Heterogeneous Catalytic Elemental Mercury Oxidation in Coal Combustion Flue Gas

摘要

The new Mercury and Air Toxics Standards issued by US EPA require the reduction of mercury emissions from coal-fired power plants by 90% starting from 2016. Oxidizing elemental mercury using the HCl that exists in the flue gas or additional halogen and catalysts, followed by oxidized mercury capture in the wet Flue Gas Desulfurizer (FGD), is a viable option for mercury removal in coal-fired power plants. The aim of this study is to develop effective and reliable mercury oxidation catalysts, advance the mechanistic understandings of heterogeneous mercury oxidation, and obtain information on heterogeneous mercury oxidation kinetics.;CuCl2 supported on gamma-Al2O3 showed excellent Hg(0) oxidation performance and SO2 resistance at 140 °C. After extensive characterizations of the CuCl2/gamma-Al 2O3 catalyst, the existence of multiple copper species was identified. It was found that CuCl2 forms inert copper aluminate on the surface of gamma-Al2O3 at lower loadings. At higher loadings, CuCl2 exists in a highly dispersed amorphous form that is active for Hg(0) oxidation by working as a redox catalyst. The CuCl 2/gamma-Al2O3 catalyst with high loadings has the potential to be used as a low temperature Hg(0) oxidation catalyst.;RuO2 catalyst was found to be an excellent Hg(0) oxidation catalyst. When rutile TiO2 was used as the catalyst support, RuO 2 formed well dispersed nano-layers due to the very similar crystal structures of RuO2 and rutile TiO2, giving higher Hg(0) oxidation activity over anatase TiO2 support. The RuO2/rutile TiO2 catalyst showed good Hg(0) oxidation performance under sub-bituminous and lignite coal simulated flue gas conditions with low concentration of HCl or HBr gas. It also showed excellent resistance to SO2. The RuO 2/rutile TiO2 catalyst can be used at the tail end section of the SCR unit for Hg(0) oxidation.;Linear combination fitting of the X-ray Absorption Near Edge Spectroscopy (XANES) was used to quantify oxidized mercury species over RuO2/TiO 2 and SCR catalysts under different simulated flue gas conditions. In the absence of halogen gas, elemental mercury can react with sulfur that is contained in both the RuO2/TiO2 and SCR catalysts to form HgS and HgSO4. In the presence of HCl or HBr gas, HgCl 2 or HgBr2 is the main oxidized mercury species. When both HCl and HBr gases are present, HgBr2 is the preferred oxidation product and no HgCl2 can be found. Other simulated flue gas components such as NO, NH3, SO2 and CO2 do not have significant effect on oxidized mercury speciation when halogen gas is present.;Mechanistic and kinetic studies of the heterogeneous oxidation of Hg(0) by HCl gas over a RuO2/rutile TiO2 catalyst were conducted. The experimental evidence of HCl adsorption was obtained using in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). Based on this result, a steady-state kinetic study was conducted to determine an intrinsic reaction kinetic expression for Hg(0) oxidation over the catalyst under HCl, NH3 and SO2 gases for the first time. The kinetic expression obtained could reasonably predict the Hg(0) oxidation performance under the competitive adsorption of NH3 and SO2 gases.