A study of the effects of NH 3 maldistribution on a urea-selective catalytic reduction system

The maldistribution of urea/NH 3 at the selective catalytic reduction inlet and the resulting NH 3 /NO x ratio variation have been reported to have negative effects on the NO x conversion efficiency and NH 3 slip in heavy-duty diesel applications. Maldistribution is caused by incomplete mixing and decomposition of the injected urea water solution within the exhaust flow in the aftertreatment system. How NH 3 and the NH 3 /NO x ratio are distributed at the selective catalytic reduction inlet in an engine aftertreatment system and how the resultant maldistribution affects the system performance and the kinetics are presented in this combined experimental and modeling study. The distribution profiles of the selective catalytic reduction inlet NH 3 and NH 3 /NO x ratio were determined through engine experiments. The NH 3 maldistribution effects on the selective catalytic reduction performance and kinetics were quantified by using a multi-channel one-dimensional model that takes into consideration the NH 3 maldistribution. It was determined that the NH 3 maldistribution is a major factor causing the difference in catalyst performance in the engine exhaust as compared to the laboratory reactor environment, and these results explain specific differences in the model calibrations determined from the laboratory reactor and engine data. Improving the selective catalytic reduction inlet NH 3 uniformity can improve the NO x conversion efficiency by up to 7% and reduce NH 3 slip by 10–20 ppm. A single-channel one-dimensional selective catalytic reduction model calibrated to the engine data that accounts for the NH 3 maldistribution phenomena can be applied for the development of on-board diagnostic and control strategies.