Effect of SCR downsizing and ammonia slip catalyst coating on the emissions from a heavy-duty diesel engine

Abstract

The combination of diesel oxidation catalyst (DOC), catalyzed diesel particulate filter (CDPF) and selective catalytic reduction (SCR) is the most effective way to reduce particulate emission and nitrogen oxide (NOx) from diesel engines. In this study, the effects of a DOC+CDPF+SCR system on the emissions including carbon monoxide (CO) total hydrocarbon (THC), NOx, particle number (PN) and particle mass (PM) from a heavy-duty diesel engine were evaluated. In addition, the influences of ammonia (NH3) slip catalyst (ASC) coating and SCR catalyst downsizing on the NOx conversion efficiency, nitrous oxide (N2O) emissions and NH3 slip were also investigated. Results showed that the installation of the after-treatment system had negligible effect on the power and BSFC of the engine. The upstream DOC reduced by an average of 97.0% of the CO and 67.5% of the THC, in combination with CDPF, the CO and THC emission further decreased with an average drop of 97.8% and 72.5%, respectively. In terms of particulate emissions, the single DOC reduced 48.0% of the PN and 50.9% of the PM, and the combination of DOC and CDPF led to a reduction of 98.0% in the PN and 96.9% in the PM on average. The SCR was effective in reducing NOx emission, but resulted in higher N2O emission by more than 3 times, meanwhile led to an average NH3 slip of 3.80 ppm. Downsizing the SCR length by 1/3 could still ensure a 91.9% conversion efficiency of NOx, and produced less N2O, but led to an increase of the NH3 slip by 2 times, reaching 7.63 ppm on average. By coating Pt-based ASC catalyst on the back end of the SCR could eliminate the NH3 slip. Considering the cost and performance, the combination of DOC, CDPF and a downsized SCR with ASC coating may be a better choice.