Influence of catalyst packing configuration on the discharge characteristics of dielectric barrier discharge reactors - a numerical investigation

Siddharth Gadkari1, 2,a) and Sai Gu1

1)_{Department of Chemical and Process Engineering, University of Surrey,}
Guildford GU2 7XH, UK

2)_{Centre for Environment and Sustainability, University of Surrey, Guildford,}
Surrey GU2 7XH, United Kingdom

A two-dimensional numerical fluid model is developed for studying the influence of packing configurations on dielectric barrier discharge (DBD) characteristics. Dis-charge current profiles, and time averaged electric field strength, electron number density and electron temperature distributions are compared for the three DBD con-figurations, plain DBD with no packing, partially packed DBD and fully packed DBD. The results show a strong change in discharge behaviour occurs when a DBD is fully packed as compared to partial packing or no packing. While the average electric field strength and electron temperature of a fully packed DBD are higher relative to the other DBD configurations, the average electron density is substantially lower and may impede the DBD reactor performance under certain operating conditions. Possible scenarios of the synergistic effect of the combination of plasma with catalysis are also discussed.

I. INTRODUCTION 1

Packed bed dielectric barrier discharge (DBD) reactors that can operate at atmospheric 2

pressure and ambient conditions are being increasingly used in remediation of a variety of 3

gaseous pollutants such as SOx, NOx, VOCs, etc1,2,3. When a catalyst is used as a packing 4

material, the synergistic effect of the so-called ‘plasma-catalysis’ helps to activate the catalyst 5

at relatively lower temperature and improve the selectivity towards desirable products2,4_{.}

6

The catalysts can either be fully packed in the entire discharge gap or be placed either 7

radially or axially covering only a fraction of the discharge gap. This second configuration 8

known as partial packing, has been shown to avoid the typical disadvantages of the “packed 9

bed effect”5. 10

While many studies with DBD in a fully packed configuration have found enhanced per-11

formance in comparison to a plasma alone system2,6,7,8,9,10_{, some studies have also reported}

12

a decrease in efficiency6,11,12,13. Experimental studies comparing the nature of interactions 13

between packing materials and plasma in fully and partially packed DBDs have observed a 14

significantly strong filamentary discharge in the partially packed DBD as compared to the 15

combination of weak filamentary discharge and surfaces discharge observed in fully packed 16

DBD5,13_{. It is postulated that suppression of filamentary discharges in a fully packed DBD}

17

may be the reason behind the negative performance observed in some cases5,13_{. Packing}

18

typically induces effective polarisation and enhances the electric field strength at the con-19

tact points, however the discharge behaviour changes depending on the particular packing 20

configuration. The changes in polarisation have a strong influence on the electron density 21

and electron energy in the discharge gap, which play a major role in deciding the DBD 22

performance. 23

However in experimental studies it is difficult to characterize the discharge parameters, 24

and the packing itself adds an extra hindrance in visibility for discharge diagnostics. Com-25

putational modelling can be used as a complimentary tool to understand the discharge char-26

acteristics and optimize the system in a directed way, providing more quantitative process-27

parameter relationships14_{. There have been some computational studies on packed bed}

28

DBD reactors in the past15,16,17,18,19,20,21,22,23,24_{. Kang et al.}16 _{developed a two dimensional}

29

(2D) model for ferroelectric packed bed barrier discharge reactor but did not include any 30

plasma chemistry. Russ et al.17 _{used the so-called ‘donor cell’ method and developed 2D}

fluid model for a packed bed DBD filled with synthetic, dry exhaust gas. However this study 32

presented very short one-directional discharge (of a few 10s of nanoseconds) with limited 33

results of spatial electric field and electron density distributions17_{. Van Laer and Bogaerts}18

