The Oxidative Power of Corona Discharge

The oxidative power of corona discharge

P. Letonja, T. Friedrich, M. Siebenhofer

Graz University of Technology, Institute of Chemical Engineering and Environmental Technology, Graz/Austria

Corona discharge, well known from electrostatic precipitation of particulate matter, has proven applicable too in wastewater treatment as well as off-gas purification.

Electrical discharge in gaseous medium depends on the basic gas composition, the temperature, the gas velocity and the type of discharge electrode. While precipitation of particulate matter needs plain electrodes to avoid formation of noxious substances, application of corona discharge in wastewater treatment is driven by needle discharge since oxidation power of corona discharge depends on the effect of very high field intensity. Brush electrodes are therefore the electrode type of choice. Because of the very different current/voltage characteristic in comparison with plain wire type electrodes brush electrodes provide significant UV-irradiation and ozone formation.

In the ongoing project oxidative precipitation of sulfur dioxide and nitrogen oxide was investigated in a wet tube type electrostatic precipitator, tube diameter was 76 mm, equipped with an 8 mm brush electrode of 0.7 m length.  Corona start up voltage was 10 kV. Close to break through operation voltage of 21 kV the maximum specific corona current of 6 mA/m was achieved. The ozone formation potential, which depends on the gas flow rate, was about 300 ppm at 0.38 m/s gas velocity. The UV₂₅₄ -irradiation intensity was equivalent 5% of the electrical power.

Nitrogen oxide is specified by extremely poor absorption properties. At low concentration it can hardly be oxidized. For oxidative absorption nitrogen oxide as well as sulphur dioxide need activation support.

Ambient air was spiked with either nitrogen oxide or sulfur dioxide and fed to the “corona discharge reactor”, which was equipped with an absorption tower in series. The concentration of both constituents was varied between 100 ppm and 400 ppm. Figure 1 shows the result of a basic experiment for NO-treatment.

Figure 1: Oxidation and absorption of nitrogen monoxide. Operation conditions: ambient air    

                 spiked wit NO, gas velocity 0.25 m/s

The final product of the constituents is either aqueous sulphuric acid or nitric acid.

Depending on the operation conditions the clean gas concentration of both substances could even be kept at < 10 ppm. Since oxidation is overall rate limiting modeling and scale up can refer to the design of plug flow reactors, considering space charge effects via the gas velocity in the reaction rate.

Based on the outcome of the project corona discharge seemingly offers a powerful tool in advanced pollution control of gaseous as well as liquid constituents.