Improvement in discharge characteristics and energy yield of ozone generation via configuration optimization of a coaxial dielectric barrier discharge reactor

Chuang Liang,Zhihao Liu,Baochang Sun,,Haikui Zou,Guangwen Chu,

1 State Key Laboratory of Organic-Inorganic Composites,Beijing University of Chemical Technology,Beijing 100029,China

2 Research Center of the Ministry of Education for High Gravity Engineering and Technology,Beijing University of Chemical Technology,Beijing 100029,China

Keywords:Coaxial DBD reactor Configuration optimization Ozone generation Discharge characteristics Energy yield

ABSTRACT Dielectric barrier discharge(DBD)has been widely employed in ozone generation.However,the technology still exhibits relatively low energy yield(EY)referring to its theoretical value.In this work,EY of ozone generation was improved by optimizing the mesh number,electrode length,and dielectric material in a coaxial DBD reactor with two wire mesh electrodes.Meanwhile,the discharge characteristics were investigated to elucidate the effect of reactor configuration on EY.Results showed that the discharge characteristics were improved by increasing the mesh number,electrode length,and relative permittivity.When the mesh number was increased from 40 to 100,an improvement of approximately 48% in EY was obtained.Additionally,higher EY values were obtained using corundum as the dielectric material relative to polytetrafluoroethylene and quartz.Ultimately,EY in the optimal DBD reactor could reach 326.77 g.(kW.h)-1.Compared with the reported DBD reactor,the coaxial DBD reactor with the mesh electrode and the dielectric material of corundum could effectively improve EY,which lays a foundation for the design of high-efficiency coaxial DBD reactor.

1.Introduction

Plasma,which is an ionized gaseous substance generated by gas discharge,exhibits prominent performance in various application fields,especially in environmental remediation [1],biotechnology[2],chemical industry [3],and material industry [4].As a typical discharge type,dielectric barrier discharge (DBD) is characterized by the presence of at least one dielectric layer in the discharge gap,which enhances the electric field intensity and improves the discharge uniformity [5,6].In the past decades,various DBD reactors have been designed to enhance ozone generation,mainly including volume [7],coplanar [8],and surface DBD reactors [9].Among these,the volume DBD reactors have been widely employed due to the simple configuration,quick heat radiation,and slow electrode corrosion [10].Ordinarily,the volume DBD reactors can be further divided into the planar and coaxial DBD reactors.Owing to the merits of the good air-tightness and the feasible industrial amplification[5,11],the coaxial DBD reactors with a stable structure are conducive to the applications involving gasphase reactions,especially in ozone generation [12-14].

When discharge plasma is employed to generate ozone,energy yield (EY) is the ratio of ozone production to discharge power,which reflects the energy consumption of ozone generation in the discharge process [15,16].The theoreticalEYof ozone generation can reach up to approximately 1200 g.(kW.h)-1in oxygen stream [17,18].However,for the commercial ozone generators designed by the non-thermal plasma technology,the practicalEYis no higher than 100 g.(kW.h)-1in air stream and 200 g.(kW.h)-1in oxygen stream [17,19],which are far below the theoretical value.Therefore,improvingEYof ozone generation in the plasma reactor is of pivotal importance and has gained ever-growing concern.

Configuration optimization of the plasma reactor was deemed as one of the most effective pathways to improveEY.On this basis,investigations on the electrode geometry [20],electrode material[21],dielectric material[22],and discharge volume[23]have been extensively carried out.To better illustrate how configuration optimization enhances ozone generation,the discharge characteristics were simultaneously studied.Takakietal.[24] investigated the discharge characteristics andEYin a DBD reactor with the electrode structures of plane,trench,and multipoint.Results showed that the multipoint electrode could form more plasma channels at the same input energy,which contributed to the improvement ofEY.Recently,wire mesh as the electrode has been employed in DBD reactors.Yeetal.[25] proposed that the discharge spots were dependent on the aperture of the mesh electrode in the DBD reactor and mainly formed on the grid node of the mesh electrode.Huietal.[26] found that the mesh electrode with smaller apertures could form more uniform dischargeviathe visualization technology.Therefore,the mesh electrode structure,especially the aperture,can significantly affect the electric discharge in the DBD reactor.However,the relationship between the mesh electrode structure and ozone generation performance needs to be further clarified.

