4.1.5 CO2/N2 mixture

Real industrial gas flows typically do not contain pure CO2 but also other gases and impurities. In most cases, N2 is the most important component. It is thus also important to study the effect of N2 on the CO2 conversion and energy efficiency, as well as which products are formed, that is, useful products or harmful NOx compounds. Hence, we developed some models for a CO2/N2 mixture, both for a MW plasma [76] and a DBD [77]. Both models predict that N2 has a beneficial effect on the CO2 splitting, but the mechanism is completely different. In a DBD, the electronically excited metastable N2(A<sup>3</sup> Σþ <sup>u</sup> ) molecules give rise to the enhanced CO2 splitting [77], while in a MW plasma, the N2 vibrational levels help to populate the CO2 vibrational levels, by VV relaxation, and this causes the enhanced CO2 splitting [76]. It should be mentioned, however, that in spite of the higher absolute CO2 conversion upon addition of N2, the effective or overall CO2 conversion will drop in both cases, because of the lower absolute fraction of CO2 in the gas mixture. The effect is minor up to about 60% N2 but more pronounced for higher N2 fractions. As the effective CO2 conversion determines the overall energy efficiency of the process, the latter also drops upon addition of more N2, as some of the energy is used for ionization, excitation, and dissociation of the N2 molecules.

Figure 7 shows that, in order to avoid the formation of NOx compounds, we

Dominant reaction pathways leading to NOx formation in a CO2/N2 DBD plasma, as obtained from the model in [77]. See details in the text. The thickness of the arrow lines corresponds to the time-integrated reaction rates,

and the O species (O, O2, or O3). Reducing the concentrations of reactive N-species in the plasma is not straightforward, so we think that a more viable option to avoid

While 0D chemical kinetic models are most suitable to elucidate the underlying chemical reaction pathways, they cannot describe detailed effects of reactor design. For this purpose, fluid modeling is more appropriate. We will show here how 2D or 3D fluid models can help to obtain a better insight in the basic characteristics of plasma reactors, using two examples of our group PLASMANT, which are of great interest for the application of CO2 conversion, that is, packed bed DBD reactors and

Packed bed DBD reactors are known to enhance the electric field and thus also

the electron temperature, at the contact points between the packing pellets or beads, due to polarization of this dielectric packing. This is illustrated in Figure 8, showing the time-averaged electric field and electron temperature distributions in a 2D representation of a packed bed DBD reactor, for a peak-to-peak voltage of 4 kV and a frequency of 23.5 kHz. These results are obtained from a so-called "channel of voids" model, where the packing beads are not in direct contact, but allow the gas flowing through the packing. This is done to allow a 2D model representing a real 3D geometry (see details in [94]). In spite of the fact that there is no real contact between the beads, the local electric field enhancement in between the beads, due to

Σþ

<sup>u</sup> ) and N)

should prevent the reaction between the reactive N-species (i.e., N2(A3

NOx formation is to remove the O atoms from the plasma, by means of O-

scavengers, or separation membranes or a catalytic system.

indicating the importance of the reactions. Adopted from [77] with permission.

Modeling for a Better Understanding of Plasma-Based CO2 Conversion

DOI: http://dx.doi.org/10.5772/intechopen.80436

4.2 2D or 3D fluid modeling

Figure 7.

reverse vortex flow GA reactors.

4.2.1 Packed bed DBD reactors

21

Both the modeling and experiments also reveal that several NOx compounds are produced in a CO2/N2 plasma, especially NO, NO2, N2O, and N2O5, as was discussed in detail in [77]. A detailed chemical kinetic analysis reveals how the NOx compounds are formed and thus also how this formation can be reduced. As illustrated in Figure 7, N2 is excited to a metastable state N2(A3 Σ<sup>þ</sup> <sup>u</sup> ), as well as dissociated into N atoms, by electron impact reactions. The N2(A3 Σ<sup>þ</sup> <sup>u</sup> ) molecules react with O atoms into NO or with O2 into N2O. The N atoms react with both O and O3 into NO. NO can be converted into NO2 upon reaction with O, but the opposite reaction, upon collision with either O or N atoms, occurs as well, making NO2 the main source of NO production and vice versa (see Figure 7).

