Study of ozone generation in an atmospheric dielectric barrier discharge reactor

Journal of Electrostatics 75 (2015) 35e42

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Study of ozone generation in an atmospheric dielectric barrier discharge reactor Shuiliang Yao a, Zuliang Wu a, *, Jingyi Han a, Xiujuan Tang a, Boqiong Jiang a, Hao Lu a, Sin Yamamoto b, Satoshi Kodama c a

School of Environmental Science and Engineering, Zhejiang Gongshang University, No. 18 Xuezheng Street, Xiasha University Town, Hangzhou, Zhejiang 310018, China Chemical Research Group, Research Institute of Innovative Technology for the Earth, 9-2 Kizugawadai, Kizugawa-shi, Kyoto 619-0292, Japan c Department of Chemical Engineering, Tokyo Institute of Technology, South Bldg 4, #401C, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 December 2014 Received in revised form 2 February 2015 Accepted 1 March 2015 Available online 13 March 2015

Ozone (O3) generation in a dielectric barrier discharge (DBD) reactor driven by a pulsed power supply was investigated at atmospheric pressure and room temperature. An O3 generation efficiency model is established in which discharge power, O2 concentration, gas flow rate, and volume of the discharge space are included. Constants in the O3 generation efficiency model were obtained by fitting the model with experiment results. O3 concentration can be simply calculated from the energy density and initial O2 concentration. Comparison on O3 concentrations from calculation with references is given. © 2015 Elsevier B.V. All rights reserved.

Keywords: O3 generation DBD Pulsed discharge Model Energy efficiency

Introduction Ozone (O3) is a useful chemical and widely used in many fields, such as advanced oxidation processes (AOPs), chemicalebiological processes (CBP), and semiconductor industry [1,2]. O3 also is a powerful chemical in food and medical treatments [3e7]. Generally, O3 is generated by applying high-voltage to a dielectric barrier discharge (DBD) reactor of a discharge space in which an oxygen (O2)-containing gas is present. It has been understood that a number of tiny breakdown channels occur in the discharge space; those channels are suggested as microdischarges having a time order of microseconds, where O3 is generated [8e18]. O3 generation reactions in microdischarges begin with the dissociation of O2 molecules to oxygen atoms (O) by the impact of O2 with energized electrons in an electric field. O atoms then combine with O2 to yield O3. The energy efficiency of O3 generation is strongly related with the production efficiency of O atoms in the microdischarges. The energy efficiency of O3 generation (x) using an AC power supply can be obtained from the approximation given by Eliasson and Kogelschatz [8], as defined by

x¼

2rD : evd E=n

(1)

Wei et al. [19] developed a numerical model which describes the influence of both electrical and discharge configuration parameters on ozone concentration in pulsed positive dielectric barrier discharge. Factors of pulse repetition frequency, difference of peak pulsed voltage and corona inception voltage, gap length, relative permittivity, gas pressure, gas flow rate, and pulse duration were taken into account. O3 concentration is given in a form in which 9 parameters are required. Related with the dissociation of O2 to O by the impact of O2 with electrons as shown in Eq. (2), the constant k1 of the dissociation process is sensitive to the amplitude of the electric field. The value of k1 in 1/(cm3 s) can be calculated using Eq. (3), when a microwave power supply is applied [20]. k1

e þ O2



! 2O þ e;  

* Corresponding author. E-mail address: [email protected] (Z. Wu). http://dx.doi.org/10.1016/j.elstat.2015.03.001 0304-3886/© 2015 Elsevier B.V. All rights reserved.

k1 ¼ 2  10

(2)

 7:8þ14:7 q

 

þ 10

 7:4þ17:1 q

;

(3)

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S. Yao et al. / Journal of Electrostatics 75 (2015) 35e42

