Increasing concerns over mercury (Hg) emissions from coal-fired power plants have led the creation of new emission standards such as the recent Mercury and Air Toxic Standards, which requires a 90% reduction of Hg emissions. These new standards have led to the implementation of various novel technologies to control Hg emissions. Selective catalytic reduction (SCR) catalysts have shown promise as bifunctional catalysts for limiting both nitrogen oxide (NOx
) and Hg emissions through simultaneous NOx
reduction and Hg oxidation. The catalytic oxidation of elemental Hg, which is difficult to capture without dedicated Hg capture systems, to Hg+2
via HCl enables the capture of Hg2+
in the existing wet flue gas desulfurization unit as it is water soluble. Previous studies have investigated the possibility of utilizing current commercial SCR catalysts to fill this role, however these catalysts often have unsatisfactory performance when it comes to reducing Hg pollution to the levels required by current emission standards. To develop the next generation of SCR catalysts, a copper (Cu) exchanged small pore zeolite (Cu-SSZ-13) was investigated for its possible application in coal-fired power plants as a new bifunctional SCR and Hg oxidation catalyst. Cu-SSZ-13 has shown promise as a SCR catalyst for diesel exhaust, but has not been studied in coal-fired power plants.
In this study, the activity of Cu-SSZ-13 was investigated for its ability to oxidize Hg under a variety of flue gas conditions. The Cu-SSZ-13 catalyst was compared to SSZ-13 to understand the effect of Cu in the Hg oxidation mechanism. The Hg oxidation performances of both zeolites were compared to that of a commercial SCR catalyst (V-Ti-W), which served as a benchmark. All catalysts required hydrogen chloride (HCl) to exhibit any Hg oxidation with the zeolites outperforming the commercial catalyst. However, the activity of all catalysts significantly decreased in the presence of either sulfur oxides (SOx) or SCR reactants (nitrogen oxide (NO)+ammonia (NH3)) in the flue gas, with SOx having a greater effect on Cu-SSZ-13 and SCR reactants having a greater one on the commercial catalyst. Additionally, it was determined that Cu was not the active site for Hg oxidation but was necessary for the SCR reaction. In fact, in the presence of SO2, Cu actually contributed to the deactivation of the catalyst for Hg oxidation, likely due to the formation of sulfates as confirmed by XPS analysis.