Effect of Polarity on Ozone Production of DC Corona Discharge with and Without Photocatalyst
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Effect of polarity on ozone production of DC corona discharge with and without photocatalyst S. Pekárek Czech Technical University in Prague, Faculty of Electrical Engineering, Prague, Czech Republic Abstract: Experimental results dealing with polarity effects of the needle to mesh DC corona discharge in air with or without TiO 2 photocatalyst on ozone production are presented. It was found that discharge without TiO 2 with the needle positive produces higher ozone concentra- tions than the discharge with the needle negative. Addition of TiO 2 increases ozone production and in case of the needle negative about six times increases ozone production yield. Keywords: Ozone, DC corona discharge, polarity, photocatalyst. 1. Introduction charge in a streamer regime. Present efforts in the research of ozone generation by Discharge ozone production also depends on presence electrical discharges are focused on obtaining higher of catalysts or ferroelectrics in the discharge region. In ozone concentrations, higher ozone generating efficien- [10] the ozone generation characteristics of a cies via better understanding of ozone formation and de- point-to-mesh DC corona discharge with ferroelectric struction plasma-chemistry, adjusting electrical parame- pellet barrier on the mesh have been experimentally in- ters of the discharge and involving new principles and vestigated. This configuration utilizes a wider plasma ideas to the process of ozone formation [1]. reacting area between the top surfaces of every pellet bar- In case of DC corona discharge the ozone production rier and the mesh electrode. It was found that the mean depends among others also on the polarity of the coronat- corona current and ozone concentration significantly in- ing electrode. Results presented in the literature dealing crease when a negative DC voltage was applied. Introduc- with the polarity effects on discharge ozone production tion of photocatalyst such as TiO 2 in the discharge region are a little bit ambiguous. Thus in [2, 3] were studied po- can increase production of reactive species. Titanium di- larity effects of the DC corona with coaxial electrode ar- oxide TiO 2 is an n-type semiconductor with the width of rangement on discharge ozone production. The discharge the forbidden gap 3.2 eV, which corresponds to the wave- took place in air stream flowing transverse to the corona length of radiation 388 nm. The TiO 2 can be activated by discharge wire. It was found that ozone production rate in ultraviolet radiation of the discharge, which for the dis- the negative corona is an order of magnitude higher than charge in air at atmospheric pressure comes mainly from in the positive corona. This result was explained by the the de-excitation of nitrogen molecules. In [11] it was effect of discharge polarity on the number of energetic found that introduction of TiO 2 in the discharge region electrons in the corona plasma. Assessment of ozone gen- and its subsequent photoexcitation contributes to the for- - eration in dc coronas was performed in [4]. Presented mation of superoxide anion O 2 , which can increase total results show that the ozone generation rate from the nega- catalytic activity. In [12] it was experimentally demon- tive corona is almost seven to ten times that from the strated that ozone concentration and corresponding energy positive corona. Corona ozone production in N 2-O2 mix- yield achieved by packed-bed reactors with glass beads ture at volume ratio 4:1 for the wire-duct and point-plane and Al 2O3 pellets are significantly higher than that reactors stressed by the negative and positive pulses was achieved by dielectric barrier discharge only. Ozone gen- treated in [5]. It was found that concentration of ozone eration by hollow needle to mesh negative corona generated by positive pulses is higher than that generated discharge enhanced by the flow of air through the needle by negative pulses. This result was explained by the in- without and with TiO 2 on the mesh was treated in [13]. It crease of the volume reaction zone in case when the reac- was found that addition of TiO 2 on the mesh shifts the tor was stressed by positive pulse voltages. Similar results transition from the glow into the streamer regime of the were obtained for the discharge in air with wire-cylinder discharge into higher currents, increases discharge ozone or wire-plane electrode configuration in [6,7]. The effect production as well as the ozone production yield. It was of polarity on DC corona discharge ozone production is also found that ozone production depends on the mass of sometimes associated with different corona regimes [8]. the TiO 2 and its location in the discharge chamber. Thus about seven times increase in ozone production in In this study we focused on the effect of polarity on negative corona discharge [9] in comparison with positive electrical parameters and ozone production of DC corona corona was found for the glow regime. Results presented discharge at atmospheric pressure enhanced by the flow in [5] for the positive corona were obtained for the dis- of air through the needle electrode with and without TiO 2. 