Research On Mechanism Of Adsorption And Catalytic Oxidation Of Elemental Mercury In Coal-fired Flue Gas

Mercury is a kind of pollutant which is very harmful and difficult to remove in the flue gas of coal-fired power plant.The research on the removal process of Hg0 and its mechanism has important application value.However,the structure characteristics and reaction mechanism of mercury removal catalysts have been greatly hindered due to the limitations of the existing experimental and characterization methods.In this paper,the adsorption and catalytic oxidation mechanism of mercury in the flue gas of coal-fired power plants are studied by using the quantum chemistry calculation based on DensityFunctionalTheory(DFT).The removal mechanism of mercury by CaO and MgO in coal-fired fly ash is studied,and the periodic CaO(100)and MgO(001)surface structure models are established.HgO is physically adsorbed on CaO(100)surface,while HCl is dissociatively absorbed,and HgCl and HgC12 is chemically adsorbed.The surf-HgCl intermediate can continue to combine with a Cl free radical to form HgCl2,which is easier to remove.Hg0,HgCl and HgCl2 are all chemisorbed on MgO(001)surface weakly,while HCl is dissociatively adsorbed,and HgCl cannot exist stably on the surface,so it has no obvious effect on the adsorption and oxidation of Hg0.The vanadium based commercial SCR denitration catalyst has been widely used in recent years.In this paper,the mechanism of Hg0 oxidation in HCl atmosphere has been investigated.The adsorption energies and geometries of Hg0,HgCl,HgCl2 and HCl on V2O5(001)surface are studied.The energy profile of oxidation reaction and the structures of relative transition states and intermediates are reviewed.The results show that Hg0,HCl and HgCl2 are physically adsorbed on V2O5(001)surface,while HgCl is chemically adsorbed on the surface,indicating that HgCl-surface is an important intermediate.The Hg0 oxidation goes through Hg0?HgCl?HgCl2.The first stage(Hg0 + HCl?HgCl)follows an Eley-Rideal mechanism,gaseous HCl reacts with Hg0 that is previously adsorbed on V2O5(001)surface to form surface HgCl.The second stage(HgCl + HCl HgCl2)follows an Langmuir-Hinshelwood mechanism,surface HgCl reacts with another HC1 that is newly adsorbed on V2O5(001)surface to form HgCl2.In the whole Hg0 oxidation reaction,the formation of HgCl2 is the rate-determining step due to its higher energy barrier.In addition,in consideration of the practical application of the SCR catalysts are supported catalysts,this paper constructs the V2O5/TiO2 and V2O5-WO3/TiO2 supported SCR catalyst models,and the adsorptions of Hg0,HgCl,HgC12 and HC1 on the surface of supported catalysts are studied.The results show that the adsorptions of Hg0 and HgCl2 are physisorptions on the surface of supported catalysts,and that of HCl is a more stable chemisorption,while the adsorption of HgCl is strong chemisorption.HCl is involved in oxidation of Hg0 on the surface of the supported catalysts,and HgCl is an important intermediate in oxidation process of Hg0.The addition of tungsten element may promote the oxidation of Hg0.