Experimental Study On The Denitrification And Demercuration Of The Mn-V-W/Ti Catalysts

NO and Hg emissions from coal-fired power plants have caused great harm to human health and the ecological environment.SCR catalysts can maintain high denitrification efficiency and oxidize Hg⁰ that is difficult to be removed.It is an economic,efficient and effective method to use SCR device to perform integrated removal of NO_x and Hg.However,the activity temperature window of existing SCR catalysts deviates from the optimum oxidation temperature of mercury,which has poor oxidation effect on mercury.Due to its high activity temperature window,the SCR device must be placed in front of the air preheater,which will cause problems such as catalysts blockage,SO₂ and alkali metal poisoning.Therefore,it is necessary to develop catalysts with high denitrification and demercuration activities at low temperature,which is also the necessities for multi-pollutants control and low boiler load.In this paper,the existing denitrification and mercury removal technologies at home and abroad are described,and the research progress of SCR catalysts is reviewed.On this basis,a series of catalysts were prepared by combining Manganese,a transition metal element with good low temperature redox effect,with vanadium having good SO₂ resistance performance.The performance experiment of denitrification and demercuration was carried out and compared with that of commercial catalysts.Furthermore,the sulfur and water resistance experiments of the catalyst were conducted under conditions as close as possible to the coal-fired flue gas atmosphere,which provides certain theoretical basis and ideas for SCR catalyst.Firstly,a certain commercialized V₂O₅-WO₃/TiO₂ catalyst was tested during 120²80?on the denitration and mercury removal performance.The effects of space velocity?GHSV?,ammonia nitrogen ratio?NSR?and oxygen?O₂%?on the properties of catalyst was explored.Results show that the best GHSV,NSR,oxygen content of the commercial catalyst is 30 000h^-1,1?[NH₃]=[NO]=500 ppm?,5%,respectively.It was found that V₂O₅-WO₃/TiO₂ catalyst has just little denitrification and mercury removal efficiency under 200?.Then,a series of V-W/Ti catalysts were prepared by impregnation method under laboratory conditions,and the influence of W and V content on the performance of catalyst for denitrification and mercury removal was studied under the conditions of GHSV,NSR and oxygen content.Physical and chemical properties of the catalysts were analyzed by BET,SEM and XRD.The results show that the specific surface area and surface microstructure of the catalyst are not significantly affected by the load of vanadium and tungsten,but its continuous increment would make the catalyst agglomerated to a certain extent in the calcination process,resulting in reduction of the specific surface area of the catalyst.The optimal load of W and V are 6?WO₃/TiO₂,wt%?and 3?V₂O₅/TiO₂,wt%?,respectively,while the low temperature effect of V-W/Ti catalyst is still very poor.In order to improve the synergistic performance of low temperature denitrification and mercury removal of the catalyst,Mn,a transition metal element,was added on the basis of the best V-W/Ti catalyst,and the effect of Mn load and calcination temperature on the performance of the catalyst was investigated further.The catalysts were systematically analyzed by means of BET,SEM,XRD,H₂-TPR,NH₃-TPD and XPS.The results show that the denitrification and demercuration efficiency of the catalyst has markedly improved in the low-temperature,the best load and calcination temperature were 0.2?Mn/Ti,mol?and 400?,respectively.Calcination temperature has a great influence on the performance of the catalyst,metal active component can be uniformly dispersed in amorphous form on the carrier while the calcination temperature is 400?.However,when the calcination temperature continues to rise,the specific surface area of the catalyst decreases significantly and the sintering and agglomeration phenomena are serious.A large amount of active component MnO₂ on the surface turns to Mn₂O₃,and meanwhile,anatase TiO₂ also turns to rutile phase,which significantly reduces the redox ability,the type and number of surface acid sites.The decrease of Mn⁴⁺,V⁴⁺and surface adsorbed oxygen indicates the decrease of catalyst performance.Finally,the sulfur-resistant and water-resistant performance of the selected Mn-V-W/Ti catalyst with the best performance was studied,and the influences of reaction temperature and concentration on the synergistic denitrification and mercury removal performance of the catalyst under the conditions of SO₂,H₂O and SO₂+H₂O were explored respectively,and the influence of HCl concentration on the performance of the catalyst under the conditions of SO₂,H₂O and SO₂+H₂O was further explored.Combined with XPS and TG characterization methods,the changes in physical and chemical properties of the catalyst before and after the reaction were analyzed.The results indicated that SO₂ and H₂O has restraint on co-removal of NO and Hg indivadualy,especially,it is worse when they exists together.As SO₂ can be oxidized by the catalyst to form SO₃,and then react with the active metal components to form metal sulfates or sulfites that are not easy to decompose,the toxicity of SO₂ on the catalyst is irreversible.Under that of SO₂+HCl and SO₂+H₂O+HCl,HCl showed a promoting effect,while under the condition of H₂O+HCl,HCl showed an inhibitory effect.This is because a large amount of H₂O covers on the surface of the catalyst,hinders the contact between the catalyst and the reaction gas,and at the same time,a large amount of ammonium halide is generated and covers the surface of the catalyst,resulting in the reduced activity of the catalyst.However,ammonium halide is easy to decompose,so when the water and HCl are cut off,the catalyst returns to the initial state,so the H₂O poisoning of the catalyst is reversible.