34

developed a fluid model for a packed bed DBD with pure helium gas. The 3D packing 35

was represented using two complementary axisymmetric 2D geometries. They highlighted 36

that addition of packing led to a higher electron field strength and electron temperature at 37

the contact points. Van Laer and Bogaerts19 also studied the effect of discharge gap size 38

and dielectric constant of the packing material on the discharge characteristics using a 2D 39

axisymmetric fluid model. In our previous work24, we have developed a 2D fluid model for 40

a partially packed DBD and showed the increase in electric field strength, electron energy 41

and electron density as compared to a DBD with no packing. 42

However at present there is no numerical study comparing the discharge characteristics 43

of DBDs in partially packed and fully packed configurations. In this work, for the first time, 44

we have developed a 2D fluid model for helium DBD in three configurations, no packing, 45

partial packing and full packing, to study how inclusion of packing material in different 46

configurations affects the discharge characteristics and the reactor performance. 47

II. MODEL DESCRIPTION

48

In this work, the 2-D fluid model is applied to a cylindrical DBD reactor with two co-49

axial metal cylinders as electrodes. The dimensions of the reactor, plasma chemistry and the 50

governing equations used in the fluid model have been described in our previous study24_{.}

51

The fully packed DBD is represented using a similar approach as described by Van Laer 52

and Bogaerts18_{. However instead of using two separate 2D geometries, we have used one}

53

geometry, as shown in Figure1. This geometry provides a comparative representation of the 54

three dimensional packing. While the the packing pellets are not touching each other or the 55

walls, the gap in between the beads and in between the walls and the pellets is optimized 56

using electrostatic models to represent a similar electric field strength as observed when 57

the contact points are included. This simplification not only saves a large computational 58

expense, it also resolves the problem of using two separate geometries to represent the 3D 59

problem. The proposed geometry also confirms with the so-called ‘channel of voids’ approach 60

recommended by Van Laer and Bogaerts18_{. The geometries used for representing the plain}

FIG. 1. 2D geometry used in the model for representing a fully packed DBD.

DBD with no packing and the DBD with partial packing have been described in our previous 62

study24_{.}

63

III. RESULTS & DISCUSSION 64

Figure 2 shows the discharge current profiles for the three DBD configurations. The 65

current profile for DBD with no packing exhibits a single distinct pulse in both the positive 66

and negative half cycle of applied potential, which is characteristic of atmospheric pressure 67

glow discharge (APGD), typically obtained for helium DBD25_{. Inclusion of packing, both}

68

partial and full, alters the discharge behaviour of the DBD, which is reflected in the change 69

in discharge current profiles. However, while partial packing leads to a very small change 70

(exhibiting two current peaks in the positive half cycle as opposed to the single peak observed 71

in DBD without packing), the fully packed DBD shows a much dramatic transition in the 72

discharge current signal and amplitude. 73

For the fully packed DBD, both the current signal profile and amplitude undergo a major 74

transition suggesting a large alteration of the plasma discharge. The current profile (figure 75

2C) shows multiple current peaks (4-5) of varying amplitude in both the positive and the 76

negative half cycle of the applied potential. Such a transition in discharge current profile has 77

been observed previously in both experiments5,13_{and numerical simulations}18_{of fully packed}

78

DBDs. It is postulated that this occurs due to the multiple breakdowns across the different 79

points in the discharge gap as the gap voltage crosses the breakdown voltage multiple times 80

FIG. 2. Discharge current profiles (red solid lines) in an atmospheric DBD in helium with (A) no packing, (B) partial packing, and (C) full packing, during one cycle of the applied potential of 3 kV peak-to-peak (black dashed lines) at a frequency of 20 kHz.

during one period of applied potential13,18 _{. It should also be noted that the amplitude of}

81

the discharge current also decreases significantly, which is in accordance to that observed 82

in experimental studies13_{. One reason for such striking change in discharge current profile}

83

signal and amplitude is the ‘packed-bed effect’, which occurs due to the significant decrease 84

in discharge volume that reduces the distance a typical microdischarge can travel in the 85

discharge gap5,13,26_{.}

86

Based on the packing configurations, the void fraction of the DBD also changes. For the 87

fully packed configuration, packed with spherical pellets, the void fraction if estimated to be 88

about 39%27_{. Such dramatic change in void fraction has been shown to cause a significant}

