Pinpointing energy losses in CO2 plasmas – Effect on CO2 conversion

Abstract

Plasma technology is gaining increasing interest for CO 2 conversion, but to maximize the energy efficiency, it is important to track the different energy transfers taking place in the plasma. In this paper, we study these mechanisms by a 0D chemical kinetics model, including the vibrational kinetics, for different conditions of reduced electric field, gas temperature and ionization degree, at a pressure of 100 mbar. Our model predicts a maximum conversion and energy efficiency of 32 % and 47 %, respectively, at conditions that are particularly beneficial for energy efficient CO 2 conversion, i.e. a low reduced electric field (10 Td) and a low gas temperature (300 K). We study the effect of the efficiency by which the vibrational energy is used to dissociate CO 2 , as well as of the activation energy of the reaction CO 2 + O → CO + O 2 , to elucidate the theoretical limitations to the energy efficiency. Our model reveals that these parameters are mainly responsible for the limitations in the energy efficiency. By varying these parameters, we can reach a maximum conversion and energy efficiency of 86 %. Finally, we derive an empirical formula to estimate the maximum possible energy efficiency that can be reached under the assumptions of the model.

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

“…The recycling of O atoms through reaction with vibrationally excited CO 2 molecules was also suggested by Silva et al (2017) in a MW post-discharge. As mentioned above, this reaction has a high activation energy, but it may indeed be overcome by either hot O atoms or the vibrationally excited CO 2 molecules (Berthelot and Bogaerts, 2018). This super-ideal quenching may explain the energy efficiencies above the thermal equilibrium limits reported in the 1980s (Legasov et al, 1978;Asisov et al, 1983).…”

“…It should be noted, however, that although the high energy efficiencies reported for low pressure MW plasmas in the 1980s (i.e., up to 80% for subsonic flow, and even up to 90% for supersonic flow conditions; e.g., Legasov et al, 1978;Asisov et al, 1983) were theoretically explained by vibrational excitation followed by vibrational ladder climbing, as depicted in Figure 6, none of the experimental or computational studies mentioned in previous section could reproduce or mimic these high energy efficiencies up to now. Berthelot and Bogaerts (2018) modeled the different energy transfers in a low pressure (100 mbar) CO 2 plasma for various conditions of reduced electric field, gas temperature, and ionization degree. At the most optimal conditions for energyefficient CO 2 conversion, a low reduced electric field (10 Td) and a low gas temperature (300 K), the model predicted a maximum conversion and energy efficiency of 32 and 47%, respectively.…”

“…Likewise, Vermeiren and Bogaerts (2019) were not able to predict the record energy efficiency of 90% for a MW plasma at 100 mbar and supersonic flow conditions (Asisov et al, 1983). The major limiting factor in the energy efficiency was pinpointed as the high activation energy of the reaction CO 2 + O → CO + O 2 (Berthelot and Bogaerts, 2018). Assuming that this activation energy can be entirely overcome, by either sufficient CO 2 vibrational energy or kinetic energy of the collision partners, the model could predict an energy efficiency of up to 86% in the ideal scenario (Berthelot and Bogaerts, 2018).…”

“…The major limiting factor in the energy efficiency was pinpointed as the high activation energy of the reaction CO 2 + O → CO + O 2 (Berthelot and Bogaerts, 2018). Assuming that this activation energy can be entirely overcome, by either sufficient CO 2 vibrational energy or kinetic energy of the collision partners, the model could predict an energy efficiency of up to 86% in the ideal scenario (Berthelot and Bogaerts, 2018). Note that the reason why it was not 100% is due to other losses in the kinetics of CO 2 conversion, i.e., the fact that not all electron energy will go to vibrational excitation of CO 2 , but some fraction is also spent to the dissociation products (CO and O 2 ).…”

“…As mentioned above, however, this reaction has a significant energy barrier, which was pinpointed as the major limiting factor for energyefficient CO 2 conversion, not only in thermal conversion, but also in case of vibration-induced dissociation; cf. above (Berthelot and Bogaerts, 2018). Thus, conditions should be explored to enhance this reaction, e.g., by vibrational excitation of CO 2 , because the latter could reduce the energy barrier of this reaction.…”