NOx Trapping

While the SCR-urea system above is of use on large diesel engines that operate xmder fuel lean conditions it faces problems when attempts are made to apply it to smaller vehicles that operate in a more urban environment and emit a range of NOx concentrations. 

These problems relate to the higher relative weight of the urea-SCR system (resulting in a fuel penalty following installation) as well as to the problems with delivering a continuously accurate dose of urea to the catalytic material (to ensure that all NOx is converted). 

Overdosing urea would result in the emission of NH3(g) which itself is a primary pollutant but which also will oxidise over time in the atmosphere to generate NO (resulting in an expensive way to emit NOx), while under dosing will result in insufficient NHj to reduce NOx present in the exhaust stream. 

These exhausts might typically contain 5-10% O, 500 ppm NOx and VOC and 1% CO. Stoichiometrically, there are sufficient reductants present to reduce the NOx to N, provided the reactions could be carried out with extremely high selectivity, i.e. each VOC molecule would react only to reduce NO and not O. This is a fundamental difference to what is required 

The Pt catalysts were active and selective at lower temperatures (as would be expected at higher temperatures combustion predominated) but they faced a second problem in that they were not product selective. While they were selective are reacting various VOC with NO in preference to with O at relatively low temperatures a major product of the reaction was N O rather than N.  

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A recent area of development is to use these NRS catalysts for the combined treatment of NOx and particulates. The combination of the NOx trap and of a particulate filter depends on various factors (1) the selection of the best suited filter technology... 

Lesage, T.,Verrier, C., Bazin, P. et al. (2004) Comparison between a Pt-Rh/Ba/Al203 and a newly formulated NOx-trap catalysts under alternate lean-rich flows, Top. Catal., 30, 31. 

Mahzoul, H., Brilhac, J.F. and Gilot, P. (1999) Experimental and Mechanistic Study of NOx Adsorption over NOx Trap Catalysts, Appl. Catal. B Environ., 20, 47. 

Another important catalytic technology for removal of NOx from lean-burn engine exhausts involves NOx storage reduction catalysis, or the lean-NOx trap . In the lean-NOx trap, the formation of N02 by NO oxidation is followed by the formation of a nitrate when the N02 is adsorbed onto the catalyst surface. Thus, the N02 is stored on the catalyst surface in the nitrate form and subsequently decomposed to N2. Lean NOx trap catalysts have shown serious deactivation in the presence of SOx because, under oxygen-rich conditions, SO, adsorbs more strongly on N02 adsorption sites than N02, and the adsorbed SOx does not desorb altogether even under fuel-rich conditions. The presence of S03 leads to the formation of sulfuric acid and sulfates that increase the particulates in the exhaust and poison the active sites on the catalyst. Furthermore, catalytic oxidation of NO to N02 can be operated in a limited temperature range. Oxidation of NO to N02 by a conventional Pt-based catalyst has a maximum at about 250°C and loses its efficiency below about 100°C and above about 400°C. 

NSR is very attractive method for NOx removal by storing NOx under lean conditions and then reducing the stored NOx to N2 under rich excursions by engine operation this technology is also referred as a lean NOx trap. In this chapter, NSR catalyst and the mechanism for NOx reduction is described. 

Lean NOx trap (LNT), in which NOx is stored on the catalyst and then the catalyst regenerated intermittently using engine control. 

Clearly, when modelling an LNT it is important to include the most important processes occurring in this relatively complex catalyst system. Kinetic and experimental studies of lean NOx trap catalysts, including those describing chemical principles, have been published previously (Brogan et al., 1995 Dou and Bailey, 1998 Fekete et al, 1997 Miyoshi et al., 1995 Takami et al., 1995). These processes can be summarised as follows ... 

From the reaction-kinetic modeling point of view, the NSRC, sometimes called lean NOx trap (LNT) or NOx adsorber, is the most complex of the currently used automobile exhaust converters. A variety of different physical and chemical processes and the number of gas and surface components participating in typical periodic lean/rich operation form a large and closely linked system. 

Sharma et al. (2005) developed a ID two-phase model for the analysis of periodic NOx storage and reduction by C3H6 in a catalytic monolith, based on a simplified kinetic scheme. They focused on the evaluation of temperature and reaction fronts along the monolith and their effect on NOx conversion. Kim et al. (2003) proposed a phenomenological control-oriented lean NOx trap model. 

A lean NOx trap (LNT) (or NOx adsorber) is similar to a three-way catalyst. However, part of the catalyst contains some sorbent components which can store NOx. Unlike catalysts, which involve continuous conversion, a trap stores NO and (primarily) N02 under lean exhaust conditions and releases and catalytically reduces them to nitrogen under rich conditions. The shift from lean to rich combustion, and vice versa, is achieved by a dedicated fuel control strategy. Typical sorbents include barium and rare earth metals (e.g. yttrium). An LNT does not require a separate reagent (urea) for NOx reduction and hence has an advantage over SCR. However, the urea infrastructure has now developed in Europe and USA, and SCR has become the system of choice for diesel vehicles because of its easier control and better long-term performance compared with LNT. NOx adsorbers have, however, found application in GDI engines where lower NOx-reduction efficiencies are required, and the switch between the lean and rich modes for regeneration is easier to achieve. 

The SCR of NOx by NH3 is the best control technology but a new breakthrough would be achieved in power plants by the SCR of NOx using methane as reductant. Regarding deNOx from mobile sources, new concepts are appearing, and NOx trap and plasma-assisted catalytic reduction seem promising. 

In the most direct application, the TWC is placed upstream from the NOx trap, providing for both the usual means of emissions control under start-up and stoichiometric conditions and a cooler environment for the trap, which has a relatively narrow, low-temperature window of efficient operation under lean conditions. If the TWC stores oxygen, the shift from lean to stoichiometric conditions, initiating the release of NOx h m the trap, will precede the breakthrough of reductants, allowing some of the NOx to escape. Limiting the OSC of the TWC mitigates this effect, but it also compromises TWC performance. 

Poisoning of the NOx trap by fuel sulfur, which as a relatively stable sulfate impedes the formation of the nitrate, presents another problem, but in this case, oxygen storage may be useful. The accumulation of sulfates can be diminished... 

Figure 16 Effect of regeneration conditions (reduction potential) on the performance of an experimental NOx trap.

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