Modelling of a combined NOxNOx storage and NH3NH3-SCR catalytic system for Diesel exhaust gas aftertreatment

A combined Diesel exhaust gas aftertreatment system is studied, consisting of the NOxNOx storage and reduction catalyst (NSRC, called also lean NOxNOx trap, LNT) and the catalyst for selective catalytic reduction of NOxNOx by NH3NH3 (NH3NH3-SCR). Most of the time the system is operated under prevailing fuel-lean conditions, enabling economical running of the engine. During this phase the NOxNOx emissions are being adsorbed in the NSRC. However, short fuel enrichments need to be applied periodically for the NSRC regeneration (reduction of the stored NOxNOx). Ammonia produced in the NSRC as a by-product of the NOxNOx reduction under controlled fuel-rich conditions is then adsorbed in the NH3NH3-SCR reactor located downstream. The adsorbed NH3NH3 is consequently utilised in selective NOxNOx reduction during the next fuel-lean period. The NSRC+SCRNSRC+SCR configuration thus eliminates the need for an external NH3NH3 source (e.g., periodically re-filled urea solution tank) that is necessary in the case of the stand-alone SCR.Development of effective mathematical models for the NSRC and SCR catalysts is discussed. Dynamic measurements in a lab mini-reactor are performed separately for the industrial NSRC (PtRh/Ba/Ce/γ-Al2O3γ-Al2O3 type) and SCR (Fe-ZSM type) catalyst samples. The experimental results are employed in the evaluation of rate parameters for the individual catalysts. Particular attention is given to the dynamic evolution of NH3NH3 during the NSRC regeneration and its dependence on temperature and length of the enrichment period. Trends in NH3NH3 selectivity with the NSRC ageing are discussed. Synergistic effects of the NSRC and NH3NH3-SCR are then studied by simulations of defined lean/rich operation and engine test driving cycles. The combined NSRC+SCRNSRC+SCR system provides higher NOxNOx conversions in comparison with the stand-alone NSRC and it prevents undesired NH3NH3 slip. The positive effects of the downstream SCR are most important at lower intermediate temperatures, and in the case of an aged NSRC that usually produces more NH3NH3.