Selective Catalytic Reduction of NO_x over V₂O₅-WO₃-TiO₂ SCR Catalysts—A Study at Elevated Pressure for Maritime Pre-turbine SCR Configuration

The selective catalytic reduction (SCR) of NO_x using NH₃ was studied at pressures up to 5 bar over a vanadium-based SCR catalyst (~1 wt% V₂O₅ and 10 wt% WO₃/TiO₂), relevant for the installation of SCR reactors upstream of the turbocharger at marine engines. Experiments were performed using both granulated catalyst in a lab-scale fixed-bed reactor and a monolith catalyst in a bench-scale setup. The residence time across the catalytic bed was kept constant, by increasing the (normalized (0 °C, 1 atm)) volumetric flow rate proportionally to the pressure. The results show that for the granulated catalyst, the NO_x conversion was independent of the pressure, indicating that the SCR kinetics are not affected by the increased pressure up to 5 bar. NH₃ temperature-programmed desorption experiments showed that the catalyst NH₃ adsorption increased with more than 30% when the pressure was increased from 1 bar to 4.5 bar. On the other hand, when the adsorption temperature was increased from 150 to 300 °C, the adsorption capacity decreased by approximately 60% independent on the pressure. The SCR reaction was unaffected by the increased NH₃ uptake caused by the increased pressure, because only a certain fraction of the sites (\( {\theta}_{N{H}_3}^{\ast } \) = 0.14) was found to be active in the SCR reaction, and these are filled up at lower NH₃ partial pressure than the total number of sites. Experiments using a monolithic catalyst showed that at temperatures above 250 °C, the NO_x conversion was lower at an increased pressure (3.1 bar) when the residence time was held constant. This decrease was ascribed to increased internal and external diffusion limitations at the elevated pressure.