Reduction of oxidized mercury over NOx selective catalytic reduction catalysts: A review

The control of mercury emission from flue gas has long been a serious task for the global environment and human health. Utilization of selective catalytic reduction (SCR) catalysts to oxidize Hg0 to form Hg2+ and subsequent capture the Hg2+ through a wet flue gas desulfurization (WFGD) scrubber is an efficient and economical technology for Hg0 removal in coal-fired power plants. Extensive studies have been conducted with regard to Hg0 oxidation over SCR catalysts. However, recent studies found that the reduction of Hg2+, an inverse process to Hg0 oxidation, also occurred even predominated under some SCR conditions. This new observation overturns the general knowledge that SCR catalysts always enhance Hg0 oxidation. To avoid the neglect of Hg2+ reduction and overcome the shortages of existing studies, a critical review of Hg2+ reduction over SCR catalysts was undertaken. The Hg2+ reduction behaviors under various SCR conditions were summarized and the effects of typical flue gas components like hydrogen chloride (HCl), sulfur dioxide (SO2), water vapor (H2O), volatile organic compounds (VOCs), and carbon monoxide (CO) on Hg2+ reduction were also presented. The involved reaction mechanisms for Hg2+ reduction were discussed in details. Moreover, feasible measures to suppress the reduction of Hg2+ in the SCR system were proposed. We expect to clearly explore the mechanism of the Hg0 redox process on SCR catalysts in this review to avoid the neglect of Hg2+ reduction, hence providing guidance for the design of SCR catalysts to achieve an efficient Hg0 oxidation.