Performance And Mechanism Study Of Simultaneous Removal Of NO And Hg~0 From Coal Combustion Flue Gas By Mn-based Composite Catalysts At Low Temperature

Selective catalytic reduction(SCR)technology is the most widely used technology to remove NO_x emitted from coal-fired power plants.Currently,commercial SCR catalyst is located upstream of air-preheater due to the limitation of the active temperature range(300-400°C),and various problems occur in practical applications,such as abrasion caused by high content of fly ash,deactivation due to alkalis,and high conversion of SO₂ to SO₃ at high temperature.The SO₃ can react with NH₃ to form ammonium bisulfate in air-preheater,which resulted in corrosion and plugging.These seriously affects the safe and stability operation of power plant.Therefore,the low temperature SCR catalyst(100-300°C)is proposed to solve the above problems.Besides,catalytic oxidation of Hg⁰ to Hg²⁺by SCR catalyst,followed by the absorption of Hg²⁺by existing air pollution control devices,which can achieve co-benefit control of coal-fired flue gas pollutants.The research for the simultaneous removal of NO and Hg⁰ over SCR catalyst at low-temperature has extremely high academic value and broad application prospects.With the objective of investigating catalyst for the simultaneous removal of NO and Hg⁰at low temperature,Mn-based MnSmCo/Ti and Mn Fe W/Ti catalysts were prepared for the simultaneous catalytic reduction of NO and the catalytic oxidation of mercury at low temperature.The catalytic performance of simultaneous removal of NO and Hg⁰,resistance to water and sulfur dioxide,physicochemical properties and the surface reaction mechanism of the catalysts were comprehensively studied.The Mn-based catalysts were prepared by ultrasonic assisted impregnation method.The performance of the catalyst for simultaneous removal of NO and Hg⁰ and the N₂ selectivity were studied on the fixed bed.The results showed that MnSmCo/Ti and Mn Fe W/Ti catalysts achieved over 80%denitration efficiency,100%mercury removal efficiency and nearly 100%N₂ selectivity at 200°C with a gas hourly space velocity(GHSV)of 100000h^-1.Besides,it can maintain high denitrification efficiency and high N₂ selectivity at above 200°C.Since the catalytic activities of the Mn-based catalysts were changed,a variety of characterization methods were used to systematically study the physical and chemical properties of the catalyst.The results showed that there was a redox cycle of Mn³⁺+Co³⁺(?)Mn⁴⁺+Co²⁺between Mn and Co in MnSmCo/Ti catalyst.Besides,the formation of Mn-O-W and Fe-O-W bonds promoted the dsipersion of active component on the surface of Mn Fe W/Ti catalyst.These can improve oxygen and electron transport capacity,which resulted in that the highly valent metal ions and chemisorbed oxygen of the MnSmCo/Ti and Mn Fe W/Ti catalysts maintained high surface concentration during the reaction process.The effects of flue gas components on the denitrification and mercury oxidation were studied.The results showed that NO can significantly improve the Hg⁰ removal ability of MnSmCo/Ti and MnFeW/Ti catalysts.NH₃ did not affect the adsorption and oxidation of Hg⁰ on the surface of MnSmCo/Ti and MnFeW/Ti catalysts.H₂O affected the adsorption of Hg⁰ on the surface of MnSmCo/Ti catalyst,but it had less influence on the adsorption of Hg⁰on the surface of Mn Fe W/Ti catalyst.SO₂ significantly reduced the Hg⁰ removal capacity of the MnSmCo/Ti and Mn Fe W/Ti catalysts.Furthermore,the competition between the SCR reaction and the Hg oxidation reaction on the surface of MnSmCo/Ti and Mn Fe W/Ti catalyst was not apparent.To reveal the reaction mechanism of SCR reaction over the modified Mn-based catalysts,the in-situ infrared technique was used to analyze the adsorption and reaction of reactants on the catalyst surface,and the thermogravimetry-mass spectrometry technique was used to analyze the form of sulfuric acid on the catalyst surface.The results showed that the surface SCR reaction over Mn Sm/Ti catalyst follows the E-R mechanism.The SCR reaction on the surface of the MnSmCo/Ti catalyst follows both L-H and E-R mechanisms,but the E-R mechanism is dominant.The SCR reaction over Mn Fe W/Ti catalyst follows E-R mechanism.The sulfate formed on the surface of the three catalysts were all metal sulfate.However,the addition of Sm and W can reduce the adsorption of SO₂ on the surface of these catalysts,besides,the existence of NH₃ in flue gas can significantly inhibit the adsorption of SO₂on the surface of these catalysts.The methods of Hg equilibrium,transient reaction,Hg-TPD and XPS were used to study the mercury removal method and oxidation mechanism.The results indicated that the chemisorption and catalytic oxidation are responsible for the removal of Hg⁰ over MnSmCo/Ti and Mn Fe W/Ti catalysts.The Hg⁰ adsorbed on the surface of the MnSmCo/Ti catalyst directly reacted with the surface chemisorbed oxygen on the surface and generate Hg O;in addition,it could react with the active intermediate NO₂ generated on the surface to generate Hg O.Furthermore,Hg⁰ can react with lattice oxygen on the catalyst surface to generate Hg O which can weakly bound with Mn O_x to form a binary metal oxide Hg Mn O_x+1.The Hg⁰ adsorbed on the surface of the Mn Fe W/Ti catalyst can react with the chemisorbed oxygen to generate Hg O.In addition,the adsorbed Hg⁰can also react with the active intermediate NO₂ to generate Hg O.