Research Of Elemental Mercury Removal From Coal-fired Flue Gas Via Morphology Adjustment

Mercury is a global pollutant which has serious effects on human health and ecological environment.Coal-fired power plants are one of the dominant sources of mercury emission in China,and mercury emission control for coal-fired power plants is of great significance.Elemental mercury(Hg0),oxidized mercury(Hg2+)and particulate-bound mercury are main existing forms of mercury in coal-fired flue gas.Hg0 and Hg2+ can be effectively removed by the air pollution control devices in power-plants while Hg0 is difficult to be removed.Therefore,enhancing Hg0 oxidation into Hg2+ play a key role in mercury control in coal-fired power plants.Previous studies suggest that there is a positive correlation between the proportion of Hg2+ in flue gas and the halogen content in coal.The low halogen content in Chinese coal results in high Hg0 concentration in flue gas.Consequently,adding extra halogen component in coal is a useful way to promote Hg0 oxidation.Additionally,Hg0 can be oxidized on SCR catalyst during flue gas denitration process,however,the oxidation rate is quite low over the widely used V-based SCR catalyst.Hence,high-efficiency Hg0 oxidation can be achieve by modifying present catalyst,which is of great significance.In this research,on the one hand,extra CaBr2?was added to coal to enhance the Hg0 oxidation in flue gas.The promotion mechanism was proposed.During the research,it was fond that CaBr2 addition could increase HBr concentration in flue gas.Therefore,the potential adverse effect of HBr was furtherly investigated in order to provide a comprehensive evaluation for industrial application.On the other hand,based on the theoretical analysis,CeO2 was selected to modify the traditional V-based SCR catalyst.The catalytic mechanism of Hg0 oxidation was investigated by activity test and characterization.The effects of operation parameters were also investigated.The main results of this research are as follows:(1)Mercury release rules in coal were studied by analyzing the mercury and chlorine contents in 19 coal samples nationwide.Lignite samples were screened out for mercury removal experiments by adding CaBr2.The results showed that the addition of CaBr2 can significantly inhibit Hg0 release.The release of Hg0 decreased by 40.4%with 200 ?g/g CaBr2 addition in coal.The addition of CaBr2 increased the concentration of HBr in flue gas.The strong negative correlation between HBr concentration and Hg0 release amount suggest that HBr can generate active Br atoms to oxidize Hge.(2)The potential negative influence of HBr generated by CaBr2 addition was study.The effect of HBr on the performance of V2O5-WO3/TiO2 SCR catalyst was studied.The results indicated that HBr inhibited the activity of catalyst when reaction temperature was below 300 ?.When 50 ppm was added in flue gas,the NO conversion rate reduced by 10%and 20%at 200 ? and 250 ?,respectively.Further studies by XPS and in-situ FTIR revealed that HBr can react with V=O sites through the following equation:HBr+V5+=O?Br-V4+-OH,which affected the redox cycle in SCR reaction,therefore inhibiting the activity of the catalyst.(3)It was innovatively proposed that enhancing NO adsorption on the catalyst will simultaneously improve SCR activity and Hg0 catalytic oxidation on catalyst.CeO2 was selected to modify traditional V2O5/TiO2 catalyst.V1Ti and V1Ce5Ti catalyst was synthesized by impregnation method and the activity of the catalysts was measured.Results show that,on the one hand,CeO2 improved the activity at low temperature.NO conversion exceed 95%within the optimum operating temperature ranging from 250 ? to 400 ?.On the other hand,Hg0 oxidation was significantly improved.Hg-TPD results indicated that Hg(NO3)2 was the oxidation product.HCl facilitated the desorption of Hg(NO3)2.A nitrate pathway for Hg0 oxidation was proposed.NO and O2 first adsorbed on catalyst and generated nitrate which furtherly react with Hg0 to form the Hg(NO3)2.Subsequently,Hg(NO3)2 desorbed from catalyst with the aid of HCl and accomplished the catalytic cycle.The bidentate nitrates generated on the catalyst plays an important role in the oxidation of Hg0.(4)The effects of operating parameters on performance of catalyst were investigated and results showed that:SO2 and H2O(g)had no significant effect on the SCR performance over VlCe5Ti while inhibited Hg0 oxidation at 400 ?.XPS analysis showed that SO2 can react with CeO2.Hg0 oxidation rate decreases with the increase of gaseous hourly space velocity(GHSV).The Hg0 oxidation rate were more than 80%even under the GHSV as high as 260000 h-1.What's more,no obvious activity reduction occurred during stability test as long as 48 h.These results suggested CeO2 modified SCR catalyst had industrial application prospect.