Nitric Oxide Reduction of Heavy-Duty Diesel Off-Gas by NH₃-SCR in Front of the Turbocharger

This paper investigates the impacts and effects of higher pressure on NH₃-SCR of NO_x corresponding to positioning the SCR catalyst in front of the exhaust gas turbine, which is an option in heavy-duty and off-road diesel engine applications (1‐4). The influence of applied pressure up to 500 kPa on the selective catalytic reduction (SCR) of nitrogen oxides by NH₃ over commercial V₂O₅/WO₃-TiO₂-type catalysts (VWT) is studied experimentally and numerically. The model includes a detailed reaction mechanism for standard and fast SCR and a two-dimensional description of the flow field and diffusion in the honeycomb-structured catalyst. The pressure effect on catalytic conversion, residence time, and mass transfer are examined for varying temperature and two different channel sizes, i.e., 25 and 300 cpsi (channels per square inch) at constant mass flux from the engine. Pre-turbo positioning of the catalyst leads to a significant increase in NO_x conversion due to higher pressure aside from the positive temperature effect. However, with increasing channel size, diffusional mass transport limits the reaction rate, and reduces the benefit of increased residence time due to the decrease in diffusion rates with increasing pressure. The achievable increase in conversion is transformed into a length reduction of the catalyst.