Plasma Chem Plasma Process (2012) 32:743–754 DOI 10.1007/s11090-012-9382-z ORIGINAL PAPER
Ozone Production Using a Power Modulated Surface Dielectric Barrier Discharge in Dry Synthetic Air M. Sˇimek • S. Peka´rek • V. Prukner
Received: 7 February 2012 / Accepted: 27 April 2012 / Published online: 13 May 2012 Springer Science+Business Media, LLC 2012
Abstract The measurements of electrical and optical characteristics of the discharge and concentrations of produced ozone and nitrogen oxides were performed to evaluate the efficiency of ozone production in an AC surface dielectric barrier discharge in dry synthetic air at atmospheric pressure. The discharge was driven in an amplitude-modulated regime with driving AC frequencies of 1, 5 and 10 kHz, variable discharge duty cycle of 0.02–0.8 and synthetic air flow rate of 2–10 slm. The experimental results show that ozone and nitrogen oxides concentrations increased with increasing AC high-voltage amplitude, increasing discharge duty cycle and with increasing residence time. The highest calculated ozone production yield reached *90 g/kWh with a corresponding energy cost of about 20 eV/molecule. The production yield was found to be independent of the driving AC frequency and specific energy density in the 10-4–10-2 Wh/l range. Keywords
Ozone Surface DBD Synthetic air Nitrogen oxides Production efficiency
Introduction Ozone is a widely used disinfecting and oxidising agent (e.g. water treatment, food processing and healthcare). When produced by electrical discharges, its concentration represents a dynamic balance which is established between the ozone formation and destruction processes [1–6]. In the case of the generation from N2–O2 mixtures, an ozone molecule is formed mainly by the reaction:
M. Sˇimek (&) V. Prukner Department of Pulse Plasma Systems, Institute of Plasma Physics v.v.i., Academy of Sciences of the Czech Republic, Za Slovankou 3, 182 00 Prague, Czech Republic e-mail:
[email protected] S. Peka´rek Faculty of Electrical Engineering, Czech Technical University in Prague, Technicka´ 2, 166 27 Prague, Czech Republic
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O þ O2 þ M ! O3 þ M;
ð1Þ
where M is a third-body collision partner (O2, N2). The oxygen atoms in discharge-based systems are mainly produced through dissociation of the ground-state O2 molecule: 0 3 3 1 1 e þ O2 ðX3 R g Þ ! e þ O( P) þ O( P; D; S);
ð2Þ
3 3 3 1 þ 3 1 1 N2 ðA3 Rþ u ; B Pg ; C Pu Þ þ O2 ðX Rg Þ ! N2 ðX Rg Þ þ 2O( P; D; S);
ð3aÞ
3 N2 ðA3 Rþ u Þ þ O2 ðX Rg Þ ! N2 O þ O:
ð3bÞ
On the other hand, there are many competing reactions leading to ozone decomposition. The most important ozone decomposition processes are: e þ O3 ! e þ O2 þ O;
ð4Þ
O þ O3 ! 2O2 :
ð5Þ
Moreover, many other species produced by the discharge and post-discharge processes reduce ozone levels. For example, when a certain amount of atomic nitrogen N(4S) is produced through electron-impact dissociation during discharge: 0 4 e þ N2 ðX1 Rþ u Þ ! e þ 2N( S);
ð6Þ
atomic nitrogen is then converted to nitric oxide: N þ O3 ! NO þ O2 ;
ð7Þ
followed by a reaction chain that reduces both oxygen atoms and ozone through: NO þ O3 ! NO2 þ O2 ;
ð8Þ
NO2 þ O ! NO þ O2 ;
ð9Þ
NO2 þ O3 ! NO3 þ O2 ;
ð10Þ
NO3 þ O ! NO2 þ O2 :
ð11Þ
NO3 þ NO2 ! N2 O5 þ O2 :
ð12Þ
Ozone generation efficiency has been widely studied in laboratory conditions for systems based on pulsed corona discharges and various dielectric barrier discharges [5–11]. The performance of an ozone generator is usually evaluated on the basis of O3 production energy efficiency (or production yield) at a given O3 concentration and on the ratio of O3/NxOy concentrations. Differences between various air-fed ozone generators generally arise due to the temperature dependence of reaction rates inhibiting O3 production at elevated gas temperatures and due to different content of nitrogen oxides consuming the produced O3 through reactions (8)–(12). The gas temperature and NOx effects are strongly influenced by the specific energy density. At low energy densities, O3 N2O*N2O5 [ NO3 species dominate in discharge P products. At high energy densities a poisoning of the ozone synthesis can be observed ( NxOy O3) [2–8]. Most frequently used ozone generators usually employ volume plasma generators such as pulsed corona discharges or volume AC barrier discharges. The basic advantage of volume discharges is their large throughput, i.e., they can easily process large gas volumes, therefore producing O3 in large quantities with high production yield. The highest ozone yields were reported for an ozone generator based on a wire-to-cylinder pulsed positive
