A LNT+SCR System for Treating the NOx Emissions from a Diesel Engine 2006-01-0210
An aftertreatment system involving a LNT followed by a SCR catalyst is proposed for treating the NOx emissions from a diesel engine. NH3 (or urea) is injected between the LNT and the SCR. The SCR is used exclusively below 400°C due to its high NOx activity at low temperatures and due to its ability to store and release NH3 below 400°C, which helps to minimize NH3 and NOx slip. Above 400°C, where the NH3 storage capacity of the SCR falls to low levels, the LNT is used to store the NOx. A potassium-based LNT is utilized due to its high temperature NOx storage capability. Periodically, hydrocarbons are oxidized on the LNT under net lean conditions to promote the thermal release of the NOx. NH3 is injected simultaneously to reduce the released NOx over the SCR.
The majority of the hydrocarbons are oxidized on the front portion of the LNT, resulting in the rapid release of stored NOx 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 NOx-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 NOx concentration into a periodic and reproducible profile that can be treated by the SCR, thus significantly simplifying the control strategy for NH3 (or urea) injection at high temperatures.