Nomenclature

Symbols d alumina spacer thickness in mm E energy injection over one pulse discharge duration in J/ Hz E0 electric field amplitude in V/cm ED energy density in J/m3 f pulse frequency in Hz F total gas flow rate at the inlet of the DBD reactor in m3/ s DF difference on total gas flow rates between the inlet and outlet of the DBD reactor in m3/s E/n reduced electric field in Td Ii discharge current in A at discharge time ti Iiþ1 discharge current in A at discharge time tiþ1 k ozone generation rate constant in mol0.5 m1.5/J k1 kinetic constant rate of O2 dissociation in 1/(cm3 s) Nm concentration of neutrals in 1/cm3 P energy injection density in W/m3 Pin energy injection power in W PINC inception energy injection power for O3 generation in W rO2 O2 consumption rate in mol/J rO3 O3 generation efficiency in mol/J Te temperature of electrons in eV ti discharge time in s

q¼

E0 $ne  1016 : 1 u2 þ n2e 2 Nm

(4)

k1 was also suggested as a constant of 2  109 1/(cm3 s) [21]. k1 can be obtained by Lee et al. using Eq. (5) in a DBD reactor [22]. k1 is given in a more complicated form in which parameters, such as the energy branch to electron, excitation rate, impact power, discharge area, channel height of gas flow path, and average electron density, are required [23]; such parameters are difficult to be obtained. However, the influence of energy on ozone generation is additionally required if k1 is given in a constant form.

  5:6 : k1 ¼ 4:2  109 exp  Te

(5)

Recently, ozone generation using DBD reactors is still an active study [24e28]. The aim of this work is to find an O3 generation efficiency model in which factors or parameters are simply obtained from experiments. At first, the O3 generation in a DBD reactor driven by a pulse power supply was experimentally investigated. Secondly, an O3 generation efficiency model was established using factors of discharge power (energy injection), O2 concentration, gas flow rate, and volume of the discharge space those are easily given or measured. Finally, constants in the O3 generation efficiency model were obtained after fitting the model with experiment results.

tiþ1 Vi Viþ1 VR x

a b h q vd

ne x rD u [O2]0 [O2] [O3] [O3]0 *

discharge time in s discharge voltage in V at discharge time ti discharge voltage in V at discharge time tiþ1 total discharge space volume in m3 O2 conversion in percentage constant constant O3 generation efficiency in g/kWh energy related factor in eV electron drift velocity in cm/s collision frequency of electrons with neutrals in Hz number of oxygen atoms produced per eV in 1/eV total O2 dissociation rate coefficient in cm3/s frequency of the microwave field in Hz initial O2 concentration in inlet gases of the DBD reactor in mol/m3 O2 concentration in mol/m3 O3 concentration in g/m3 O3 concentration in mol/m3 excited state

Abbreviations CT current transformer DBD dielectric barrier discharge e electron HV high-voltage OSC oscilloscope

reactor. The discharge voltage and current waveforms were measured using a voltage probe (V-P, P6015A, bandwidth DC75 MHz, Tektronix, USA) and a current transformer (CT, TCP0030, bandwidth DC120 MHz, Tektronix, USA), respectively. The signals from the voltage probe and current transformer were digitized and recorded using a digital phosphor oscilloscope (OSC, DPO 3034, bandwidth 300 MHz, Tektronix, USA). The DBD reactor consists of two alumina plates (purity 96%, 50  50  1 mm3, Kyocera, Japan) and two metal electrodes (aluminum tapes, 31  31 mm2), those plates and electrodes were sandwiched closely. The distance between two alumina plates was adjusted using two alumina spacers of different thicknesses (d ¼ 0.29, 0.85, 1.48, 2.03, or 2.36 mm). Microdischarges occur in a discharge space between two alumina plates when high-voltage is applied to two metal electrodes. A gas mixture of nitrogen (N2, purity 99.9999%) and O2 (purity 99.9999%) was supplied to the inlet of the DBD reactor using two mass flow controllers (MFC) at a fixed

Experimental setup Fig. 1 shows the experimental setup for the investigation of O3 generation in a DBD reactor. A pulsed high-voltage (HV) from a power supply (DP-12K5-SCR, PECC, Japan) was applied to the DBD

Fig. 1. Schematic diagram of O3 generation in a DBD reactor.