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corona (PPC) discharge driven with ultra-short high-voltage pulses (*50–60 kV of nanosecond duration): 544 and 239 g/kWh in the oxygen-fed and air-fed discharges, respectively [12]. Extremely high production yield is of importance in the case of largescale installations; however, some specific applications requiring local application of O3 (e.g. surface cleaning or sterilization) might not necessarily need either large ozone quantities or extremely high O3 concentrations. For such applications, surface dielectric barrier discharge (SDBD), which produces only a very thin layer (\1 mm) of highly reactive species [13–15], might be a quite reasonable and more economic choice. Recently, we proved that an AC SDBD operated at a sufficiently low discharge duty cycle can be used to produce ozone from pure oxygen with a high production yield [14] or to deposit bio-compatible PEO-like plasma polymer films [15]. In this work, we investigate the efficiency of ozone and nitrogen oxides production in the case of air-fed amplitudemodulated AC SDBD for three different AC frequencies (1, 5 and 10 kHz) over quite a wide range of energy densities (10-4–10-1 Wh/l).
Experimental Set-Up The experimental set-up is shown in Fig. 1. It includes the SDBD reactor, gas feeding unit, discharge power supply with electro-optical diagnostics, temperature sensors and O3/NOx/ N2O monitors. The SDBD reactor consists of a planar SDBD electrode system placed in a polymethylmethacrylate (Plexiglas) chamber equipped with gas feed input/output ports and a high-voltage interface [14]. The SDBD was powered by an AC high-voltage power supply composed of the TG1010A Function Generator (TTi), Powertron Model 250A RF Amplifier and a highvoltage step-up transformer. Applied AC high-voltage waveform (fAC = 1, 5, 10 kHz) was amplitude-modulated by a square-wave modulation waveform (fM = 0.1 9 fAC or fM = 0.01 9 fAC) producing sine-wave TON and TOFF periods with a variable duty cycle D = TON/(TON ? TOFF). The duty cycle D was controlled through the number N of consecutive sine-wave cycles (N = 1–64) defining the duration of TON period. A fast digitizing oscilloscope (Tektronix DPO4034, 350 MHz, 2.5 GS/s) was used to record the voltage–charge and optical characteristics of the discharge. The discharge high-voltage waveforms were sampled by a Tektronix P6015 high-voltage probe (1,000:1@100 MX). A Tektronix P6139A high-voltage probe (10:1@10 MX, bandwidth 500 MHz) was used to measure a transferred charge through the measuring capacitor (C = 0.5 lF) inserted
Fig. 1 Schematic diagram of the experimental set-up
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between the induction electrode and ground. A fast Hamamatsu R2949 photomultiplier (PMT) was used to detect the surface-averaged plasma-induced emission (PIE) produced by the discharge and collected by the quartz optical fibre bundle through the upper Plexiglas cap. The reactor was fed with synthetic air (99.999 %) (containing H2O \ 2 ppm, CO ? CO2 \ 0.4 ppm impurities) through a Bronkhorst HI-TEC model mass-flow controller (flow rate Q = 2–10 slm). The whole surface of the rear side induction electrode was flushed with technical air (fixed flow rate 2 slm) to avoid excessive temperature drifts of the alumina plate with various duty cycles. The temperatures of synthetic air and cooling air at the input ports were stable (21 ± 0.5 C) throughout all measurements. The temperatures of the discharge products and cooling air were independently and continuously monitored for