Research On Simultaneous Removal Of Hg⁰ And NO By Low Temperature SCR Catalysts Mn/Co-Ce-ZrO₂ In Coal-fired Flue Gas

A lot of the coals were used for power generation in china,and NOx and Hg⁰ produced by coal combustion have serious harm to human body and environment.It is a potential technology to catalytic oxidation of water-insoluble Hg⁰ to water-soluble Hg2+.SCR catalysts can oxidize Hg⁰ to Hg2+.Using the existing SCR pollution control device to simultaneously remove NO and Hg⁰ is a very promising technology,which is not only simple but also economical.Catalyst is the major factor for simultaneously removing Hg⁰ and NO.Aiming at solving the problems of the SCR catalysts which showed a poor Hg⁰ removal efficiency,the Co doped Ce-ZrO₂ catalysts were firstly used for Hg⁰ adsorption and oxidation.After that Mn was loaded on Co_0.3-Ce-ZrO₂ to improve the SCR activity,according to the separated removal performance of NO and Hg⁰,the catalyst with excellent performance was selected to simultaneously remove Hg⁰ and NO.1.The catalysts Cox-Ce-ZrO₂ were prepared by co-precipitation method,and the Hg⁰ adsorption and Hg⁰ oxidation capacity of the catalysts Cox-Ce-ZrO₂ with different Co contents were investigated.The result showed that cobalt addition increased the BET surface area,improved redox activity and increased the amount of surface chemisorbed oxygen resulting in higher adsorption capacity and Hg⁰ oxidation efficiency.When the molar ratio of Co to Co-Ce-ZrO₂ was 0.3,the catalyst Co_0.3-Ce-ZrO₂ had the highest adsorption and oxidation activity.The Hg⁰ removal efficiency of Co_0.3-Ce-ZrO₂ was nearly 100% in the initial 120 h and it still maintained above 80% after 164.5 h reaction,and the Hg⁰ oxidation efficiency of Co_0.3-Ce-ZrO₂ was 100% in the temperature range of 100-250 oC.2.The Mn was loaded on Co_0.3-Ce-ZrO₂ through impregnation method,and the separated capacity of Hg⁰ oxidation and NO conversion over the catalysts Mnx/Co_0.3-Ce-ZrO₂ were investigated.The enhanced catalytic activity of Mnx/Co_0.3-Ce-ZrO₂ for NO reduction was due to the existence of MnO₂,and the NO conversion efficiencies of Mn0.1/Co_0.3-Ce-ZrO₂ and Mn_0.2/Co_0.3-Ce-ZrO₂ both increased from 0 to 96.3% with Mn loading.However,the Hg⁰ oxidation efficiency was decreased after Mn addition,which was due to the decrease of chemisorbed oxygen,cobalt and Co³⁺.3.Considering the performance of separated activity of NO conversion and Hg⁰ oxidarion over the catalysts,Mn0.1/Co_0.3-Ce-ZrO₂ was determined for simultaneously removing NO and Hg⁰.The Hg⁰ oxidation efficiency of 83.6% and NO conversion efficiency of 89.4% were simultaneously obtained in SCR+Hg⁰ atmosphere under a high GHSV 180,000 h-1.It was found that the promoting effects of NO and O₂ on Hg⁰ oxidation could not offset the inhibitive effect of NH3 on Hg⁰ oxidation in SCR+Hg⁰ atmosphere,resulting in the Hg⁰ oxidation efficiency in the process of simultaneously removing NO and Hg⁰ was lower than the efficiency of separated Hg⁰ oxidation;HgO accumulated on the catalyst surface bring about less available active sites,resulting in the NO conversion efficiency in the process of simultaneously removing NO and Hg⁰ was lower than the efficiency of separated NO conversion.4.The effects of some other gases such as O₂,H2 O and SO₂ on simultaneous Hg⁰ oxidation and NO conversion were investigated in SCR+Hg⁰ atmosphere.O₂ could improve the catalytic activity of Mn0.1/Co_0.3-Ce-ZrO₂ for simultaneous Hg⁰ oxidation and NO conversion.While,H2 O and SO₂ showed prohibitive effects on the simultaneous Hg⁰ oxidation and NO conversion.