Multi-effect SCR Catalyst For Synergistic Oxidation Of Elemental Mercury And Slip Ammonia

With the increase of service life of ammonia selective catalytic reduction(SCR)denitration catalysts,the problem of ammonia slip from coal-fired power plants has become increasingly prominent,which has been considered as an important factor affecting the normal operation of flue gas systems.In addition,coal-fired power plants are the main anthropogenic source of mercury emission.The mercury emitted from flue gas can cause global pollution and seriously endanger human health.In this situation,strict control of slip ammonia and flue gas mercury emission is imminent.Catalytic oxidation of elemental mercury(Hg0)to oxidized mercury(Hg2+),which is easily removed by wet desulfurization units(WFGD),is an important measure to effectively reduce mercury emissions from flue gas.However,due to the inhibitory effect of NH3,most of the Hg0 is oxidized at the tail of SCR system.On the other hand,using ammonia selective catalytic oxidatiori(NH3-SCO)catalyst downstream of SCR catalyst could probably remove the slip ammonia as maintaining high denitration efficiency.Therefore,installing a small amount of catalyst with both NH3-SCO and Hg0 oxidation activity at the end of SCR device would maximally using the existing equipment to remove slip ammonia and elemental mercury,which could achieve the synergistic removal of multiple pollutants.Based on this,the paper focuses on experimental and theoretical researches on the oxidation of ammonia and elemental mercury over copper-modified SCR catalyst and copper-modified attapulgite catalyst.The purpose of this study is to provide valuable experimental data and theoretical basis for the application of SCR device to synergistically control multiple pollutants.In this thesis,the traditional V2O5-WO3/TiO2 catalyst was modified by using an improved wet impregnation method to prepare a series of Cu-SCR catalysts.Their NH3-SCO,NH3-SCR and Hg0 oxidation performance were investigated at 350℃ to screen the optimum copper loading.Next,the influence of reaction temperature and different flue gas colponents on the removal properties of NH3,NOx and Hg0 were investigated.The results showed that 1%Cu-SCR catalyst exhibited excellent NH3 oxidation and denitrification activities at 350℃.The increase in temperature was beneficial to NH3 oxidation,but decreased NO removal efficiency and promoted N2O formation.The presence of NO further promoted NH3 conversion,while SO2 and H2O had a certain inhibitory effect on ammonia oxidation and denitration reaction.O2 played an important role in Hg0 oxidation over 1%Cu-SCR catalyst,excellent Hg0 oxidation activity was maintai1ed within 150～400 ℃.Further research found that 1%Cu-SCR catalyst effectively removed a small amount of slip ammonia,residual NOX and Hg0 under simulated SCR tail conditions.In order to ascertain the active components for each reaction in 1%Cu-SCR catalyst and the relevant reaction mechanisms,characterization methods including BET,XRD,UV-vis,XPS,NH3-TPD,H2-TPR and in situ DRIFT were conducted.The results found that copper species and vanadium species reacted synergistically in the form of Cu2++V4+(?)V5+Cu+ redox pair in both ammonia oxidation and mercury oxidation.According to in situ DRIFT results,the ammonia oxidation followed the iSCR mechanism.XPS results found the surface chemisorbed oxygen acted as the oxidant for Hg0.In order to reveal the influencing mechanism of copper modification on mercury oxidation activity of SCR catalyst,the adsorption and oxidation of a Hg atom over Cu-SCR catalyst surface models were also calculated based on density functional theory.The results showed that the mercury oxidation activity of surface VOX species was slightly improved,but CuOx provided the preferential adsorption and oxidation active sites for Hg.The Hg atom tended to combine with copper terminal oxygen to form HgO and desorbed from the surface.This reaction energy barrier was significantly lower than that of the mercury oxidation over the surface model of SCR catalyst.On the other hand,a series of environmental-friendly copper modified attapulgite(Cu-ATP)catalysts were prepared.Their NH3-SCO,NH3-SCR and Hg0 oxidation performance were investigated and the influencing factors were analyzed.The results showed that Cu-ATP catalysts all exhibited excellent NH3-SCO activity,and their N2 selectivity was improved compared with Cu-SCR catalysts.The increase in copper content and temperature was beneficial to ammonia oxidation,but was not conducive to maintain high N2 selectivity.NO promoted NH3 conversion,while SO2 and H2O had a certain inhibitory effect on ammonia oxidation and denitration activity.Cu-ATP catalysts had excellent Hg0 oxidation performance and sulfur resistance under HCl+O2 atmosphere.Furthermore,the simulation experiment under SCR tail flue gas conditions showed that 3%Cu-ATP can effectively oxidize the slip ammonia and Hg0,and the presence of NOx further improved their removal efficiency.The physical and chemical properties of Cu-ATP catalysts were characterized by BET,XRD,UV-vis,XPS,SEM,TEM,NH3-TPD and H2-TPR methods.Physical interactions and chemical reactions of various flue gas components,including NH3,NO,O2,Hg0,HCl and SO2,on 3%Cu-ATP catalyst were investigated by in-situ DRIFT and transient response experiments.The results showed that Cu was mainly dispersed on ATP surface as Cu2+ions at low copper loading,and the increase in copper loading significantly increased the content of CuO species.According to in situ DRIFT results,the ammonia oxidation over 3%Cu-ATP catalyst also followed the iSCR mechanism.The transient response experiments found that the mercury oxidation over the catalyst could follow both Deacon mechanism and Eley-Rideal mechanism under the atmosphere of HCI+O2.While in sulfur-containing flue gas,Hg0 was mainly oxidized by surface active chlorine to form HgCl2 and desorbed from the surface.