| China was one of the largest anthropogenic emission sources of mercury in the world. Our country was first to specified the emission limits of mercury compounds in The Coal-fired Power Plant Air Pollutants Emission Standards in2011. Flue gas mercury pollution control technology has become the urgent needs of the reality, especially the pollution control technology for the elemental mercury in flue gas. Photocatalytic oxidation&adsorption method for elemental mercury removal had a synergistic effect between adsorption and photocatalytic oxidation because the HgO generated by photocatalytic oxidation on the catalyst surface can improve the Hg0adsorption performance of photocatalyst, and showed a broad prospect for the industrial application of Hg0removal. Aimed at the industrial application of Hg0removal in the flue gas, the energy-efficient white light LED lamp was chosen as the light source in our experiments because of its lower energy consumption and longer lifetime. In this paper, the researches about photocatalytic-adsorption removal performance of elemental mercury by iodine, nitrogen and other non-metal doped TiO2were made, the preparation conditions of photocatalysts were optimized, and the effects of flue gas environment on adsorption-photocatalytic removal efficiency of elemental mercury were also investigated. In short, a useful exploration was made in this study for the development of visible-light activated photocatalyst for the removal of elemental mercury and for the application of photocatalytic-adsorption technology in elemental mercury removal.Firstly, iodine-doped TiO2was synthesized by hydrolysis precipitation method to improve the adsorption performance of TiO2and its photocatalytic activity under visible light. The existing state of iodine in the samples was studied, and the influences of calcination temperature and doping concentrations to the mercury removal efficiency were investigated. The results showed that the light absorption spectra of iodine doped samples were extended to the visible light region, and the doping of iodine was the main reason for these visible light absorptions. Samples calcined at400℃had the highest photocatalytic oxidation capability among the tested samples, and also showed the wonderful adsorption performance for elemental mercury.Secondly, N-doped TiO2was synthesized by using ammonia as the nitrogen source. The influences of calcination temperature and nitrogen doping concentrations to the mercury removal efficiency were investigated. Both the photocatalytic activity and adsorption efficiency of N-TiO2calcined at300℃can be up to90%. The increase of calcination temperature would lead to the loss of nitrogen element, the enlargement of TiO2crystal, and the decrease of surface area, and resulted in the decline of activity. The doped nitrogen existed in three forms:O-Ti-N and Ti-N-O with N as substitute for O, and intermittent doping.Thirdly, N, Cl co-doped TiO2was prepared by using ammonium chloride as nitrogen and chlorine source. The effects of doping ratio and calcination temperature on photocatalytic removal activity were both investigated. For the samples calcined at300℃, the photocatalytic activity was improved with the increasing of doping concentration. However, the adsorption performance was declined with the rise of doping concentration. Chlorine deposited on the surface of sample was the main reason of the decrease of adsorption activity. For the samples calcined at400℃and500℃, the photocatalytic oxidation and adsorption performance were both improved with the rise of doping concentration.Finally, the effects of flue gas compositions such as O2, NO, SO2concentration and residence time on the photocatalytic removal efficiency of elemental mercury were studied by using carbon modified titanium nanotubes as catalysts. Results showed that the effect of oxygen concentration on mercury removal efficiency was not significant. NO and SO2both had negative effect on the removal of elemental mercury. The main reasons for inhibitory effect were mainly due to their competitive adsorption with elemental mercury, free radical consumption during the photocatalytic reactions and the by-product generated on the catalysts surface. |
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