DeNOx Performance of Combined LNT-SCR Catalyst Systems: A Simulation Study

Abstract:

Transient studies of combined LNT-SCR catalyst systems are investigated and their performance is evaluated against that of individual LNT or SCR systems. A one-dimensional two-phase model with global rate expressions for various reactions is used in the simulations. The LNT system modeled is that of Pt/BaO/Al₂O₃ catalyst with two different BaO storage sites, designated as fast and slow sites. LNT model includes reactions that describe NO oxidation, storage of NO₂ in the form of nitrates during lean storage cycle and reduction to N₂ in the subsequent rich cycle with H₂ as a reductant. The SCR system considered is that of Cu-ZSM5 and the model includes NO oxidation, ammonia storage and oxidation, selective catalytic reduction of NO, NO₂ and formation of N₂O. Model predicts the spatio-temporal concentration profiles of reactants and products inside the channel which gives insight into the DeNO_x performance of combined systems. The effect of various design and operating parameters on DeNO_x efficiency have been examined which includes number of LNT and SCR catalysts in series, length of each catalyst, catalyst loading, rich cycle time and composition.  It has been found that series arrangement of alternate LNT and SCR catalyst, by dividing them into equal halves, substantially improves NO_x conversion. For fixed total catalyst length as the number of divisions increase, NO_x conversion increases and reaches a limit which is theoretically equivalent to that of mixed catalyst. It is also observed that for fixed length of LNT catalyst better DeNO_x performance is obtained by increasing the length of SCR catalyst in the alternate arrangement. Spatio-temporal profiles indicate complete storage of NH₃ on SCR catalyst which can be efficiently used to reduce NO_x slipping out of LNT.