89

change in discharge behaviour5_{. Discharge of molecular gases in a fully packed DBD is}

90

modified from the typical filamentary mode of discharge to a prevalent surface discharge 91

on the packing surface and spatially limited microdischarges5,28,29_{. Comparatively, the void}

FIG. 3. Time averaged logarithm to the base 10 of electric field strength (V/m) over one period of applied potential 4.0 kV peak-to-peak for (A) plain DBD with no packing, (B) partially packed DBD and (C) fully packed DBD.

fraction of partially packed DBD is 99.5 %, almost same as the DBD with no packing, which 93

is also reflected in the similar discharge current profiles obtained for the two discharges 94

(figure 2A and B). 95

More information on the exact nature of the discharge mode can be obtained by observing 96

the distributions of electric field, electron density and electron temperature (energy) in the 97

discharge gap. Figures 3, 4, and 5 show the time averaged electric field strength, electron 98

number density, and electron temperature distributions over one period of applied potential 99

for the three DBD configurations. The time averaged electric field distributions for the three 100

DBD configurations are quite different from each other (figure 3). As expected there is an 101

electric field enhancement at the contact points between pellet-pellet and pellet-dielectric 102

barrier. Such an enhancement is typically attributed to increased charge deposition due to 103

more effective polarisation at the contact points13,18_{.}

104

The electric field strength for partially packed and fully packed DBD are in the similar 105

range i.e. between 4.5 to 7. Comparatively, the same value for an empty DBD is far less, 106

reaching to a maximum of only 4.9. The packing in both the configurations, distorts and 107

enhances the electric field strength, mainly at places where there is close contact between 108

packing pellets or between packing and dielectric barrier. The distortion of the electric field 109

distribution in the discharge gap depends on the shape of the packing material. For spherical 110

objects, intensification of the electric field occurs at the poles of the solid object with a 111

local minima observed at the equatorial plane30,31. For the particle packing arrangements 112

described in this work, this enhancement in electric field occurs at the vertical poles (top 113

surface of the particles), which can be clearly seen in case of fully packed DBD from the 114

time averaged distribution plot shown in figure 3C. Enhanced electric field can be observed 115

at the vertical poles and in the contact region between the two pellets. However there is 116

also a local minima in the equatorial plane, which falls along the mid section of all pellets 117

parallel to the dielectric barrier. Thus we obtain regions of low electric field strength between 118

the two pellets along these equatorial planes. Similarly for the partially packed DBD, we 119

obtain electric field strength enhancement at the contact points and at the top surface of 120

the packing, and the local minima in the region between the two pellets along the equatorial 121

plane. The partially packed DBD is discussed in more detail in our previous study24. These 122

results on electric field strength enhancement at the poles of packing material and at the 123

contact points between packing and dielectric layer are in accordance with that reported 124

previously in experimental and numerical studies on packed bed DBDs13,16,18,22. 125

The time averaged electric field strength of fully packed DBD is of similar magnitude to 126

that of the partially packed DBD. However it should be noted that there are more regions 127

of high electric field strength in the discharge gap (i.e. values > 6 ) for fully packed DBD 128

compared to partially packed DBD. This factor needs to be seen in the context of discharge 129

volume and the high energy electron density distribution in that volume. As discussed in the 130

previous study24_{, partial packing does not lead to any major reduction in discharge volume}

131

or the total electron count compared to a plain DBD with no packing. On the other fully 132

packed configuration leads to a drastic reduction in discharge volume13_{. To reflect on this}

more, we need to study the time averaged electron density distributions, which are shown 134

in figure 4. 135

As can be seen from figure4, the time averaged electron density for plain DBD and DBD 136

with partial packing are of similar magnitude ( ∼ 1010_{), which falls within the typical range}
137

for APGD. For the fully packed DBD however, the time averaged electron density is three 138

orders of magnitude smaller (∼ 107_{), and this value is characteristic of atmospheric pressure}
139

Townsend discharge (APTD)32_{.} _{Based on the magnitude of electron density, it can be}

