Study On Catalytic Oxidation Of Elemental Mercury With Titanium Based Composite Catalysts

Mercury pollution caused by coal-firing has become more and more serious, and it has attracted widespread attention all over the world. Most of China's coal contains high level mercury and low chloride, which would result in serious Hg0 control problems. Therefore, mercury emissions from coal-fired flue gas control become a key issue in recent years.Catalytic conversion of elemental mercury (Hg0) to its oxidized form has been considered as an effective way to enhance mercury removal from coal-fired power plants. Conventional SCR catalyst showed a poor catalytic oxidation efficiency of elemental mercury (Hg0) emitted from coal-fired flue gas. In order to make good use of the existing selective catalytic reduction of NOx (SCR) catalysts as a co-benefit of Hg0 conversion at lower level HCl in flue gas, various catalysts supported on titanium dioxide (TiO₂) and conventional SCR catalyst were investigated at various temperatures. Among the tested catalysts, ruthenium oxides(RuO₂) shows an excellent performance on Hg0 conversion with the insignificant effect on NOx reduction, which appears to be a promising component to modify the conventional SCR catalyst for Hg0 conversion.In order to enhance the co-benefit of SCR catalyst for both NO removal and Hg0conversion, SCR catalyst were doped with various elements. RuO₂ modified SCR catalyst shows excellent tolerant to SO₂ and ammonia. The demanded HCl concentration can be reduced dramatically, and Hg0 oxidation efficiency over Ru doped SCR catalyst is over 90% at about 10ppm HCl in the simulated gases.In addition, Mn was found to be the better, and the cooperation of Mo on Mn doped SCR catalyst showed the highest efficiency among the employed catalysts. Under the condition of the temperature 350℃and 10ppm HCl, Hg0 oxidation efficiency over Mo-Mn/SCR catalyst was up to 90%, but it was only about 35% with the original one. The results also indicated that doping Mo-Mn on SCR displayed insignificant effect on NOx removal efficiency.Analysed and discussed the possible mechanisms of catalytic reactions by characterization methods such as XRD, XPS, TEM. Through experimental research on Deacon reaction and discussion of mechanism of elemental mercury transformation, it was found that elemental mercury catalytic oxidation is not directly dependent on the Cl2 yielded by Deacon reaction, while the chlorine atom (Cl) is considered to be the key factor. The V and W composition of Ru/SCR catalyst are likely to be helpful for Deacon process under the condition of 500ppm SO₂. The multi-metal composite structure may make Ru / SCR catalyst has better sulfur resistance.Through the experiments and the characterization methods in this study, it was found that the Ru doped SCR catalyst and Mo-Mn doped SCR catalyst had excellent catalytic oxidation efficiency with the insignificant effect on NOx reduction. These catalysts also had low requirements of chlorine concentration of the flue gas and had good resistance to sulfur and ammonia. So it had good prospects for industrial use. Moreover, the research results of this paper were helpful to the control of mercury pollution and industrial use in the coal-fired flue gas.