Mechanism Study Of Elemental Mercury Oxidation In The Flue Gas Over Mn Based Low Temperature Catalysts

Coal-fired power plant is one of the most important anthropogenic mercury emission sources.Three forms of mercury exist in coal-fired flue gas,including elemental mercury(Hg0),oxidized mercury(Hg2+)and particulate mercury(HgP).Hg0 is difficult to remove by the existing air pollution control devices.In this work,catalytic oxidation was used to remove Hg0,by which the Hg0 was converted into soluable Hg2-and then removed by wet flue gas desulfurization devices.Based on the shortcomings,e.g.deactivated by fly ash,of high temperature commercial SCR catalyst,the objective of this thesis is developing low temperature catalyst for Hg0 oxidation,which can be placed after the ESP.The optimal temperature of low temperature catalysts is hope to be 100-200 ?.Mn based materials are recognized as promising catalysts for Hg0 removal.but the optimal temperature is?250 ?.Otherwise,the activity is strictly dependent on HCl concentration in the flue gas and the materials are with poor sulfur resistance.Taking the above into consideration,a series of Mn based catalysts were developed and low temperature activity,influence factors and reaction mechanism were studied for Hg0 removal in present work.The catalytic activity of a traditional MnOx based catalyst was investigated to determine the active species for Hg0 oxidation.A novel MnOx based catalyst was produced based on the deactivated commercial SCR catalyst.The optimal temperature for NO reduction and Hg0 oxidation is-250 0C.As the optimal temperature is?250 ?,the catalyst cannot be used after ESP and it will be deactivated as well.At 150-250 ?,Mn4+ and chemical adsorbed oxygen species were mainly responsible for Hg0 oxidation.To solve the issues of MnOx based catalysts,some other Mn-based materials with special structures should be proposed and studied.One candidate is Mn based perovskite oxides.Sr doped perovskite oxides(Lai-xSrxMnO3)showed much better activity at 100-200 ?.As there were abundant of active oxygen on the catalyst surface and good mobility of reactive oxygen species,the perovskite catalyst exhibited superior activity even though in low concentration of O2 and HCl.However,sulfur resistance of the sample was not good and the catalyst was easily deactivated due to the sulphate reaction and competitive adsorption.Mn doped ceria zirconia solid solution(CZMx)exhibited similar low temperature activity to perovskite catalyst,due to the aboudant active oxygen species and their mobility.The activity tests show that the optimal temperature is 150 ?,and the dependence of the catalytic activity on HCl was low.However,CZMx showed better sulfur resistance.In the pure N2,SO2 iinhibited catalytic activity due to competing with the reactant for active oxygen species consumption.In presence of O2 and SO2,the inhibitory effect disappeared,because the consumed active oxygen could be replenished by the gaseous O2.At low temperature,the humidity of flue gas will be higher,and MnOx would be hydrated.Therefore,the effects of water vapor on the physical and chemical properties of the catalyst and on the Hg0 catalytic oxidation process should be clarified.The results indicated the hydration reaction changed the property of MnOx.The concentration of Mn4+and chemical adsorbed oxygen species are decreased.In addition,the existence of water vapor in the flue gas inhibited active chloriine species formantion,which is significant for Hg0 oxidation.Nowadays,Hg0 removal in the coal-fired power plant was mainly dependent on the existing air pollution control devices(APCDs),but the stability of the performance is rather poor.Therefore,running condition of the APCDs should be optimized to improve mercury removal efficiency.Some sampling works were carried out in two coal-fired power plants.The effect of load variation on Hg0 removal over SCR catalyst was studied.As load varied,the lflue gas temperature,oxygen content,space velocity,etc.changed simultaneously.Based on grey correlation analysis,flue gas temperature is the most sensitive factor to the Hg0 removal efficiency.Installation of a low temperature economizer could significantly enhance the Hg0 removal performance over the electrostatic precipitator.