Modeling of a combined exhaust gas aftertreatment system: NOx storage and reduction + NH3-SCR
Petr Koci, Institute of Chemical Technology, Prague
The combination of a NOx storage and reduction catalyst (NSRC, called also lean NOx trap, LNT) and a catalyst for the selective catalytic reduction of NOx by NH3 (NH3-SCR) offers a potential to significantly increase the efficiency of the 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 NOx reduction in the course of the consequent lean phase. This synergy becomes more important with the aged NSRC, where generally lower NOx conversions and higher NH3 yields are observed in a wider range of operating temperatures (in comparison with the fresh or de-greened NSRC).
Effective global kinetic models for the NSRC (Pt/Ba/Ce/&#947;-Al2O3 catalyst type) and NH3-SCR (Fe-ZSM5 catalyst type) are presented in this contribution. 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 NOx 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. Differences in NOx conversion and NH3 selectivity between the fresh and aged NSRC are discussed.
The performance of the NSRC+SCR system is simulated in dependence on temperature and rich phase length, and their effects on integral NOx conversions and NH3 yields are discussed. Defined periodic lean/rich operation is simulated as well as the test driving cycles FTP and NEDC. The simulation results show that the added SCR can significantly
improve the deNOx efficiency in a wide range of operating conditions.
 Chatterjee D., Ko&#269;í P., Schmeißer V., Marek M., Weibel M., Krutzsch B. Modelling of a combined NOx storage and NH3-SCR catalytic system for Diesel exhaust gas aftertreatment. Catal. Today (2010) in press, http://dx.doi.org/10.1016/j.cattod.2010.01.014