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

Abstract

• The Hg 2+ reduction process over SCR catalysts were reviewed for the first time. • The effects of typical flue gases on Hg 2+ reduction were presented. • The involved reaction mechanisms for Hg 2+ reduction were discussed in details. • Suggestions to alleviate the reduction of Hg 2+ over SCR catalysts were proposed. 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 Hg 0 to form Hg 2+ and subsequent capture the Hg 2+ through a wet flue gas desulfurization (WFGD) scrubber is an efficient and economical technology for Hg 0 removal in coal-fired power plants. Extensive studies have been conducted with regard to Hg 0 oxidation over SCR catalysts. However, recent studies found that the reduction of Hg 2+ , an inverse process to Hg 0 oxidation, also occurred even predominated under some SCR conditions. This new observation overturns the general knowledge that SCR catalysts always enhance Hg 0 oxidation. To avoid the neglect of Hg 2+ reduction and overcome the shortages of existing studies, a critical review of Hg 2+ reduction over SCR catalysts was undertaken. The Hg 2+ reduction behaviors under various SCR conditions were summarized and the effects of typical flue gas components like hydrogen chloride (HCl), sulfur dioxide (SO 2 ), water vapor (H 2 O), volatile organic compounds (VOCs), and carbon monoxide (CO) on Hg 2+ reduction were also presented. The involved reaction mechanisms for Hg 2+ reduction were discussed in details. Moreover, feasible measures to suppress the reduction of Hg 2+ in the SCR system were proposed. We expect to clearly explore the mechanism of the Hg 0 redox process on SCR catalysts in this review to avoid the neglect of Hg 2+ reduction, hence providing guidance for the design of SCR catalysts to achieve an efficient Hg 0 oxidation.