Removal of NOx from Diesel Exhausts: The New “Enhanced NH3-SCR” Reaction

Pio Forzatti; Isabella Nova; Enrico Tronconi

doi:10.4271/2010-01-1181

Ammonia/urea-SCR is a mature technology, applied worldwide for the control of NOx emissions in combustion exhausts from thermal power plants, cogeneration units, incinerators and stationary diesel engines and more recently also from mobile sources. However a greater DeNOx activity at low temperatures is desired in order to meet more and more restrictive legislations.

In this paper we report transient and steady state data collected over commercial Fe-ZSM-5 and V₂O₅-WO₃/TiO₂ catalysts showing high NOx reduction efficiencies in the 200 - 350°C T-range when NO and ammonia react with nitrates, e.g., in the form of an aqueous solution of ammonium nitrate. Under such conditions a new reaction occurs, the so-called "Enhanced SCR" reaction, 2 NH₃ + 2 NO + NH₄NO₃ → 3 N₂ + 5 H₂O.

Under the conditions of our experiments the new reaction resulted in total conversion both of the nitrates additives and of ammonia, when they were fed in sub-stoichiometric amounts: thus, the "Enhanced SCR" chemistry is also compatible with limitations on the ammonia slip.

These results open new pathways for low-T NOx reduction, wherein the amount of boosting agent added to the SCR reactor feed is no longer determined by the activity of the upstream oxidation catalyst, like in the case of NO₂ for the well known Fast SCR reaction, but can be optimally controlled by dosing an aqueous solution of the oxidizing additive (nitrates), possibly by means of the same injection systems currently employed for dosage of ammonia/urea solutions.

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Removal of NOx from Diesel Exhausts: The New “Enhanced NH3-SCR” Reaction 2010-01-1181

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