Experimental and Modeling Studies of Cycle Frequency, Reductant Type and Non-Isothermal Effect on the Performance of a Lean NOx Trap
Allen Wei-Lun Ting, University of Houston
Experimental and Modeling Studies of Cycle Frequency, Reductant Type and Non-Isothermal Effect on the Performance of a Lean NOx Trap
Allen Wei-Lun Ting, Michael P. Harold and Vemuri Balakotaiah
Department of Chemical & Biomolecular Engineering, University of Houston, Houston, TX 77204, USA
Abstract
The “Di-Air” system proposed by Toyota researchers shows an overall increase in NOx conversion when using propene as the reductant with a short injection interval [1]. The HC-intermediate pathway from which the short-lived HC intermediates generated is thought to play important role in this novel system. In order to clarify this mechanism, a combined experimental and modeling study of fast cycling NOx storage and reduction for emissions control of lean burn gasoline and diesel vehicles is conducted. LNT (lean NOx trap) catalysts containing Pt/BaO/CeO2/Al2O3 is used in the monolithic reactor experiments. A 1-D two phase model that includes washcoat diffusional effects is used to simulate the cyclic experimental results and to provide more details, such as the spatio-temporal concentration, site saturation, and temperature profiles.
By feeding H2 or C3H6 as the reductant agents with the same reductivity in stoichiometry, the traditional H2-NH3 NOx regeneration pathway and the HC NOx regeneration pathway can be compared systematically. Thermal effect resulting from different molecular diffusivity between H2 and C3H6 plays an important role under aerobic slow cycling condition. This is observed by measuring the temperature in the front and last quarter of the monolith reactor. Higher temperature rises reduce the NOx storage capacity during the early lean period and results in a 10 to 15 % lower NOx conversion under slow cycling when using H2. Fast cycling exhibits a more efficient usage of storage sites and a near-steady-state axial temperature profile comparing to slow cycling [2]. Therefore, the enhancement in the NOx conversion of 5-10 % under fast cycling may contributed from the HC-intermediate mechanism.
In addition to the thermal effect, steam reforming of C3H6 may also play an important role in NOx reduction pathways. Gas reforming selectivity of H2 from C3H6 can may be very high at temperatures higher than 400 oC with the presence of H2O. This indicates that NOx reduction pathways are complex under realistic environment that H2O and O2 cannot be excluded. Therefore, experiments under anaerobic condition with no H2O in feed were conducted to minimize the formation of H2. Results shows that NOx conversions are very similar at temperatures higher than C3H6 ignition temperature. Under fast cycling, the HC NOx reduction pathway only reduce 5 % more NOx, while the fast cycling results in at least 20% NOx increase regardless of the reducing species.
This work concludes that the NOx conversion enhancement of Di-air system mainly results from the fast cycling, and HC-intermediate mechanism is relatively a minor factor.
Reference:
[1] Y. Bisaiji, K. Yoshida, M. Inoue, N. Takagi, T. Fukuma, Development of Di-Air – A New Diesel deNOx System by Adsorbed Intermediate Reductants, SAE Int. J. Fuels Lubr. 5 (2012) 380–388.
[2] Ting et al., Fast Cycling in a non-isothermal monolithic lean NOx trap using H2 as reductant: Experiments and Modeling, Chemical Engineering Journam, 326 (2017) 419-435.