Medium And Low-temperature Flue Gas Catalytic Denitration And Mechanism Research

Coal-fired flue gas is the main reason that causes the air pollution in China,and the flue gas mainly comes from the medium and low temperature coal-fired boiler.Hence,the control of flue gas from medium and low temperature coal-fired boiler becomes the key point to control the air pollution.Based on the traditional VWTi catalysts,this work focused on the modification of VWTi catalysts to obtain the high de-NO_x catalytic activity,and meanwhile the reaction mechanism was investigated.For the low load coal-fired units,this work also studied the catalytic oxidation process of NO₂^-and SO₃^2-which were the ammonia process of desulfurization and denitrification products.Establishing a bubbling reaction device,the oxidation process of the S???with +4 valence was investigated,which was the by-product in the ammonia process of desulfurization.The decomposition rules of H₂O₂ itself and the oxidation process of the main pollutant in the flue gas were studied,and meanwhile the reaction that SO₂ and NO_x adsorbed by NH3 at the same time in the bubble reactor was investigated.V₂O₅/TiO₂ and V₂O₅/Al₂O₃ catalysts were prepared via the impregnation process,and their de-NO_x catalytic acitivity was investigated.The results showed that V₂O₅/TiO₂ exhibited better activity than V₂O₅/Al₂O₃.Combined with the characterizations of XRD,BET,UV-vis,DFT theory calculation,XPS,EPR and in-situ DRIFTS,the structure and surface properties of the catalysts were studied,showing that both two supports exhibited the ability to react with V₂O₅.The interaction between support and V₂O₅ made the transfer of electron from support to V₂O₅ easily,leading to the increase of surface reduced-state V₂O₅.Compared to V₂O₅/Al₂O₃,the superoxide radical formed on the surface of V₂O₅/TiO₂ enhanced the ability to oxidize NO,thus leading to the improve of de-NO_x activity over V₂O₅/TiO₂.V₂O₅ and W03 were loaded on the TiO₂ support to prepare commercial de-NO_x catalysts V₂O₅-WO₃/TiO₂.The doping of WO₃ broadened the reaction temperature window,and this trend was the most obvious when the doping amount was 6%.Adopted with XRD,XPS,PL and EPR technologies,the influence of WO₃ doping on the structure and surface properties of the catalysts was stuied.The results showed that W03 could interacted with the oxygen vacancy on the TiO₂ surface,suppressing the formation of rutile TiO₂,and thus enhancing the catalytic stability at high temperature.WO₃ possessed the ability to store and transfer electron,improving the interaction between V₂O₅ and TiO₂.This made the transfer of electron from the support to V₂O₅ easier,thus increasing the surface reduced-state V₂O₅ further.The surface reduced-state V₂O₅ was the active site to generate superoxide radical.Hence,the existence of WO₃ increased the amount of superoxide radical,improving the selectivity catalytic reduction?SCR?activity over V₂O₅-WO₃/TiO₂.The above metioned SCR method can not be applied in the low-temperature boilers?lower than 180?C?especially for industrial boilers,sincering machine and catalytic cracking unit.Hence,in the present study,de-NO_x with H₂O₂ as the oxidizing agent and with a-Fe₂O₃,Fe/Al₂O₃ and Fe/TiO₂ as the catalyst was proposed.Under the catalysis of Fe-based catalysts,H₂O₂ was decomposed and-OH radicals were gene^ated,which could oxidize NO_x at low temperature.Combined with the characterizations of XRD,SEM,BET,NH3-TPD,FTIR and XPS,the decomposition mechanism of H₂O₂ on the surface of Fe-based catalysts was mainly investigated.The mechanism to remove NO_x via the oxidation process was also studied.In the process of simultaneous desulfurization and denitrification,the unstable ammonium sulfite and nitrite could be obtained in the hybrid solutions,which is easily decomposed back into SO₂ and NO_x.In order to fixed the sulfur and nitrogen,it is necessary to oxidize unstable sulfite and nitrite into sulfate and nitrate.Hence,the oxidation kinetics of ammonium sulfite and ammonium nitrite was studied systematically.The results showed that the nitrite and sulfite can be cooxidized in the hybrid solutions with oxygen.