Microwave plasmas are one of the most promising techniques for CO 2 conversion into value-added chemicals and fuels, since they are very energy-efficient. Nevertheless, experiments show that this high energy efficiency is only reached at low pressures, and significantly drops towards atmospheric pressure, which is a clear limitation for industrial applications. In this paper, we use a zero-dimensional reaction kinetics model to simulate a CO 2 microwave plasma in a pressure range from 50 mbar to 1 bar, in order to evaluate the reasons for this decrease in energy efficiency at atmospheric pressure. The code includes a detailed description of the vibrational kinetics of CO 2 , CO and O 2 as well as the energy exchanges between them, because the vibrational kinetics is known to be crucial for energy efficient CO 2 splitting. First, we use a self-consistent gas temperature calculation in order to assess the key performance indicators for CO 2 splitting, i.e., the CO 2 conversion and corresponding energy efficiency. Our results indicate that lower pressures and higher power densities lead to more vibrational excitation, which is beneficial for the conversion. We also demonstrate the key role of the gas temperature. The model predicts the highest conversion and energy efficiencies at pressures around 300 mbar, which is in agreement with experiments from literature. We also show the beneficial aspect of fast gas cooling in the afterglow at high pressure. In a second step, we study in more detail the effects of pressure, gas temperature and power density on the vibrational distribution function and on the dissociation and recombination mechanisms of CO 2 , which define the CO 2 splitting efficiency. This study allows us to identify the limiting factors of CO 2 conversion and to propose potential solutions to improve the process.
Plasma-based CO 2 conversion is worldwide gaining increasing interest. A large research effort is devoted to improving the energy efficiency. For this purpose, it is very important to understand the underlying mechanisms of the CO 2 conversion. The latter can be obtained by computer modeling, describing in detail the behavior of the various plasma species and all relevant chemical processes. However, the accuracy of the modeling results critically depends on the accuracy of the assumed input data, like cross sections. This is especially true for the cross section of electron impact dissocation, as the latter process is believed to proceed through electron impact excitation, but it is not clear from literature which excitation channels effectively lead to dissocation. Therefore, the present paper discusses the effect of different electron impact dissociation cross sections reported in literature on the calculated CO 2 conversion, for a dielectric barrier discharge (DBD) and a microwave (MW) plasma. Comparison is made to experimental data for the DBD case, to elucidate which cross section might be the most realistic. This comparison reveals that the cross sections proposed by Itikawa and by Polak and Slovetsky both seem to underestimate the CO2 conversion. The cross sections recommended by Phelps with threshold of 7 eV and 10.5 eV yield a CO2 conversion only slightly lower than the experimental data, but the sum of both cross sections overestimates the values, indicating that these cross sections represent dissociation, but most probably also include other (pure excitation) channels. Our calculations indicate that the choice of the electron impact dissociation cross section is crucial for the DBD, where this process is the dominant mechanism for CO2 conversion. In the MW plasma, it is only significant at pressures up to 100 mbar, while it is of minor importance for higher pressures, when dissociation proceeds mainly through collisions of CO2 with heavy particles.
Although CO 2 conversion by plasma technology is gaining increasing interest, the underlying mechanisms for an energy-efficient process are still far from understood. In this work, a reduced non-equilibrium CO 2 plasma chemistry set, based on level lumping of the vibrational levels, is proposed and the reliability of this levellumping method is tested by a self-consistent zero-dimensional code. A severe reduction of the number of equations to be solved is achieved, which is crucial to be able to model non-equilibrium CO 2 plasmas by 2-dimensional models. Typical conditions of pressure and power used in a microwave plasma for CO 2 conversion are investigated. Several different sets, using different numbers of lumped groups, are considered. The lumped models with 1, 2 or 3 groups are able to reproduce the gas temperature, electron density and electron temperature profiles, as calculated by the full model treating all individual excited levels, in the entire pressure range investigated. Furthermore, a 3-groups model is also able to reproduce the shape of the vibrational distribution function (VDF) and gives the most reliable prediction of the CO 2 conversion. A strong influence of the vibrational excitation on the plasma characteristics is observed. Finally, the limitations of the lumped-levels method are discussed.
CO 2 conversion by a gliding arc plasma is gaining increasing interest, but the underlying mechanisms for an energy-efficient process are still far from understood. Indeed, the chemical complexity of the non-equilibrium plasma poses a challenge for plasma modeling due to the huge computational load. In this paper, a one-dimensional (1D) gliding arc model is developed in a cylindrical frame, with a detailed non-equilibrium CO 2 plasma chemistry set, including the CO 2 vibrational kinetics up to the dissociation limit. The model solves a set of timedependent continuity equations based on the chemical reactions, as well as the electron energy balance equation, and it assumes quasi-neutrality in the plasma. The loss of plasma species and heat due to convection by the transverse gas flow is accounted for by using a characteristic frequency of convective cooling, which depends on the gliding arc radius, the relative velocity of the gas flow with respect to the arc and on the arc elongation rate. The calculated values for plasma density and plasma temperature within this work are comparable with experimental data on gliding arc plasma reactors in the literature. Our calculation results indicate that excitation to the vibrational levels promotes efficient dissociation in the gliding arc, and this is consistent with experimental investigations of the gliding arc based CO 2 conversion in the literature. Additionally, the dissociation of CO 2 through collisions with O atoms has the largest contribution to CO 2 splitting under the conditions studied. In addition to the above results, we also demonstrate that lumping the CO 2 vibrational states can bring a significant reduction of the computational load. The latter opens up the way for 2D or 3D models with an accurate description of the CO 2 vibrational kinetics.
In recent years there is growing interest in the use of plasma technology for CO2 conversion. To improve this application, a good insight in the underlying mechanisms is of great importance. This can be obtained from modeling of the detailed plasma chemistry, to understand the chemical reaction pathways leading to CO 2 conversion (either in pure form or mixed with another gas). Moreover, in practice several plasma reactor types are being investigated for CO 2 conversion, so in addition it is essential to model these reactor geometries, in order to be able to improve their design, and to achieve the most energy efficient CO2 conversion. Modeling the detailed plasma chemistry of CO2 conversion in complex reactors is, however, very time-consuming. This problem can be overcome by using a combination of two different types of models. 0D chemical reaction kinetics models are very suitable for describing the detailed plasma chemistry, while the characteristic features of different reactor geometries can be studied by 2D or 3D fluid models. The latter can in first instance be developed in argon or helium, with a simple chemistry, to limit the calculation time, but the ultimate aim is to implement the more complex CO2 chemistry in these models. In the present paper, examples will be given of both 0D plasma chemistry models and 2D and 3D fluid models for the most common plasma reactors used for CO2 conversion, to emphasize the complementarity of both approaches. Furthermore, based on the modeling insights, the paper discusses the possibilities and limitations of plasma-based CO 2 conversion in the different types of plasma reactors, and what would be needed to make further progress in this field.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.