A LNT+SCR System for Treating the NOx Emissions from a Diesel Engine

An aftertreatment system involving a LNT followed by a SCR catalyst is proposed for treating the NO_x 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 NO_x activity at low temperatures and due to its ability to store and release NH₃ below 400°C, which helps to minimize NH₃ and NO_x slip. Above 400°C, where the NH₃ 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. NH₃ 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 NH₃ (or urea) injection at high temperatures.