| Coal is the most abundant fossil fuel. It is still and will be the main energy source in a long time for China. However, coal combustion is the greatest anthropogenic source of toxic air pollution. SO2, NOx and trace metal mercury are the main toxic pollutants from coal combustion. Coal-fired boiler of power plants releases large amounts of these pollutants. At present, the coal-fired power plants in our country are being installed desulphurization equipments and denitrification equipments. At the same time, the control of mercury emission from coal-fired power plants has also caused significant concern. The release of multiple pollutants leads to the increasing of flue gas purification devices and the rising of investment and operating cost. Thus, development of novel catalysts and integration removal of multiple pollutants from coal-fired flue gas have great significance.In this study, a novel TiO2-aluminum silicate fiber (TAS) nanocomposite, synthesized by a sol-gel method, is proposed to use as a photocatalyst for the removal of SO2、NO and Hg0. The photocatalyst has been characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersing X-ray (EDX), Brunauer-Emmett-Teller (BET), and UV-Vis spectra (UV-Vis). The results showed that TAS calcined at500℃exhibited the highest crystallinity, highest photocatalytic activity and greatest BET surface area.The TAS calcined at500℃was used as the photocatalyst for the removal of SO2、NO and Hg0from simulated coal combustion flue gas. In this work, systematic experiments were conducted to investigate the removal efficiencies for SO2, NO and Hg0over TAS, the effects of SO2and NO on mercury removal and also the effects of temperature, O2, H2O and UV intensity on the photocatalytic removal efficiency. The results showed that the mercury removal efficiency over TAS is the highest, which is84%. During the simultaneous removal of SO2, NO and Hg0over TAS, O2served as an oxidant exhibited a promotional effect on the photocatalytic desulfurization, denitrification and mercury removal. However, the addition of water vapor to the simulated flue gas inhibited the oxidation of SO2, NO and Hg0. Higher temperatures weakened the depositions and adsorptions of SO2, NO and Hg0on the catalyst surface, which resulted in a reduction of pollutants removal efficiency. In addition, the UV intensity was the most important factor in the photocatalytic oxidation over TAS. There was a reduction of photoexcited active species with the decrease of UV intensity, which resulted in the reduction of photocatalytic removal efficiency for SO2, NO and Hg0.In order to extend the light response range of TiO2to visible light region and enhance its optical activity, various metal oxides (CuO, In2O3, V2O5and WO3) and precious metal silver were doped into the TiO2nanofibers. These nanofibers prepared by an electrospinning method were used to remove elemental mercury from simulated coal combustion flue gas. The catalysts have been characterized by XRD, SEM, EDX, UV-Vis spectra and transmission electron microscopy (TEM). The results showed that the TiO2in the fibers all existed as anatase. The diameter of the TiO2-based nanofibers was200±50nm. The fibers have uniform thickness and they were composed of a large number of nanoparticles with diameter of around10nm. Hg0removal efficiencies over the TiO2-based nanofibers were tested under dark, visible light irradiation and UV irradiation, respectively. The results showed that the Hg0removal efficiency over TiO2-V2O5increased from6%to63%under visible light irradiation due to the electron transition. Additionally, WO3doped TiO2exhibited the highest Hg0removal efficiency of almost100%under UV irradiation, which was attributed to the increase of surface acidity and better photoelectron-hole separation. Moreover, Ag doped TiO2showed a steady Hg0removal efficiency of around95%without any light due to the formation of silver amalgam.In order to effective removal of Hg0at higher temperature, TiO2-V2O5-Ag (TVA) nanofibers prepared by an electrospinning method were proposed to use as the novel SCR catalysts for simultaneous removal of NO and Hg0at typical SCR operating temperature. In this work, systematic experiments were conducted to investigate the NO, Hg0and SO2removal efficiencies over TVA; the effects of Ag doping amount, NH3/NO molar ratio, and individual flue gas components on NO removal; and the effects of Ag doping amount, operating temperature, and individual flue gas components on Hg0removal. The results showed that TVA had wide temperature window for selective catalytic reduction. The best mass ratio of V2O5/TiO2was5%, and the best molar ratio of NH3/NO was1.2%O2has met the demands of the SCR reaction. Further increasing of O2concentration had no significant effect on NO removal. The introduction of SO2into the gas flow did not change the NO removal efficiency. However, H2O had a prohibitive effect on NO removal due to the adsorption of H2O on the active sites. Moreover, Hg0removal rate over TVA could reach98%at370℃. The TVA nanofibers had wide operating temperature range to remove Hg0. Therefore, the TVA nanofibers could realize the simultaneous removel of NO and Hg0at typical SCR operating temperature.The pollutants removal over TiO2-based nanomaterials follows the Langmuir-Hinshelwood mechanism. The photocatalytic kinetic model of desulfurization and denitrification and mercury removal over TAS was built. There was fine coincidence between fitting function of mercury removal over TAS and experimental data. In addition, the photocatalytic kinetic model of mercury removal over TiO2-based nanofibers was also built. Experimental data matched with the kinetic model very well. Such knowledge is of fundamental importance in developing effective catalysts for pollution control technologies. |
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