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37

where Vi, Viþ1, Ii, Iiþ1, ti, and tiþ1 are from the datum sequences of discharge voltage and current waveforms. The energy injection power Pin in W is the product of E and pulse frequency f in Hz. The energy injection power was adjusted by changing the output level of pulse voltage from the pulse power supply at a fixed pulse frequency of 51 Hz. Results and discussion Typical discharge waveforms

Fig. 2. Typical waveforms of discharge voltage and current over one pulse discharge duration.

flow rate of 500 ml/min and at room temperature (298 K) and atmospheric pressure. The residence times of the gas mixture in the discharge space were 0.033, 0.098, 0.17, 0.23, and 0.27 s for spacer thicknesses (d) of 0.29, 0.85, 1.48, 2.03, and 2.36 mm, respectively. O3 concentration in the outlet gases of the DBD reactor was measured using an O3 meter (UV-100, Eco Sensors, USA). All experiments were conducted at atmospheric pressure and room temperature (298 K). All percent gas purities and concentrations are the percentage by volume. The energy injection to the DBD reactor results in the increase in the temperature of the gas in the discharge space. In order to decrease the influence of temperature increase on O3 generation, all discharge experiments were carried out with a time around 5 min after a rest time (without discharges) more than 10 min. The energy injection over one pulse discharge duration from the pulse power supply to the DBD reactor (E in J/Hz) was calculated using Eq. (6) [8,14].

E¼

 XVi þ Viþ1 Ii þ Iiþ1 ðtiþ1  ti Þ ; 2 2 i

(6)

The discharge properties using the DBD reactor and the pulse power supply were measured using the voltage probe and current transformer. The typical pulse voltage is shown in Fig. 2a using alumina spacer (d ¼ 1.48 mm) and initial O2 concentration 21%. The peak voltage is 12.6 kV. The rise time of the pulse voltage and the pulse width are, respectively, 1.55 ms and 3.19 ms. The voltage rise rate is calculated to be 6.53 kV/ms. Discharge current increased with the increase in pulse voltage and peaked at 1.02 A while the pulse voltage peaked (Fig. 2b). The current pulse has a pulse width of 90 ns, indicating that the pulsed microdischarge happens within 90 ns although the voltage pulse has a pulse width of 1.55 ms. There is a negative current pulse with the lowest value of 0.45 A at which the discharge voltage is lowest (4 kV). This fact shows that the discharge current appears to be bipolar, similar with those reported by other researchers [29e31]. The energy injection E was calculated with Eq. (6) using the voltage and current datum sequences shown in Fig. 2. E trends to a constant value of 2.35 mJ/Hz, indicating the energy injection to the DBD reactor is 2.35 mJ/Hz. Ozone generation The typical O3 concentration in the outlet gases of the DBD reactor is shown in Fig. 3a as a function of energy injection power Pin. O3 concentration is zero when energy injection power is less than PINC, here PINC is defined as the inception value of energy injection power for O3 generation. O3 concentration increased with increasing energy injection power almost linearly at an energy injection power higher than PINC. As O3 is generated at the energy injection power higher than PINC, the energy injection power higher than PINC becomes important. Here, net energy injection power is defined as the difference between the energy injection power Pin and PINC. O3 concentration as a function of net energy injection power is shown in Fig. 3b. O3 concentration increases with increasing net energy injection power linearly. Relations of O3 concentrations and net energy injection power (Pin  PINC) at various spacer thicknesses and initial O2

Fig. 3. Typical O3 concentrations versus energy injection power Pin (a) and net energy injections (Pin-PINC) (b).

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S. Yao et al. / Journal of Electrostatics 75 (2015) 35e42

Fig. 4. Relations of O3 concentrations and net energy injection power (Pin  PINC) at various spacer thicknesses (d) and initial O2 concentrations ([O2]0).

