PNNL

Abstract

Significant reduction of NOx in synthetic light duty diesel exhaust has been achieved over a broad temperature window by combining atmospheric plasma with appropriate catalysts. The technique relies on the addition of hydrocarbon reductant prior to passing the exhaust simulant through a non-thermal plasma and a catalyst bed. The observed chemistry in the plasma includes conversion of NO to NO2 as well as the partial oxidation of the hydrocarbon. Despite driving the NO oxidation to completion, the overall NOx reduction has a maximum of less than 80%, with this maximum obtained only at high energy input into the plasma, high concentration of hydrocarbon reductant and low space velocity. We present data in this paper illustrating that a multiple stage treatment strategy, whereby two or more plasma-catalyst reactor stages are utilized in series, can increase the maximum NOx conversion obtainable. Alternatively this technique can reduce the energy and/or hydrocarbon requirements for a fixed conversion efficiency. When propene is used as a reductant, the limiting reagent for the overall process is most likely acetaldehyde. The data suggest that acetaldehyde is formed in concert with NO oxidation to NO2 in the plasma stage. The limited NOx reduction efficiency attained in a single stage, even with excess energy, oxygen content and/or hydrocarbon to NOx ration is well explained by this hypothesis, as is the effectiveness of the multiple stage treatment strategy. We present data here illustrating the advantage of this approach under a wide variety of conditions.