A modeling study of an advanced ultra-low NOx aftertreatment system

Document Type

Article

Publication Date

1-9-2020

Department

Department of Mechanical Engineering-Engineering Mechanics

Abstract

The 2010 Environmental Protection Agency (EPA) Emission Standard for heavy-duty engines required 0.2 g/bhp-hr over certification cycles (cold and hot Federal Test Procedure [FTP]), and the California Air Resources Board (CARB) standards require upto 90% reduction of overall oxides of nitrogen (NOx) emissions. Similar reductions may be considered by the EPA through its Cleaner Trucks Initiative program. In this article, aftertreatment system components consisting of a diesel oxidation catalyst (DOC); a selective catalytic reduction catalyst on a diesel particulate filter (DPF), or SCR-F; a second DOC (DOC2); and a SCR along with two urea injectors have been analyzed, which could be part of an aftertreatment system that can achieve the 0.02 g/bhp-hr standard. The system performance was evaluated using validated one-dimensional (1D) DOC, two-dimensional (2D) SCR-F, and 1D SCR models at various combinations of inlet ammonia (NH3)-to-NOx ratio (ANR) values for the SCR-F and the SCR to determine the injection rates required to achieve an optimum nitrogen dioxide (NO2)/NOx ratio at the inlets of both the SCR-F and the SCR. A strategy was developed that yielded 99.5% NOx conversion at inlet temperatures from 203° to 450°C, while maximizing particulate matter (PM) oxidation rate in the SCR-F and minimizing the urea consumption rate. These system components have the potential to be robust to variations in the inlet NOx and NH3 concentrations and the NOx conversion performance of the system components. NOx conversions greater than 95% in the SCR-F and SCR were determined to be primarily due to the fast SCR reaction. The two urea injectors were used to maximize NOx reduction in both devices and SCR-F PM oxidation. For the case with ANR1 = 0, a 90%-100% increase in NO2-assisted PM oxidation in the SCR-F was determined compared to a system without the second DOC and urea injector. Further development of the system components should be pursued in terms of catalyst type, catalyst loading, and external heating along with a close-coupled SCR/DOC or passive NOx adsorbers (PNA) to reduce the light-off time for cold-start emissions control.

Publication Title

SAE International Journal of Fuels and Lubricants

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