The Boosting Action of Ammonium Nitrate

6 Feeding Nitrates The Enhanced SCR Reaction 9.6.1 The Boosting Action of Ammonium Nitrate 

The role of nitrates as key intermediates in the Fast SCR mechanism has been fully confirmed in recent years by the discovery of the so-caUed Enhanced SCR reaction [3, 31], 

Reaction (9.12) involves the selective catalytic reduction of NO by its reaction with ammonia and with nitrate species, supplied, e.g., in the form of aqueous solution of ammonium nitrate sprayed into the gaseous feed stream to the SCR catalyst. Reaction (9.12) results in very high DeNO efficiencies at low temperatures, similar to those of Fast SCR, even though no NO2 is fed to the SCR catalyst. Accordingly, it can in principle replace reaction (9.6) in boosting the DeNOx 

It is clearly apparent from Fig. 9.10 that addition of ammonium nitrate to NO-NH3-O2 dramatically increased the low temperature NO reduction activity over Fe-ZSM-5. Furthermore, the added NH4NO3 itself was totally and selectively converted according to reaction (9.12). The substantial enhancement of the NOx removal eflhciency due to NH4NO3 addition as compared to the Standard SCR was confirmed also at higher temperatures, though it progressively decreased with growing temperature. 

The Enhanced SCR chemistry was further examined in a similar transient experiment at 205 °C (GHSV = 36,000 h ), wherein the Fe-ZSM-5 catalyst was first exposed to a feed containing 500 ppm NO, 750 ppm NH3, 0 % O2 in N2, to which an aqueous solution of nitric acid, corresponding to feeding 250 ppm HNO3 and 1 % H2O, was added in a second stage. In this case a steady-state conversion of both NO and NH3 close to 90 % was observed, consistent with the stoichiometry of 

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