Furthermore, the N atoms are trapped in two reaction loops, that is, between NO, NO2, and N2O3 and between NO2, NO3, and N2O5. The only way to escape from these loops is by the reaction of NO2 to N2O (which can react back to N2 and N upon collision with N2(A<sup>3</sup> Σ<sup>þ</sup> <sup>u</sup> ) and N2 + ) or by the reaction of NO with either N atoms or N2(a' 1 Σ� <sup>u</sup> ) molecules, forming again N atoms or N2 molecules (see Figure 7).

Modeling for a Better Understanding of Plasma-Based CO2 Conversion DOI: http://dx.doi.org/10.5772/intechopen.80436

#### Figure 7.

above), so the above mechanism explains the drop in CO2 conversion upon addition of H2O, as the OH radicals created upon H2O dissociation give rise to the back

The above mechanism can also explain why no (significant) methanol (or other oxygenated hydrocarbons) is formed in the CO2/H2O mixture, because all the H atoms needed to form CH and CHO fragments for the formation of methanol are steered to OH and subsequently H2O again. Hence, this chemical kinetic analysis indicates that H2O might not be a suitable H-source for the formation of oxygenated hydrocarbons in a one-step process, because of the abundance of O atoms, O2

It should be noted that this fast reaction between H and O atoms was demonstrated to be useful for the O-trapping in the case of pure CO2 conversion, thus providing a solution for the separation of the CO2 splitting products [120], but in the present case, it is clearly the limiting factor for the formation of oxygenated

Real industrial gas flows typically do not contain pure CO2 but also other gases and impurities. In most cases, N2 is the most important component. It is thus also important to study the effect of N2 on the CO2 conversion and energy efficiency, as well as which products are formed, that is, useful products or harmful NOx compounds. Hence, we developed some models for a CO2/N2 mixture, both for a MW plasma [76] and a DBD [77]. Both models predict that N2 has a beneficial effect on the CO2 splitting, but the mechanism is completely different. In a DBD, the elec-

<sup>u</sup> ) molecules give rise to the enhanced CO2

Σ<sup>þ</sup>

) or by the reaction of NO with either N

Σ<sup>þ</sup>

<sup>u</sup> ), as well as dissociated into

<sup>u</sup> ) molecules react with O atoms

Σþ

for ionization, excitation, and dissociation of the N2 molecules.

in Figure 7, N2 is excited to a metastable state N2(A3

Σ<sup>þ</sup>

<sup>u</sup> ) and N2

N atoms, by electron impact reactions. The N2(A3

NO production and vice versa (see Figure 7).

upon collision with N2(A<sup>3</sup>

1 Σ�

atoms or N2(a'

Figure 7).

20

splitting [77], while in a MW plasma, the N2 vibrational levels help to populate the CO2 vibrational levels, by VV relaxation, and this causes the enhanced CO2 splitting [76]. It should be mentioned, however, that in spite of the higher absolute CO2 conversion upon addition of N2, the effective or overall CO2 conversion will drop in both cases, because of the lower absolute fraction of CO2 in the gas mixture. The effect is minor up to about 60% N2 but more pronounced for higher N2 fractions. As the effective CO2 conversion determines the overall energy efficiency of the process, the latter also drops upon addition of more N2, as some of the energy is used

Both the modeling and experiments also reveal that several NOx compounds are produced in a CO2/N2 plasma, especially NO, NO2, N2O, and N2O5, as was discussed in detail in [77]. A detailed chemical kinetic analysis reveals how the NOx compounds are formed and thus also how this formation can be reduced. As illustrated

into NO or with O2 into N2O. The N atoms react with both O and O3 into NO. NO can be converted into NO2 upon reaction with O, but the opposite reaction, upon collision with either O or N atoms, occurs as well, making NO2 the main source of

Furthermore, the N atoms are trapped in two reaction loops, that is, between NO, NO2, and N2O3 and between NO2, NO3, and N2O5. The only way to escape from these loops is by the reaction of NO2 to N2O (which can react back to N2 and N

<sup>u</sup> ) molecules, forming again N atoms or N2 molecules (see

+

reaction, creating CO2 out of CO.

