Research on the Mechanism of Elemental Mercury Removal over Mn-Based SCR Catalysts by a Developed Hg-TPD Method

Using an selective catalytic reduction (SCR) catalyst to oxidize and remove elemental mercury (Hg-0) synergistically is a promising method for mercury emission control in coal combustion power plants and is currently attracting widespread interest. In this study, a developed mercury thermal desorption (Hg-TPD) approach, combined with other associated methods, was employed for the identification of mercury species in the Mn-based SCR catalysts that have been used for synergistic Hg-0 removal. The analysis results demonstrated that the Hg-0 adsorption on the catalysts was a necessary process for the Hg-0 removal, though the SCR catalysts removed Hg-0 mainly through catalytic oxidation and the amount of the adsorbed mercury contributed only a little to the He removal efficiency. This essential adsorption process was mainly realized through chemisorption. (HgO)-O-0 was the prime species that was identified to form in the catalysts, and a little amount of adsorbed HgCl2, Hg(NO3)(2), and Hg-OM was detected as well in the samples spent in the simulated coal-fired flue gases. O-2, HC1, NO, and high concentration of CO2 in the flue gas all promoted the adsorption capacity and the generation of the related Hg compounds, so that they were conductive to Hg-0 removal efficiency, whereas a similar phenomenon was not observed in the presence of SO2, NH3, and H2O, and meanwhile the amount of adsorbed HgO was decreased, so that the efficiency was inhibited. The mobility testing by a sequential extraction procedure indicated that most of the retained Hg belonged to the mobile fraction, which was in accordance with the identification results of the Hg-TPD analysis. The consequences of this research would create a better understanding of the Hg-0 removal mechanism over the Mn-based SCR catalysts and provide additional information on the disposal of the retired catalysts in the environment.