Study On Modification And Mercury Removal Performance Of Commercial SCR Catalysts

Mercury is a heavy metal that is not only harmful to the human body,but also has great harm to the environment.The emission of mercury from coal-fired power plants is receiving more and more attention.The form of mercury in coal-fired flue gas is mainly:particulate mercury?Hg^p?,elemental mercury?Hg⁰?,and oxidized mercury(Hg²⁺),of which only Hg⁰ is difficult to be removed by existing air pollution treatment equipment of coal-fired power plants.Some scholars have found that the current SCR denitration catalyst of coal-fired power plants has a certain Hg⁰ oxidation effect,but the oxidation efficiency is low,and the presence of HCl in coal-fired flue gas greatly promotes the removal of Hg⁰.Therefore,in view of the low oxidation efficiency of the conventional commercial SCR catalyst Hg⁰,this paper modified it.The oxidation performance of Hg⁰was studied.The catalysts were characterized by BET,XRD,SEM,EDS and XPS to explore the possible mechanism of oxidation Hg⁰.1.The modified SCR catalyst was prepared by impregnating the active material CeCl₃,CuCl₂,FeCl₃,MnCl₂ on a conventional commercial V₂O₅-WO₃/TiO₂ SCR catalyst.A simple catalyst selection experiment was then carried out on the above four groups of modified SCR catalysts.The experimental results show that the CeCl₃ modified catalyst has the best Hg⁰ oxidation effect,and the CeCl₃ modified catalyst also has better Hg⁰oxidation rate at lower temperature.Therefore,the subsequent mercury removal performance experiment was carried out by selecting a CeCl₃ modified catalyst.The modified SCR catalyst was subsequently subjected to a characteristic experiment of Hg⁰oxidation.The effects of modified SCR catalysts on the oxidation performance of Hg⁰under different active material loadings,different fixed bed reaction temperatures,different space velocities and different flue gas components?O₂,SO₂,NO,NH₃?were investigated.The results show that the loading of 5%CeCl₃ has a good Hg⁰ oxidation rate,and the optimum reaction temperature is 300°C.When the space velocity is greater than25000 h^-1,the Hg⁰ oxidation rate of the catalyst decreases drastically.O₂ and NO promoted the oxidation of Hg⁰ by the catalyst,but NO is weaker.SO₂ and NH₃ have an inhibitory effect on the oxidation of Hg⁰ by the catalyst and NH₃ has a stronger inhibitory effect on the oxidation of Hg⁰ by the catalyst.2.It was found that in the 5%CeCl₃/SCR catalyst,the active material CeCl₃ was supported on the surface of the catalyst without blocking the pores of the SCR catalyst.The Ce element in the catalyst is mainly present in the form of CeCl₃.The 5%CeCl₃/SCR catalyst has better catalytic redox performance than the conventional V₂O₅-WO₃/TiO₂SCR catalyst.The catalytic oxidation of Hg⁰ occurs on the surface of 5%CeCl₃/SCR catalyst.After combining the catalyst performance experiments and characterization experimental data,it is speculated that the Hg⁰ oxidation mechanism of 5%CeCl₃/SCR catalyst belongs to Mars-Maessen mechanism,that is to say,Hg⁰?g?is first adsorbed on the surface of 5%CeCl₃/SCR catalyst to become Hg⁰?ads?.Hg⁰?ads?is then reacted with a lattice oxidant?such as O or Cl?in the 5%CeCl₃/SCR catalyst.Generate HgO?ads?or HgCl₂?ads?.Then,due to the high temperature of the system,HgO?ads?or HgCl₂?ads?will desorb and finally form HgO?g?or HgCl₂?g?.