Rapidly Pulsed Reductants in Diesel NOx Reduction with Lean NOx Traps

Amin  Reihani, University of Michigan

Rapidly Pulsed Reductants in Diesel NOx Reduction with Lean NOx Traps
Amin Reihani*, Galen B. Fisher, John Hoard
University of Michigan
Joseph R. Theis, Christine K. Lambert, Evgeny Smirnov, Dirk Roemer
Ford Motor Company
The use of Rapidly Pulsed Reductants (RPR) provides a method for injecting hydrocarbons in rapid pulses ahead of a lean NOx trap (LNT) with the goal of achieving greater than 90% NOx conversion at temperatures as high as 600℃. This approach has recently been discussed by Toyota and named Di-Air. It may be useful for diesel vehicles with smaller engines that do not conveniently accommodate SCR technologies. A further goal of RPR is to reduce the fuel penalty associated with the hydrocarbon injections used to regenerate LNTs. In our work we have also found RPR to be beneficial at temperatures lower than 250-300°C but the focus of this study has been on understanding this approach in the high temperature range of 450°C to 600°C.

In general, four sets of parameters have been found important in RPR operation: 1- flow field and reductant mixing uniformity, 2- pulsing parameters including pulse frequency, amplitude and duty cycle, 3- the reductant type, and 4- catalyst composition, including precious metal type/loading, NOx storage material, and oxygen storage capacity (OSC). To study the pulsing parameters, a novel injection system was designed that allows for the investigation of previously unexplored areas of high frequencies (up to f = 50Hz) and reduced pulse durations as low as a millisecond [1, 2]. It was found that there is an optimal pulsing frequency for each flow condition that produces the highest NOx conversion at a constant fuel penalty. The optimal pulsing frequency, on the order of 1 Hz, is dependent on a number of parameters including; flow velocity, partial pressure of injected hydrocarbons, operating temperature, and type of reductant.

The effect of reductant type was investigated in a previous study [2], which were focused mainly on light gaseous hydrocarbons as reducing agents. In the current study, longer hydrocarbon chain molecules, e.g. dodecane, were used to better represent diesel fuel. Liquid hydrocarbons were vaporized and injected into simulated exhaust gases to generate rich pulses on a research LNT catalyst. The performance of RPR with liquid hydrocarbons as the reducing agent was tested under varying operating conditions, and a comparison was made with the results obtained from benchmark reductants, i.e. propane, ethylene, propylene and carbon monoxide. It was found that dodecane, and saturated hydrocarbons in general, are not as effective as alkenes for RPR operation. The alkanes studied resulted in lower NOx conversions (maximum of 65% at 600°C) as compared to alkenes, which in some cases resulted in 90+% NOx conversion at a 600°C flow temperature. A fuller examination of various fuel components will amplify this study.

[1] Reihani, A., Fisher, G.B., Hoard, J.W., Theis, J., Lambert, C., and Smirnov E. “Numerical and Experimental Investigation of Mixing Quality for Pulsed Mixing Flows with Application”, CLEERS Workshop, April 27-29, 2015
[2] Reihani, A., Corson, B., Hoard, J.W., Fisher, G.B., Smirnov, E., Roemer, D., Theis, J. and Lambert, C., 2016. “Rapidly Pulsed Reductants in Diesel NOx Reduction by Lean NOx Traps: Effects of Mixing Uniformity and Reductant Type”. SAE International Journal of Engines, 9 (2016-01-0956).