[CCoE Notice] PhD Defense: Spatio-temporal Features of Combined NOx Storage and Reduction and Selective Catalytic Reduction

[Grayson, Audrey A]

PhD DEFENSE STUDENT: Mengmeng Li
DATE: Friday,June 2, 2017
TIME: 2:00 PM
PLACE:  Electrical & Computer Engineering Conference Room (N 328)
DISSERTATION CHAIR: Dr. Michael P. Harold
________________________________
TITLE:
Spatio-temporal Features of Combined NOx Storage and Reduction and Selective Catalytic Reduction

Emerging more stringent vehicle emissions rules for non-methane organic gases (NMOG) plus NOx and demanding improvement of vehicle fuel economy have increased the application of lean-burn engine technology. Therefore, the research and development of the commercial deNOx technology, i.e. NOx storage and reduction (NSR, also referred as LNT), selective catalytic reduction (SCR) and passive SCR system (synergy of LNT and SCR), has been promoted. This dissertation focuses on understanding NOx reduction and NH3 generation for the standalone LNT and a passive SCR system spanning kinetics and reactor performance studies. Systematic bench reactor experiments were conducted to study the impact of catalyst composition, reductant agents, and operating strategies. Furthermore, a global kinetic model combined with a low-dimensional monolithic reactor model was developed aiming to optimize the design and operation strategies of the similar three-way NOx storage catalyst (TWNSC).
Starting with performance evaluation of model LNT, SCR, and LNT+SCR catalysts, simulated exhaust gas containing C3H6 or CO reductant was used over a range of feed concentrations and temperatures, and gas hourly space velocities (GHSV). Spatially-resolved mass spectrometry (SpaciMS) was applied to construct spatio-temporal concentration and temperature profiles spanning LNT+SCR system. The spatial profiles are useful in pinpointing the LNT length that gives a product with NH3/NOx = 1 stoichiometry beneficial for promoting NOx reduction in the SCR. The combination of an overall GHSV = 135k h-1 and Tf = 350 oC generates a SCR feed from the LNT that results in the highest incremental cycle-averaged NOx conversion in the SCR (~30%) and high cycle-averaged conversions of NOx (~90%) and C3H6 (~80%). The temperature measurements reveal a large exotherm caused by the C3H6 oxidation manifested as a propagating temperature front.
The impact of reductant type, specifically carbon-free (H2), olefin (C3H6) and alkane (C3H8), in rapid lean-rich cycling over a LNT catalyst was investigated over a range of feed temperatures and cycle times. The NOx conversion is enhanced at elevated temperatures (Tf > 350 oC) independent of reductant type by decreasing the cycle time. While the opposite trend was observed at intermediate temperatures (Tf = 250 – 325 oC) with C3H8 attributed to kinetic limitations of the C3H8 dehydrogenation. The contrasting features obtained with H2, C3H6, and C3H8 during cyclic operation and steady-state in-situ diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) help provide a deeper understanding of the underlying mechanism.
Finally, a systematic kinetic study was carried out to develop the global kinetic model to capture the NOx reduction performance based on the parallel experimental study conducted by Malamis et al. [1]. The simulation of the cyclic condition has well captured the cycle-averaged NOx conversion and NH3 selectivity, the transient species concentrations and temperature profiles over a wide range of total cycle time (10s - 200s) at 50% duty cycle rich.
Reference
[1]      S. A. Malamis, M. Li, W. S. Epling, M. P. Harold, C. Dimaggio, and K. Premchan, “Steady State and Lean-Rich Cycling Characterization of a Three-Way NOX Storage Catalyst (TWNSC).” . In preparation, 2017.

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