Study of ozone generation in an atmospheric dielectric barrierdischarge 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, Zhejiang310018, Chinab Chemical Research Group, Research Institute of Innovative Technology for the Earth, 9-2 Kizugawadai, Kizugawa-shi, Kyoto 619-0292, Japanc Department of Chemical Engineering, Tokyo Institute of Technology, South Bldg 4, #401C, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan
a r t i c l e i n f o
Article history:Received 16 December 2014Received in revised form2 February 2015Accepted 1 March 2015Available online 13 March 2015
Ozone (O3) generation in a dielectric barrier discharge (DBD) reactor driven by a pulsed power supplywas investigated at atmospheric pressure and room temperature. An O3 generation efficiency model isestablished in which discharge power, O2 concentration, gas flow rate, and volume of the discharge spaceare included. Constants in the O3 generation efficiency model were obtained by fitting the model withexperiment results. O3 concentration can be simply calculated from the energy density and initial O2
Ozone (O3) is a useful chemical and widely used in many fields,such as advanced oxidation processes (AOPs), chemicalebiologicalprocesses (CBP), and semiconductor industry [1,2]. O3 also is apowerful chemical in food and medical treatments [3e7]. Gener-ally, O3 is generated by applying high-voltage to a dielectric barrierdischarge (DBD) reactor of a discharge space in which an oxygen(O2)-containing gas is present. It has been understood that anumber of tiny breakdown channels occur in the discharge space;those channels are suggested as microdischarges having a timeorder of microseconds, where O3 is generated [8e18]. O3 genera-tion reactions in microdischarges begin with the dissociation of O2molecules to oxygen atoms (O) by the impact of O2 with energizedelectrons in an electric field. O atoms then combine with O2 to yieldO3. The energy efficiency of O3 generation is strongly related withthe production efficiency of O atoms in the microdischarges.
The energy efficiency of O3 generation (x) using an AC powersupply can be obtained from the approximation given by Eliassonand Kogelschatz [8], as defined by
x ¼ 2rDevdE=n
: (1)
Wei et al. [19] developed a numerical model which describes theinfluence of both electrical and discharge configuration parameterson ozone concentration in pulsed positive dielectric barrierdischarge. Factors of pulse repetition frequency, difference of peakpulsed voltage and corona inception voltage, gap length, relativepermittivity, gas pressure, gas flow rate, and pulse duration weretaken into account. O3 concentration is given in a form in which 9parameters are required.
Related with the dissociation of O2 to O by the impact of O2 withelectrons as shown in Eq. (2), the constant k1 of the dissociationprocess is sensitive to the amplitude of the electric field. The valueof k1 in 1/(cm3 s) can be calculated using Eq. (3), when a microwavepower supply is applied [20].
Symbolsd alumina spacer thickness in mmE energy injection over one pulse discharge duration in J/
HzE0 electric field amplitude in V/cmED energy density in J/m3
f pulse frequency in HzF total gas flow rate at the inlet of the DBD reactor in m3/
sDF difference on total gas flow rates between the inlet and
outlet of the DBD reactor in m3/sE/n reduced electric field in TdIi discharge current in A at discharge time tiIiþ1 discharge current in A at discharge time tiþ1
k ozone generation rate constant in mol0.5 m1.5/Jk1 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 WPINC inception energy injection power for O3 generation in
WrO2
O2 consumption rate in mol/JrO3
O3 generation efficiency in mol/JTe temperature of electrons in eVti discharge time in s
tiþ1 discharge time in sVi discharge voltage in V at discharge time tiViþ1 discharge voltage in V at discharge time tiþ1
VR total discharge space volume in m3
x O2 conversion in percentagea constantb constanth O3 generation efficiency in g/kWhq energy related factor in eVvd electron drift velocity in cm/sne collision frequency of electrons with neutrals in Hzx number of oxygen atoms produced per eV in 1/eVrD total O2 dissociation rate coefficient in cm3/su frequency of the microwave field in Hz[O2]0 initial O2 concentration in inlet gases of the DBD
reactor in mol/m3
[O2] O2 concentration in mol/m3
[O3] O3 concentration in g/m3
[O3]0 O3 concentration in mol/m3
* excited state
AbbreviationsCT current transformerDBD dielectric barrier dischargee electronHV high-voltageOSC oscilloscope
S. Yao et al. / Journal of Electrostatics 75 (2015) 35e4236
q ¼ E0$ne�u2 þ n2e
�12 Nm
� 1016: (4)
k1 was also suggested as a constant of 2 � 10�9 1/(cm3 s) [21]. k1can be obtained by Lee et al. using Eq. (5) in a DBD reactor [22]. k1 isgiven in a more complicated form inwhich parameters, such as theenergy branch to electron, excitation rate, impact power, dischargearea, 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 addi-tionally required if k1 is given in a constant form.