all discharge conditions at the output ports. The gas pressure in the discharge chamber was measured by a pressure gauge and fixed at 760 ± 5 Torr. The discharge products were sampled at the SDBD reactor output port. Two nondispersive UV absorption ozone monitors API 450 and API 450 M, chemiluminescence NO/NOX analyser API 200EM and infrared N2O analyzer API 320E (all Teledyne Instruments) were used to quantify both initial mixture composition and stable discharge products (total sample flow rate fixed at 2 slm). Production of ozone is strongly affected by the residence time of species passing through the discharge area. To evaluate the mean residence time we considered the geometry of the internal volume of the SDBD reactor and flow-field velocity. Flow-field patterns were calculated (Comsol Multiphysics) considering the geometry of the discharge chamber and air flow rate of 1 slm. Results are shown in Fig. 2. The calculated flow patterns are shown (Fig. 2a) in two perpendicular planes (vertical x–y cut for z = 0, and horizontal x–z cut for y = 10 mm). The horizontal cut shows the flow velocity inside the air-feeding system. The vertical cut showing flow velocity in the middle of the reaction chamber (z = 0) is separately displayed in Fig. 2b. The calculated velocity shows the maximum of 0.11 m/s in the middle of the gap between the upper surface of the SDBD electrode (y = 0) and the surface of the Plexiglas cap (y = 2 mm). The mean residence time corresponding to the average velocity in the gap is *1.3 s (Q = 1 slm).
Fig. 2 Flow-field patterns a air flow velocity in two perpendicular planes cutting the SDBD reactor (vertical x–y cut for z = 0, and horizontal x–z cut for y = 10 mm) and b vertical cut showing flow velocity in the middle of the reaction chamber (z = 0). Calculated for air flow rate Q = 1 slm at 300 K
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Experimental Results and Discussion The discharge driven in the amplitude-modulated AC regime at variable duty cycle produces a large number of micro-discharges that cover the surface of the SDBD electrode. For the whole range of experimental conditions we observed short (*0.5 mm) bright and blueish micro-discharge filaments distributed around the edges of the strip electrodes, as evidenced by the photograph taken for fAC = 10 kHz in Fig. 3. As there were no observable differences for photos registered for the other two AC frequencies (fAC = 1 and 5 kHz), we conclude that for all the investigated conditions, the discharge was working in a filamentary (streamer) regime. Electrical and Optical Characteristics Standard optical and electrical (high-voltage and transferred charge) measurements were performed to track the SDBD emission and to determine the average energy deposited into the discharge during the TON period [14]. Figure 4 shows typical averaged PIE waveforms for all three AC frequencies as a function of the AC high-voltage phase at fixed duty cycle (D = 0.4). The PIE waveforms were acquired without any spectral filter and the PMT signal is, therefore, given by an integral discharge radiation limited only by the wavelength dependence of the Plexiglas transmittance and the PMT’s photocathode efficiency. The PIE waveform tracks all individual micro-discharges occurring in the field of view of the PMT. When averaged over many TON periods, the registered PIE signal reflects the different statistical distribution of micro-discharges (each lasting several tens of nanoseconds). Micro-discharges occur at a certain phase u with some statistical distribution depending on the amplitude/frequency of the HV sine-wave [16]. For fAC = 1 kHz (uAC = 11.3 kV), the discharge onset occurs close to zero voltage (u = 0, p, 2p, 3p…), and with increasing fAC (and decreasing uAC), the onset is shifted towards AC maxima/ minima (u = p/2, 3p/2, 5p/2…). The average discharge power evaluated from the voltage–charge characteristics is shown for fAC = 1 kHz and for the TON period composed of the two AC cycles as a function of the high-voltage uAC in Fig. 5, together with measured ozone concentrations.
