CuCl2 contents,reaction temperatures,flue gas components and GHSV on Hg0 catalytic oxidation activity were investigated.Several characterization techniques including BET,XRD,XRF.Hg0-TPD were used to characterize the catalysts.The results indicated that CuCl2 could release active Cl species in the presence of O2,which enhanced the Hg0 oxidation activity.The consumed Cl species could be replenished from the gas phase HCl.The Hg0-TPD results confirmed that the oxidized mercury compounds mainly existed in the form of HgCl2.Further study revealed that Hg0 oxidation over CuCl2-SCR catalyst followed the Mars-Maessen mechanism.In order to improve the Hg0 oxidation activity of conventional V2O5-WO3/TiO2 catalyst in the absence of HCl,a series of CuO-V2O5-WO3/TiO2 catalysts(Cux-VyWTi)and CuO modified commercial SCR catalysts(Cux-SCR)were prepared by the improved impregnation method.The effects of reaction temperatures,flue gas components and GHSV on Hg0 catalytic oxidation activity over Cu3-V0.8 WTi catalyst were systematically studied.Simultaneously,the effects of Hg0,NO+NH3,SO2 and H2O on the catalytic activity of Cu3-SCR catalyst were also investigated.Moreover,the BET,XRD and XPS were used to characterize the CuO modified SCR catalysts.The results indicated that the existence of the redox cycle of V4++Cu2+(?)V5++Cu+ on the CuO modified SCR catalysts yielded abundant chemisorbed oxygen,which enhanced the Hg0 oxidation activity.The Hg0 oxidation activity of NaClO2 solution was studied in the simulated limestone slurry.The effects of NaClO2 concentrations,slurry temperatures,pH,Cl-concentrations and acid gases on the Hg0 removal efficiency were investigated.The mercury removal products were characterized by the XRD and XPS.which suggested that SO2-NO and NaClO2 were participated in the Hg0 oxidation process,and the oxidized mercury compounds mainly existed in the form of HgCl2,Hg(NO3)2 and HgS04.The kinetics experiments of mercury re-emission inhibited by the additive were carried out to evaluate the mercury re-emission performance in desulfurization slurry.Mercury re-emission activity before and after adding DTCR was investigated in the simulated desulfurization slurry.Moreover,the reaction order,reaction rate constant and activation energy of mercury re-emission were calculated according to the kinetics experiment results.The effects of reaction conditions on the reaction rate constants were also investigated in the simulated desulfurization slurry.Finally,the mercury removal activity was evaluated based on SCR and WFGD system.The effects of SCR conditions and WFGD conditions on the mercury removal activity were investigated,respectively.The results showed that the synergistic mercury removal system using Cu3-SCR catalyst exhibited stable and efficient mercury removal activity,and DTCR could significantly inhibit the mercury re-emission,thus enhancing the mercury removal efficiency based on SCR and WFGD system.These conclusions provide the theoretical foundation for the practical industrial application in future.