The Effect of NO₂/NOₓ 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 (NOₓ) 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 NOₓ 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 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 one dimensional (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 ammonia-to-NOₓ ratio (ANR). 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 particulate matter (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 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/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.


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  • Accession Number: 01732115
  • Record Type: Publication
  • Source Agency: SAE International
  • Report/Paper Numbers: 04-12-02-0008
  • Files: TRIS, SAE
  • Created Date: Feb 26 2020 11:13AM