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 Hg 0 removal efficiency. This essential adsorption process was mainly realized through chemisorption. HgO was the prime species that was identified to form in the catalysts, and a little amount of adsorbed HgCl 2, Hg(NO 3 ) 2, and Hg-OM was detected as well in the samples spent in the simulated coal-fired flue gases. O 2, HCl, NO, and high concentration of CO 2 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 SO 2, NH 3, and H 2 O, 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.