Plasma Chemistry and Gas Conversion

hydrocarbons.

4.1.5 CO2/N2 mixture

tronically excited metastable N2(A<sup>3</sup>

molecules, and OH radicals, trapping the H atoms.

Dominant reaction pathways leading to NOx formation in a CO2/N2 DBD plasma, as obtained from the model in [77]. See details in the text. The thickness of the arrow lines corresponds to the time-integrated reaction rates, indicating the importance of the reactions. Adopted from [77] with permission.

Figure 7 shows that, in order to avoid the formation of NOx compounds, we should prevent the reaction between the reactive N-species (i.e., N2(A3 Σþ <sup>u</sup> ) and N) and the O species (O, O2, or O3). Reducing the concentrations of reactive N-species in the plasma is not straightforward, so we think that a more viable option to avoid NOx formation is to remove the O atoms from the plasma, by means of Oscavengers, or separation membranes or a catalytic system.

#### 4.2 2D or 3D fluid modeling

While 0D chemical kinetic models are most suitable to elucidate the underlying chemical reaction pathways, they cannot describe detailed effects of reactor design. For this purpose, fluid modeling is more appropriate. We will show here how 2D or 3D fluid models can help to obtain a better insight in the basic characteristics of plasma reactors, using two examples of our group PLASMANT, which are of great interest for the application of CO2 conversion, that is, packed bed DBD reactors and reverse vortex flow GA reactors.

#### 4.2.1 Packed bed DBD reactors

Packed bed DBD reactors are known to enhance the electric field and thus also the electron temperature, at the contact points between the packing pellets or beads, due to polarization of this dielectric packing. This is illustrated in Figure 8, showing the time-averaged electric field and electron temperature distributions in a 2D representation of a packed bed DBD reactor, for a peak-to-peak voltage of 4 kV and a frequency of 23.5 kHz. These results are obtained from a so-called "channel of voids" model, where the packing beads are not in direct contact, but allow the gas flowing through the packing. This is done to allow a 2D model representing a real 3D geometry (see details in [94]). In spite of the fact that there is no real contact between the beads, the local electric field enhancement in between the beads, due to

#### Figure 8.

Calculated time-averaged 2D profiles of the electric field and electron temperature in a packed bed DBD reactor, at a peak-to-peak voltage of 4 kV and a frequency of 23.5 kHz, as obtained from the model in [94]. Adopted from [94] with permission.

their polarization, is still visible, although it must be mentioned that the effect is more pronounced in a so-called "contact point" model (see [94]). This enhanced electric field gives rise to more electron heating and thus to a higher electron temperature in between the beads (see right panel of Figure 8). At this relatively low applied voltage of 4 kV, the plasma is initiated at the contact points and remains in this region, reflecting the properties of a Townsend discharge, while at higher applied voltage, for example, 7.5 kV (peak-to-peak), the discharge will spread out more into the bulk of the reactor, from one void space to the other, ultimately covering the whole gas gap [94]. Such behavior was also reported from experiments. Indeed, by means of an intensified charge-coupled device (ICCD) camera [121, 122], Kim and coworkers also observed that at low applied potential, the discharge stays local at the contact points, while at higher potential, it spreads across the surface of the packing material, and similar observations were also made by Tu et al. [123].

dielectric constant of the packing (ε<sup>r</sup> = 5), plasma ignition between the beads occurs directly in the mode of surface discharges (or surface ionization waves), which can connect with the surface of the adjacent bead; see Figure 9. On the other hand, at high dielectric constants (ε<sup>r</sup> = 1000), no surface streamer jumping toward the

Calculated electron number density distribution as a function of time, for a packed bed DBD reactor in dry air,

microdischarges, so-called local discharges, are generated between the beads; see Figure 10. For intermediate dielectric constants, a mixed mode of surface discharges and local discharges exists [97]. Good qualitative agreement with experi-