140

inferred that inclusion of full packing inside the discharge gap of a helium DBD operating 141

at applied potential 4.0 kV peak-to-peak and 20 kHz, leads to a transition of discharge 142

mode from APGD to APTD. This also explains the significantly different discharge current 143

signal and amplitude, as observed in figure 2 and is in accordance with the experimental 144

observations of a significant transition in discharge behaviour reported previously for fully 145

packed DBDs5,13. 146

147

The electron density distribution shows that for the DBD with no packing, the maximum 148

electrons are located in the vertical central plane (parallel to the dielectric barrier), whereas 149

for DBD with partial packing, the maximum electron density is concentrated along the 150

horizontal central plane (perpendicular to the dielectric barrier). While the electron density 151

of the fully packed DBD is significantly lower compared to the other two DBD configurations, 152

the distribution is more spread out across the discharge gap in between the pellets (figure 153

4C). 154

Electron temperature is another important parameter of a DBD. This is directly corre-155

lated with the decomposition efficiency of the reactor, as higher the electron temperature, 156

more will be the energy to break down chemical bonds of molecular pollutants. As can be 157

seen from figure 5, the maximum electron temperature observed for DBD with no packing 158

(∼ 3 eV) is substantially less than that obtained for DBDs with packing. 159

Comparing between partially and fully packed DBDs, the time averaged electron tem-160

perature is higher for fully packed DBD with a maximum of ∼ 18-19 eV, while the same 161

for partially packed DBD reaching only up to ∼ 14 eV. It should also be noted that there 162

are more regions of high electron temperature intensity in the discharge gap of the fully 163

packed DBD, resembling the same points which showed high electric field strength, as seen 164

in figure 3. Whereas for partially packed DBD, the high electron temperature is restricted 165

to the contact points between pellets and dielectric barrier. Looking only at the electron 166

FIG. 4. Time averaged electron density (cm−3) distribution over one period of applied potential 4.0 kV peak-to-peak for (A) plain DBD with no packing, (B) partially packed DBD and (C) fully packed DBD.

temperature distribution, one can conclude that the decomposition efficiency of the fully 167

packed DBD would be higher than that compared to both partially packed DBD and DBD 168

with no packing. However we also need to account for the significant reduction in electron 169

density and the dramatic change in discharge behaviour caused by a reduction in discharge 170

volume in fully packed DBDs. It should also be noted that this work only accounts for the 171

change in discharge behaviour based on the electrical properties of the packing material, 172

whereas the influence of the chemical and physical characteristics of the packing and how 173

they would change under the influence of discharge have not been considered. It has been 174

observed through experimental studies that the catalytic activity of a packing material can 175

be enhanced under exposure of plasma, and this factor would also play an important role in 176

FIG. 5. Time averaged electron temperature (eV) distribution over one period of applied potential 4.0 kV peak-to-peak for (A) plain DBD with no packing, (B) partially packed DBD and (C) fully packed DBD.

calculating the performance efficiency of the DBD. We have shown here numerically that a 177

helium DBD discharge behaviour would undergo a substantial change in fully packed con-178

figuration as compared to DBD with no packing or partial packing. This is in accordance 179

with previous experimental studies that have also shown a significant change in discharge 180

behaviour of fully packed DBDs when compared to DBDs with no packing5,13. 181

Another important parameter that governs the performance of a DBD reactor is the 182

dissipated power density. Table I shows the spatially and time averaged dissipated power 183

density, electron density and electron energy for one voltage cycle, at applied potential 4.0 184

kV peak-to-peak for the three DBD configurations. 185

As we can see from table I, as the packing inside the discharge gap of DBD increases, 186

TABLE I. Spatially and time averaged dissipated power density (W m−3), electron density (cm−3) and electron temperature (eV) over one period of applied potential 4.0 kV peak-to-peak with applied frequency 20 kHz for different DBD configurations

DBD configuration Power density Electron density Electron energy

(W m−3) (cm−3) (eV)

