Fuel and System Interaction Effects on Urea-SCR Control of NOx in Diesel Exhaust Aftertreatment
Date Published: 2006-04-03
Paper Number:2006-01-0638
DOI: 10.4271/2006-01-0638
Citation:
Acharya, R., Alam, M., and Boehman, A., "Fuel and System Interaction Effects on Urea-SCR Control of NOx in Diesel Exhaust Aftertreatment," SAE Technical Paper 2006-01-0638, 2006, doi:10.4271/2006-01-0638.
This work considers the performance of a NO
x
control system on a diesel engine and the interaction between the NO
x
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 (NO
2
) on the ammonia consumption, production of nitrous oxide (N
2
O) and relative selectivity of the urea-SCR catalyst for NO
2
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 NO
2
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 NO
2
. Further, it uses NO
2
to oxidize particulate matter (PM). But depending on the catalyst temperature and PM accumulation, the NO
2
generation and consumption varies. Therefore, the net NO
2
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 NO
2
, as well as, to observe the impact of NO
2
on NH
3
consumption. The results show the significant effect of NO
2
on the overall performance and efficiency of the SCR system. It is shown that the urea-SCR catalyst has higher selectivity for NO
2
than NO and in presence of NO
2
, the NH
3
requirement for complete removal of NO
x
changes. The results show that the urea-SCR catalytic process does not form N
2
O, which can be an undesired byproduct of catalytic NO
x
control.
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