Dual-bed catalytic system for NOx–N2O removal: a practical application for lean-burn deNOx HC-SCR

A dual-bed catalytic system has been developed by which NOx and N2O are successively removed from flue gases, simulating lean-burn engine conditions. NOx is removed in the first stage by selective catalytic reduction with propene over a Pt supported on activated carbon catalyst. The second bed decomposes the N2O formed in the first bed into N2 and O2. Catalysts tested in this second stage comprise ex-Co-Rh,Al-HTlc and ex-Co,Pd-La,Al-HTlc mixed oxides derived from hydrotalcite-like compounds and ion-exchanged Fe-ZSM-5 and Pd-ZSM-5 zeolites. Variables studied for the most promising deN2O catalysts are the effect of the pretreatment, the sodium content of the ex-HTlc-catalysts, the presence of inhibitors in the feed stream, the addition of hydrocarbons to enhance the N2O reduction in the second bed, and both time-on-stream and thermal stability of the catalytic system. Ex-Co-Rh,Al-HTlc proved to be the most active and stable catalyst, followed by Fe-ZSM-5, even when propene was added as reducing agent, which enhanced the conversion over the latter catalyst only. In an optimal configuration, at a temperature of 475 and 700 K in the first and second stage, respectively, molar conversions of 90 and 100% for NOx and N2O were achieved in a stable operation during 50 h.