k1 ¼ 4:2� 10�9 exp��5:6Te
�: (5)
Recently, ozone generation using DBD reactors is still an activestudy [24e28]. The aim of this work is to find an O3 generationefficiency model in which factors or parameters are simply ob-tained from experiments. At first, the O3 generation in a DBDreactor driven by a pulse power supply was experimentally inves-tigated. Secondly, an O3 generation efficiency model was estab-lished using factors of discharge power (energy injection), O2concentration, gas flow rate, and volume of the discharge spacethose are easily given or measured. Finally, constants in the O3generation efficiency model were obtained after fitting the modelwith experiment results.
Fig. 1. Schematic diagram of O3 generation in a DBD reactor.
Experimental setup
Fig. 1 shows the experimental setup for the investigation of O3generation in a DBD reactor. A pulsed high-voltage (HV) from apower supply (DP-12K5-SCR, PECC, Japan) was applied to the DBD
reactor. The discharge voltage and current waveforms weremeasured using a voltage probe (V-P, P6015A, bandwidthDC�75 MHz, Tektronix, USA) and a current transformer (CT,TCP0030, bandwidth DC�120 MHz, Tektronix, USA), respectively.The signals from the voltage probe and current transformer weredigitized 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 weresandwiched closely. The distance between two alumina plates wasadjusted using two alumina spacers of different thicknesses(d ¼ 0.29, 0.85, 1.48, 2.03, or 2.36 mm). Microdischarges occur in adischarge space between two alumina plates when high-voltage isapplied to two metal electrodes. A gas mixture of nitrogen (N2,purity 99.9999%) and O2 (purity 99.9999%) was supplied to the inletof the DBD reactor using two mass flow controllers (MFC) at a fixed
Fig. 2. Typical waveforms of discharge voltage and current over one pulse dischargeduration.
S. Yao et al. / Journal of Electrostatics 75 (2015) 35e42 37
flow rate of 500 ml/min and at room temperature (298 K) and at-mospheric pressure. The residence times of the gas mixture in thedischarge space were 0.033, 0.098, 0.17, 0.23, and 0.27 s for spacerthicknesses (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 wasmeasured using an O3 meter (UV-100, Eco Sensors, USA). All ex-periments were conducted at atmospheric pressure and roomtemperature (298 K). All percent gas purities and concentrationsare the percentage by volume. The energy injection to the DBDreactor results in the increase in the temperature of the gas in thedischarge space. In order to decrease the influence of temperatureincrease on O3 generation, all discharge experiments were carriedout 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 thepulse power supply to the DBD reactor (E in J/Hz) was calculatedusing Eq. (6) [8,14].
E ¼Xi
�Vi þ Viþ1
2Ii þ Iiþ1
2ðtiþ1 � tiÞ
�; (6)
Fig. 3. Typical O3 concentrations versus energy injection
where Vi, Viþ1, Ii, Iiþ1, ti, and tiþ1 are from the datum sequences ofdischarge voltage and current waveforms.
The energy injection power Pin inW is the product of E and pulsefrequency f in Hz. The energy injection power was adjusted bychanging the output level of pulse voltage from the pulse powersupply at a fixed pulse frequency of 51 Hz.
