A Modeling Study of an Advanced Ultra-Low NOx Aftertreatment System
Venkata Rajesh Chundru, Michigan Technological University
The 2010 Environmental Protection Agency (EPA) NOx standard for heavy-duty engines is 0.2 g/bhp-hr. Further reduction of outlet NOx to 0.02 g/bhp-hr. is being proposed by the California Air Resources Board (CARB) and by the EPA through its Cleaner Truck Initiative program. In this paper, aftertreatment system components consisting of a DOC, a SCR-F, a second DOC (DOC2) and a SCR along with two urea injectors have been analyzed which could be part of a aftertreatment system that can achieve the 0.02 g/bhp-hr. standard. The system performance was evaluated using validated 1D DOC, 2D SCR-F, and 1D SCR models at various combinations of inlet ANR values for the SCR-F and the SCR to determine the injection rates required to achieve an optimum 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 – 450C, while maximizing 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 PNA to reduce the light off time for cold start emissions control.