CO₂ Conversion in Nonuniform Discharges: Disentangling Dissociation and Recombination Mechanisms

Abstract

Motivated by environmental applications such as synthetic fuel synthesis, plasma-driven conversion shows promise for efficient and scalable gas conversion of CO 2 to CO. Both discharge contraction and turbulent transport have a significant impact on the plasma processing conditions, but are, nevertheless, poorly understood. This work combines experiments and modeling to investigate how these aspects influence the CO production and destruction mechanisms in the vortex-stabilized CO 2 microwave plasma reactor. For this, a two-dimensional axisymmetric tubular chemical kinetics model of the reactor is developed, with careful consideration of the nonuniform nature of the plasma and the vortexinduced radial turbulent transport. Energy efficiency and conversion of the dissociation process show a good agreement with the numerical results over a broad pressure range from 80 to 600 mbar. The occurrence of an energy efficiency peak between 100 and 200 mbar is associated with a discharge mode transition. The net CO production rate is inhibited at low pressure by the plasma temperature, whereas recombination of CO to CO 2 dominates at high pressure. Turbulence-induced cooling and dilution of plasma products limit the extent of the latter. The maxima in energy efficiency observed experimentally around 40% are related to limits imposed by production and recombination processes. Based on these insights, feasible approaches for optimization of the plasma dissociation process are discussed.

Advances in non-equilibrium $$\hbox {CO}_2$$ plasma kinetics: a theoretical and experimental review

“…In practice, VT relaxation appears quite important in both MW plasmas (at (sub)atmospheric pressure) and GA plasmas, explaining their high gas temperature (3000 K or higher). For this reason, the CO 2 conversion proceeds mainly by thermal reactions in MW and GA plasmas at practical operating conditions, as demonstrated by several models [271,294,[298][299][300][301][302]. However, Pietanza et al recently studied in detail the kinetics vs thermodynamics effects on CO 2 dissociation in high-temperature MW plasmas, with a self-consistent model of the vibrational kinetics of CO 2 , CO and O 2 , and the electron Boltzmann equation, and they concluded that the assumption of thermodynamic equilibrium in MW CO 2 high temperature plasmas has to be considered with caution, as there are still non-equilibrium effects at play, even at temperatures of 3500-5500 K [271].…”

“…The authors showed the calculated 2D profiles of electron temperature, electron density, gas temperature and vibrational temperature at different moments in time, providing useful insights in the arc behavior of the CO 2 GA plasma. [300]. In this model, the power deposition was assumed to lead to direct gas heating, so vibrational kinetics was neglected, which was justified by the fast thermalization and the fact that thermal conversion dominates over vibrationinduced dissociation at the high temperatures under study.…”

Resolving discharge parameters from atomic oxygen emission

“…The contraction of a vortex-stabilised CO 2 MW plasma has been characterised in Wolf et al (2019) in relation to its dielectric properties. Then, in Wolf et al (2020b), two distinct contracted discharge modes have been identified and described at pressures above 85 mbar: a low-confinement L-mode at lower pressures and at temperatures between 3000 and 5000 K and a high-confinement Hmode at higher pressures and at temperatures above 5500 K. The works in Viegas et al (2020) and Wolf et al (2020a) have investigated the contracted modes numerically. The distinct changes in spatial profiles of n e and P abs with discharge modes have been shown to be important to obtain accurate descriptions of the reactivity within the plasma, that determines the spatial structure of the plasma itself (Viegas et al 2020), as well as of the follow-up reactions of plasma products in its periphery, that influence the reactor performance (Wolf et al 2020a).…”

Redefining the Microwave Plasma-Mediated CO2 Reduction Efficiency Limit: The Role of O–CO2 Association

“…It has been hypothesized that this limit can be increased significantly by controlling the quenching trajectory of the atomic oxygen generated in reaction R1. Association of O with CO 2 yields an additional CO for each O 2 which, would increase the efficiency limit to 70% 6,8 by valorizing the enthalpy of formation of nascent O atoms:…”