Fig. 3 Photograph of the discharge in air (uAC = 4.5 kV, fAC = 10 kHz, fM = 1 kHz, D = 0.04, Q = 5 slm). Exposure time was 1 s
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Fig. 4 Optical characteristics of the discharge for a fAC = 1 kHz, uAC = 11.3 kV, b fAC = 5 kHz, uAC = 6.5 kV and c fAC = 10 kHz, uAC = 4.5 kV (fM = 0.1 9 fAC, D = 0.4, Q = 5 slm). The photomultiplier signal is given by integral discharge radiation limited by the transmittance of Plexiglas and PMT’s photocathode efficiency
Fig. 5 Average power delivered to the discharge and ozone concentration as a function of an AC highvoltage peak-to-peak amplitude uAC (fAC = 1 kHz, fM = 10, D = 0.02 and Q = 5 slm)
The power increases (best-fitted with the polynomial of the second order) faster than the O3 concentration. An increase of the discharge power is probably due to the total number of micro-discharges produced during the TON period. The higher the AC amplitude the longer is the part of the positive/negative half-cycle in which micro-discharges occur on the SDBD surface.
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SDBD Stability and the Discharge Onset Figure 6 shows the dependences of the ozone and nitrogen dioxide concentrations on the AC high voltage uAC (fAC = 1 kHz, fM = 10 Hz, D = 0.02, Q = 5 slm). Their linear extrapolations for the lowest uAC voltages (\7.5 kV) were used to determine the discharge onset voltage threshold (u0AC = 3.2 kV). The linear increase in ozone concentration with the AC high voltage between 10 and 25 kV is linked to an increase of the total number of individual micro-discharges produced on the SDBD surface. Furthermore, the stability of the SDBD was tested through the long-term measurement of the ozone and NO2 production for fixed fAC, fM, uAC. D, and Q. A typical long-term run is shown in Fig. 7. After switching on the SDBD, ozone production becomes well stabilised after *1 h. Nitrogen dioxide concentration, however, shows very slow drifting to the stationary value which is reached after many hours of the discharge operation. This slow drift is probably due to small temperature variations of surfaces of the reactor chamber and output tubing. Products Concentrations Ozone, nitrogen monoxide, nitrogen dioxide and nitrous oxide concentrations were further monitored for three different AC frequencies fAC = 1, 5 and 10 kHz. For each fAC, these concentrations were registered at three fixed uAC’s (*6.7, 11 and 15 kV peak-to-peak). For each uAC, we varied the energy density by combining two modulation frequencies (fM = 0.1 9 fAC or fM = 0.01 9 fAC) with a variable discharge duty cycle (D = 0.02–0.64), all at a fixed air flow rate of 5 slm. Furthermore, to inspect the effect of residence time on discharge products concentrations, one set of data (fAC = 1 kHz, fM = 10 Hz, uAC *6.7 kV) was taken at a variable air flow rate (Q = 2–10 slm). Figure 8 shows the dependence of the O3 and NO2 concentrations on air flow rate or average residence time for two duty cycles (D = 0.02 and 0.16) at fixed fAC, fM and uAC. Average residence time was simply evaluated using results of numerical modelling shown in Fig. 2. The symbols in Fig. 8 correspond to the experimental values, whereas the dotted
Fig. 6 Ozone and nitrogen dioxide concentrations as a function of an AC high-voltage peak-to-peak amplitude uAC (fAC = 1 kHz, fM = 10, D = 0.02 and Q = 5 slm)
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Fig. 7 Ozone and nitrogen dioxide concentrations as a function of time (fAC = 1 kHz, fM = 10, D = 0.02 and Q = 5 slm). The time represents the interval between switching on the discharge and the reading of the ozone monitor
Fig. 8 Ozone and nitrogen dioxide concentrations as a function of air flow for duty cycles D = 0.02 and 0.16 (fAC = 1 kHz, fM = 10 and uAC = 11.3 kV). The symbols correspond to the experimental values; dotted curves show their least-square fits
