Modeling of atomic oxygen in the effluent of a CO2 microwave discharge

This study examines the kinetics of low-pressure 2.45 GHz microwave CO₂ plasmas under conditions aimed at optimizing CO₂ dissociation for energy-efficient gas reforming-based applications on Earth and Mars. It focuses on the behavior of ground-state atomic oxygen, O(³P). The LisbOn KInetics (LoKI) framework composed of a Boltzmann solver (LoKI-B) and chemistry solver (LoKI-C), modified to include extensions for the vibrational sets of CO₂ and CO; and the excited state O(¹S), was used to describe electron kinetics, heavy-species interactions, and surface recombination in both plasma and post-discharge regions.

The modeling study developed capitalizes on recent experimental data with operating parameters varied as follows: pressures of 120-500 Pa, absorbed powers of 600-1200 W, and flow rates of 74-370 sccm. Results align well with experimental data on temperature and conversion, though discrepancies remain, particularly with the highest dissociation of 90.3%.

While attempting to describe the experimental data, various potential sources for the O(³P) concentration peak observed in the post-discharge were explored, including the vibrational dissociation of CO₂, O₂, and CO, and reactions with electronically excited species. However, simulations indicate that most highly excited vibrational and electronic states are depleted early in the post-discharge.

Thermal contraction was inspected for its role in the increase of O(³P) concentration. Simulations reproduced the peak this way, although with some differences in shape and position. Findings suggest that this thermal contraction results from non-uniform cooling, not collisional mechanisms driven by excited states. O(³P) destruction was analyzed by parametric studies on oxygen recombination mechanisms, especially at the reactor walls.