The Photocatalytic Removal of Mercury from Coal-Fired Flue Gas

Abstract

Mercury in the flue gas of coal combustion exists in three chemical forms: elemental mercury (Hg0), particle-bounded mercury (Hgp), and oxidized mercury (Hg2+). Hg2+ is water-soluble, so it can be removed by wet scrubbers and the Hg2+ removal efficiency can reach up to 90%. In addition, Hgp can be easily removed by electrostatic precipitator (ESP) or fabric filter (FF). However, elemental mercury is difficult to remove because of its volatility, insolubility and chemical stability. So the most critical step is to control elemental mercury. There are two main methods for elemental mercury control: adsorption and oxidation. Activated carbon injection has proven to be an effective adsorption method for elemental mercury removal in flue gas. However, its high operation cost and negative effect on fly ash quality may restrict its industrial application. The drawbacks associated with the use of particulate adsorbents make the oxidation method more attractive. In recent years, the progress in the photocatalytic oxidation of elemental mercury has aroused wide interests among researchers. Compared with other methods, photocatalytic oxidation possesses higher oxidation ability and no secondary pollution, therefore it is a promising technology for oxidation of elemental mercury. We set up a chemical reactor system to evaluate the photocatalytic performance on mercury in the simulated flue gas, and studied the mercury oxidation efficiency of different photocatalysts, including morphology controlled photocatalysts, metal or nonmetal modified titanium dioxide photocatalysts and others photocatalysts, such as BiOIO3 and ZnO. The morphology controlled photocatalysts include hollow titanium dioxide photocatalyst and anatase titanium dioxide with co-exposed (001) and (101) facets. The metal modified titanium dioxide photocatalysts include CuO/titanium dioxide and V2O5/titanium dioxide photocatalysts, and the nonmetal modified titanium dioxide photocatalysts include carbon spheres supported CuO/titanium dioxide photocatalysts and carbon decorated In2O3/titanium dioxide photocatalysts. These studies are helpful to observe the characteristics and performance of different kinds of photocatalysts.