Spatially Resolved in Situ Study of Lean NOx Reduction in the NSR+SCR Sequential Configuration

Spatio-temporal features of lean NOx reduction in the NSR+SCR sequential configuration

 

Mengmeng Li, Vencon G. Easterling, Michael P. Harold*

Dept. of Chemical & Biomolecular Engineering, University of Houston, Houston, Texas

*corresponding author e-mail: mpharold@central.uh.edu

NOx storage and reduction (NSR) and selective catalytic reduction (SCR) are the two leading technologies utilized to achieve the stringent NOx emission limits for lean-burn gasoline and diesel engines. SCR catalysts utilize NH₃ as the reductant which requires a urea dosing system. An alternative approach that is attractive for light-duty vehicles involves locating the lean NOx trap (LNT) catalyst upstream of a SCR which enables the generation of NH₃ during the rich phase of periodic (lean-rich switching) operation [1]. Although previous studies have demonstrated the synergy of combining a LNT with a SCR, most of these studies have utilized H₂ and CO as the rich feed components while real diesel exhaust reductants are primarily hydrocarbons (HCs). Previous studies have shown that unconverted HCs from the LNT can also reduce NOx over SCR catalysts [2]. In order to reduce the volume of the more expensive LNT, ideally half the NOx should be converted to NH₃ to give the 1:1 NH₃:NOx ratio (ANR) that is most suitable for the downstream SCR. Thus it is of interest to determine the optimal LNT+SCR configuration and operating conditions using realistic hydrocarbon reductants.

In this study, we conduct bench flow reactor experiments involving monolith LNT+SCR catalysts to determine the conditions which approach ANR ~1 for CO and propylene as reductants. We also utilize spatially-resolved capillary inlet mass spectrometry (SpaciMS) to construct the spatial and temporal dependencies of reacting species concentrations and temperature spanning the two reactors. The experiments provide insight into the synergy between the two catalysts in terms of the NOx storage and reduction including: NH₃ generation, storage and consumption; supplemental reduction provided by the propylene; and nonisothermal effects from HC oxidation.

The experiments indicate that NOx reduction with propylene becomes comparable with CO when the feed temperature is above 300 °C; both reductants in fact can achieve >90% NOx cycle-average conversion across the LNT+SCR. Typical data is shown in Fig. 1 for an experiment comprising a 60 s lean, 5 s rich cycle using propylene as the reductant. An NH₃ yield of ~40% is achieved in the LNT, which is then consumed through reaction with NOx in the SCR. Over a range of temperatures, with increasing space velocity (GHSV), the NOx conversion expectedly decreases. The blue bar in Fig. 1 indicates the NH₃ yield. For example, at 400 °C and GHSV between135,000 h^-1 and 180,000 h^-1 the NOx conversion is ~50%, while the NH₃ yield is ~43% resulting in an ANR value approaching the desired value of 1. Although the NOx conversion in the LNT decreases at higher space velocities, the SCR is able to compensate due to the favorable NH₃ to NOx feed ratio. The optimal GHSV for this set of experimental conditions was determined to be between 135,000 h^-1 and 180,000 h^-1for the sequential LNT+SCR.

The spatial dependence of the NOx concentration was measured by SpaciMS to elucidate its reduction features. Fig. 2 shows a 3-D plot of NOx concentration as a function of time and position in the LNT+SCR catalyst system during the 65s combined cycle at 350 °C. The rich phase started at 60s and lasted 5s. Both LNT and SCR monoliths are 2 cm long; hence, the 0 cm position corresponds to the front face of the LNT catalyst (the front face of the SCR is at 2cm). All subsequent positions are in reference to the distance from the front face of the LNT catalyst. At the front face of the LNT, a small NOx spike or "puff" is apparent during the rich phase which is likely the result of stored NOx that is released due to the exotherm from the oxidation of the C₃H₆, which results in a ~90 °C temperature increase at the front face of LNT catalyst. Fig. 3 shows the reaction temperature along LNT+SCR catalyst system length as a function of time. It indicates that most of the C₃H₆ is consumed at the front portion of LNT catalyst which observed to be the highest temperature difference. The cycle-averaged temperature increased 20 °C along SCR catalyst. The decrease in this NOx peak along the length of the SCR section indicates that NH₃ breakthrough from the upstream LNT contributes to NOx reduction over SCR catalyst, especially at the front portion of SCR where most of NOx breakthrough from LNT during the lean phase was eliminated. It is interesting to note that during NOx reduction, formaldehyde was observed in the reactor system effluent by FTIR, which also reported by Kim et al. [2]. Fig. 4 compares the m/z 30 intensity profile at the reaction condition with (Fig. 4a) or without NO (Fig. 4b). Two different peaks were observed in both lean and early rich phase in Fig. 4(b) (without NO in the reaction conditions). The HCHO formation pathway over SCR catalyst was still ongoing.

Fig. 1. Cycle-averaged NOx conversion (red+ blue) and NH₃ yield (blue) over LNT catalyst LNT+SCR catalysts (Lean: 200 ppm NO, 6% O₂, bal. Ar. Rich: 500 ppm NO, 1% O₂, 6055 ppmC₃H₆, bal. Ar).

 

Fig. 2. Spatially-resolved profiles of NOx during cycling conditions at 350 °C. (Lean: 200 ppm NO, 6% O₂, 9% CO₂ and 7% H₂O, bal. Ar, Rich: 6055 ppm C₃H₆, 1% O₂, 9% CO₂ and 7% H₂O, GHSV: 90,000 h^-1).

 

Fig. 3. Spatially-resolved profiles of reaction temperature as a function of time during the cyclic reduction of NOx by C₃H₆ at 350 °C (Lean: 200 ppm NO, 6% O₂, 9% CO₂ and 7% H₂O, bal. Ar, Rich: 6055 ppm C₃H₆, 1% O₂, 9% CO₂ and 7% H₂O, bal. Ar, GHSV: 90,000 h^-1).

Fig. 4. Temporal profiles of m/z 30 a) with NO in lean phase (Lean: 200 ppm NO, 6% O₂, 9% CO₂ and 7% H₂O, Rich: 9300 ppm C₃H₆, 1% O₂, 9% CO₂ and 7% H₂O, bal. Ar, GHSV: 135,000 h^-1) and b) without NO in lean phase (Lean: 6% O₂, 9% CO₂ and 7% H₂O, Rich: 9300 ppm C₃H₆, 1% O₂, 9% CO₂ and  7% H₂O, bal. Ar, GHSV: 135,000 h^-1) over LNT+SCR catalyst at 350 °C.

 

References

1.      U.S. Patent 6176079

2.      M.Y. Kim, J. S. Choi, and M. Crocker, Catal. Today, 231 (2014) 90-98.

3.      R. D. Clayton, M. P. Harold, V. Balakotaiah and C. Z. Wan, Appl. Catal. B, 90 (2009) 662-676.