A Simulation Study on Urea Maldistribution and Implications for NOx Reduction with a Multi-Channel Modelling Approach

Urea-based selective catalytic reduction (SCR) is highly efficient for NOx abatement within a diesel aftertreatment system. However, abnormally high NOx emissions in the aftertreatment system tailpipe during WHSC (World Harmonized Steady-State Cycle) evaluation have been observed due to insufficient urea decomposition or mixing, which cannot be predicted by the current uniform 1D (one-dimensional) modelling approach with different urea dosing ratios. As a result, a multi-channel model has been developed to investigate the effect of urea maldistribution on aftertreatment system performance, where the uniformity index (UI) is used as a characteristic parameter to describe urea mixing efficiency. It was found that NOx emissions at the tailpipe can be successfully described with the multi-channel model even with a relatively high UI (UI = 0.95). Additionally, an improved segment UI factor as a function of mass flow rate has also been applied for maldistribution description, wherein better correlation with the measured NOx emission can be obtained.