Results and discussion
Typical discharge waveforms
The discharge properties using the DBD reactor and the pulsepower supply were measured using the voltage probe and currenttransformer. The typical pulse voltage is shown in Fig. 2a usingalumina spacer (d¼ 1.48 mm) and initial O2 concentration 21%. Thepeak voltage is 12.6 kV. The rise time of the pulse voltage and thepulse width are, respectively, 1.55 ms and 3.19 ms. The voltage riserate is calculated to be 6.53 kV/ms. Discharge current increased withthe increase in pulse voltage and peaked at 1.02 A while the pulsevoltage peaked (Fig. 2b). The current pulse has a pulse width of90 ns, indicating that the pulsed microdischarge happens within90 ns although the voltage pulse has a pulse width of 1.55 ms. Thereis a negative current pulse with the lowest value of �0.45 A atwhich the discharge voltage is lowest (�4 kV). This fact shows thatthe discharge current appears to be bipolar, similar with those re-ported by other researchers [29e31]. The energy injection E wascalculated with Eq. (6) using the voltage and current datum se-quences 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 DBDreactor is shown in Fig. 3a as a function of energy injection powerPin. O3 concentration is zero when energy injection power is lessthan PINC, here PINC is defined as the inception value of energy in-jection power for O3 generation. O3 concentration increased withincreasing energy injection power almost linearly at an energy in-jection power higher than PINC. As O3 is generated at the energyinjection power higher than PINC, the energy injection power higherthan PINC becomes important. Here, net energy injection power isdefined as the difference between the energy injection power Pinand PINC. O3 concentration as a function of net energy injectionpower is shown in Fig. 3b. O3 concentration increases withincreasing net energy injection power linearly.
Relations of O3 concentrations and net energy injection power(Pin � PINC) at various spacer thicknesses and initial O2
power Pin (a) and net energy injections (Pin-PINC) (b).
Fig. 4. Relations of O3 concentrations and net energy injection power (Pin � PINC) at various spacer thicknesses (d) and initial O2 concentrations ([O2]0).
S. Yao et al. / Journal of Electrostatics 75 (2015) 35e4238
S. Yao et al. / Journal of Electrostatics 75 (2015) 35e42 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 theinlet gases of the DBD reactor were controlled to be 10%, 21%, or100%. Net energy injection power varied up to 280 mW. The inceptvalues of the energy injection power PINC for O3 generation are inthe range of 4e20 mW.
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 concentra-tions are from Fig. 4 at a net energy injection power of 50 mW.
The most important reactions for O3 generation and decompo-sition are as follows [32]:
Oþ O2 ����!k2 O3; (7)
Oþ O3 ����!k3 2O2; (8)
Oþ O ����!k4 O2: (9)
When concentrations of O and O3 are low, the O atoms areconverted to O3. Here, O3 generation reaction is the combination ofReactions (2) and (7), resulting in Reaction (10) in total, where0.5 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 reactionsfor O atom production, such as Reactions (13) and (15), if N2 ispresent in the discharge space [32e34]. About half of O3 is fromthose reactions in air discharges [35,36].
0:5O2 þ energyþ O2 /k
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)
N2* þ O2 ¼ N2O þ O. (15)
The O3 generation efficiency that was given by Yagi and Tanaka[37] is defined as a ratio of the amount of O3 generated to thedischarge energy injected to the DBD reactor. This definition is nowwidely used for the evaluation of O3 generation efficiency in variousO3 generation processes (such as [28,38]). From the fact that O3generation using a fixed DBD reactor at a constant gas temperatureand 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 andpressure are uniform, rO3
is generally given in Eqs. (16)�(18).
rO3¼ F
VR
d½O3�0dP
¼ k½O2�aPb; (16)
�rO2¼ F
VR
d½O2�dP
; (17)
rO3¼ � 1
1:5rO2
: (18)
When a s 1, the integration of Eqs. (16)�(18) gives Eq. (19),where x is O2 conversion and calculated using Eq. (20).
½O2�1�a0
h1� ð1� xÞ1�a
i¼ 1:5ð1� aÞkP1þbVR
ð1þ bÞF ; (19)
x ¼ F½O2�0 � ðF� DFÞ½O2�F½O2�0
¼ 1:5½O3�48½O2�0
; (20)
where 48 is molecular weight of O3 for converting O3 concentrationfrom g/m3 to mol/m3. DF is negligible as the O3 concentration islow.
