Simultaneous Removal Of SO₂ And NO_x Via Metal Sulfides Co-catalyzed Fenton-Na₂SO₃ System In Exhaust Gas Of Cement Kiln

Cement industry is one of the main emission sources of SO₂ and NO_x,which can exert adverse effects on the human health and environmental disaster in both direct and indirect ways.It is urgent to significantly reduce SO₂ and NO_x emissions from the cement industry when coal power industry has achieved ultra-low emissions.Numerous desulfurization（de-SO₂）and denitrification（de-NO_x）technologies have been developed,mainly including flue gas desulfurization（FGD）,selective catalytic reduction and selective non-catalytic reduction.Traditional de-SO₂ and de-NO_x technologies are mainly designed for single pollutant removal,and can hardly meet the stringent environmental regulations.Numerous simultaneous de-SO₂and de-NO_x methods have been developed over the past decades by involving various catalyzers.The Fenton catalytic oxidation method is one of the effective ways to achieve simultaneous de-SO₂ and de-NO_x due to the high oxidation of hydroxyl radical（HO·）in the system.HO·can oxidize SO₂ and NO into higher solubility valence states,thus accelerating the liquid phase absorption rate.However,the competition between SO₂ and NO_x in flue gas for HO·forces the increase of the amount of H₂O₂ and Fe²⁺,which leads to the increase of cost,and the formation of"Fe sludge".Inorganic co-catalysts such as Mo S₂,WS₂,Cr₂S₃,Co S₂,Pb S and Zn S accelerate the conversion of Fe³⁺/Fe²⁺,because the explosion of reducing metal reduction sites,thus can accelerate the yield of hydroxyl radical（HO?）and superoxide radical（O₂?^-）which are responsible for efficient oxidation of SO₂ and NO_x.Therefore,the potential of de-SO₂ and de-NO_x in Fenton system assisted by metal sulfide is greater theoretically.In order to reduce the competition between SO₂ and NO_x for HO?,a large amount of SO₂ in flue gas was first removed by absorbent.Then,the oxidation and absorption of NOx with low solubility can be achieved through the co-catalytic Fenton system to achieve the purpose of de-SO₂ and de-NO_xsimultaneously.The research significances are summarized in detail as follows:A series of sulfites were used as absorbents for simultaneous removal of SO₂ and NO_x in flue gas.The influence of different process variables,such as temperature,gas flow rate,solution p H,feed concentrations of NO and SO₂,and the presence of O₂ and CO₂ in the feed stream on the absorption of SO₂ and NO_x were evaluated.Sodium sulfite（Na₂SO₃）had the best absorption effect and is most affected by the p H of the solution.When the p H was higher than8,the de-SO₂ and de-NO_x efficiency can reach 100%and 60%respectively;but when the p H of the solution was lower than 4,the sulfite was tending to decompose to SO₂ and the removal efficiency of NO_x will be as low as 35%.A higher concentration of SO₂ can promote the removal of NO₂ but inhibit the removal of NO.With the increase of NO concentration,NO removal efficiency first decreased and then increased.The removal efficiency of NO_x was obviously promoted by O₂,while the effects of CO₂ on NO_x removal efficiency was not obvious.The optimal working range of Na₂SO₃ absorbent is as follows:Na₂SO₃ concentration is 0.100 mol/L,p H=10,and the temperature of the absorbent is 20℃,the gas flow rate is 1.0L/min,the gas composition concentrations were NO（100ppm）,SO₂（500ppm）,CO₂（20%）and O₂（10%）.Under this condition,the de-SO₂ and de-NO_x efficiency can reach 100%and 64%.The effect of WS₂,Mo S₂ and other single metal sulfide as co-catalysts on the efficiency of simultaneous removal of SO₂ and NO_x in Fenton system was studied,and the mechanism of simultaneous removal of SO₂ and NO_x in co-catalyzed Fenton system coupled with Na₂SO₃was explored.Among all the metal sulfides tested,WS₂ has the highest co-catalytic de-NO_xefficiency（34.7%）for Fenton system.The temperature,p H and concentration of Fe²⁺in Fenton system have dual effects on de-SO₂ and de-NO_x efficiency.In the range of Fe²⁺concentration≤0.0020 mol/L,temperature≤60°C,p H≤4,the content of ROS in Fenton system will enhance with the increase of the three variables,which promotes the removal of NO_x.However,beyond this range,ROS in the system will be annihilated and de-NO_x efficiency will reduce.The key step to the simultaneous de-SO₂ and de-NO_x of WS₂/Fe²⁺/H₂O₂ system is the accumulation of ROS（HO·,O₂?^-,etc.）,which is related to the active site of W⁴⁺on WS₂ surface.The enhancement role resulted from generating a large amount of HO?by the activation of H₂O₂ by sufficient Fe²⁺which were promoted by the exposed metal active sites(e.g.,W⁴⁺)of WS₂.Finally,Na₂SO₃ coupled WS₂/Fe²⁺/H₂O₂ system simultaneously de-SO₂ and de-NO_x,while completely removing SO₂,the de-NO_x can be maintained at an average of 85-90%.A series of core-shell polymetallic sulfur oxides cocatalysts were designed and prepared by hydrothermal method.Co-W,Co-Mo and Co-Mo-W with uniform core shell structures could be obtained at 5 h at 150°C,and we applied Co-Mo-W into the Fenton system to de-SO₂and de-NO_x and the removal efficiency of SO₂ and NO_x can be achieved at 100%and 92%,respectively,effectively improving the co-catalytic effect of WS₂.The change of the amount of ammonium molybdate(（NH₄）₆Mo₇O₂₄·4H₂O)will affect the formation of Co₄O₁₉Mo₄ at the initial crystallization stage,but with the increase of hydrothermal temperature and time,the phase and structure of the final product of Co-Mo-W generated are the same.The core-shell structure of Co-W and Co-Mo-W co-catalysts can effectively increase the number of reduced reducing metal reduction sites on the surface thus increasing the accumulation of HO?in the Fenton system which are mainly responsible for the high removal efficiency of SO₂（100%）and NO_x（95%）.However,the collapse of Co-Mo-W’s core-shell structure led to the decrease of de-NO_xefficiency after 5 h co-catalysis.Fortunately,the removal efficiency of NO_x decreased to 81.2%after 5 h reaction with coupled Na₂SO₃ solution.Therefore,the coupling of Na₂SO₃ can effectively alleviate the reduction of de-NO_x efficiency and prolong the life of co-catalyst Co-Mo-W.This article mainly studies the mechanism of simultaneous removal of SO₂ and NO_xfrom cement kiln tail flue gas under the action of Fenton-sulfite system,and discusses the effects of the physicochemical properties of the liquid phase and the removal reaction conditions on the simultaneous removal efficiency.It provides new ideas and technical reference for cement industry flue gas treatment,and provides a theoretical basis for future research and development of catalytic oxidation flue gas de-SO₂ and de-NO_x technology applicable to cement industry,which has extremely significant economic,social and environmental benefits for promoting cement industry to achieve ultra-low emission and sustainable development.