In this work,the effects of mesh number,electrode length,and dielectric material on the discharge characteristics andEYwere systematically investigated in a coaxial DBD reactor with two wire mesh electrodes.Subsequently,the comparison of ozone generation performance between the optimal coaxial DBD reactor and the reported coaxial DBD reactors was carried out.This work aims to lay a foundation for the design of high-efficiency coaxial DBD reactor.

2.Experimental

2.1.Experimental setup

Fig.1(a)and(b)show the schematic diagram of the coaxial DBD reactor,which contained two dielectric layers.Two stainless steel wire meshes were located outside the outer dielectric layer as the grounding electrode and inside the inner dielectric layer as the high-voltage electrode,respectively.The discharge gap between the two dielectric layers was fixed at 5 mm.The temperature of the DBD reactor increased with the increase of discharge time,which was disadvantageous to ozone generation [27,28].Therefore,a gas cooling system was set in the reactor to control the gas temperature.Cooling gas flowed through the reactor and was exitedviaa hollow polytetrafluoroethylene (PTFE) pipe located in the interior of the inner dielectric tube.Gas temperature in the reactor was maintained at approximately 25°C.The detailed parameters of the coaxial DBD reactor are listed in Table 1.

Table 1 Detailed parameters of the coaxial DBD reactor

Fig.1 (c) shows the schematic diagram of the experimental setup,including the coaxial DBD reactor,power supply (CTP-2000 KP,Nanjing Suman Electronic Co.,Ltd.,China),oscilloscope(DS1052E,Beijing RIGOL Technology Co.,Ltd.,China),ozone analyzer (UV-2300C,Shandong ZHIPRER M&C Technology Co.,Ltd.,China),and ozone absorber.The oxygen stream (purity: 99.5%)with a flow rate of 1.5 m3.h-1was introduced into the coaxial DBD reactorviaa gas distributor,flowed from the top to bottom of the discharge gap,and then was exited from the reactorviaa gas outlet.Ozone was generated during this process,and its concentration was detected in the ozone analyzer.The tail gas was treatedviaMnO2catalyst to decompose ozone [29].All experiments were conducted at normal temperature and pressure.Before each experiment,the residual gas in the experimental setup needed to be evacuatedviathe oxygen stream.To minimize the experimental error,the ozone concentration was recorded every 10 seconds for five times.

2.2.Electrical measurements and analysis

The discharge signals were recorded using the digital oscilloscope.The discharge voltage was obtainedviaa capacitive divider(the divider ratio was 1000:1) using a passive probe (TPP0100,10:1,Tektronix).The electric charge(Q)was obtained by detecting the voltage(Vm)of a non-inductive capacitor(Cm=0.47 μF)which was connected to the discharge circuit in series using another passive probe (Eq.(1)).Subsequently,theV-QLissajous diagram was obtained.A typical Lissajous diagram in the coaxial DBD reactor is shown in Fig.2.The discharge power (P) could be calculated from the Lissajous diagram by Eq.(2) [30,31]:

wherefPandSdenote the pulse frequency and the area of theV-QLissajous diagram,respectively.Additionally,the peak voltage (Vp)and transferred electric charge (Qd,Eq.(3)) could also be obtained by the Lissajous diagram [32,33].

The specific input energy (SIE) was directly related to the energy consumption and formation of reactive species in the discharge process and was calculated by Eq.(4) [34].Ozone production () was calculated by Eq.(5) [35].As aforementioned,EYof ozone generation denoted the ozone production per electrical power consumed by discharge plasma and was calculated by Eq.(6) [36]:

3.Results and Discussion

3.1.Effect of mesh number on the discharge characteristics and EY

Mesh number (the number of mesh openings per inch) was an essential parameter to reflect the characteristic size of wire mesh[37].Hence,the effects of wire mesh electrodes with different mesh numbers on the discharge characteristics and ozone generation performance were investigated.Schematic diagram of the mesh electrode was shown in Fig.3.The characteristic sizes of the mesh electrode,including the diameter and aperture,were exhibited in Table 2.Considering the stiffness and structure of the wire mesh,the diameter and aperture decreased with the increase of mesh number.As shown in Fig.4,whenVpwas fixed,PandQdincreased with the increase of mesh number from 40 to 100.Then,a distinct decline was observed when the mesh number was further increased to 150.Owing to the uneven surface structure of wire mesh,an inhomogeneous electric field distribution was formed,which could cause the corona discharge around the grid node of wire mesh in the initial stage of gas discharge [38].Abundant electrons were accumulated on the dielectric surface to further trigger the gas breakdown in the discharge gap.Subsequently,the streamer channels appeared and mainly existed in the grid node of the wire mesh.Therefore,the filamentous discharge could be observed in this coaxial DBD reactor.When the mesh number was increased,the number of grid nodes also increased,which contributed to the increase in the number of discharge filaments.Hence,PandQdincreased with the increase of mesh number at a fixedVp.Nevertheless,the streamer channels were randomly distributed when the mesh number was excessive,resulting in inferior discharge characteristics.