No Packing 4.00x105 2.3x1010 2.19

Partial Packing 5.64x105 2.20x1010 3.21

Full Packing 7.13x105 5.8x106 7.33

the power density also increases. Thus while the DBD with no packing has a power den-187

sity of 4.00x105_{, inclusion of partial packing increases it ∼ 40 % to 5.64x10}5_{, and the full}
188

packing increases the power density by ∼ 80 % to 7.13x105. Similarly we see an increase 189

in average electron temperature as the volume fraction of packing material increases. How-190

ever the increase in electron temperature is much more pronounced for a fully packed DBD, 191

which shows ∼ 230 % increase when compared to DBD without any packing. These values 192

contribute to the higher decomposition efficiency of full packed DBDs as observed in sev-193

eral experimental studies2,6,7,8,9. However the dramatic change in discharge behaviour in a 194

fully packed DBD also decreases the electron density in the discharge, which shows a drop 195

of ∼ 100 % when compared to DBD with no packing. On the other hand, the partially 196

packed DBD does not show any significant drop in electron density (given negligible change 197

in discharge volume and discharge behaviour ), while showing a ∼ 40 % increase in power 198

density and ∼ 46 % increase in electron temperature. This explains the relatively higher 199

performance efficiency of partially packed DBD as compared to plain DBDs with no packing 200

as observed in some experimental studies5,33,34,35_{.}

201

The discharge characteristics in a DBD are closely linked to the available void volume 202

in the discharge gap. Compared to the DBD with no packing, the void volume of partially 203

packed and fully packed DBD are 99.5% and ∼ 39% respectively. The smaller void fraction 204

of fully packed DBD reduces the electron density, however the close packing also intensifies 205

the electric field strength near the several contact points between the pellets. The higher 206

electric field strength increases the electron energy and the power density as the amount of 207

packing is increased. 208

The overall performance of the DBD in a fully packed configuration would depend on the 209

trade-off between the reduction in efficiency due to the packed bed effect (leading to change 210

in discharge mode and drastic reduction in electron number density), and the enhancement 211

in electric field strength leading to the increase in electron temperature and the possible 212

improvement in the catalyst activity of the packing under exposure of plasma discharge. 213

If it is assumed there is no influence of plasma on the catalytic activity of the packing 214

material, the major factor that can reduce the efficiency of fully packed DBD compared to 215

plain DBD would be the reduction in electron density. This factor is related to the chemical 216

composition and the residence time of the pollutant gases in the DBD. If the high energy 217

electron count is proportionate enough to disintegrate the pollutants in the given operating 218

conditions, the fully packed DBD would show a better decomposition efficiency compared 219

to other configurations. However if the high energy electron count is inadequate, the fully 220

packed DBD may show no change or even reduction in efficiency compared to DBD with no 221

packing. 222

IV. CONCLUSION

223

To summarize, we have used a 2D fluid model to understand the influence of different 224

packing configurations on the discharge characteristics of a helium DBD. For the operating 225

parameters used in this study, we have found that there is a complete change in discharge 226

mode in the fully packed DBD when compared to either partially packed DBD or DBD 227

without any packing. However at a given applied potential, a fully packed DBD shows 228

more effective polarisation, leading to enhanced electric field strength and higher average 229

electron temperatures compared to the other DBD configurations. The interactions between 230

plasma and packing are very complex and instead of a general rule, the synergistic effect 231

of plasma-packing interaction needs to be understood individually for each specific case. 232

For packing material that also acts as a catalyst, it would be essential to evaluate both 233

the chemical and physical changes that packing undergoes due to plasma discharge and the 234

change in electrical discharge behaviour induced by the packing. Influence of plasma on the 235

catalytic activity can be studied using atomic scale simulations based on molecular dynamics 236

or density functional theory36_{.}

ACKNOWLEDGMENTS 238

The authors would like to gratefully acknowledge the Engineering and Physical Sciences 239

Research Council (EPSRC) UK for the financial support of this work through EPSRC grants 240

EP/M013162/1 and EP/K036548/2. 241

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