Implications of thermo-chemical instability on the contracted modes in CO₂ microwave plasmas

Abstract

Understanding and control over plasma instabilities and contraction phenomena in reactive flows is essential to optimize the discharge parameters for plasma processing applications such as fuel reforming and gas conversion. In this work, we describe the characteristic discharge modes in a CO 2 microwave plasma and assess the impact of wave coupling and thermal reactivity on the contraction dynamics. The plasma shape and gas temperature are obtained from the emission profile and the Doppler broadening of the 777 nm Op 5 S Ð 5 Pq oxygen triplet, respectively. Based on these observations, three distinct discharge modes are identified in the pressure range of 10 mbar to atmospheric pressure. We find that discharge contraction is suppressed by an absorption cut-off of the microwave field at the critical electron density, resulting in a homogeneous discharge mode below the critical transition pressure of 85 mbar. Further increase in the pressure leads to two contracted discharge modes, one emerging at a temperature of 3000 to 4000 K and one at a temperature of 6000 to 7000 K, which correspond to the thermal dissociation thresholds of CO 2 and CO respectively. The transition dynamics are explained by a thermo-chemical instability, which arises from the coupling of the thermalionization instability to heat transfer resulting from thermally driven endothermic CO 2 dissociation reactions. These results highlight the impact of thermal chemistry on the contraction dynamics of reactive molecular plasmas.

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 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.…”

“…However, in recent years, there is growing insight that optimizing the vibration-induced dissociation pathway may not be the most realistic strategy. In fact, experiments indicate that thermal CO 2 conversion gives rise to quite high energy efficiencies (up to 40-50%) (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. Indeed, the rate coefficients of thermal reactions increase with gas temperature, thus enhancing the conversion.…”

“…The above suggestions to improve the energy efficiency by promoting the vibrational dissociation pathway are not straightforward to realize in practice. Indeed, recent insights based on both modeling and experiments have revealed that the CO 2 conversion in (sub)atmospheric pressure MW and GA plasmas proceeds by thermal reactions (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, and this can also give rise to quite high energy efficiencies (up to 40-50%). As explained above, the rate coefficients of the thermal reactions between various plasma species rise upon higher gas temperature, thus enhancing the conversion.…”

“…As explained above, the rate coefficients of the thermal reactions between various plasma species rise upon higher gas temperature, thus enhancing the conversion. Temperatures around 3,000-4,000 K should be optimal for thermal conversion to realize significant CO 2 conversion and still limit thermal losses (Wolf et al, 2020). In this section, we provide some suggestions how to further improve the energy efficiency of thermal CO 2 conversion beyond the thermal equilibrium limits of 45-50%.…”