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39

concentrations are given in Fig. 4. The spacer thickness d was 0.29, 0.85, 1.48, 2.03, or 2.36 mm, and initial O2 concentrations in the inlet gases of the DBD reactor were controlled to be 10%, 21%, or 100%. Net energy injection power varied up to 280 mW. The incept values of the energy injection power PINC for O3 generation are in the range of 4e20 mW. Ozone generation model The most important reactions for O3 generation and decomposition are as follows [32]: k2

O þ O2



!O3 ;

(7)

k3

O þ O3



!2O2 ;

(8)

k4

O þ O



!O2 :

(9)

When concentrations of O and O3 are low, the O atoms are converted to O3. Here, O3 generation reaction is the combination of Reactions (2) and (7), resulting in Reaction (10) in total, where 0.5 mol of O2 and energy are used to generate one mole of O atoms; another one mole of O2 is used to react with O atoms to yield O3; “heat” is the energy used to heat the gas mixture. Beside the decomposition of O2 to O atoms, there are reactions for O atom production, such as Reactions (13) and (15), if N2 is present in the discharge space [32e34]. About half of O3 is from those reactions in air discharges [35,36]. k

Fig. 5. Typical calculation results at various initial O2 concentrations, where d, (Pin  PINC), and F are 1.48 mm, 50 mW, and 500 ml/min, respectively; O3 concentrations are from Fig. 4 at a net energy injection power of 50 mW.

h i 1:5ð1  aÞkP 1þb V R 1  ð1  xÞ1a ¼ ; ½O2 1a 0 ð1 þ bÞF x¼

(19)

F½O2 0  ðF  DFÞ½O2  1:5½O3  ¼ ; F½O2 0 48½O2 0

(20)

where 48 is molecular weight of O3 for converting O3 concentration from g/m3 to mol/m3. DF is negligible as the O3 concentration is low. Due to Reactions (13) and (15), nitrogen oxide (such as NO2 and N2O) can be found as products when N2 gas is presented in the discharge space. For simplication, conversion of O2 to nitrogen oxide is omitted as nitrogen oxide is at a level less than 10% of O3 [39]. It must be noted that the case when a ¼ 1 does not satisfy our experimental results.

0:5O2 þ energy þ O2 / O3 þ heat;

(10)

e þ energy ¼ e*

(11)

N2 þ e* ¼ N þ N* þ e,

(12)

N* þ O2 ¼ NO þ O,

(13)

N2 þ e* ¼ N2*,

(14)

a calculation

N2* þ O2 ¼ N2O þ O.

(15)

Considering Eq. (19), if P, VR, and F are constant, a can be calculated from a function of a as shown in Eq. (21), where a must be a constant at different [O2]0 and x.

The O3 generation efficiency that was given by Yagi and Tanaka [37] is defined as a ratio of the amount of O3 generated to the discharge energy injected to the DBD reactor. This definition is now widely used for the evaluation of O3 generation efficiency in various O3 generation processes (such as [28,38]). From the fact that O3 generation using a fixed DBD reactor at a constant gas temperature and a constant gas pressure, rO3 is the function of O2 concentration ([O2]) and energy injection density (P, P ¼ (Pin  PINC)/VR) in W/m3, If the energy injection to the discharge space, gas temperature and pressure are uniform, rO3 is generally given in Eqs. (16)(18).

rO3 ¼

F d½O3 0 ¼ k½O2 a P b ; VR dP

(16)

h i 1  ð1  xÞ1a ¼ constant: f ðaÞ ¼ ½O2 1a 0

Fig. 5 shows a typical calculation result of f(a) as a function of a. There are four points T1eT4 (except that a equals 1) partially satisfying Eq. (21). Table 1 shows a values for each point T1eT4 at various spacer thicknesses and initial O2 concentrations and at 50 mW net energy injection powers. a is in a range of 0.09e0.83 Table 1 a values at various spacer thicknesses. d (mm)

rO2

F d½O2  ; ¼ VR dP

1 r : rO3 ¼  1:5 O2

(17)

(18)

When a s 1, the integration of Eqs. (16)(18) gives Eq. (19), where x is O2 conversion and calculated using Eq. (20).

