Mode resolved heating dynamics in pulsed microwave CO₂ plasma from laser Raman scattering

Abstract

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Plasma Technology for CO2 Conversion: A Personal Perspective on Prospects and Gaps

“…It was indeed demonstrated, both experimentally (e.g., van Rooij et al, 2015;Bongers et al, 2017;den Harder et al, 2017;van den Bekerom et al, 2019;Wolf et al, 2019Wolf et al, , 2020 and computationally (e.g., Berthelot and Bogaerts, 2017;Heijkers and Bogaerts, 2017;Kotov and Koelman, 2019), that in practice, the vibrational non-equilibrium is not the leading mechanism in (sub)atmospheric MW or GA CO 2 plasmas, and that the CO 2 conversion is thermal, although Pietanza et al (2020) suggested to act with caution on this assumption, as their model still showed clear deviations from equilibrium, even at temperatures of 3,500-5,500 K.…”

“…In the past decade, many papers, both experimental (e.g., Silva et al, 2014Silva et al, , 2017Klarenaar et al, 2017Klarenaar et al, , 2018Klarenaar et al, , 2019Stewig et al, 2020;Terraz et al, 2020;van den Bekerom et al, 2020) and computational (e.g., Bogaerts, 2014, 2015;Pietanza et al, 2015Pietanza et al, , 2017aPietanza et al, ,b, 2018aPietanza et al, ,b, 2020Bogaerts, 2016, 2018;Capitelli et al, 2017, Heijkers andGrofulović et al, 2018;Silva et al, 2018;Bogaerts, 2018, 2019;Kotov and Koelman, 2019;Terraz et al, 2020), focused on the role of the vibrational kinetics in plasmas, but mainly at low pressure conditions (few mbar up to a few hundred mbar). Most of these studies revealed the importance of the vibrational kinetics at specific conditions (low gas temperature, low pressure) and their contribution to a high energy efficiency.…”

“…It is an important measure to distinguish the operating conditions of different plasma types. A DBD exhibits a reduced electric field around 200 Td and above, whereas MW and GA plasmas (and some other plasma types, such as APGD) typically operate around 50-100 Td (Bongers et al, 2017;Pietanza et al, 2020;van den Bekerom et al, 2020), although a recent model for determining the reduced electric field in CO 2 MW plasma derived by the principle of impedance matching (Groen et al, 2019) predicted somewhat higher E/n values, i.e., 10-60 Td for the so-called contracted regime (higher pressure and power), but 80-180 Td for the so-called diffuse regime (lower pressure and power). As depicted in Figure 5, for reduced electric fields around 200 Td and above, the largest part of the electron energy (70-80%) goes to electronic excitation, while limited amounts are spent for dissociation and ionization (ca.…”

“…In MW and GA plasmas, the electron temperature is still higher than the gas temperature (hence, they are not really thermal plasmas), but the vibrational and gas temperature are often close to each other (i.e., VT equilibrium), so they can also be considered as quasi-thermal plasmas. For this reason, the CO 2 conversion proceeds mainly by thermal reactions in MW and GA plasmas at practical operating conditions (van Rooij et al, 2015;Berthelot and Bogaerts, 2017;Bongers et al, 2017;den Harder et al, 2017;Heijkers and Bogaerts, 2017;Kotov and Koelman, 2019;van den Bekerom et al, 2019;Wolf et al, 2019Wolf et al, , 2020.…”

“…This vibrational pathway requires strong VT non-equilibrium, i.e., much higher vibrational levels than gas (translational) temperature, or in other words, a strong overpopulation of the higher vibrational levels, compared to a thermal vibrational distribution function (VDF). Such behavior can be exploited under specific conditions, e.g., MW plasmas at low pressure and supersonic expansion (Asisov et al, 1983) or pulsed MW plasmas at low pressure, depending on the pulse on-off times, as demonstrated both computationally and experimentally (Vermeiren and Bogaerts, 2019;van den Bekerom et al, 2020). Generally, modeling has revealed that this VT nonequilibrium can be realized in plasmas at low pressure, high power density, and low gas temperature (Berthelot and Bogaerts, 2017); see below.…”