Effective generation of atmospheric pressure plasma in a sandwich-type honeycomb monolith reactor by humidity control

Large-volume atmospheric-pressure plasma based on corona discharge was successfully generated in a sandwich-type plasma reactor by controlling the humidity of the feed gas. The reactor comprised a 5-cm-high and 9-cm-wide commercial honeycomb monolith and two perforated stainless-steel disk electrodes with 3-mm holes, which covered up both sides of the monolith. The high-voltage applied disk electrode was placed 2 similar to 6 mm away from the monolith, whereas the counter ground electrode typically touched it. The characteristics of air plasma such as power delivery, breakdown voltage, current, and impedance were found to strongly depend on the water vapor content in the feed gas as well as in the monolith, exhibiting an exponential increase of discharge power with increasing the humidity. The discharge characteristics also depended on the gas flow rate and the electrode-monolith distance. Better discharge performance was observed as the electrode was closer to the monolith, and as the air flow rate was higher. Discharge power above 30 W was achieved under the condition of gas flow rate 75 L/min, applied voltage 25 kV, and humidity 2.1%(v/v) when the high-voltage electrode was 2 mm away from the monolith, and the ground electrode kept up against it. The air plasma produced reactive species, being demonstrated by the formation of ozone at several tens of ppm, depending on specific energy input. The plasma source has potential applications in the abatements of odor, organic compounds, and NOx with high throughputs.