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Towards a fundamental understanding of a gliding arc discharge for the purpose of greenhouse gas conversion into value-added chemicals

Periodic Reporting for period 1 - GlidArc (Towards a fundamental understanding of a gliding arc discharge for the purpose of greenhouse gas conversion into value-added chemicals)

Periodo di rendicontazione: 2015-06-22 al 2017-06-21

The conversion of greenhouse gases to value-added chemicals is an effective strategy to reduce the emissions and an interesting process both from economic and ecological point of view. Moreover, the conversion of atmospheric nitrogen into valuable compounds, i.e. so-called nitrogen fixation, is gaining increasing interest, owing to the essential role in the nitrogen cycle of the biosphere. A gliding arc (GlidArc) plasma offers unique perspectives for activating inert molecules at mild conditions and allows the gas conversion with limited energy cost. A GlidArc is, however, very complex and poorly understood. Therefore, this project intends to obtain more fundamental insight in the plasma mechanisms of the GlidArc for gas conversion, by means of extensive modeling, validated by experimental diagnostics. This project intends to unravel the fundamental processes and mechanisms of the GlidArc and give rise to unprecedented energy-efficient reaction chemistry and plasma dynamic behaviour in GlidArc assisted conversion of gases into value-added chemicals. This project is expected to provide a theoretical foundation and guidance for research and industrial applications of the GlidArc.
1. Study on the crucial role of the dissociation cross section in modelling plasma-based CO2 conversion
The accuracy of the modelling results of plasma-based CO2 conversion critically depends on the accuracy of the assumed input data. This is especially true for the cross section of electron impact dissociation, but it is not clear from literature which excitation channels effectively lead to dissociation. Therefore, this work discusses the effect of different electron impact dissociation cross sections reported in literature on the calculated CO2 conversion, for a dielectric barrier discharge (DBD) and a microwave (MW) plasma. Our calculations indicate that the choice of this cross section is crucial for the DBD, where this process is the dominant mechanism for CO2 conversion. Comparison is made to experimental data to elucidate which cross section might be the most realistic. (See Fig.1). This work also provides important reference for modelling of the gliding arc and is published in .

2. Study on the effect of excited states and ion kinetics on the critical breakdown of hot CO2
Understanding the dielectric properties of a hot gas is required for various practical applications, including a gliding arc used for CO2 conversion. The gliding arc discharge reignites itself at the shortest electrode gap separation, where a residual warm environment facilitates the start of a new cycle. This work discusses some overlooked physics and clarifies inaccuracies in the evaluation of the effective ionization coefficients and the critical reduced breakdown electric field of CO2 at elevated temperature, considering the influence of excited states and ion kinetics. Our results indicate that the excited species lead to a greater population of high-energy electrons at higher gas temperature and this affects the Townsend rate coefficients, but the critical reduced breakdown electric field strength of CO2 is only affected when also properly accounting for the ion kinetics (See Fig.2). This work is published in and honorably recommended as a Featured article and selected as a LabTalk.

3. Study on gliding arc based CO2 conversion
In order to reduce the computation time, we developed a non-quasineutral plasma description which is firstly evaluated for reliability and applied to a one-dimensional as well as a two-dimensional gliding arc model (See Fig.3) and validated with experiments. Our calculated values of the electron number density in the plasma, the CO2 conversion and energy efficiency show reasonable agreement with the experiments, indicating that the model can provide a realistic picture of the plasma chemistry. As the model provides a realistic picture of the plasma behaviour, we use it first to investigate the plasma characteristics, which is necessary to understand the underlying mechanisms. Subsequently, we perform a chemical kinetics analysis, to investigate the different pathways for CO2 loss and formation. Based on the revealed discharge properties and the underlying CO2 plasma chemistry, the model allows us to propose solutions on how to further improve the CO2 conversion and energy efficiency by a gliding arc plasma. This work is published in:
S Kolev, S Sun, G Trenchev, W Z Wang, H Wang, 2016 Plasma Process. Polym. 14,1600110; W Z Wang, A Berthelot, S Kolev, X Tu, A Bogaerts, 2016 Plasma Sources Sci. Technol. 25, 065012;
A Bogaerts, A Berthelot, S Heijkers, S Kolev, R Snoeckx, S Sun, G Trenchev, K Van Laer and W Z Wang, 2017 Plasma Sources Sci. Technol. 26, 063001;
M Ramakers, G Trenchev, S Heijkers, W Z Wang and A Bogaerts, 2017 ChemSusChem 10, 2642-2652; W Z Wang, D H Mei, X Tu, A Bogaerts, 2017 Chem. Eng. J 330, 11–25.

4. Study on nitrogen fixation by gliding arc plasma
Gliding arc plasma also has great potential in nitrogen fixation, but little is known about the underlying mechanisms. Therefore, we developed a chemical kinetics model for a gliding arc reactor for nitrogen oxide synthesis. Experiments are carried out to validate the model. We use the model to investigate the reaction pathways for the formation and loss of NOx. The results indicate that vibrational excitation of N2 in the gliding arc contributes significantly to activating the N2 molecules, and leads to an energy efficient way of NOx production, compared to the thermal process. Based on the underlying chemistry, the model allows us to propose solutions on how to further improve the NOx formation by gliding arc technology. This work is published in and selected as very important paper, for the cover profile story and as cover figure of the journal (see Fig.4).
This project is expected to provide us a better understanding of the fundamental processes and mechanisms and a much more generic nature of warm plasma which is required for building, controlling and steering an energy efficient GlidArc setup for the conversion of gases to value-added chemicals. If electricity from sustainable energy sources is used, utilization of the greenhouse gases and converting them into a new feedstock would successfully mimic the natural photosynthetic process. This does not only comply with the framework of sustainable/green chemistry but also fits within the “cradle-to-cradle” concept. By generating useful products out of CO2 we create the possibility to effectively close the carbon loop. Furthermore, this conversion is also envisaged as one of the solutions to the growing energy problem, because it allows to directly convert electrical energy into chemicals and/or fuels. Similarly, the intrinsic potential of gliding arc plasma-based nitrogen fixation can provide a promising opportunity for producing nitrogenous fertilizer in remote locations by just using small-scale plants, which offer farmers a new source of revenue from their land. This helps to come up with realistic scenarios of entering a cutting-edge innovation in new business cases of plasma agriculture, in which low-temperature plasma technology might play an important role.
CO2 conversion in the DBD plasma, calculated with the different dissociation cross sections, as a fu
Calculated values of the critical reduced breakdown electric field in CO2 (and its mixture of dissoc
Time evolution of the electron number density (in m-3), electron temperature, gas temperature and CO
Cover figure of Gliding arc based nitrogen fixation