Fig.4.Effects of Vp on (a) P and (b) Qd for different mesh numbers.

Fig.5.Effect of SIE on (a) PO3 and (b) EY for different mesh numbers.

3.2.Effect of electrode length on the discharge characteristics and EY

To reveal the effect of electrode length on the discharge characteristics andEY,three different lengths were investigated in this work: 10.0,12.5,and 15.0 cm.As shown in Fig.6,PandQdincreased with the increase of electrode length at a fixedVp.The electrode length was related to the discharge volume.The discharge volume increased with the increase of electrode length,resulting in the increase of energy used for ionization of the gas molecules in the discharge gap.Therefore,when increasing the electrode length,PandQdincreased at a fixedVp.Besides,under the samePandQd,the larger the electrode length,the smaller theVpvalues,indicating that gas breakdown was more easily triggered by increasing the electrode length.

Fig.6.Effects of Vp on (a) P and (b) Qd for different electrode lengths.

Fig.7.Effects of SIE on (a) PO3 and (b) EY for different electrode lengths.

3.3.Effect of dielectric material on the discharge characteristics and EY

The dielectric was the core component of the DBD reactor,and its material had a significant effect on ozone generation.In this work,the effects of PTFE,quartz,and corundum on the discharge characteristics andEYwere investigated.As shown in Fig.8,under a fixedVp,thePandQdvalues using corundum as the dielectric material were higher than those using PTFE and quartz as the dielectric material.It could be explained by the following aspects:(i)The relative permittivity(εr)was employed to describe the ability of dielectric material to store the electric charges.εrof PTFE,quartz,and corundum was 2.0,3.7,and 9.8,respectively.The dielectric material with higher εrpossessed larger electric conductivity,which could reduce the reactor impedance and enlarge the discharge current.(ii)In the DBD process,the electric charges were accumulated on the dielectric surface to form an inverse electric field,and then the discharge was gradually extinguished.Whereas the inverse electric field was weakened with the increase of εr[42].Hence,abundant electric charges were accumulated to make the discharge extinguish for the dielectric material with a large εr,indicating the dissociation degree of gas in the discharge gap was higher when using corundum as the dielectric material.(iii) As shown in Fig.S1,large values ofCdcould be obtained when using corundum as the dielectric material.It was attributed to the factor that the accumulated charges could be distributed to a large area on the surface of corundum,which contributed to the improvement in the electric discharge [43].Based on the above analysis,better discharge characteristics could be obtained when using corundum as the dielectric material compared with PTFE and quartz.The conclusions were consistent with those in the literature.Liuetal.[28]investigated the effects of glass,quartz,polycarbonate,and PTFE on the discharge characteristics in a DBD reactor.Better discharge uniformity and larger discharge power could be obtained when using quartz as the dielectric material relative to PTFE.Wangetal.[44] found that larger dielectric barrier capacitance and higher NO removal efficiency were obtained in the DBD reactor with the dielectric material of corundum compared with ceramic and quartz.

Fig.8.Effect of Vp on (a) P and (b) Qd for different dielectric materials.

Fig.9 shows the effect of SIE onandEYfor different dielectric materials,indicating that corundum exhibited outstanding ozone generation performance in the coaxial DBD reactor.As shown in Fig.9(a),increased and then decreased with the increase of SIE using PTFE and corundum,which was different from quartz.The increase ofwas attributed to the enhancement in electric field intensity with increasing SIE values.More streamer channels were formed in the discharge gap,which was conducive to the dissociation of oxygen molecules.However,the thermal decomposition of ozone was also strengthened owing to the increase in the number of high-temperature streamer channels[21].In this work,the negative effect of the thermal decomposition of ozone gradually surpassed the positive effect of the intense plasma-induced emission with increasing SIE constantly,leading to a decrease in

Fig.9.Effect of SIE on (a) PO3 and (b) EY for different dielectric materials.

