The invention relates to a device and a process for the treatment of flowing gases, and in particular flowing exhaust gases. The invention can be used in all fields of technology, where flowing gases are subjected to an aftertreatment in order to reduce hazardous substances, for example in the automobile industry, in power plants or waste incinerating plants or where gases are to be converted by plasma chemistry into new substances, such as in the production of ozone from oxygen.
The present invention is an advantageous advancement over DE 195 18 970 C1.
To increase the efficiency in cleaning flowing gases by means of barrier discharges, DE 195 18 970 C1 proposed that the electrode surface exhibit nonhomogeneity along the flow direction of the gas. This nonhomogeneity, namely thickening, can increase the probability of the discharge filaments igniting even in those gas volumes where no discharge has taken place yet. In the thick spots, the electrodes are spaced apart at shorter distance. Thus, the electric field strengths are higher between the electrodes. Therefore, the discharge ignites exclusively in the thick spots. If there are altogether adequate number of thick spots, more volume elements can be exposed altogether to a discharge filament, a feature that increases the efficiency.
Our own studies have revealed that the solution proposed in DE 195 18 970 C1 exhibits the drawback that electric charge carriers in the thick spot decrease the local ignition voltage. The nonhomogeneity of the electrode surface also results namely in a nonhomogeneity of the distribution of the electric charge carriers. Since the discharge ignites exclusively in the thick spots, the result in these areas is an accumulation of electric charge carriers and thus a decrease in the ignition field strength.
This negative effect results in not only the filaments igniting on the surface of the thickening facing the counter electrode when charge carriers are present in the thick spot, but also in the filaments now igniting by a longer path, for example on the side faces of the thickenings.
In those spatial areas, which lie behind the thickenings based on the direction of flow, there is hardly any gas flow. Thus, the result of the thickenings is a volume area V1, where no significant gas exchange takes place any more.
The ignition of filaments on the side faces of the thickenings, and the circumstance that in volume V1 no significant gas exchange takes place any more, result in the gas discharge in volume V1 having virtually no effect. Depending on the geometric shape of the thickenings, their number and their configuration in the reactor, a significant portion of the energy coupled into the gas discharge is unused. Thus, the potential for the efficiency of the barrier discharge is not exhausted.
Another drawback of the thickenings lies in the fact that the gas flow is severely impeded by the discharge slit, which is only a few millimeters large and is formed by the electrodes. The increased flow resistance is a drawback for all aforementioned applications and can result, for example in internal combustion engines, in a backup of the exhaust gases and thus to a deterioration of the engine performance. For this reason it is desirable to dimension the discharge slit as large as possible. However, the resulting required enlargement of the electrode spacing makes it necessary to increase the voltage amplitude generated by the voltage source. However, the increased cost for insulation and voltage generators does not support this step.
The invention is based on the problem of overcoming the drawbacks of DE 195 18 970 C1 and providing an exhaust gas reactor wherein the flow of exhaust gas through the volume occupied by the gas discharge is as comprehensive as possible.
Furthermore, the object of the invention is to decrease the flow resistance of the discharge slit. In so doing, the flow resistance is supposed to be decreased without enlarging the discharge slit.
It was recognized, according to the invention, that the problems arising from DE 195 18 970 C1 can be solved with thickenings that exhibit at least one opening through which the exhaust gas can flow.
The openings are affixed in such a manner that the exhaust gas can flow through the opening. Thus, there is spatially behind the thickenings a gas flow or gas exchange, based on the direction of flow. In this space, flowing gas is exposed to the discharge filaments. Thus, there is virtually comprehensive exhaust gas flow through the volume occupied by the gas discharge and the cleaning effect is improved. In exhaust gas reactors in everyday practice, the volume swept by the discharge filaments is approximately doubled and the cleaning effect increases proportionally by approximately 100%.
In the context of the present invention it can be desirable to provide only one part of the thickenings with openings, when the flow behavior in the respective application demands it. One or more openings are possible per thickening. For optimal gas flow, it is expedient to align the openings parallel to the direction of flow. This can be done, for example, by drilling with a laser one or several holes, whose axis of symmetry lies largely parallel to the direction of flow, into the thickening. The width of the openings is dimensioned in such a manner that it is certain that the discharge filaments will also ignite over the path elongated by this width.
Should the flow resistance and the cleaning effect be adequate for the application, the invention can also serve to use smaller voltage amplitudes than in a prior art reactor. For a cleaning effect that is just as good compared to the prior art, the discharge slit can be decreased to the point that the cross sectional area, through which the gas flows and which is formed by the sum of the cross sectional areas of the discharge slit and the openings, remains constant. This feature is especially advantageous for cleaning the exhaust gas of internal combustion engines, where it is then possible to work advantageously with lower voltages so that less expensive and more compact voltage sources can be chosen.
Without restricting the general idea of the invention the inventive advancement shall be explained with reference to the embodiments.