Fast SCR

The fast SCR reaction , which involves both NO and N02, exhibits a reaction rate at least 10 times higher than that of the well-known standard SCR reaction with pure NO ... 

Another possibility for the plasma device is the generation of N02 for the enhancement of NH3-SCR at low temperature, (the so-called fast SCR reaction) which occurs if there is a 1 1 N0 N02 stoichiometry over V-Ti02 type catalysts and with variable stoichiometry over Fe-beta zeolite. Being able to switch on and tune N02 production over a limited temperature range will help to avoid N02 slip issues, that can be an issue for oxidation catalysts. Also, if tuned correctly, plasma can do the NO oxidation without in turn doing S02 oxidation and so generate N02 without making sulphates (and associated particulates). 

The most effective way to improve the SCR performance is to increase the rate of reaction itself by the help of N02 [28,36,37], When equimolar amounts of NO and N02 are used, a very high DeNO, is observed ( fast-SCR reaction), as the very potent oxidizing agent N02 reoxidizes the catalyst much faster than oxygen [38] ... 

The decline of the DeNO. curve for N02 fractions above 50% is much stronger than the incline below 50% due to the different reaction rates of standard- and N02-SCR. The latter is much slower than the fast-SCR reaction and even slower than the standard-SCR reaction. The promoting effect of N02 levels off above 350°C, because the rate constants of standard-, fast- and N02-SCR reactions are converging at higher temperatures. 

The N0/N02-SCR chemistry on a typical V205AV03—TiOz catalyst was revealed by Koebel et al. and is summarized in the reaction scheme depicted in Figure 9.12 [32,41], Please note that the sum of the reactions in Figure 9.12 results in the reaction equation of the fast-SCR reaction. 

Figure 9.26. N20 formation under fast-SCR conditions. ( ) fresh, and ( ) wet aged Fe-ZSM5, (A) fresh Cu-ZSM5, (.) V205/W03-Ti02.

The main reactions, which have to be considered on SCR catalysts, are the standard-SCR, fast-SCR, and the N02-SCR reactions, beside the ammonia oxidation and the formation of N20. The fast-SCR reaction is promoted by N02 in the feed that can be generated from NO in a pre-oxidation catalyst. However, the right dimensioning of the oxidation catalyst is critical in order to prevent the production of an excess of hazardous N02. This problem is further aggravated if a continuous regenerating DPF is installed in front of the SCR system, as part of the N02 produced by the oxidation catalyst is always consumed in the filter for soot oxidation. 

Ciardelli, C., Nova, I., Tronconi, E., et al. (2004) A Nitrate Route for the Low Temperature Fast SCR Reaction over a y205-W03/Ti02 Commercial Catalyst, Chem. Commun. 2718. 

The SCR technology is also considered for the control of NO emission in diesel vehicles. Here the SCR catalyst is typically placed after the diesel oxidation catalyst (DOC), which is used to oxidize CO and UHCs and to convert part of the NO to NO2. In this way, the SCR catalysts can take advantage of the fast SCR reaction to enhance significantly the de-NO efficiency at low temperature (Figure 13.4). The fast SCR reaction is based on the following stoichiometry ... 

Therefore, there is a strong motivation to develop a dynamic model of the SCR monolithic reactor suitable for extended temperature operation and to study the fast SCR reaction in view of future possible applications. In the following, we will focus on these two issues. 

Also, the MR kinetics provided a much better description than the modified ER kinetics of fast SCR transients originated by high-frequency NHj feed pulses in a stream of 1000 ppm NO, 2% v/v O2 and 1% v/v H2O and similar to those associated with the operation of SCR after treatment devices for vehicles [56]. Indeed, the MR model is definitively more chemically consistent than the modified ER model in view of the redox character of the standard SCR reaction. 

To reduce the emissions during cold start of the engine and extended operation at low loads in vehicles, it has been proposed to use the so-called fast SCR reaction in which NH3 reacts with NO and NO2 according to... 

The study of the mechanism of the fast SCR over V-W-Ti-0 catalysts was addressed first by Koebel and co-workers [65-68]. They suggested that (i) the reoxidation ofthe catalyst is rate determining at low temperature in the redox cycle of standard SCR catalyst, (ii) NO2 reoxidizes the catalyst faster than O2 the NO2-enhanced reoxidation of the catalyst was demonstrated by in situ Raman experiments, (hi) the reaction occurs via the nitrosamide intermediate in both standard and fast SCR and (iv) ammonium nitrate is considered an undesired side-product. 

The mechanism of fast SCR over a zeolite-based catalyst has also been addressed by Sachtler and co-workers using an IR technique [69, 70]. They concluded that nitrogen is produced through fast decomposition of ammonium nitrite (the hydrated form of nitrosamide), vhich is formed from equimolar NO/NO2 feeds via N2O3 and its reaction vith water and ammonia ... 

Ammonium nitrite is unstable above 100 °C and the sum of reactions (13.25) and (13.26) results in the fast SCR reaction (13.24). This reaction scheme can explain the optimal 1 1 NO NO2 feed ratio of the fast SCR on the basis of well-known chemistry however, it cannot explain all of the several products (N2, NH4NO3, N2O) observed in experiments covering the full range of NO NO feed ratios [71]. 

Tronconi and co-workers, through an extensive study of the reactivity of NH3-NO/ NO2 mixtures with different NO NO2 ratios over V-W-Ti-O SCR catalysts, proposed a novel mechanism for the fast SCR reaction that has been validated step by step by dedicated experiments [71-74]. 

The stoichiometry of the fast SCR reaction (Equation 13.24), is recovered once the overall readion summarizing the chemistry of the NO2-NH3 system is coupled with that describing the addition of NO to this reacting system. The proposed readion scheme accounts for the optimal equimolar NO NO2 feed ratio and also for the... 

It is worth mentioning that the same chemistry is involved when surface nitrite and nitrate are considered instead of gas-phase nitrous and nitric add. Indeed, surface nitrate has been suggested to take part in the reoxidation of the reduced catalyst sites, thus accounting for the higher rate of the fast SCR reaction compared with the standard SCR reaction [74]. 

The advances in the knowledge of fast SCR and in its implementation may also offer a challenging opportunity for the use of urea-ammonia SCR at low temperatures in some stationary applications. 

In order to develop a suitable kinetic model of the full NH3-N0-N02/02 SCR reacting system, first the active reactions depending on N0/N02 feed ratio and temperature were identified then a dedicated study was performed aimed at clarifying the catalytic mechanism of the fast SCR reaction on the basis of such a reaction chemistry a detailed kinetic model was eventually derived, whose intrinsic rate parameters were estimated from global non-linear regression of a large set of experimental transient runs. 

The depletion of NH4N03 is responsible for the subsequent drop of NO conversion observed after t — 4,150 s, and indeed at the end of the experiment steady-state concentrations of reactants and products were eventually in agreement with the occurrence of the fast SCR reaction only. It appears thus that the fast SCR reaction was proceeding only before t — 4,000 s while, as it became limited by the N02 feed concentration, the reaction (—R9 + R10,... 

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