NUMERICAL-SIMULATION OF A REVERSE-FLOW NOX-SCR REACTOR WITH SIDE-STREAM AMMONIA ADDITION

A heterogeneous model of a reverse-flow NOx-SCR (selective catalytic r eduction) reactor was developed to demonstrate ammonia trapping and ef ficient nitric oxide reduction with side-stream ammonia introduction. The flue gas containing nitric oxide is cycled between the reactor end s while the ammonia flux is distributed among one or more side streams . Similar to energy trapping, a pseudo-steady state is achieved after several how reversals when the spatiotemporal ammonia adsorbate profil es become invariant, displaying regenerative mass exchange and mass tr apping characteristics. The nitric oxide conversion occurs predominant ly at the trailing edge of the adsorbate front. Analogous to energy tr apping, more adsorbate is trapped at higher reversal frequencies. Howe ver, unlike energy trapping wherein increasing the power input increas es temperature-peak heights, increasing the gas-phase ammonia concentr ation broadens the adsorbate profile due to the equilibrium limitation s on ammonia adsorption. For stoichiometric amounts of ammonia introdu ced, sufficient adsorbate is trapped in the catalyst at a pseudo-stead y state to convert nitric oxide for tens of minutes. Thus, transient i ncreases in nitric oxide concentration can be easily accommodated with virtually complete nitric oxide conversion and little ammonia slippag e from the reactor. Periodic flow reversal with side-stream feed addit ion is thus an attractive concept for maximizing the utilization of re active species that adsorb strongly on catalysts. (C) 1998 Elsevier Sc ience Ltd. All rights reserved.