Abstract
A combined Diesel exhaust gas aftertreatment system is studied, consisting of the NO x storage and reduction catalyst (NSRC, called also lean NO x trap, LNT) and the catalyst for selective catalytic reduction of NO x by NH 3 ( NH 3 -SCR). Most of the time the system is operated under prevailing fuel-lean conditions, enabling economical running of the engine. During this phase the NO x emissions are being adsorbed in the NSRC. However, short fuel enrichments need to be applied periodically for the NSRC regeneration (reduction of the stored NO x ). Ammonia produced in the NSRC as a by-product of the NO x reduction under controlled fuel-rich conditions is then adsorbed in the NH 3 -SCR reactor located downstream. The adsorbed NH 3 is consequently utilised in selective NO x reduction during the next fuel-lean period. The NSRC + SCR configuration thus eliminates the need for an external NH 3 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 -Al 2 O 3 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 NH 3 during the NSRC regeneration and its dependence on temperature and length of the enrichment period. Trends in NH 3 selectivity with the NSRC ageing are discussed. Synergistic effects of the NSRC and NH 3 -SCR are then studied by simulations of defined lean/rich operation and engine test driving cycles. The combined NSRC + SCR system provides higher NO x conversions in comparison with the stand-alone NSRC and it prevents undesired NH 3 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 NH 3 .
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