Abstract
We have measured NOx conversions and N2O productions over Fe-BEA and Cu-SAPO catalysts and over their sequential arrangements under Enhanced SCR conditions, resulting from the addition of an aqueous solution of ammonium nitrate (AN) to the typical Standard SCR feed stream, and we have compared them to those observed under Standard and Fast SCR conditions. The expected strong enhancement of the poor low temperature activity of the Fe-BEA catalyst was confirmed: both NH3 and NOx conversions and N2O formations similar to those of the Fast SCR reaction were achieved when cofeeding ammonium nitrate. On the other hand, the Cu-SAPO efficiency was drastically decreased by the addition of AN at low temperatures, possibly due to trapping of the ammonium nitrate salt within the SAPO zeolite, characterized by smaller pores than those of the BEA zeolite. The Cu-SAPO performances were recovered only at T > 250 °C with a huge release of N2O due to the thermal decomposition of AN. The combined system with the Fe-zeolite sample placed upstream of the Cu-zeolite also exhibited outstanding low temperature deNOx performances, with even lower N2O production than over the Fe-zeolite only at the same Enhanced SCR (E-SCR) conditions.
Highlights
The current well-established solution for the NOx emission control on Diesel and/or natural gas lean-burn engines is represented by the NH3 selective catalytic reduction (NH3 SCR) [1, 2]
We demonstrated that the sequential arrangement of the two Fe- and Cu- monolith catalysts allowed to overcome the issues connected to the use of only one of these metal-promoted zeolite catalysts under Standard SCR conditions, namely the poor low-T deNOx activity typical of Fe-zeolite catalysts and the lower selectivity in the high-T range of Cu-zeolites
The cofeed of ammonium nitrate according to the Enhanced SCR reaction was demonstrated to be an effective method to improve the typically poor low-T activity of NH3SCR Fe-zeolite catalysts
Summary
The current well-established solution for the NOx emission control on Diesel and/or natural gas lean-burn engines is represented by the NH3 selective catalytic reduction (NH3 SCR) [1, 2]. Ricardo UK Ltd, Shoreham Technical Centre, Shoreham-by-Sea BN43 5FG, UK producing harmless N2 and H2O, according to the main SCR reactions. One big challenge is the improvement of the efficiency when the temperatures are too low for the SCR catalysts to be active and for the urea to effectively decompose. The development of more efficient Diesel engines is leading to operating conditions with high contents of oxygen. Such increasingly efficient engines produce higher amounts of NOx as well as exhaust gases with increasingly lower temperatures, aggravating the task of the SCR converter. Since the NOx gases, produced during the combustion, are composed mainly by NO (95%), the Standard SCR reaction (1) is the main reaction involved in the NOx
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