An Experimental And Theoretical Research On Simultaneous Removal Of Elemental Mercury And NO In Simulated Flue Gas By The Modified Selective Catalytic Reduction Catalysts

Nitrogen oxides?NO,NO₂,and N2O?are major causes of photochemical smog,acid rain ozone depletion and greenhouse effects,which are harmful to human health and environment.Coal-fired power plants are the biggest pollution source among the main anthropogenic mercury emission sources.Mercury has been listed as one of the most hazardous and toxic pollutants by the laws and regulations of more and more countries or regions because of the extreme toxicity,persistence,and bioaccumulation of methyl mercury transformed from emitted mercury.Significant efforts have been made to effectively control NO_x and mercury emissions by the researchers.The selective catalytic reduction?SCR?of NO_x by NH3 is an effective method for elimination of NO_x in flue gases.It has been proven that a lot of commercial catalysts used in selective catalytic reduction?SCR?are favorable for the oxidation of Hg0 along with NO emission control.Accordingly,based on the situation of our country and existing pollution control devices in coal-fired power plants,it is the most economical way to use currently available pollution control devices?APCDs?such as SCR to remove Hg from the flue gas.However,the conventional SCR catalysts were not effective enough for Hg0 oxidation under flue gas without or with low HCl concentration.Therefore,developing the SCR catalysts which could promotes the removal of NO and Hg would be of great scientific and practical values.In this work,the modified SCR catalysts were developed to simultaneous control the emission of NO and elemental mercury?Hg0?from coal-fired flue gas and the catalytic performance of SCR catalysts for the simultaneous removal of NO and Hg0 in simulated flue gas was studied.After compared the physicochemical properties of SCR catalysts before and after the reaction,the removal mechanism of NO and Hg0 on SCR catalysts were studied.Then,the interactioI mechanism between the Hg0 oxidation and NO conversion was built.Based on the interaction mechanism,a nove SCR catalyst exhibited excellent Hg0 oxidation efficiency and NO conversion efficiency was developed.The important conclusions of this work are summarized as follows:?1?The recycle of spent TiO₂-based SCR-DeNOX-catalysts were employed as a potential catalytic support material for elemental mercury?Hg0?oxidation in simulated coal-fired flue gas.The catalytic mechanism for simultaneous removal of Hg0 and NO was also investigated.Results indicated that CeMn/SCR,catalyst was highly active for Hg0 oxidation at low temperatures,approximately 92.8%mercury oxidation efficiency was obtained at 150?.The inhibition effect of NH3 on Hg0 oxidation was confirmed in that NH3 consumed the surface oxygen.Moreover,H₂O inhibited Hg0 oxidation while SO₂ had a promotional effect with the aid of O₂.Furthermore,the surface oxygen was responsible for Hg0 oxidation and NO conversion which were also thought to be aided by synergistic effect between the manganese and cerium oxides.?2?To inerease the surface area of conventional vanadium base SCR catalyst,the modification of TiO₂ support by Ce-doping was undertaken by sol-gel method and WO3 and V2O5 was introduced into the support by wet impregnation.It was found that a novel V2O5-WO3/TiO₂-CeO₂ catalyst exhibited excellent Hg0 oxidation efficiency?87.9%?and NO conversion efficiency?88.5%?at 250?.Furthermore,CeO₂-modified V2O5-WO3/TiO₂ not only enhanced catalytic activity but also exhibited good resistance in SO₂ and H₂O.Meanwhile,we found that,with lower crystallinity,more reduced species and better texture properties presented,which were all ascribed to CeO₂ doping.And the redox cycle?V4++Ce4+???V5++Ce3+?played a key role in promoting the Hg0 oxidation and NO conversion.In concert with the experimental results,a mechanism for the simultaneous removal of Hg0 and NO was proposed.?3?A series of V/ZrCex catalyst synthesized by a co-precipitation method was employed to investigate the simultaneous removal of NO and Hg0 in simulated flue gas.V/ZrCe0.6 showed excellent SCR activity?87.3%?and high Hg0 oxidation efficiency?77.6%?in SCR atmosphere?NH3/NO=1?.The Hg0 had little impact on NO conversion,while the coexistence of NO and O₂ would be beneficial for the Hg0 oxidation.Hg0 oxidation was inhibited in SCR atmosphere owing to the presence of NH3.Besides,the superior performance of V/ZrCeo.6 catalyst might be attributed to lower crystallinity,better texture properties,strong redox ability and high reactive nitrate together with NH3 species.The redox equilibrium?Ce3++V5+???Ce4++V4+?contributed to the NO conversion and Hg0 oxidation.The bidentate sulfates formed by adsorbed SO₂ could provide new acid sites for NH3 adsorption and increase the amount of N4+,which reduced the poisoning effect of SO₂ and H₂O.A mechanism for the simultaneous removal of NO and Hg0 was proposed for the V/ZrCe0.6 catalysts:2NH3/NH4+?ad?+N02?ad?+NO?g??2N2+3H₂0+/2H+,Hg?ad?+ O??HgO?ad?.?4?CoO_x was introduction into V/ZrCe catalyst to improve its Hgo oxidaiton and NO conversion.The combination of CoO_x and V2O5 resulted in significant for the simultaneous removal of NO and Hg0 in simulated flue gas.VCo0.10/ZrCe catalyst was highly active for the Hg0 oxidation?88.9%?and NO conversion?89.6%?at 250 ?.Gas hourly space velocity significantly influences the catalytic activity of catalyst.Low space velocity would be beneficial to the Hg0 oxidation and NO conversion.Gas-phase O₂ regenerated the lattice oxygen and replenished the chemisorbed oxygen to facilitate Hg0 oxidation and NO conversion.The addition of cobalt species promoted the reduetion of the vanadium oxide,and improved the activity of vanadium oxide supported on ZrCe because of the interaction between dispersed vanadium oxide and cobalt oxide?V5++Co2+???V4+Co3+?.Inadditon,the interaction between cobalt oxide and support could assist the support in supplying oxygen.