(629ac) 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.