Study On Synergetic Deactivation Mechanism Of Mercury,Arsenic And Potassium In Coal-fired Flue Gas On V₂O₅-WO₃/TiO₂ Catalyst

Coal is an important primary energy in our country,and as its combustion,NO_x,one of the major pollutants,is emitted.Selective catalytic reduction?SCR?of NO_x with NH₃is the main method for denitrification.Catalyst,as the core of SCR system,its properties decide the run of the De-NO_x system.V₂O₅-WO₃/TiO₂ is the most widely used catalyst and is usually setted in the flue gas containing high concentrations of dust,alkali metals and heavy metals,which easily result in the catalyst deactivation.Based on these factors,this research systematically studied the influences of different mercury,arsenic and potassium species on the properties of V₂O₅-WO₃/TiO₂ catalyst,and also analyzed their synergetic mechanisms between arsenic and potassium,as well as mercury and potassium.It will provide a rationale for developing mercury,arsenic and alkali metals resistance catalysts.From the results of distribution of mercury in coal-fired flue gas by thermodynamic equilibrium calculation and its adsorption behavior on the catalyst,it was found that elemental mercury?Hg⁰?and mercuric chloride?HgCl₂?are the major species in the flue gas in the catalyst working temperature range.The influences of Hg⁰ and HgCl₂ on catalyst properties were studied.The catalytic activitydecreasedwith Hg⁰ adsorption amount increasing and temperature elevating.Hg⁰ could reduce the catalyst surface acid sites,chemisorbed oxygen and active V=O bonds,which are responsible for its declined activity.HgCl₂ decreased the amount of acid sites,lattice oxygen and V=O bonds,and Cl-V-O-H was newly formed.Therefore,the catalytic activity decreased with HgCl₂ loadings increasing,and the decreased extent of the activity weakened as temperature increasing.Studies on the arsenic species in flue gas and corresponding adsorption behavior showed that gaseous As₂O₃ are the major forms adsorbed on the catalyst.Vapor deposition and wet impregnation methods were used to study the impact of arsenic on catalyst properties.The influences of vapor arsenic,as well as liquid arsenic,on catalyst were analyzed in the research.Results presented that As³⁺was oxidized to As⁵⁺under both conditions and the catalyst reducibility was also enhanced.However,arsenic destroyed the V-OH acid sites to deactivate the catalyst,and simultaneously formed less active As-OH.A dense As₂O₅filmwas formed when vapor arsenic was adsorbed on the catalyst and the catalyst pores were blocked,which resulted in the suppression of De-NO_x reactions.Therefore,gaseous As₂O₃ performed more serious deactivation impact than liquid As₂O₃.Deactivation mechanisms of three potassium species?KCl,K₂SO₄,K₂O?in coal-fired flue gas on the catalyst were studied.The results showed that K⁺ could decrease the acidity and stability of the surface Br?nsted and Lewis acid sites,as well as the reducibility and chemisorbed oxygen,which resulted in the catalytic activity decline.The order of the deactivation extent followed:K₂SO₄<K₂O<KCl.This phenomenon could be explained by that the role of K₂O on the catalytic activity mainly decided by K⁺.Except for K⁺,the introduction of SO₄^2-performed advantages to promote catalyst activity in K₂SO₄-poisoned catalyst,which is the reason for its weakest deactivation.Both Cl^- and K⁺are disadvantageous for NO reduction,so KCl resulted in the strongest deactivation.From the results of synergetic mechanism of arsenic and potassium on the catalyst,it was found that thecatalyst presented enhanced deactivation.It can be attributed to the further decrease of catalyst surface chemisorbed oxygen and acid sites,as well as the catalysts reducibility and N₂ selectivity,compared with the arsenic-poisoned or potassium-poisoned catalyst.Moreover,K⁺could neutralize the newly formed As-OH acid site.The synergetic mechanism of mercury and potassium on the catalyst was studied,and the results showed that elemental mercury could weaken the deactivation of KCl-poisoned catalyst.When both mercury and KCl were adsorbed on the catalyst,KCl could react with catalyst to form Cl-V-O-H and V-O-K,resulting in V=O bonds further decrease,and the catalytic activity decline.However,the adsorbed Hg⁰ could combine with-Cl to form HgCl and then replace the potassium on the vanadium active sites to form V-O-Hg-Cl.Meanwhile,HgCl and NH₃ would compete for the active sites and NH₃ performed stronger adsorption ability than HgCl,which inhibiting the adsorption of HgCl and KCl.It is contributed to the catalyst activity promotion.