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

Abstract

The maldistribution of urea/NH₃ at the selective catalytic reduction inlet and the resulting NH₃/NO_x ratio variation have been reported to have negative effects on the NO_x conversion efficiency and NH₃ 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 NH₃ and the NH₃/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 NH₃ and NH₃/NO_x ratio were determined through engine experiments. The NH₃ 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 NH₃ maldistribution. It was determined that the NH₃ 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 NH₃ uniformity can improve the NO_x conversion efficiency by up to 7% and reduce NH₃ slip by 10–20 ppm. A single-channel one-dimensional selective catalytic reduction model calibrated to the engine data that accounts for the NH₃ maldistribution phenomena can be applied for the development of on-board diagnostic and control strategies.