The positive restrikes, local discharges, and surface discharges all give rise to the production of reactive species, because they exhibit an enhanced electric field and thus they create a burst of energetic electrons, which produce reactive species by electron impact dissociation. Packed bed reactors are often used for plasma catalysis, where packing beads with different dielectric constants can act as supports for the catalytic materials. Therefore, this study is important to gain a better insight on how different packing materials can influence the performance of packed bed DBD reactors for plasma catalysis. As our results indicate that a higher dielectric constant constrains the discharge to the contact points of the beads, this may limit the catalyst activation due to the limited catalyst surface area in contact with the discharge, and thus it may have implications for the efficiency of plasma catalytic CO2 conversion. Indeed, the best results are not always reached for the highest

adjacent bead surface is observed, and spatially limited filamentary

ments was obtained, as detailed in [97].

with packing beads of ε<sup>r</sup> = 5. Adopted from [97] with permission.

Modeling for a Better Understanding of Plasma-Based CO2 Conversion

DOI: http://dx.doi.org/10.5772/intechopen.80436

Figure 9.

dielectric constant [9, 10].

23

Although the above model was developed for helium, we expect a similar behavior in a CO2 plasma. The higher electron temperature will give rise to more electron impact ionization, excitation, and dissociation of the CO2 molecules, for the same applied power, and this can explain why a packed bed DBD gives a higher CO2 conversion and energy efficiency than an empty reactor.

We also developed a model for a packed bed DBD reactor in dry air, to study the propagation of a plasma streamer [97], as illustrated in Figures 9 and 10. Our calculations reveal that the plasma formation in a packed bed DBD reactor in dry air may exhibit three types of discharge behavior, that is, positive restrikes; filamentary microdischarges, also localized between the packing beads; and surface ionization waves, in agreement with the model by Kruszelnicki et al. [93]. Positive restrikes between the dielectrics result in the formation of filamentary microdischarges. Surface charging creates electric field components parallel to the dielectric surface and leads to the formation of surface ionization waves. At a low

Modeling for a Better Understanding of Plasma-Based CO2 Conversion DOI: http://dx.doi.org/10.5772/intechopen.80436

#### Figure 9.

their polarization, is still visible, although it must be mentioned that the effect is more pronounced in a so-called "contact point" model (see [94]). This enhanced electric field gives rise to more electron heating and thus to a higher electron temperature in between the beads (see right panel of Figure 8). At this relatively low applied voltage of 4 kV, the plasma is initiated at the contact points and remains in this region, reflecting the properties of a Townsend discharge, while at higher applied voltage, for example, 7.5 kV (peak-to-peak), the discharge will spread out more into the bulk of the reactor, from one void space to the other, ultimately covering the whole gas gap [94]. Such behavior was also reported from experiments. Indeed, by means of an intensified charge-coupled device (ICCD) camera [121, 122], Kim and coworkers also observed that at low applied potential, the discharge stays local at the contact points, while at higher potential, it spreads across the surface of the packing material, and similar observations were also made

Calculated time-averaged 2D profiles of the electric field and electron temperature in a packed bed DBD reactor, at a peak-to-peak voltage of 4 kV and a frequency of 23.5 kHz, as obtained from the model in [94].

Although the above model was developed for helium, we expect a similar behavior in a CO2 plasma. The higher electron temperature will give rise to more electron impact ionization, excitation, and dissociation of the CO2 molecules, for the same applied power, and this can explain why a packed bed DBD gives a higher

propagation of a plasma streamer [97], as illustrated in Figures 9 and 10. Our calculations reveal that the plasma formation in a packed bed DBD reactor in dry air may exhibit three types of discharge behavior, that is, positive restrikes; filamentary microdischarges, also localized between the packing beads; and surface ionization waves, in agreement with the model by Kruszelnicki et al. [93]. Positive restrikes between the dielectrics result in the formation of filamentary

microdischarges. Surface charging creates electric field components parallel to the dielectric surface and leads to the formation of surface ionization waves. At a low

We also developed a model for a packed bed DBD reactor in dry air, to study the

CO2 conversion and energy efficiency than an empty reactor.

by Tu et al. [123].

22

Figure 8.

Adopted from [94] with permission.

