Kinetic study of the selective catalytic reduction of nitric oxides with hydrocarbon in diesel exhausts

摘要

The kinetics of the selective catalytic reduction of nitric oxides (NO_x) on a proprietary high temperature catalyst with diesel as the reductant have been studied. The objective was to derive a kinetic model that can be used for real time simulation of the catalyst. In the extension, the real time simualtion will be used when controlling the injection of reductant. This is a requirement for achieving a high efficiency and a low fuel penalty. The response time and the NO_x conversion level upon transient diesel injection was found to be dependent on the temperature. At temperatures below 570 K very low or no NO_x conversion was observed. Above 570 K a small conversion was observed. No direct response upon diesel injection could be distinguished and the NO_x conversion was independent on the hydrocarbon concentration. As the temperature was increased the response became apparent and then faster and the conversion level gradually became more dependent on the hydrocarbon concentration. Above 700 K the response was immediate (response time less than 15 s) and the covnersion level was directly dependent on the hydrocarbon concentration. It was concluded that the NO_x reduction proceeds via the formation of a hydrocarbon intermediate and the successive reaction between the hydrocarbon intermediate and NO_x. When this reaction mechanism was modeled many features of the catalyst behaviour were reproduced.