A LNT+SCR System for Treating the NOx Emissions from a Diesel Engine
Date Published: 2006-04-03
Paper Number:2006-01-0210
DOI: 10.4271/2006-01-0210
Citation:
Theis, J. and Gulari, E., "A LNT+SCR System for Treating the NOx Emissions from a Diesel Engine," SAE Technical Paper 2006-01-0210, 2006, doi:10.4271/2006-01-0210.
An aftertreatment system involving a LNT followed by a SCR catalyst is proposed for treating the NO
x
emissions from a diesel engine. NH
3
(or urea) is injected between the LNT and the SCR. The SCR is used exclusively below 400°C due to its high NO
x
activity at low temperatures and due to its ability to store and release NH
3
below 400°C, which helps to minimize NH
3
and NO
x
slip. Above 400°C, where the NH
3
storage capacity of the SCR falls to low levels, the LNT is used to store the NO
x
. A potassium-based LNT is utilized due to its high temperature NO
x
storage capability. Periodically, hydrocarbons are oxidized on the LNT under net lean conditions to promote the thermal release of the NO
x
. NH
3
is injected simultaneously to reduce the released NO
x
over the SCR.
The majority of the hydrocarbons are oxidized on the front portion of the LNT, resulting in the rapid release of stored NO
x
from that portion of the LNT. To provide time for the exothermic heat to propagate back and purge the rear portion of the LNT, a wait period of 60 seconds with NO
x
-free flow passing through the LNT is needed after each HC injection. A reversing-flow dual-LNT system is proposed that satisfies these requirements. The system is shown to transform a fluctuating NO
x
concentration into a periodic and reproducible profile that can be treated by the SCR, thus significantly simplifying the control strategy for NH
3
(or urea) injection at high temperatures.
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