(21)

a Value T1

0.29 0.16 0.85 0.16 1.48 0.26 2.03 0.09 2.36 0.09 Average Standard deviation

T2

T3

T4

Average

0.36 0.46 0.52 0.43 0.43

0.46 0.58 0.59 0.58 0.58

0.80 0.82 0.77 0.83 0.83

0.45 0.51 0.54 0.48 0.48 0.49 3.00  102

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Table 2 k calculation results at various spacer thicknesses and initial O2 concentrations. k [107 (mol0.5 m1.5/J)]

d (mm)

[O2]0 ¼ 10% 0.29 0.85 1.48 2.03 2.36

[O2]0 ¼ 21%

1.25 1.10 0.95 1.19 1.24 1.17 1.27 1.07 1.21 1.09 Average k value: 1.12  107 Standard deviation: 1.16  108

[O2]0 ¼ 100% 0.86 1.26 1.05 1.05 1.07

2 ½O2 0 4 1 ½O3  ¼ 1:5

b calculation In order to get b value, Eq. (16) is changed to Eq. (22),

(22)

From the fact that O3 concentration is linear to net energy injection power (Fig. 4) when the net energy injection powers are low. The slope of the linear relation of O3 concentration and net energy injection power is averaged to be 0.977 with a standard deviation of 4.21  102, which indicated that b is 0.023. The average squared correlation coefficient (R2) is 0.996. Therefore, O3 generation efficiency can be presented as:

rO3 ¼

F d½O3 0 ¼ k½O2 0:49 P 0:023 : VR dP

(23)

For a convince, a and b are set to 0.5 and 0.0, respectively, then Eq. (23) is converted to

rO3 ¼

F d½O3 0 ¼ k½O2 0:5 : VR dP

!2 3 ED 1  1:12  107  0:75pffiffiffiffiffiffiffiffiffiffiffi 5  48; ½O2 0 (27)

and has an average value of 0.49 and standard deviation of 3.0  102. Using the O3 concentrations at 100 mW net energy injection powers as those in Fig. 4, a has an average value of 0.48 and standard deviation of 2.56  102. Those two a average values are almost same at different net energy injection powers; suggesting that the a values are reasonable.

d½O3 0 kVR ¼ ½O2 a P b : dP F

(28) represent the simulation formulas for O3 concentration in g/m3 and O3 generation efficiency h in g/kWh; where 48 is molecule weights of O3, 3.6  106 is a conversion factor of J to kWh. Obviously, O3 concentration and O3 generation efficiency are a function of initial O2 concentration ([O2]0) and energy density (ED). ED is defined in Eq. (29).

(24)

k calculation

h¼

½O3   3:6  106 ; ED

ED ¼

Pin  PINC : F

(28)

(29)

Fig. 6 shows O3 concentrations from experiments and simulation results at various initial O2 concentrations and net energy injection powers. The simulation results agree with experiment results well. Fig. 7 illustrates O3 generation efficiencies from experiment and simulation results using Eqs. (27)(29) at various initial O2 concentrations and spacer thicknesses. O3 generation efficiencies from simulation are constant. O3 generation efficiencies from experiments decrease slightly with increasing energy density at various spacer thicknesses. This finding is similar with the O3 generation using an AC surface DBD reactor, but the level of O3 generation efficiencies are generally lower than those using the AC surface DBD reactor equipped with a water-cooling unit [32]. Those facts implied that the gas temperature is an important factor in order to get a higher O3 generation efficiency. The O3 generation efficiencies with 1.48 mm spacer thickness at an energy density lower than 10,000 J/m3, indicating that the spacer thickness has influence on O3 generation efficiency but the influence becomes small when the energy density is higher than 10,000 J/m3. The differences between O3 efficiencies from experiments and simulation at an energy density less than 20,000 J/m3 is bigger than those at an energy density higher than 20,000 J/m3; those differences are possibly due to the measurement error (about 40%) of O3 meter at a low O3 concentration (around 0.01 g/m3). O3 generation efficiencies from experiment results vary around those from simulation. The O3 generation efficiencies from simulation satisfy experimental results at initial O2 concentrations of 21% better than those at initial O2 concentrations of 10% and 100%. O3 generation

From Eq. (19), one gets k calculation Eq. (25). Table 2 shows the calculation results at various spacer thicknesses and initial O2 concentrations. k has an average value of 1.12  107 mol0.5 m1.5/J and a standard deviation of 1.16  108. Thus, as shown in Eq. (26), O3 generation efficiency is just a function of initial O2 concentration in a fixed DBD reactor.

k¼

pffiffiffiffiffiffiffiffiffiffiffi  pffiffiffiffiffiffiffiffiffiffiffi

½O2 0 1  1  x

rO3 ¼

INC 0:75 Pin P F

;

F d½O3 0 ¼ 1:12  107 ½O2 0:5 : VR dP

(25)

(26)

Simulation of O3 generation O3 concentration and O3 generation efficiency are important factors to show the characteristics of the O3 generator. Eqs. (27) and

Fig. 6. O3 concentrations from experiments and simulation results at various initial O2 concentrations and net energy injection powers (Pin  PINC), where d ¼ 2.36 mm; d: simulation results.

