Modelling of NOx Storage + SCR Exhaust Gas Aftertreatment System with Internal Generation of Ammonia

Combination of an NO_x storage and reduction catalyst (NSRC, called also lean NO_x trap, LNT) and a catalyst for the selective catalytic reduction of NO_x by NH₃ (NH₃-SCR) offers a potential to significantly increase the efficiency of NSRC-based exhaust gas aftertreatment systems. Under most situations the SCR catalyst is able to adsorb the NH₃ peaks generated in the NSRC during the regeneration and utilize it for additional NO_x reduction in the course of the consequent lean phase. This synergy becomes more important with the aged NSRC, where generally lower NO_x conversions and higher NH₃ yields in wider range of operating temperatures are observed (in comparison with the fresh or de-greened NSRC).

In this paper we present global kinetic models for the NSRC (Pt/Ba/Ce/gγ-Al₂O₃ catalyst type) and NH₃-SCR (Fe-ZSM5 catalyst type). The NSRC regeneration by a mixture of CO, H₂ and HC is considered with a differentiated activity and selectivity of individual reducing agents in the NO_x reduction. The oxygen storage effects and NH₃ oxidation reactions are also included in the NSRC model. A non-equilibrium spill-over of the adsorbed NH₃ between the non-reactive and reactive sites is involved in the SCR model. The kinetic parameters are evaluated from transient lab experiments with synthetic gases, individually for the NSRC and SCR, and the models are then validated by engine test data.

The performance of the NSRC+SCR system is simulated in dependence on temperature and rich phase length, and their effects on integral NO_x conversions and NH₃ yields are discussed. Defined periodic lean/rich operation is simulated as well as the test driving cycle FTP. The simulation results show that the added SCR can significantly improve the deNO_x efficiency in a wide range of operating conditions. The mechanistic understanding and modeling of the relevant chemical and physical processes in the NSRC and SCR played a key role in the development of the combined NSRC+SCR system and enabled to bring this technology to series production within the BlueTec I system (2006).