The Effect of NO<sub>2</sub>/NO<sub>x</sub> Ratio on the Performance of a SCR Downstream of a SCR Catalyst on a DPF

Abstract

<div>Different aftertreatment systems consisting of a combination of selective catalytic reduction (SCR) and SCR catalyst on a diesel particulate filter (DPF) (SCR-F) are being developed to meet future oxides of nitrogen (NO<sub>x</sub>) emissions standards being set by the Environmental Protection Agency (EPA) and the California Air Resources Board (CARB). One such system consisting of a SCRF<sup>®</sup> with a downstream SCR was used in this research to determine the system NO<sub>x</sub> reduction performance using experimental data from a 2013 Cummins 6.7L ISB (Interact System B) diesel engine and model data. The contribution of the three SCR reactions on NO<sub>x</sub> reduction performance in the SCR-F and the SCR was determined based on the modeling work. The performance of a SCR was simulated with a one-dimensional (1D) SCR model. A NO<sub>2</sub>/NO<sub>x</sub> ratio of 0.5 was found to be optimum for maximizing the NO<sub>x</sub> reduction and minimizing NH<sub>3</sub> slip for the SCR for a given value of ammonia-to-NO<sub>x</sub> ratio (ANR). The SCRF<sup>®</sup> + SCR system was simulated using the 2D SCR-F + 1D SCR system model. For all the test conditions, the NO<sub>2</sub>/NO<sub>x</sub> ratio downstream of the SCRF<sup>®</sup> was found to be 0 due to NO<sub>2</sub> consumption by the NO<sub>2</sub> assisted particulate matter (PM) oxidation and the SCR reactions in the SCRF<sup>®</sup>. Due to this low NO<sub>2</sub>/NO<sub>x</sub> ratio, the NO<sub>x</sub> conversion performance of the downstream SCR was limited to a maximum of 70% and the system performance to a maximum of 97%. The low SCR performance is due to low fast SCR (<10%) and high standard SCR reaction (>85%) rates in the downstream SCR. Also, high NH<sub>3</sub> slip due to lower utilization by the SCR reactions was observed from the SCR. Improved NO<sub>2</sub>/NO<sub>x</sub> ratio at the SCRF<sup>®</sup> inlet results in NO<sub>2</sub> slip at the SCRF<sup>®</sup> outlet, which then leads to better NO<sub>x</sub> reduction performance of this system.</div>