Catalytic Oxidation Of Element Mercury From Coal-Fired Flue Gas Over OMS-2 Under Low Temperature

Mercury has attracted much attention due to its toxicity,persistence,long-range environmental migration and bioaccumulation,posing a threat to human health and ecological safety.Coal combustion is the largest anthropogenic source of mercury emissions.Existing air pollution control devices in coal-fired power plants can effectively capture oxidized mercury(Hg²⁺),but their capture efficiency for elemental mercury（Hg⁰）is low.To control mercury emissions in coal-fired flue gas,a feasible method is to first denitrate and remove dust from the flue gas,then catalytically oxidize Hg⁰ to Hg²⁺,and finally remove it.However,the presence of SO₂ in the flue gas can poison the catalyst for Hg⁰ oxidation,making it essential to develop sulfur-resistant catalysts suitable for low-temperature catalytic oxidation of mercury.Catalysts suitable for low-temperature catalytic oxidation of mercury and with strong sulfur resistance have become research hotspots.In this paper,a series of highly active sulfur-resistant manganese oxide octahedral molecular sieve（OMS-2）catalysts for mercury removal were prepared by hydrothermal method based on manganese-based catalysts with high activity at low temperature.The crystal particle size is uniform,and a large surface area with a specific surface area of221.32m²/g.When the temperature was 200 ^oC and no SO₂ was present in the flue gas,the oxidation efficiency of the catalyst for Hg⁰ reached 94%.At the same time,it was found that O₂ and NO facilitated the formation of active oxygen,NO₂,NO^2-,and NO^-on the surface of OMS-2,promoting the catalytic oxidation of elemental mercury.However,when there is 500 ppm SO₂ in the flue gas,SO₂ will react with the surface active component Mn of the OMS-2 catalyst and generate Mn SO₄,preventing the catalytic oxidation of Hg⁰,with Hg⁰ removal efficiency of only 57.5%.In order to improve the sulfur resistance of OMS-2 catalyst,a Cu doped manganese oxide octahedral molecular sieve catalyst（Cu_x-OMS-2）was prepared based on the excellent affinity of CuO to SO₂.The results showed that when the molar ratio of copper to manganese was 0.05,the surface active Mn⁴⁺content and specific surface area of OMS-2 catalyst increased by 18%and 9.3%,respectively.The optimal reaction temperature decreased from 200 ^oC to 150 ^oC,making the catalyst more suitable for Hg⁰removal after ESP.When there is 500 ppm SO₂ in flue gas,the oxidation efficiency of Cu_0.05-OMS-2 for Hg⁰ is as high as 92%.Even in the presence of 1500 ppm SO₂,the Hg⁰ oxidation efficiency of Cu_0.05-OMS-2 can still reach 77.6%.Furthermore,it was found that Fe doping could also improve the low-temperature activity and sulfur resistance of the OMS-2 catalyst,and the optimal temperature for Fe_0.05-OMS-2 was also reduced to 150 ^oC.In the absence of SO₂,the Hg⁰ oxidation efficiency reached 98%.Even in the presence of 500 ppm and 1500 ppm SO₂,the Hg⁰oxidation efficiency of Fe_0.05-OMS-2 can reach 90%and 89.2%.Characterization showed that the adsorbed oxygen content(O_ad)on the surface of Fe_0.05-OMS-2exceeded 60%,and its Fe and Mn would form Fe-O-Mn double active centers and catalyze the oxidation of Hg⁰.In order to further optimize the catalytic activity of Fe_0.05-OMS-2,a Cu-Fe bimetallic doped Cux-Fe_0.05-OMS-2 catalyst with a bimodal pore structure was prepared combining the advantages of Cu and Fe doping,with a maximum specific surface area of 213.09m²/g and an optimal reaction temperature of 130 ^oC.The catalyst not only utilizes the strong adsorption of Cu on SO₂ to protect the active manganese component in the catalyst,but also utilizes the Fe-O-Mn double active center formed by addiing of Fe.At the optimal reaction temperature,even in the presence of 500 and 1500 ppm SO₂,the Hg⁰ oxidation efficiency of Cu_0.05-Fe_0.05-OMS-2 can reach 97.1%and 95.6%.the mechanism analysis of catalytic oxidation of Hg⁰ show that the reaction path of elemental mercury on the surface of OMS-2 catalyst is that Hg⁰ is oxidized by the active site on the surface of the catalyst to generate Hg O.The doping of Fe and Cu causes more oxygen vacancies to form on the surface of OMS-2 catalyst,increasing the content of chemically adsorbed oxygen,and reducing the optimal reaction temperature,which is conducive to the low-temperature oxidation of Hg⁰.Both CuO and Fe₂O₃ can adsorb SO₂ to protect the active components,However,improving the sulfur resistance characteristics of the catalyst.Fe-O-Mn double active centers provide more active sites and promote the catalytic oxidation of Hg⁰.