Fuel and System Interaction Effects on Urea-SCR Control of NOx in Diesel Exhaust Aftertreatment 2006-01-0638
This work considers the performance of a NOx control system on a diesel engine and the interaction between the NOx and particulate control devices. A commercial urea-selective catalytic reduction (SCR) catalyst (twin catalytic reactors used in series) was characterized for the impact of nitrogen dioxide (NO2) on the ammonia consumption, production of nitrous oxide (N2O) and relative selectivity of the urea-SCR catalyst for NO2 versus NO when the SCR reactors were positioned downstream of a catalyzed diesel particulate filter (DPF). The aqueous urea solution was injected into the exhaust by using a twin fluid, air-assisted atomizer. It was possible to observe the role of NO2 due to the catalyzed diesel particulate filter (DPF) upstream of the SCR catalyst. This catalyzed DPF oxidizes nitric oxide (NO) in the engine-out emissions to NO2. Further, it uses NO2 to oxidize particulate matter (PM). But depending on the catalyst temperature and PM accumulation, the NO2 generation and consumption varies. Therefore, the net NO2 in the exhaust feed that enters the SCR catalyst can be significantly higher than the baseline engine-out emissions. This phenomenon provided an excellent opportunity to examine the relative selectivity of the SCR catalyst for both nitrogen oxides, NO and NO2, as well as, to observe the impact of NO2 on NH3 consumption. The results show the significant effect of NO2 on the overall performance and efficiency of the SCR system. It is shown that the urea-SCR catalyst has higher selectivity for NO2 than NO and in presence of NO2, the NH3 requirement for complete removal of NOx changes. The results show that the urea-SCR catalytic process does not form N2O, which can be an undesired byproduct of catalytic NOx control.