The effect of NO2/NOx ratio on the performance of a SCR downstream of a SCR catalyst on a DPF

Venkata Rajesh  Chundru, Michigan Technological University

Different after treatment systems consisting of a combination of selective catalytic reduction (SCR) and SCR catalyst on a DPF (SCR-F) are being developed to meet future NOx emissions standards being set by the Environmental Protection Agency (EPA) and the California Air Resources Board (CARB). One such system consisting of an SCRF® with a downstream SCR was used in this research to determine the system NOx reduction performance using experimental data from a 2013 Cummins 6.7 liter ISB diesel engine and model data. The contribution of the three SCR reactions on NOx 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 1D SCR model. A NO2/NOx ratio of 0.5 was found to be optimum for maximizing the NOx reduction and minimizing NH3 slip for the SCR for a given value of ANR (ammonia to NOx ratio). The SCRF®+SCR system was simulated using the 2D SCR-F+1D SCR system model. For all the test conditions, the NO2/NOx ratio downstream of the SCRF® was found to be 0 due to NO2 consumption by the NO2 assisted PM oxidation and the SCR reactions in the SCRF®. Due to this low NO2/NOx ratio, the NOx 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 (<10%) and high standard SCR reaction (>85%) rates in the downstream SCR. Also, a high NH3 slip due to lower utilization by the SCR reactions was observed from the SCR. Improved NO2/NOx ratio at the SCRF® inlet results in NO2 slip at the SCRF® outlet which then leads to better NOx reduction performance of this system.