2. Experimental set up characteristics of the breakdown and the formation of the The experimental set up is shown in Fig. 1. The elec- discharge were sensitive to the value of the ballast resistor, trode arrangement consisted from a hollow stainless steel we used for the needle negative ballast resistor R = 3.92 needle and a stainless steel mesh. MΩ and for the needle positive R = 17.86 M Ω. The mesh was situated perpendicularly to the needle. Results concerning ozone production are presented as The electrodes were placed in a circular glass discharge a function of energy density, which is defined as a ratio of chamber of the inner diameter 32.1 mm. The needle had the power delivered to the discharge divided by the air- an inner and outer diameter 0.7 mm and 1.2 mm respec- flow through the needle. The energetic efficiency of the tively. The tip of the needle was sharpened at the angle discharge ozone production is described by the ozone 15 o. The mesh had rhombus cells dimensions 0.60×0.50 production yield α, which is defined in the following way: mm and thickness 0.15 mm. For experiments we used hollow needle to mesh elec- 21 4. ×(Ozone conc .)× Airflow × 6 ×10 −3 trodes or the same electrodes with the globules of Aero- α = [g/kWh], U × I lyst 7706 TiO 2 photocatalyst on the mesh. The anatase content of this photocatalyst is minimum 70 %, the den- where ozone concentration is substituted in ppm, airflow sity is ~3.8 g/cm 3 and the surface area is 40-50 m 2/g. The in slm, discharge voltage U and current I in kV and mA cylindrical globules had the diameter ~3 mm and the respectively. height ~4 mm. Air from a cylinder was supplied into the needle. Mass 3.1. Electrical parameters of the discharge flow controller MFC adjusted the airflow through the Discharge ozone production could not be separated needle. At the output of the discharge tube, was placed the from electrical parameters of the discharge. Fig. 2 shows sensor of temperature T and the sensor of relative humid- the discharge voltage-current characteristics (V-A) for ity RH. A fan cooled the discharge tube. both polarities of the coronating needle electrode and for For study of polarity effects on discharge ozone pro- the discharge without and with TiO 2 photocatalyst on the duction we used two regulated DC HV power supplies. mesh. It is seen that for the needle biased negatively for The first one with negative output terminal provided particular current addition of TiO 2 photocatalyst on the voltage up to 30 kV. The second one with positive output mesh electrode substantially decreases discharge voltage. terminal provided voltage up to 25 kV. Thus for the current 0.25 mA addition of TiO 2 photocata- Ozone concentration was measured by the absorption of lyst on the mesh decreases the discharge voltage from the 254 nm U.V. spectral line with API 450 ozone monitor. 20.2 to 12.7 kV. On the other hand for the needle biased positively for MFC lower currents addition of TiO 2 photocatalyst on the mesh electrode slightly increases discharge voltage and for higher currents addition of this photocatalyst on the mesh Needle R V does not influence discharge voltage. Thus for the current Catalyst 0.25 mA addition of the same mass of TiO 2 photocatalyst Discharge chamber as for the needle negative on the mesh increases discharge mA Mesh voltage from 14.8 kV to 15.3 kV. T RH API 450 22,5 20,0 Mesh without TiO2, negative Output Mesh+TiO2, 17,5 positive Fig. 1. Experimental set up. 15,0 12,5 Discharge [kV] voltage 3. Experimental results and discussion Mesh without TiO2, positive Experiments were performed with the needle biased 10,0 Mesh+TiO2, negative either negatively or positively, with the airflow through 7,5 the needle electrode 1.5 slm, the distance between the 5,0 needle and the mesh 12 mm and relative humidity of air 0,0 0,1 0,2 0,3 0,4 0,5 0,6 3.7 %. Temperature of air at the output from the discharge Discharge current [mA] chamber varied from 21 to 25 ºC. Mass of the TiO 2 photocatalyst globules arranged in one layer on the mesh Fig. 2. Effect of polarity and TiO photocatalyst on was 2.070 g. 2 voltage-current characteristics of the discharge. As far as the stability of the discharge as well as the The differences in V-A characteristics for the needle needle is biased negatively for particular energy density 3 biased negatively and positively are caused by the differ- (e.g.