Plasma Chemistry and Gas Conversion

Calculated electron number density distribution as a function of time, for a packed bed DBD reactor in dry air, with packing beads of ε<sup>r</sup> = 5. Adopted from [97] with permission.

dielectric constant of the packing (ε<sup>r</sup> = 5), plasma ignition between the beads occurs directly in the mode of surface discharges (or surface ionization waves), which can connect with the surface of the adjacent bead; see Figure 9. On the other hand, at high dielectric constants (ε<sup>r</sup> = 1000), no surface streamer jumping toward the adjacent bead surface is observed, and spatially limited filamentary microdischarges, so-called local discharges, are generated between the beads; see Figure 10. For intermediate dielectric constants, a mixed mode of surface discharges and local discharges exists [97]. Good qualitative agreement with experiments was obtained, as detailed in [97].

The positive restrikes, local discharges, and surface discharges all give rise to the production of reactive species, because they exhibit an enhanced electric field and thus they create a burst of energetic electrons, which produce reactive species by electron impact dissociation. Packed bed reactors are often used for plasma catalysis, where packing beads with different dielectric constants can act as supports for the catalytic materials. Therefore, this study is important to gain a better insight on how different packing materials can influence the performance of packed bed DBD reactors for plasma catalysis. As our results indicate that a higher dielectric constant constrains the discharge to the contact points of the beads, this may limit the catalyst activation due to the limited catalyst surface area in contact with the discharge, and thus it may have implications for the efficiency of plasma catalytic CO2 conversion. Indeed, the best results are not always reached for the highest dielectric constant [9, 10].

depicts the formation of a reverse vortex flow. The gas is forced into a tangential motion due to the tangential inlets and travels in this way, close to the sidewalls, toward the closed cathode side at the end (= back of Figure 11(a)) with a velocity around 30–40 m/s. After it has reached the closed cathode end, it moves in the opposite direction, in a smaller inner (reverse) vortex toward the outlet, with much lower velocity, and it exits the reactor with a velocity around 20 m/s (see also the

Modeling for a Better Understanding of Plasma-Based CO2 Conversion

DOI: http://dx.doi.org/10.5772/intechopen.80436

The arc plasma reacts to this gas flow pattern, in the sense that when the gas stream is forced to the center, the arc channel will also move to the center (due to convection), and it will stay in this position as long as the gas keeps it stabilized. Hence, the arc plasma is effectively stabilized in the center, as illustrated in Figure 11(b). Furthermore, as the mass transfer is directed toward the center, the walls are thermally insulated from the hot plasma arc column. The fact that no heat is lost to the reactor walls or other parts of the reactor means that more power can be consumed by the discharge, that is, the plasma generation is more effective. Furthermore, keeping the walls insulated (cold) is also beneficial for the reactor materials itself. The calculated plasma density, for an arc current of 240 mA, is

It should be noted that the results of Figure 11 are for an argon plasma, but we also developed a similar model for a CO2 plasma, but this was only possible in 2D,

We can conclude from Figure 11 that the gas, when moving in the inner vortex flow will largely pass through the arc column. This result is very interesting for the application of CO2 conversion, as it shows that the design of this GAP allows more gas to pass through the arc zone than in a classical (diverging electrodes) GA. Nevertheless, our combined simulations and experiments reveal that the fraction of gas that passes through the arc is still somewhat limited, thus limiting the overall CO2 conversion [19, 20]. By means of this type of 3D fluid dynamics modeling, we aim to predict a more optimized design, to further improve the application of CO2

Plasma-based CO2 conversion is gaining increasing interest, but to improve this application, we need to obtain a better insight in the underlying mechanisms. The latter can be obtained by both plasma chemistry modeling and plasma reactor modeling. This chapter shows some examples of both modeling approaches from our own group, to illustrate what type of information can be obtained from such models and how this modeling can contribute to a better insight, in order to

0D chemical reaction kinetic modeling is very suitable for describing the underlying plasma chemical reaction pathways of the conversion process. We have illustrated this for pure CO2 splitting, showing the difference between a DBD and MW/ GA plasma. Indeed, in a DBD, the CO2 conversion is mainly due to electron impact electronic excitation followed by dissociation with the CO2 ground-state molecules,

"downgraded" the 3D argon model into 2D, and a comparison between both indicated that the difference between the 3D and 2D argon models was limited. Therefore, the 2D CO2 model provides data with reasonable accuracy. We calculated a typical plasma density of 4 <sup>10</sup><sup>19</sup> <sup>m</sup><sup>3</sup> in the arc center, which is about one order of

because of computation time [87]. However, in the same paper, we also

magnitude lower than in argon (4<sup>10</sup><sup>20</sup> <sup>m</sup><sup>3</sup>

, which is a typical value for GA plasmas at atmospheric pressure.