curves show their least-square fits (const. 9 Q-1). Decrease in O3 and NO2 concentrations with an increasing air flow rate Q is related to the reduction of the residence time at fixed discharge parameters (uAC, fAC, fM and D). Because the residence time changes as Q-1 and experimental data shown in the figure can be fitted by the same dependence, we can conclude that the SDBD produces a fixed amount of O3 and NOx precursors independently of the air flow rate. Therefore, within the inspected Q range, the discharge properties do not depend on air flow. Similar 1/Q dependence was observed in the case of the oxygen-fed
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SDBD [14]. We therefore conclude that at low duty cycles heating effects do not affect the kinetics of O3 and NxOy formation. Figure 9 shows the dependence of product concentrations on energy density (specific energy) at fixed air flow rate Q = 5 slm and for fAC = 1, 5 and 10 kHz. The energy density is defined as the ratio of average power delivered to the discharge and flow rate of air through the reactor. The energy density was controlled through the variation of the discharge duty cycle D and modulation frequency fM at fixed uAC (*6.7, 11 and 15 kV peak-to-peak). In order to compare simply our results with other authors, we show three equivalent x-axis units (Wh/l, kJ/m3 and eV/molecule). Ozone concentration increases linearly with energy density reaching a maximum of *750 ppm for energy density of (2–4) 9 10-2 Wh/l. With further energy density increase, the concentration slightly decreases. Concentration of nitrous oxide increased linearly with energy density independently of D, fM, fAC and uAC. Both O3 and N2O versus energy density dependences are in qualitative agreement with the work of Eliasson and Kogelschatz [3, 4], including the onset of the discharge poisoning (ozoneless mode) for energy density of C0.1 Wh/l (specific energy %0.1 eV/molecule). O3, N2O and N2O5 should be the main stable SDBD products under normal conditions (no discharge poisoning) [4–6]. Nitrous oxide is mainly produced by O2 dissociating processes involving electronically excited N2(A3R). The stationary state of N2O is therefore reached within hundreds of nanoseconds after the microdischarge extinction [4–6]. By contrast, dinitrogen pentoxide is formed over a much longer time-scale (tens-to-hundreds of seconds) through the sequence of reactions (7)–(12). Consequently, stationary concentrations of NO and NO2 should become negligible with respect to both N2O and N2O5
Fig. 9 Ozone, nitrogen dioxide and nitrous oxide concentrations as a function of the energy density or specific energy at various high-voltage frequencies fAC (Q = 5 slm). The triangles, squares and circles correspond to the fAC of 1, 5 and 10 kHz, respectively
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concentrations after quite a long time, which should be compared with the transit time necessary to transport samples to the analyser. We should note here that the API 200EM NO/NOx analyser monitors NO and NO2 simultaneously and that for the whole range of investigated discharge parameters, measured NO concentrations were always below the instrument’s detection limit. However, our measured concentrations of NO2 exceed those of N2O, with a maximum registered NO2 concentration of about 20 ppm (limited by the API 200EM range) at energy density *2 9 10-2 Wh/l. This apparent contradiction can be explained on the basis of the transit time of the discharge products from the SDBD surface to the analyser (\1 s). Very probably, the NO/NOx analyser gives NO2 concentrations close to a maximum due to the post-discharge chemistry. This maximum should in principle determine N2O5 stationary values (approximately one-third of the maximum NO2 concentration) [4]. Dividing experimental NO2 concentrations by a factor of 3, estimated stationary N2O5 concentrations could be roughly equal to the measured N2O values for the intermediate range of energy densities (10-3–10-2 Wh/l), which is consistent with predictions made by the homogeneous kinetic model of Eliasson and Kogelschatz [4]. Production Yield and Energy Cost From the