Rapidly Pulsed Reductants for Diesel NO_x Reduction With Lean NO_x Traps: Comparison of Alkanes and Alkenes as the Reducing Agent

Lean NOx Traps (LNTs) are often used to reduce NOx on smaller diesel passenger cars where urea-based Selective Catalytic Reduction (SCR) systems may be difficult to package. However, the performance of LNTs at temperatures above 400°C needs to be improved. The use of Rapidly Pulsed Reductants (RPR) is a process in which hydrocarbons are injected in rapid pulses ahead of the LNT in order to improve its performance at higher temperatures and space velocities. This approach was developed by Toyota and was originally called Di-Air (Diesel NOx aftertreatment by Adsorbed Intermediate Reductants) [1].

There is a vast parameter space that needs to be explored in order to maximize the NOx conversion at high temperatures and flow rates while minimizing the fuel penalty associated with the hydrocarbon injections. Four parameters were identified as important for RPR operation: (1) the flow field and reductant mixing uniformity; (2) the pulsing parameters including the pulse frequency, duty cycle, and rich magnitude; (3) the reductant type; and (4) the catalyst composition, including the type and loading of precious metal, the type and loading of NOx storage material, and the amount of oxygen storage capacity (OSC). In this study, RPR performance was assessed between 150°C and 650°C with several reductants including dodecane, propane, ethylene, propylene, H₂, and CO. A novel injection and mixer system was designed that allowed for the investigation of previously unexplored areas of high frequency injections up to f = 100Hz.

Under RPR conditions, H₂, CO, dodecane, and C₂H₄ provided approximately 80% NOx conversion at 500°C, but at 600°C the conversions were significantly lower, ranging from 40 to 55%. The NO_x conversion with C₃H₈ was low across the entire temperature range, with a maximum conversion of 25% near 300°C and essentially no conversion at 600°C. In contrast, C₃H₆ provided greater than 90% NOx conversion over a broad range of temperature between 280°C and 630°C. Among the hydrocarbons, this suggested that the high temperature NO_x conversion with RPR improves as the reactivity of the hydrocarbon increases.