Non-thermal plasma application to the abatement of noxious emissions in automotive exhaust gases

Experiments and numerical model calculations on non-thermal plasma treatment of lean combustion exhaust gases were reviewed. It was found that because of the oxygen concentration of several per cent, oxidation of noxious compounds is the prevailing non-thermal plasma-induced process. Therefore nitric oxides cannot be reduced directly, but hybrid processes combining non-thermal plasma pre-treatment with catalytic reduction using either hydrocarbons or ammonia-based reducing agents have to be applied.

Plasma-enhanced selective catalytic reduction (PE-SCR) of the nitric oxides emitted from a modern car's diesel engine for values of more than 60% was demonstrated in test bench experiments. For these experiments, a compact dielectric barrier discharge reactor with a flow cross section of 15 cm² excited by a semiconductor switched pulse voltage source and a urea-based selective catalytic reduction system were applied. The average fuel penalty for this process under urban driving conditions was estimated to be around 2%.

Thus PE-SCR has the potential to reduce the NO_x emission of diesel cars to values well below future emission standards to be set in force in 2007. A number of investigations on the non-thermal plasma-induced oxidation of diesel soot showed very encouraging results.