Proposed vehicle emissions regulations for the near future have prompted automotive manufactures and component suppliers to focus heavily on developing more efficient exhaust aftertreatment devices to lower emissions from spark and compression ignition engines. One of the primary pollutants from lean-burn engines, especially from diesels, are oxides of nitrogen (NOx). Current three-way catalytic converters will not have adequate performance to meet future emission reduction requirements. Therefore, there is a need for researchers and engineers to develop efficient exhaust aftertreatment devices that will reduce NOx emissions from lean-burn engines. These devices must have very high conversion of NOx gases, be unaffected by exhaust-gas impurity such as sulfur, and have minimal impact on vehicle operations and fuel economy. An effective technology for NOx control that is currently receiving a lot of attention is a non-thermal plasma system. This system is comprised of a two-stage plasma reactor and reduction catalyst that reduces nitric oxide and nitrogen dioxide emissions to nitrogen.
The plasma reactor in such a system is used to oxidize nitric oxide that is present in the exhaust stream, to nitrogen dioxide. The nitrogen dioxide is then reduced to nitrogen in a specifically formulated catalyst, using the hydrocarbons present in the exhaust stream as a reductant. An investigation of different reactor geometries was conducted, to determine which design is the most effective as a nitric oxide oxidation device. There are a number of non-thermal plasma reactor geometries that can be used for producing a plasma and achieving the desired nitric oxide conversion chemistry. Most units used for non-thermal plasma reactors are dielectric barrier discharge reactors. Three basic non-thermal plasma reactor geometries were evaluated, parallel plate, concentric cylinder, and tube array designs. The focus of the study was to determine the energy requirement and plasma chemistry associated with each of the geometries, and select the appropriate design for further development, based on performance and energy efficiency. While each reactor design had its pros and cons, the parallel plate geometry seemed to have the most potential and benefit as a non-thermal plasma reactor for an exhaust aftertreatment device. This paper will present the results of the study, compare each of the three designs, and draw some conclusions as to the best candidate for an automotive non-thermal plasma reactor.