), due to other chemical processes, not

color scale in Figure 11(a)).

around 1020 m<sup>3</sup>

all leading to ionization.

conversion.

25

5. Conclusion

improve this application.

Figure 10.

Calculated electron number density distribution as a function of time, for a packed bed DBD reactor in dry air, with packing beads of ε<sup>r</sup> = 1000. Adopted from [97] with permission.

### 4.2.2 Gliding arc plasmatron (GAP)

Figure 11 illustrates a typical 3D gas flow pattern (a), as well as the calculated electron density profile (b), in a reverse vortex flow (RVF) GA plasma reactor, also called gliding arc plasmatron (GAP), operating in argon. The stream line plot clearly

#### Figure 11.

Calculated steady-state gas flow stream lines (a) and electron density at a time of 5.3 ms, when the arc is stabilized in the center (b), for a reverse vortex flow GA plasma reactor at an arc current of 240 mA, as obtained from the model in [87].

### Modeling for a Better Understanding of Plasma-Based CO2 Conversion DOI: http://dx.doi.org/10.5772/intechopen.80436

depicts the formation of a reverse vortex flow. The gas is forced into a tangential motion due to the tangential inlets and travels in this way, close to the sidewalls, toward the closed cathode side at the end (= back of Figure 11(a)) with a velocity around 30–40 m/s. After it has reached the closed cathode end, it moves in the opposite direction, in a smaller inner (reverse) vortex toward the outlet, with much lower velocity, and it exits the reactor with a velocity around 20 m/s (see also the color scale in Figure 11(a)).

The arc plasma reacts to this gas flow pattern, in the sense that when the gas stream is forced to the center, the arc channel will also move to the center (due to convection), and it will stay in this position as long as the gas keeps it stabilized. Hence, the arc plasma is effectively stabilized in the center, as illustrated in Figure 11(b). Furthermore, as the mass transfer is directed toward the center, the walls are thermally insulated from the hot plasma arc column. The fact that no heat is lost to the reactor walls or other parts of the reactor means that more power can be consumed by the discharge, that is, the plasma generation is more effective. Furthermore, keeping the walls insulated (cold) is also beneficial for the reactor materials itself. The calculated plasma density, for an arc current of 240 mA, is around 1020 m<sup>3</sup> , which is a typical value for GA plasmas at atmospheric pressure.

It should be noted that the results of Figure 11 are for an argon plasma, but we also developed a similar model for a CO2 plasma, but this was only possible in 2D, because of computation time [87]. However, in the same paper, we also "downgraded" the 3D argon model into 2D, and a comparison between both indicated that the difference between the 3D and 2D argon models was limited. Therefore, the 2D CO2 model provides data with reasonable accuracy. We calculated a typical plasma density of 4 <sup>10</sup><sup>19</sup> <sup>m</sup><sup>3</sup> in the arc center, which is about one order of magnitude lower than in argon (4<sup>10</sup><sup>20</sup> <sup>m</sup><sup>3</sup> ), due to other chemical processes, not all leading to ionization.

We can conclude from Figure 11 that the gas, when moving in the inner vortex flow will largely pass through the arc column. This result is very interesting for the application of CO2 conversion, as it shows that the design of this GAP allows more gas to pass through the arc zone than in a classical (diverging electrodes) GA. Nevertheless, our combined simulations and experiments reveal that the fraction of gas that passes through the arc is still somewhat limited, thus limiting the overall CO2 conversion [19, 20]. By means of this type of 3D fluid dynamics modeling, we aim to predict a more optimized design, to further improve the application of CO2 conversion.