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41

Fig. 8. Comparison of simulated O3 generation efficiencies with experimental results from references. [O3] from simulation was the O3 concentrations calculated with Eq. (23) using the same energy density and initial O2 concentration in each reference. [O3] from reference was the O3 concentrations from experiments reported in marked references. Line presents that O3 from simulation is equal to that from reference. The words “Pulse” and “AC” are pulse and alternative current types of high-voltage power supplies. DBD type reactors were used except of [46].

simulated using Eqs. (27)e(29) when O3 concentration is less than 1 g/m3. The simulation results are generally higher than the experiment results when the O3 concentration is higher than 1 g/ m3; this is possibly due to the O3 decomposition as it is not considered in the model. There is also a requirement to modify the model in which the influences of not only O2 initial concentration and energy density but also the kinds of power supplies, geometries of DBD reactors should be considered. Furthermore, influence of N2 gas on O3 generation via Reactions (11)e(15) should be considered as those reactions result in O3 generation in a different way from O2 gas. Fig. 7. O3 generation efficiency (h) as a function of energy density ED at various initial O2 concentrations. D: d ¼ 0.29 mm,  : d ¼ 0.85 mm, B: d ¼ 1.48, þ: d ¼ 2.03 mm, ◊: d ¼ 2.36 mm. d: Simulation results.

efficiency from simulation is about 5 g/kWh lower than those from experiments at 10% initial O2 concentration, this is possibly due to that there are other O3 generation reactions such as Reactions (13) and (15) those are not included in the O3 generation efficiency model. O3 generation efficiency from simulation is about 10 g/kWh higher than those from experiments at 100% initial O2 concentration and an energy density higher than 10,000 J/m3. The difference at 100% initial O2 concentration is possibly due to the decomposition reaction of O3 (such as Reaction (8)) in the discharge space as O3 concentration at 100% initial O2 concentration is higher than those at lower O2 concentrations. This finding suggests that O3 decomposition reactions and other O3 generation reactions should be considered in order to get a satisfied simulation. Fig. 8 is the comparison of O3 generation efficiencies of the simulated results with experiment conditions given in Refs. [14,28,32,38e49] for 20%e100% initial O2 concentrations using DBD or non-DBD reactors and AC or pulse power supplies. Despite of the remarkable differences in initial O2 concentrations, geometries of the discharge reactors, and kinds of the highvoltage power supplies, our simulation results are generally surround the line on which O3 concentration from simulation is equal to that from experiments in an O3 concentration range below 1 g/ m3. This finding suggested that O3 generation efficiency can be

Conclusions In this study, O3 generation from O2 in a DBD reactor driven by a pulsed power supply was investigated. The influence of initial O2 concentration, discharge gap distances, and energy injection powers on O3 generation has been studied. It has been demonstrated that O3 concentration is linear to energy injection power. O3 generation efficiency is given as:

rO3 ¼

F d½O3 0 ¼ 1:12  107 ½O2 0:5 : VR dP

O3 concentration and O3 generation efficiency can be simply simulated using Eqs. (27)(29), where only the energy density and initial O2 concentration are required. Despite the simplicity of the model, the simulation results agree surprisingly well with the experiments when O3 concentration is less than 1 g/m3. O3 decomposition should be considered when the simulation is carried out at an O3 concentration higher than 1 g/m3.

Acknowledgments Financial supports are provided by Zhejiang Provincial Natural Science Foundation of China (No. LY13B070004), the Program for Zhejiang Leading Team of S&T Innovation (No. 2013TD07), and the Natural Science Foundation Key Program of Zhejiang Province (No. Z5100294).

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