Mercury Oxidation over Cu-SSZ-13 Catalysts Under SCR Conditions for Power Plant Applications

Many combustion-based systems, such as power plants and automobiles, utilize selective catalytic reduction (SCR) as a pollution control technology to reduce the emissions of nitrogen oxide (NO_x) species. In power plants, the reduction of the NO_x occurs via the injection of NH₃ over a V-(Mo or W)/Ti oxide catalyst. Recently, the promulgation of the Mercury and Air Toxic Standards regulates mercury, requiring a 90% reduction of Hg emissions. While activated carbon injection is one solution to limit Hg pollution, it can be costly. An alternative is the catalytic oxidation of the elemental Hg to Hg⁺² via HCl, where the oxidized pollutant is readily water soluble and easily captured downstream in the flue gas desulfurization unit. In this way, the traditional SCR material acts as a bifunctional catalyst limiting both NO_x and Hg emissions. While previous studies have examined the oxidation activity of the commercial SCR catalyst, they often display limited activity, falling short of the required emissions limits. To address this, multiple new materials have been proposed for Hg oxidation. In this study, 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.

In this study, SSZ-13 and Cu-exchanged SSZ-13 are compared to a commercial SCR catalyst with respect to their Hg oxidation activity under a variety of flue gas compositions and temperatures. A simulated flue gas environment is created through the combustion of methane in air in a high-temperature furnace. Other pollutants common to coal-combustion flue gas, such as HCl, SO₂, NO, and NH₃ are injected downstream of the furnace but upstream of the packed-bed reactor housing the catalyst. Concentrations of elemental and oxidized mercury were measured using a PS Analytical mercury analyzer to determine the amount of mercury oxidation. Initially, the flue gas composition was varied to examine the effect of individual flue gas components on Hg oxidation with the catalysts kept at 375°C, an intermediate SCR temperature. The compositions investigated include flue gas only (CO₂+H₂O+N₂+O₂), flue gas with HCl, with HCl+SO₂ and with HCl+NO+NH₃. Finally, the temperature effect was studied by varying the temperature between 300-450°C, representative of standard operating temperatures in SCR units for coal-fired power plants.