A Predictive Mechanism for Mercury Oxidation on Selective Catalytic Reduction Catalysts under Coal-Derived Flue Gas

Abstract

This paper introduces a predictive mechanism for elemental mercury (Hg 0 ) oxidation on selective catalytic reduction (SCR) catalysts in coal-fired utility gas cleaning systems, given the ammonia (NH 3 )/nitric oxide (NO) ratio and concentrations of Hg 0 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 0 oxidation behavior on SCRs: that hydrochloric acid (HCl) competes for surface sites with NH 3 and that Hg 0 contacts these chlorinated sites either from the gas phase or as a weakly adsorbed species. This mechanism explicitly accounts for the inhibition of Hg 0 oxidation by NH 3 , so that the monolith sustains two chemically distinct regions. In the inlet region, strong NH 3 adsorption minimizes the coverage of chlorinated surface sites, so NO reduction inhibits Hg 0 oxidation. But once NH 3 has been consumed, the Hg 0 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 NH 3 /NO ratios, promote Hg 0 oxidation. This mechanism accurately interprets the reported tendencies for greater extents of Hg 0 oxidation on honeycomb monoliths with smaller channel pitches and hotter temperatures and the tendency for lower extents of Hg 0 oxidation for hotter temperatures on plate monoliths. The mechanism also depicts the inhibition of Hg 0 oxidation by NH 3 for NH 3 /NO ratios from zero to 0.9. Perhaps most important for practical applications, the mechanism reproduces the reported extents of Hg 0 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.

Oxidation of Mercury in Products of Coal Combustion

“…The steep rise in mercury oxidation at low HCl volume fractions is in contrast to the behavior predicted by Niksa and Fujiwara (2005b) in the presence of both NO and NH 3 . In a square honeycomb catalyst at 364 o C, at an NH 3 /NO ratio of 0.9 and space velocity of 3930 h -1 , those authors' calculations showed that the fraction of mercury oxidized at the exit from the catalyst increased approximately linearly from 0 at low HCl volume fractions, to only 14% of mercury oxidized in the presence of 25 ppmv HCl.…”

“…As previously noted in work by others, mercury oxidation in SCR catalyst was highly sensitive to the concentration of HCl at low levels of HCl (< 10 ppmv) (Eswaran and Stenger, 2005), and is suppressed in the presence of NH 3 (Machalek et al, 2003;Niksa and Fujiwara, 2005b;Senior, 2006;Gale et al, 2006b), because NH 3 competes with one of more of the reactants (Hg, HCl) for active sites on the surface of the catalyst.…”

“…Measurements in the absence of NO and NH 3 , in the presence of NO and absence of NH 3 , and in the presence of both NO and NH 3 (NH 3 /NO ~ 0.8 to 1 mol/mol) are compared in Figure 2.3.1. As shown in the figure, the extent of mercury oxidation is highly sensitive to the concentration of HCl at low levels of HCl (Eswaran and Stenger, 2005), and is suppressed in the presence of NH 3 (Machalek et al, 2003;Niksa and Fujiwara, 2005b;Senior, 2006;Gale et al, 2006b), because ammonia competes with at least one of the reactants (Hg, HCl) for surface sites toward the entrance to the monolith, before the NH 3 is consumed by reaction with NO and where it is still present at significant concentration (Niksa and Fujiwara, 2005b). Of the three conditions shown in Figure 2.3.1, the extent of mercury oxidation is most variable in the presence of both NO and NH 3 at NH 3 /NO ratios near stoichiometric (Figure 2.3.1c, NH 3 /NO ~ 0.8 to 1), when small changes in conditions, such as the inlet levels of NO and NH 3 , can cause large changes in the concentration of unreacted NH 3 toward the catalyst exit.…”

DFT Study of Hg Oxidation across Vanadia-Titania SCR Catalyst under Flue Gas Conditions

“…However, when supported vanadia systems were studied by Vittadini et al, 20 the unit cell used was small, causing vanadia dimers to cover almost the entire surface, which is not a reasonable representation of the low loading associated with SCR systems. Even for loadings higher than a theoretical V 2 O 5 monolayer, only a small fraction (≈10%) of the TiO 2 surface is covered by "towers" (i.e., vertical-growth) of polycrystalline V 2 O 5 . 33 In addition, their surfaces contain few free surface Ti atoms, which likely exist in real systems leading to possible interactions with the water vapor present in the flue gas.…”

Analytical management of SCR catalyst lifetimes and multipollutant performance

“…This section describes each element in turn. Niksa and Fujiwara (2005) introduced the first reaction mechanism for simultaneous NO reduction and Hg 0 oxidation along SCRs in terms of lumped catalyst reactivities that resolved film transport of reactants onto the external catalyst channel wall from an overall chemical reactivity. The key features were a competition between NH 3 and HCl for surface sites, and an Hg°oxidation reaction on chlorinated sites.…”