A Modeling Study of an Advanced Ultra-low NO _x Aftertreatment System

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-NO_x ratio (ANR) values for the SCR-F and the SCR to determine the injection rates required to achieve an optimum nitrogen dioxide (NO₂)/NO_x ratio at the inlets of both the SCR-F and the SCR. A strategy was developed that yielded 99.5% NO_x 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 NO_x and NH₃ concentrations and the NO_x conversion performance of the system components.

NO_x 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 NO_x 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 NO_x adsorbers (PNA) to reduce the light-off time for cold-start emissions control.