Nonthermal Plasma Chemistry and Physics
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On the other hand, the discharge gap appears to affect the discharge behaviour, and thus also the CO 2 conversion. Aerts et al. Furthermore, Aerts et al. However, the plasma appears more filamentary at high frequency 75 kHz compared to at low frequency 6 kHz. Ozkan et al.
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This was explained by Paulussen et al. In thermochemical reactions, the gas temperature is one of the most important parameters governing the reaction rates. For the plasma-based conversion in a DBD, however, the effect of temperature is not so clear. Paulussen et al. In addition, Wang et al. Brehmer et al. The effect of the dielectric material used in the DBD reactor is another topic of debate. By studying the formation of a conductive coating, Belov et al.
However, Aerts et al. Wang et al. However, from the calculation of the plasma power in both studies, it appears that the more sophisticated dielectrics mainly increased the efficiency between the input power and plasma power, and not the effective plasma-conversion energy efficiency. Nevertheless, these dielectrics allowed operation under lower voltages, which could be beneficial for certain processes.
In summary, different dielectrics may allow for easier igniting and streamer formation, but not necessarily more energy-efficient plasma-conversion chemistry. Besides the dielectric material, also the electrode material can be varied.
The Cu and Au electrodes yielded a relative increase in the conversion by a factor 1. Furthermore, the maximum energy efficiency of the Au electrode was almost three times higher than the energy efficiency of the Rh electrode under the same conditions.
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However, besides the fact that some of these electrodes or coatings are more expensive, compared to the inert stainless steel electrode, they also are susceptible to chemical erosion i. As mentioned above, some researchers have also added inert gases, such as N 2 , Ar and He, to ignite the plasma more easily. This also has several effects on the discharge characteristics, conversion, energy efficiency and even by-product formation in the case of N 2.
The addition of He and Ar leads to an increase in the CO 2 conversion, but the effective conversion decreases, since there is less CO 2 present in the mixture and the increased conversion is not sufficient to counteract this drop in the CO 2 fraction. As a result, the energy efficiency decreases as well. Snoeckx et al. Indeed, N 2 enhances the absolute CO 2 conversion, due to the dissociation of CO 2 upon collision with N 2 metastable molecules, and this effect is strong enough to compensate for the lower CO 2 content in the mixture.
However, the presence of N 2 in the mixture leads to the formation of unwanted by-products, i. N 2 O and several NO x compounds, with concentrations in the range of several ppm, which can give rise to severe air pollution problems. Besides all the experimental work, great advances have also been made in modelling the plasma chemistry for CO 2 conversion in a DBD. Electron-impact ionization is also important, but is compensated by the fact that a large fraction of the formed ions will eventually recombine, resulting in the formation of CO 2.
Splitting from the vibrationally excited states is found to be of minor importance in a DBD. A 1D fluid model, with roughly the same chemistry, was developed by Ponduri et al. Finally, as mentioned in Section 4. In the case of pure CO 2 splitting, the addition of a packing will not influence the formation of products, since no hydrogen source is available.
Hence, most of research work focuses on increasing the conversion and energy efficiency by physical effects. Generally, when tested, the CO 2 conversion in packed-bed DBD reactors is always higher than in the corresponding empty reactors, albeit Fig. Nevertheless, at high conversions, we see that the packed-bed reactors are generally more efficient.
As such, in general, adding a packing seems to allow the system to operate at the same energy efficiency, but significantly increases the conversion. One of the more recent and interesting works is from Butterworth et al. However, they also increase the reactor breakdown voltage and they lead to partial discharging, i. Comparison with the works of other researchers shows that quite often, insufficient electric field strengths are applied for complete reactor discharging to occur.
Hence, packing materials for plasma-catalysis should be tested with equivalent reactor operating conditions. It is therefore important to ensure that either: a complete discharging occurs in the reactor, or b the partial reactor discharging is quantified. DBDs have the advantage of being very scalable and easy to operate, but their current energy efficiency makes it doubtful that they will be the most suitable technology for pure CO 2 splitting. The main set-ups used for CO 2 conversion are the surfaguide MW discharge 2. More details about these set-ups can be found in Section 4.
To obtain the best values for the conversion and energy efficiency, several approaches have already been proposed, including changing the applied power, gas flow rate, flow type, reactor geometry, gas temperature, and admixture gases, or by introducing catalytic packing materials, as well as by carrying out extensive plasma chemistry modelling to gain a better insight into the underlying mechanisms. The authors concluded that the applied values for a reduced electric field, i. It should be noted that all the experiments reporting such high energy efficiencies were performed at reduced pressures, which might not be beneficial for high-throughput industrial implementation, despite the high flow rates of up to 75 SLPM.
Again, like for the DBD results above, it is important to note that some of the data have been recalculated to represent coherent values for the conversions and energy efficiencies. As already mentioned, pressure has one of the most important influences on the MW discharge and on its performance for CO 2 conversion, and most studies are carried out at reduced pressure. Several more complex flow types and geometries have already been studied to optimize the MW discharge performance for CO 2 conversion.
