Removal of elemental mercury from simulated coal-combustion flue gas using a SiO2–TiO2 nanocomposite

A novel silica–titania (SiO2–TiO2) nanocomposite has been developed to effectively capture elemental mercury (Hg0) under UV irradiation. Previous studies under room conditions showed over 99% Hg0 removal efficiency using this nanocomposite. In this work, the performance of the nanocomposite on Hg0 removal was tested in simulated coal-fired power plant flue gas, where water vapor concentration is much higher and various acid gases, such as HCl, SO2, and NOx, are present. Experiments were carried out in a fix-bed reactor operated at 135 °C with a baseline gas mixture containing 4% O2, 12% CO2, and 8% H2O balanced with N2. Results of Hg speciation data at the reactor outlet demonstrated that Hg0 was photocatalytically oxidized and captured on the nanocomposite. The removal efficiency of Hg0 was found to be significantly affected by the flue gas components. Increased water vapor concentration inhibited Hg0 capture, due to the competitive adsorption of water vapor. Both HCl and SO2 promoted the oxidation of Hg0 to Hg(II), resulting in higher removal efficiencies. NO was found to have a dramatic inhibitory effect on Hg0 removal, very likely due to the scavenging of hydroxyl radicals by NO. The effect of NO2 was found to be insignificant. Hg removal in flue gases simulating low rank coal combustion products was found to be less than that from high rank coals, possibly due to the higher H2O concentration and lower HCl and SO2 concentrations of the low rank coals. It is essential, however, to minimize the adverse effect of NO to improve the overall performance of the SiO2–TiO2 nanocomposite.