An Experimental and Modeling Study of a Diesel Oxidation Catalyst and a Catalyzed Diesel Particulate Filter Using a 1-D 2-Layer Model 2006-01-0466

Modeling of diesel exhaust after-treatment devices is a valuable tool in the development and performance evaluation of these devices in a cost effective manner. Results from steady state loading experiments on a catalyzed particulate filter (CPF) in a Johnson Matthey CCRT®, performed with and without the upstream diesel oxidation catalyst (DOC) are described in this paper. The experiments were performed at 20, 40, 60 and 75% of full load (1120 Nm) at rated speed (2100 rpm) on a Cummins ISM 2002 heavy duty diesel engine. The data obtained were used to calibrate one dimensional (1-D) DOC and CPF models developed at Michigan Technological University (MTU). The 1-D 2-layer single channel CPF model helped evaluate the filtration and passive oxidation performance of the CPF. DOC modeling results of the pressure drop and gaseous emission oxidation performance using a previously developed model are also presented.

The DOC modeling showed that HC, CO and NO oxidation kinetics can be represented by one set of ‘apparent’ kinetic parameters across the entire temperature and flow rate range encountered in this research. CPF model results showed that oxidation in the pores of the filter wall can cause a disproportionate decrease in the pressure drop with respect to the particulate matter (PM) mass present in the CPF. The CPF models oxidation kinetics were calibrated using thermal and NO₂/Temperature assisted oxidation effects, and did not require any ‘catalyst’ effect with O₂. The oxidation rates were described using the same kinetic parameters, irrespective of the temperature and CPF-inlet NO₂ concentrations (with or without the DOC). NO₂/Temperature was the dominant means of PM oxidation in the temperature range of 280°C - 460°C. Separate rates of PM oxidation were found to exist on and in the filter wall. The values of the PM density in the filter wall and PM cake permeability determined from the model are in agreement with those reported in the literature.

In line with other reported studies, the CPF model results showed that the DOC significantly increases the PM oxidation rates due to the increased concentrations of NO₂ entering the CPF. After 5 hours of running at 20 and 75% load at rated speed, 9.9 and 83.4% of the inlet PM mass was oxidized in CCRT® configuration. The NO₂ assisted oxidation was found to be much more effective at temperatures greater than 340°C. Model results showed that due to low concentrations of NO₂ entering the filter in CPF-only configuration, the NO₂ generated in the filter increased oxidation rates by 27.6 - 75.9%. The model also showed that in CCRT® configuration, the majority of PM oxidized was by NO₂ entering the filter, while the NO₂ generated in the filter only increased oxidation rates by 3.9 - 9.6%. Thus the catalyst loading in this CPF could possibly be reduced without significantly decreasing the passive regeneration performance of the CCRT®.