Supersonic flows in MW discharges have proven to reduce the losses of vibrational levels upon collision with ground-state molecules i. Similarly, the addition of a vortex gas flow, more specifically a reverse vortex, led to a significant improvement in CO 2 conversion and energy efficiency compared to a forward vortex flow. For MW discharges, the effect of gas temperature is quite complicated. A signature of the desired non-equilibrium conditions in a MW plasma is a low or moderate gas temperature in the order of — K , while vibrational and electron temperatures are higher i.
However, when increasing pressure at an SEI of about 0. It is thus clear that the gas temperature should be kept as low as possible to reduce vibrational energy losses via VT relaxation. Ar, ,, He and N 2. Indeed, while in the DBD, the metastable electronically excited N 2 molecules give rise to enhanced CO 2 conversion see the previous section , while in the MW plasma, the improvement is due to the vibrationally excited N 2 molecules. On the other hand, the vibrationally excited N 2 molecules can also react with O atoms, leading to the production of NO x in undesirable concentrations, as was also observed for a DBD see previous section.
Just like for DBDs, big leaps forward have been made in the past few years regarding modelling the plasma chemistry to better understand and improve CO 2 conversion in MW discharges.
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This means that all the vibrational levels up to the dissociation limit need to be taken into account in accurate models for a CO 2 MW plasma, as well as all the reactions with these different vibrationally excited levels. The models predict that, besides electron-impact dissociation of the vibrationally excited levels, also collisions with neutrals will become important as dissociation mechanisms, as was also suggested by one optical characterization study, and this is the key to achieving maximum energy efficiency of the CO 2 splitting process.
To be able to perform multidimensional modelling investigations for these type of discharges, i.
Finally, in contrast to the DBD research, no work has been performed on adding a catalyst packing in the MW discharge zone, and only a few papers have reported on adding a post-discharge catalytic packing. Spencer et al. On the other hand, Chen et al. However, this increase was suggested to arise from the dissociation of CO 2 at the catalyst surface with oxygen vacancies through dissociative electron attachment, which is an inherently less efficient dissociation process than the step-wise vibrational excitation as shown in Section 4.
This observation, together with the reported gas temperatures, makes it questionable whether vibrational excitation plays the major role here. The main disadvantage of MW discharges is, however, their current requirement to operate at low pressures, in order to reach this strong non-equilibrium, and thus these high energy efficiencies.
As explained in Section 4.
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The first one relies on simple two-dimensional 2D electrode blades. However, this configuration has a few disadvantages: the residence time in the plasma is quite short, flow rates are more limited and, due to its geometry, only a limited fraction of the gas flow is processed by the discharge e. The gas in the reverse inner vortex flow passes exactly through the arc in the longitudinal direction, which ensures longer residence times in the discharge zone, even at high flow rates.
Beside these geometry variations, other work has focused on changing the applied power, gas flow rate, flow type, interelectrode gap, admixture gases and plasma chemistry modelling. Again, all the data available in the literature are plotted in Fig. The only exception is the work of Indarto et al.
As such, compared to DBDs, which also work at atmospheric pressures, GA plasmas deliver about the same conversion, but the energy efficiency is in general 3—4 times higher. A more detailed discussion on the influence of the different parameters is given below. It should be no surprise that again a trade-off between the energy efficiency and conversion as a function of SEI is observed.
For a regular GA, the conversion clearly increases and the energy efficiency decreases when more power is supplied. This was probably the result of reaching an optimum SEI 0. Besides the gas flow rate, an important parameter for the GAP is the vortex flow type, which can be modulated by adjusting the reactor geometry. For the regular GA, the interelectrode gap can be varied to improve the CO 2 conversion, and the best result was observed for the smallest interelectrode distance.
Indeed, increasing this distance leads to a larger arc volume and a corresponding drop in plasma power and electron density, and consequently also a drop in CO 2 conversion. Only one research group has reported the use of additive gases, other than CH 4 , H 2 O or H 2 ; more specifically, the addition of N 2 , O 2 and air. Furthermore, the presence of N 2 leads to the unwanted production of NO X. The addition of O 2 and air showed a decrease in the conversion, indicating the possible strong negative effect of the presence of O 2 impurities , presumably due to Le Chatelier's principle.
Due to the more complex behaviour of the gas flow and of the arc movement, and also the relationship between both in a GA, modelling work is more limited in the literature. For the regular GA, a simple plasma kinetic model has been developed, while recently a more detailed 0D chemical kinetic model 97 and a 1D quasi-gliding arc mode have been presented. The results from these modelling studies show that the electron-impact dissociation of vibrationally excited CO 2 is predominant for an arc temperature of K and the recombination between CO and O atoms is the main conversion limiting reaction.
In summary, the data in the literature show that GA discharges succeed in exploiting the most energy-efficient CO 2 dissociation channel based on vibrational excitation, while operating at atmospheric pressure. Just like for MW discharges, operating GA plasmas at lower gas temperatures might be the key for achieving this.
In addition, the main limiting factor compared to MW discharges appears to be the conversion, due to the limited fraction of the gas flow that is currently processed by the discharge.