measured O3, NO2 and N2O concentrations we finally calculated the corresponding production yield (Fig. 10) and, in the case of ozone, also the corresponding energy cost (Fig. 11). The O3 yield varied in the range 20–90 g/kWh, with the highest yield obtained at the lowest energy density (linked with the lowest discharge duty cycle). The highest production yield is associated with the lowest energy cost of *20 eV/molecule. Compared with the oxygen-fed SDBD [14], the energy cost to produce one ozone molecule in equivalent conditions (i.e., identical reactor and O3 analyser) is approximately doubled. Compared with the efficiency achieved in the case of the air-fed PPC discharge (*75 kV high-voltage pulses of *150 ns duration applied in the wire-cylinder reactor geometry) [9], we find much higher efficiency of the air-fed SDBD (yield *90 g/kWh with respect to *50 g/kWh). The nearly two times higher efficiency of the SDBD with
Fig. 10 Ozone, nitrogen dioxide and nitrous oxide production yield as a function of the energy density at various high-voltage frequencies fAC (Q = 5 slm). The triangles, squares and circles correspond to the fAC of 1, 5 and 10 kHz, respectively
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Fig. 11 Ozone production energy cost as a function of the energy density at various high-voltage frequencies fAC (Q = 5 slm). The triangles, squares and circles correspond to the fAC of 1, 5 and 10 kHz, respectively
respect to the PPC is probably due to easier destruction of O3 by volume streamers through (4)–(5) boosted by the longer residence time of ozone formed in the PPC reactor (typical residence time 3–30 s). Similar efficiency (yields up to 110 g/kWh) has been achieved in a spiral wire-cylinder streamer discharge by Samaranayake et al. [17]. Extremely high efficiency (yield 239 g/kWh which is equivalent to the energy cost of *8 eV/O3 molecule) achieved in wire-to-cylinder PPC discharge driven with ultra-short high-voltage pulses of nanosecond duration was recently reported by Wang et al. [12]. Ratio of Ozone and Nitrogen Oxides Concentrations It has been pointed out by Deryugin et al. [8] that in an ideal dry-air operating ozonator, the [O3]/[NxOy] ratio rapidly decreases when increasing the reduced electric field E/N, while production yield increases. An extremely high production yield of *240 g/kWh is predicted for E/N % 200 Td with the [O3]/[NxOy] \ 100. Considering the concentrations shown in Fig. 9 and the estimated stationary concentrations of N2O5, we evaluated the ratio of ozone and sum of nitrogen oxides (N2O ? N2O5) concentrations. This ratio in our case equals 40 ± 8 in the intermediate range of energy densities (10-3–10-2 Wh/l) and confirms O3 production in a discharge with high average E/N.
Conclusions We extended our previous study of ozone production efficiency by the oxygen-fed SDBD to the production efficiency of ozone and nitrogen oxides using the air-fed SDBD. Besides the influence of power modulation, we explored also the influence of an AC high-voltage frequency. We found that the energy efficiency of ozone generation by the SDBD does not
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depend on fAC (1–10 kHz) which is consistent with the fact that the discharge was always working in a filamentary streamer regime. As in the case of air-fed SDBD, variation in the basic device operational parameters (modulation waveform frequency fM, AC high voltage uAC and total flow rate Q) provides a simple tool to control precisely the required output ozone levels. Besides ozone production, we registered minor concentrations of nitrogen dioxide and nitrous oxide with estimated characteristic ratio [O3]/[NxOy] % 40 ± 8. The best energy efficiency for ozone synthesis (production yield 90 ± 10 g/kWh; energy cost 20 ± 2 eV/ molec) was obtained in wide range of specific energy densities (10-4–10-2 Wh/l). ˇ R contract no. Acknowledgments This work was supported by the Czech Science Foundation (GAC 202/09/0176).
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