(216e) Rapidly Pulsed Reductants in Diesel NOx Reduction with Lean NOx Traps:  Spatial Distribution of Species

Lean NOx Trap (LNT) is a deNOx method using a NOx storage and reduction (NSR) catalyst that can be a desirable choice for NOx reduction on light duty diesel vehicles with smaller engines (< 2.0 L). A LNT used in a conventional way with long lean (60 s) â?? short rich (5 s) cycles obtains poor NOx conversions at high temperatures (T>400°C) and space velocities. Rapidly Pulsed Reductants (RPR) is a method for injecting hydrocarbons in rapid pulses ahead of a LNT which has shown the potential to achieve greater than 90% NOx conversion at temperatures as high as 600°C. This approach has recently been discussed by Toyota and named Di-Air [1]; it appears to be useful for smaller diesel vehicles that do not easily accommodate SCR technologies.

Four sets of parameters have been found important in RPR operation: 1- flow field and reductant mixing uniformity [2], 2- pulsing parameters, 3- the reductant type [2], and 4- catalyst composition, PGM type/loading, NOx storage material, and oxygen storage capacity (OSC). This parameter space has been explored using a synthetic bench reactor equipped with a novel high frequency injection and mixing system capable of pulsing reductants over the catalyst at up to 100 Hz.

The main findings from our previous studies on the effect of aforementioned parameters are; (a) The NOx conversion is maximized using nearly perfect plug flow pulses with minimal axial mixing [2], (b) The pulses of injected hydrocarbon need to be large enough in magnitude to generate a rich mixture during the pulse in order to achieve high NOx conversion. However, the cycle-averaged overall A/F ratio was always net lean. (c) The NOx conversion is dependent on the type of reductant at different operating temperatures [2]. (d) There exists an optimal operating condition, in terms of pulse amplitude and frequency that maximizes NOx conversion and minimize the fuel penalty associated the pulsed reductant.

In the current study, the spatial distribution of species along the catalyst have been explored using spatially resolved capillary inlet mass spectrometry (Spaci-MS) [3]. The experiments were conducted at near optimal operating conditions found from previous studies, with maximal NOx conversion and minimal byproduct (CO, N₂O, â?¦) formation. Ethylene was used as the main reductant on a typical Pt/Rh/BaO/Al₂O₃ LNT sample (no Ce). Experiments were conducted under reductant pulsing conditions to find the spatial distribution of the catalyst bed temperature (T/C) and the concentration of some of the important reactants and products, such as, C₂H₆, O₂, CO, CO₂, H₂O, H₂, NO, and NH₃ in the temperature range of 150°C to 600°C. Due to the limited response time of the instruments and high frequency pulsing approach, transient measurements were not possible of products and cycle-averaged measurements were performed. As an example, Fig.1 shows the cycle-averaged NO concentration vs temperature and axial location.

Figure 1 NO concentration vs temperature, measured at different axial locations of a LNT monolith with 25mm of length. Experimental conditions are: 30,000h^-1 space velocity, P = 2s, 15% pulse duty cycle, and λ_pulse = 0.73.

As can be seen, the length of catalyst necessary to achieve high or complete NOx conversion is dependent on the operating temperature regime. At 300°C significant NOx conversion is seen 2 mm into the catalyst and complete conversion occurs 18 mm into the catalyst. At 420°C NOx conversion is high only 2 mm into the catalyst and complete at 6 mm into the catalyst, about one fourth of its length. At 540°C we find a new regime, where NOx storage capacity becomes the limiting factor over much of the length of this catalyst.

It should be mentioned that, the Spaci measurements are prone to variation due to the restriction effects of the capillary inside the channel. We have observed considerable deviation (10-20%) of these measurements from the bulk concentration at the outlet of catalyst. Thus, while these data are not perfectly quantitative, they are very useful in determining where in the catalyst product distributions change.

As described in the caption of Fig. 1, the spatial distribution provides insight into the limiting factors and different operating regimes. In addition, it enables us to locate the formation and consumption of important reaction byproducts, such as NH₃, CO and H₂. For instance, it was observed that NH₃ was formed within the first few millimeters of the catalyst, and then consumed moving downstream. Furthermore, formation of CO and H₂ was observed, around 350-400°C as an indication of steam reforming, which occurred within the first 2-6mm of the catalyst. A simple balance showed that the generated CO was then consumed moving downstream, mainly by the water gas shift reaction, forming H₂ which was observed at the catalyst outlet.

In summary, Spaci-MS is a useful technique which provides spatially resolved concentration and temperature distributions to help better understand the overall reactivity data. In the context of the RPR process, we have employed this information for determining the governing factors at different operating regimes, as well as important reaction pathways.

References

[1]Bisaiji, Yuki, et al. "Development of Di-Air-A New Diesel deNOx System by Adsorbed Intermediate Reductants."

[2] Reihani, A; Corson, B; Hoard, JW; Fisher, GB; Smirnov, E; Roemer, D; Theis, J; Lambert, C. â??Rapidly Pulsed Reductants in Diesel NOx Reduction by Lean NOx Traps: Effects of Mixing Uniformity and Reductant Type.â?

[3] Choi, Jae-Soon, William P. Partridge, and C. Stuart Daw. "Spatially resolved in situ measurements of transient species breakthrough during cyclic, low-temperature regeneration of a monolithic Pt/K/Al 2 O 3 NO x storage-reduction catalyst."