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.