Due to Reactions (13) and (15), nitrogen oxide (such as NO2 andN2O) can be found as products when N2 gas is presented in thedischarge space. For simplication, conversion of O2 to nitrogenoxide 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 ourexperimental results.
a calculation
Considering Eq. (19), if P, VR, and F are constant, a can becalculated from a function of a as shown in Eq. (21), where a mustbe a constant at different [O2]0 and x.
f ðaÞ ¼ ½O2�1�a0
h1� ð1� xÞ1�a
i¼ constant: (21)
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) partiallysatisfying Eq. (21). Table 1 shows a values for each point T1eT4 atvarious spacer thicknesses and initial O2 concentrations and at50 mW net energy injection powers. a is in a range of 0.09e0.83
Table 2k calculation results at various spacer thicknesses and initial O2 concentrations.
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.
S. Yao et al. / Journal of Electrostatics 75 (2015) 35e4240
and has an average value of 0.49 and standard deviation of3.0 � 10�2. Using the O3 concentrations at 100 mW net energyinjection powers as those in Fig. 4, a has an average value of 0.48and standard deviation of 2.56 � 10�2. Those two a average valuesare almost same at different net energy injection powers; sug-gesting that the a values are reasonable.
b calculation
In order to get b value, Eq. (16) is changed to Eq. (22),
d½O3�0dP
¼ kVR
F½O2�aPb: (22)
From the fact that O3 concentration is linear to net energy in-jection power (Fig. 4) when the net energy injection powers arelow. The slope of the linear relation of O3 concentration and netenergy injection power is averaged to be 0.977 with a standarddeviation of 4.21 � 10�2, which indicated that b is 0.023. Theaverage squared correlation coefficient (R2) is 0.996. Therefore, O3generation efficiency can be presented as:
rO3¼ F
VR
d½O3�0dP
¼ k½O2�0:49P0:023: (23)
For a convince, a and b are set to 0.5 and 0.0, respectively, thenEq. (23) is converted to
rO3¼ F
VR
d½O3�0dP
¼ k½O2�0:5: (24)
k calculation
From Eq. (19), one gets k calculation Eq. (25). Table 2 shows thecalculation results at various spacer thicknesses and initial O2concentrations. k has an average value of 1.12 � 10�7 mol0.5 m1.5/Jand a standard deviation of 1.16 � 10�8. Thus, as shown in Eq. (26),O3 generation efficiency is just a function of initial O2 concentrationin a fixed DBD reactor.
k ¼ffiffiffiffiffiffiffiffiffiffiffi½O2�0
p 1�
ffiffiffiffiffiffiffiffiffiffiffi1� x
p �0:75 Pin�PINC
F
; (25)
rO3¼ F
VR
d½O3�0dP
¼ 1:12� 10�7½O2�0:5: (26)
Simulation of O3 generation
O3 concentration and O3 generation efficiency are importantfactors to show the characteristics of the O3 generator. Eqs. (27) and
(28) represent the simulation formulas for O3 concentration in g/m3
and O3 generation efficiency h in g/kWh; where 48 is moleculeweights of O3, 3.6 � 106 is a conversion factor of J to kWh. Obvi-ously, O3 concentration and O3 generation efficiency are a functionof initial O2 concentration ([O2]0) and energy density (ED). ED isdefined in Eq. (29).
½O3� ¼½O2�01:5
241�
1� 1:12� 10�7 � 0:75
EDffiffiffiffiffiffiffiffiffiffiffi½O2�0p
!235� 48;
(27)
h ¼ ½O3�ED
� 3:6� 106; (28)
ED ¼ Pin � PINCF
: (29)
Fig. 6 shows O3 concentrations from experiments and simula-tion results at various initial O2 concentrations and net energy in-jection powers. The simulation results agree with experimentresults well. Fig. 7 illustrates O3 generation efficiencies fromexperiment and simulation results using Eqs. (27)�(29) at variousinitial O2 concentrations and spacer thicknesses. O3 generationefficiencies from simulation are constant. O3 generation efficienciesfrom experiments decrease slightly with increasing energy densityat various spacer thicknesses. This finding is similar with the O3generation using an AC surface DBD reactor, but the level of O3generation efficiencies are generally lower than those using the ACsurface DBD reactor equipped with a water-cooling unit [32]. Thosefacts implied that the gas temperature is an important factor inorder to get a higher O3 generation efficiency. The O3 generationefficiencies with 1.48 mm spacer thickness at an energy densitylower than 10,000 J/m3, indicating that the spacer thickness hasinfluence on O3 generation efficiency but the influence becomessmall when the energy density is higher than 10,000 J/m3.
