A predictive mechanism for mercury oxidation on selective catalytic reduction catalysts under coal-derived flue gas

This paper introduces a predictive mechanism for elemental mercury (Hg(o)) oxidation on selective catalytic reduction (SCR) catalysts in coal-fired utility gas cleaning systems, given the ammonia (NH3)/nitric oxide (NO) ratio and concentrations of Hg(o) and HCl at the monolith inlet, the monolith pitch and channel shape, and the SCR temperature and space velocity. A simple premise connects the established mechanism for catalytic NO reduction to the Hg(o) oxidation behavior on SCRs: that hydrochloric acid (HCl) competes for surface sites with NH3 and that Hg(o) contacts these chlorinated sites either from the gas phase or as a weakly adsorbed species. This mechanism explicitly accounts for the inhibition of Hg(o) oxidation by NH3, so that the monolith sustains two chemically distinct regions. In the inlet region, strong NH3 adsorption minimizes the coverage of chlorinated surface sites, so NO reduction inhibits Hg(o) oxidation. But once NH3 has been consumed, the Hg(o) oxidation rate rapidly accelerates, even while the HCl concentration in the gas phase is uniform. Factors that shorten the length of the NO reduction region, such as smaller channel pitches and converting from square to circular channels, and factors that enhance surface chlorination, such as higher inlet HCl concentrations and lower NH3/NO ratios, promote Hg(o) oxidation. This mechanism accurately interprets the reported tendencies for greater extents of Hg(o) oxidation on honeycomb monoliths with smaller channel pitches and hotter temperatures and the tendency for lower extents of Hg(o) oxidation for hotter temperatures on plate monoliths. The mechanism also depicts the inhibition of Hg(o) oxidation by NH3 for NH3/NO ratios from zero to 0.9. Perhaps most important for practical applications, the mechanism reproduces the reported extents of Hg(o) oxidation on a single catalyst for four coals that generated HCl concentrations from 8 to 241 ppm, which covers the entire range encountered in the U.S. utility industry. Similar performance is also demonstrated for full-scale SCRs with diverse coal types and operating conditions.