Study Of Regeneration Of SO₂-captured Vanadium-AC Catalyst And Poisoning Of Vanadium-Titanium Based SCR Catalysts

SO2 and NOx,as the most common pollutants in air,would be produced from the combustion of fossil fuels.They are harmful to our socioeconomic and ecological environment.The wet process technology and selective catalytic reduction(SCR)technology are widely utilized for desulfurization and denitration accordingly.However,these technologies are facing with more challenges as the requirements of environmental protection increasing.More effective desulfurization technologies are needing to be developed and more perfect denitration technologies are needing to be implemented.Activated carbon/coke not only can be used for desulfurization without the disadvantages of wet fule gas desulfurization technology but also can be used for the removal of SO2,NOx,Hg,VOCs and fly ash simultaneously when NH3 is added.And the resources of sulfur can be recycled when activated carbon/coke is used for desulfurization or combined desulfurization and denitration.The advantages of the integrated removal technology is becoming more obvious as the increase of the requirement of environmental protection.Based on the traditional AC technology,V2O5/AC catalyst was developed,which could improve the efficiency of desulfurization and denitrification,optimize the optimum reaction temperature(150-200?)that could match the temperature of flue gas better,and have promising potential for application.The catalyst was studied extensively in the past decade including the preparation techniques of the catalyst,the mechanism and kinetic behaviors of desulfurization and denitrification reactions,the influences of flue gas components on the desulfurization and denitrification reactions,the thermostability of the catalyst,the mechanism of thermal regeneration,the effects of regeneration conditions,and the coupling of regeneration process with sulfur recovery etc.However,the kinetic behaviors of thermal regeneration of the SO2-adsorbed V2O5/AC and AC catalysts,which are very significant to design thermal regeneration reactors,have not been well understood.Selective catalytic reduction(SCR)of NOx by NH3 or urea is the most widespread and effective technology for NOx abatement.V2O5/TiO2 catalyst with W03 or MoO3 as additives has been widely used in this technology.The V2O5/TiO2 catalyst is always located in complex flue gas before dust remover in order to avoid SO2 poisoning.The catalyst would be deactivated gradually because of physical deactivation and chemical deactivation.The physical deactivation includes blocking,abrade and sintering and the chemical deactivation include alkaline and alkaline earth metals poisoning,As poisoning,P poisoning etc.The alkaline and alkaline earth metals poisoning is recognized as the major reason leads to chemical deactivation and has been studied extensively.However,it is regret that the influence of flue gas components,especially acid gases,such as SO2 and CO2,was neglected in most previous studies,which would lead their conclusions to be different from the realistic deactivation mechanism.In addition,the content of W03 in V2O5/TiO2 catalyst is always much more than that of V2O5 in the catalyst.The interaction between alkaline/earth metals and W03 and the effect of this interaction on the poison tolerance of the catalyst have not been reported in literature.The systemic studies were carried out on the kinetic behavior of the thermal regeneration of SO2-adsorbed V2O5/AC catalyst,the poisoning effect of alkaline earth metals on the V2O5-W03/TiO2 catalyst in the presence of SO2 and CO2,the effect of W03 and alkaline earth metal on the V2O5-WO3/TiO2 catalyst.The major conclusions are as follows:(1)SO2,CO2 and H2O,as the major products during thermal regeneration of SO2-captured V2O5/AC,are produced from the reaction between H2SO4 and C.The regeneration kinetic process could be divided into two stages.At H2SO4 conversion of 0.1-0.4,the activation energy of the reaction is about 85.7 kJ/mol and the reaction is first-order with respect to the unreacted H2SO4 and could be expressed as f(a)=1-a.At H2SO4 conversion of 0.5-0.8,the activation energy increases from 88.9 to 112.1 kJ/mol,and Jander's 2 l three-dimensional diffusion model expressed as f(a)=1.5(1-a)2/3[1-(1-a)1/3]-1 could describe this process very well.This means that the reaction in this stage may be controlled by diffusion of the unreacted H2SO4 to the reactive carbon.(2)The single CO2,similar to the single SO2,promotes the SCR activity of un-poisoned catalyst due to the increased surface acidity.In the absence of SO2(i.e.the cases of single CaCl2 and CaCl2+CO2),Ca2+interacts with WO3 to form CaW04 and possibly interacts with vanadium species.These interactions result in significant reduction of surface acidity and oxidability of the catalyst.In the presence of SO2(i.e.the cases of single SO2 and SO2+CO2),formation of CaW04 is inhibited and CaSO4 is formed.The SO42-alleviates the reduction of surface acidity caused by Ca2+,but CaSO4 covers or blocks the vanadium species and W03,causing a complete loss of oxidabilities of vanadium and W03 during H2-TPR.The loss of oxidability is pseudo for the SCR reaction and the covered vanadium is still active for activation of the adsorbed ammonia.The synergetic role of sulfates and vanadium accounts for the slight Ca2+-induced deactivation in the presence of SO2.(3)Ca loaded on the catalyst leads to reduction of the number and strength of acid sites,especially Lewis acid sites on tungsten species.At low temperature(300?),NH3 adsorbed on the catalyst could not be activated by V5+=O due to the effect of Ca.The oxidability of the catalysts increases with the rise of temperature,which lead to reduction of the poisoning effect of Ca.W03 is beneficial to increase micropores,surface areas,surface acidity and redox properties of the catalysts.And W03 could interact with V2O5 to form various compounds with different redox properties.W03 could interact with Ca to reduce the poisoning effect of Ca,especially at low temperature(300?),leading to obvious increase of SCR activity of the Ca-loaded catalysts.However,too much W03 in the catalyst would be crystallized which could reduce the amount of acid sites and the surface area and go against improving the SCR activity of the catalyst.(4)The poisoning effect of MgCl2 was studied by contrast of that of CaC12.In the presence of single CO2,the interactions between Mg and active compounds in the catalyst are weak.MgW04 could not be found and this is different from Ca-doped catalyst(CaW04 could be formed).And the proportion of V3+ in Mg-doped catalyst is lower than that in Ca-doped catalyst.All these lead to stronger surface acidity and oxidability and higher SCR activity of Mg-doped catalyst than that of Ca-doped catalyst.In the presence of SO2 and CO2,Mg2Si04 was found in Mg-doped catalyst and this indicating that Mg could diffuse into interior of the catalyst easily.Sulfates formed in Mg-doped catalyst increases surface acidity and SCR activity of the catalyst.However,the SCR activity of Ca-doped catalyst was increased more obviously,due to the reduction or even disappearance of V3+ and the increase of surface acidity,than that of Mg-doped catalyst in the presence of SO2 and CO2.