The Effect of NO₂/NO_x Ratio on the Performance of a SCR Downstream of a SCR Catalyst on a DPF

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_x) emissions standards being set by the Environmental Protection Agency (EPA) and the California Air Resources Board (CARB). One such system consisting of a SCRF^® with a downstream SCR was used in this research to determine the system NO_x 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_x 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 NO₂/NO_x ratio of 0.5 was found to be optimum for maximizing the NO_x reduction and minimizing NH₃ slip for the SCR for a given value of ammonia-to-NO_x ratio (ANR). The SCRF^® + SCR system was simulated using the 2D SCR-F + 1D SCR system model. For all the test conditions, the NO₂/NO_x ratio downstream of the SCRF^® was found to be 0 due to NO₂ consumption by the NO₂ assisted particulate matter (PM) oxidation and the SCR reactions in the SCRF^®. Due to this low NO₂/NO_x ratio, the NO_x 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 NH₃ slip due to lower utilization by the SCR reactions was observed from the SCR. Improved NO₂/NO_x ratio at the SCRF^® inlet results in NO₂ slip at the SCRF^® outlet, which then leads to better NO_x reduction performance of this system.