Fig.9(b)exhibitsEYas the function of SIE for different dielectric materials.Results showed thatEYdecreased with the increase of SIE and was in the order of corundum ＞quartz ＞PTFE.Relative low values ofEYwere obtained when using PTFE as the dielectric material.The possible reason was that some polymer particles with low dissociation energy were released after colliding with the high-energy electrons,and then converted into small molecules by consuming the reactive species[45].Hence,the dielectric material of PTFE was unsuitable for ozone generation.

A tremendous improvement inEYwas achieved when using corundum as the dielectric material relative to quartz and PTFE,andEYcould reach up to 326.77 g.(kW.h)-1.As described above,better discharge characteristics were obtained when using corundum as the dielectric material,which was conducive to the dissociation of oxygen molecules to generate ozone.Besides,the porous structure on the corundum surface was more abundant than that on the quartz surface,which facilitated the absorption of oxygen molecules to extend its residence time [46].However,a dramatic drop inEYwas observed for corundum from Fig.9(b),which could be attributed to the following analysis.On one hand,more energy in the electric discharge was consumed to convert into heat,photons,and vibration energy with the increase of SIE.On the other hand,Eqs.(4) and (6) indicated that the increase of SIE resulted in the decrease ofEY.Simultaneously,as aforementioned,the values ofincreased and then decreased with the increase of SIE when using corundum as the dielectric material.Eqs.(5) and (6)revealed that the decrease ofalso contributed to the decrease ofEY.On this basis,a dramatic drop inEYwas observed when using corundum as the dielectric material in this work.

3.4.Comparison of ozone generation performance between the

optimalcoaxialDBDreactorandthereportedcoaxialDBDreactors

Table 3 shows the comparison of ozone generation performance between the optimal coaxial DBD reactor and the reported coaxial DBD reactors.andEYin this work could reach 30.75 g.h-1and 326.77 g.(kW.h)-1,respectively,which were higher than those shown in the literature[47-49].In this coaxial DBD reactor,a large discharge gap (5 mm) was designed,which could achieve a large capacity to accommodate oxygen on the premise of the same residence time.Hence,large values ofPO3were obtained in this work.Besides,compared with other electrode structures and dielectric materials,mesh electrode and corundum could significantly improve the discharge characteristics,thereby effectively enhancing ozone generation in the coaxial DBD reactor.

Table 3 Comparison of ozone generation performance between the coaxial DBD reactor and the reported coaxial DBD reactors

However,it could be seen that the value ofEYin this work was smaller than that in the work of Layatietal.[50].It might be attributed to the fact that the large discharge gap could result in the instability of electric discharge [51].The coaxial DBD reactor with such a large discharge gap not only could be used to generate ozone but also possessed the potential to treat the liquid phase in situ by forming a liquid film in the discharge gap,such as water purification and material modification.

4.Conclusions

In this work,the effects of mesh number,electrode length,and dielectric material on the discharge characteristics andEYof ozone generationwere systematicallyinvestigatedin a coaxial DBD reactor with two wire mesh electrodes.Results showed that the discharge characteristics were improved,and an increase of approximately 48% inEYwas obtained by increasing the mesh numbers from 40 to 100.When the electrode length was increased,the dischargecharacteristics were improved,whereas the values ofEYdecreased.Furthermore,the dielectric material of corundum demonstrated better discharge characteristics and ozone generation performance compared with PTFE and quartz.Ultimately,EYcould reach up to 326.7 7 g.(kW.h)-1in the coaxial DBD reactor with the mesh electrode and the dielectric material of corundum.This work lays a foundation for the design of high-efficiency coaxial DBD reactor.

Data Availability

Data will be made available on request.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21725601 and 2187081058).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2022.11.016.

Nomenclatures

Cddielectric barrier capacitance,pF

Cmcapacitance of non-inductive capacitor,μF

CO3ozone concentration,g.m-3

Egelectric field intensity

EYenergy yield,g.(kW.h)-1

fPpulse frequency,Hz

ldthickness of the outer dielectric layer,mm

lgwidth of discharge gap,mm

Pdischarge power,W

PO3ozone production,g.h-1

Qelectric charge,nC

Qdtransferred electric charge,nC

QGgas flow rate,m3.h-1

rd1inner radius of the outer dielectric layer,mm

rd2outer radius of the outer dielectric layer,mm

rcouter radius of the inner dielectric layer,mm

Vapplied voltage,kV

Vmvoltage of non-inductive capacitor,V

Vppeak voltage,kV

εgrelative permittivity of the air

εrrelative permittivity of the dielectric layer