The differences between O3 efficiencies from experiments andsimulation at an energy density less than 20,000 J/m3 is bigger thanthose at an energy density higher than 20,000 J/m3; those differ-ences are possibly due to the measurement error (about 40%) of O3meter at a low O3 concentration (around 0.01 g/m3). O3 generationefficiencies from experiment results vary around those fromsimulation. The O3 generation efficiencies from simulation satisfyexperimental results at initial O2 concentrations of 21% better thanthose at initial O2 concentrations of 10% and 100%. O3 generation
Fig. 7. O3 generation efficiency (h) as a function of energy density ED at various initialO2 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.
Fig. 8. Comparison of simulated O3 generation efficiencies with experimental resultsfrom 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 ref-erences. Line presents that O3 from simulation is equal to that from reference. Thewords “Pulse” and “AC” are pulse and alternative current types of high-voltage powersupplies. DBD type reactors were used except of [46].
S. Yao et al. / Journal of Electrostatics 75 (2015) 35e42 41
efficiency from simulation is about 5 g/kWh lower than those fromexperiments at 10% initial O2 concentration, this is possibly due tothat there are other O3 generation reactions such as Reactions (13)and (15) those are not included in the O3 generation efficiencymodel. O3 generation efficiency from simulation is about 10 g/kWhhigher than those from experiments at 100% initial O2 concentra-tion and an energy density higher than 10,000 J/m3. The differenceat 100% initial O2 concentration is possibly due to the decomposi-tion reaction of O3 (such as Reaction (8)) in the discharge space asO3 concentration at 100% initial O2 concentration is higher thanthose at lower O2 concentrations. This finding suggests that O3decomposition reactions and other O3 generation reactions shouldbe considered in order to get a satisfied simulation.
Fig. 8 is the comparison of O3 generation efficiencies of thesimulated results with experiment conditions given inRefs. [14,28,32,38e49] for 20%e100% initial O2 concentrationsusing 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 high-voltage power supplies, our simulation results are generally sur-round the line on which O3 concentration from simulation is equalto that from experiments in an O3 concentration range below 1 g/m3. This finding suggested that O3 generation efficiency can be
simulated using Eqs. (27)e(29) when O3 concentration is less than1 g/m3. The simulation results are generally higher than theexperiment results when the O3 concentration is higher than 1 g/m3; this is possibly due to the O3 decomposition as it is notconsidered in the model. There is also a requirement to modify themodel in which the influences of not only O2 initial concentrationand energy density but also the kinds of power supplies, geom-etries of DBD reactors should be considered. Furthermore, influ-ence of N2 gas on O3 generation via Reactions (11)e(15) should beconsidered as those reactions result in O3 generation in a differentway from O2 gas.
Conclusions
In this study, O3 generation from O2 in a DBD reactor driven by apulsed power supply was investigated. The influence of initial O2concentration, discharge gap distances, and energy injectionpowers on O3 generation has been studied. It has been demon-strated that O3 concentration is linear to energy injection power. O3generation efficiency is given as:
rO3¼ F
VR
d½O3�0dP
¼ 1:12� 10�7½O2�0:5:
O3 concentration and O3 generation efficiency can be simplysimulated using Eqs. (27)�(29), where only the energy density andinitial O2 concentration are required.
Despite the simplicity of the model, the simulation results agreesurprisingly well with the experiments when O3 concentration isless than 1 g/m3. O3 decomposition should be considered when thesimulation is carried out at an O3 concentration higher than 1 g/m3.
Acknowledgments
Financial supports are provided by Zhejiang Provincial NaturalScience Foundation of China (No. LY13B070004), the Program forZhejiang Leading Team of S&T Innovation (No. 2013TD07), and theNatural Science Foundation Key Program of Zhejiang Province (No.Z5100294).
S. Yao et al. / Journal of Electrostatics 75 (2015) 35e4242
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