Validation of non-equilibrium kinetics in CO₂–N₂plasmas

Abstract

This work explores the effect of N2 addition on CO2 dissociation and on the vibrational kinetics of CO2 and CO under various non-equilibrium plasma conditions. A self-consistent kinetic model, previously validated for pure CO2 and CO2-O2 discharges, is further extended by adding the kinetics of N2. The vibrational kinetics considered include levels up to v = 10 for CO, v = 59 for N2 and up to v 1 = 2 and v 2 = v 3 = 5, respectively for the symmetric stretch, bending and asymmetric stretch modes of CO2, and account for electron-impact excitation and de-excitation (e-V), vibration-to-translation (V-T) and vibration-to-vibration energy exchange (V-V) processes. The kinetic scheme is validated by comparing the model predictions with recent experimental data measured in a DC glow discharge operating in pure CO2 and in CO2-N2 mixtures, at pressures in the range 0.6 - 4 Torr (80.00 - 533.33 Pa) and a current of 50 mA. The experimental results show a higher vibrational temperature of the different modes of CO2 and CO and an increased dissociation fraction of CO2, that can reach values as high as 70 %, when N2 is added to the plasma. On the one hand, the simulations suggest that the former effect is the result of the CO2-N2 and CO-N2 V-V transfers and the reduction of quenching due to the decrease of atomic oxygen concentration; on the other hand, the dilution of CO2 and dissociation products, CO and O2, reduces the importance of back reactions and contributes to the higher CO2 dissociation fraction with increased N2 content in the mixture, while the N2(B3Πg) electronically excited state further enhances the CO2 dissociation.

On CO2 conversion in discharges in CO2–N2 mixtures

“…* . An agreement between the measured [6] and calculated values of the conversion factor versus the N 2 molar fraction is obtained under the assumption that the yield of CO 2 dissociation channel in the reaction of collisional quenching of N 2 in low-pressure CO 2 -N 2 discharges [7]. On the other hand, according to the results of simulation [9] for conditions of nanosecond pulse dielectric barrier discharge in the mixture CO 2 :N 2 = 5:95, for reaching agreement with experiment, it is necessary to take the dissociation yield much lower, around 25%.…”

“…With the addition of nitrogen, another channel of CO 2 conversion comes into play-quenching by CO 2 molecules of electronically excited states of N 2 molecules. In conditions of experiments [6,7], at the nitrogen molar fraction X N2 varying in the range from 0 to 0.75, the nitrogen states most important for CO production are the triplet levels N 2 (B,B ′ ,W) efficiently produced in the discharge by electron impact. The rate of CO production at quenching of the combined triplet state N 2 (B) in collisions with CO 2 is governed by the yield γ B of dissociation channel N 2 (B) + CO 2 → N 2 + CO + O of this reaction.…”

“…Effects on the conversion efficiency of the addition to CO 2 of various gases have been considered. In particular, it has been shown that the CO 2 conversion factor, both in barrier discharges [4,5] and in low-pressure glow discharges [6,7], increases substantially with the addition of nitrogen.…”

A reaction mechanism for oxygen plasmas

“…The calculations in this work are performed with the LisbOn KInetics (LoKI) 0D simulation tool, constituted by two modules: a Boltzmann solver (LoKI-B) for the two-term electron Boltzmann equation [39,40] and a chemical solver (LoKI-C) for the heavy-species kinetics [41,42]. In past works, LoKI has been used to describe with success the positive column of DC glow discharges in different gas mixtures [41,[43][44][45]. However, other authors have employed different models to describe the kinetics of glow discharges.…”

CO2/CH4 glow discharge plasma. Part I: Experimental and numerical study of the reaction pathways

“…Third, the rates coefficients of VV and VT processes between vibrationally excited CO 2 or CO and the various by-products of CO 2 -CH 4 mixtures are very poorly known, with values in literature ranging over several orders of magnitude ( [47,48]), making it difficult to accurately simulate the populations of the various levels. Note that a model accounting for the vibrational kinetics of CO 2 and CO was recently presented in [49]. For the CO 2 -CH 4 chemistry including only single-carbon molecules, the added chemistry totalizes 140 neutralneutral reactions, 40 electron impact reactions and 40 ion-neutral reactions.…”