Numerical simulation study on the reaction mechanism of atmospheric pressure non-equilibrium CO₂–H₂O plasma discharge

Understanding the reaction mechanisms in low‑temperature CO₂–H₂O plasmas is essential for developing efficient CO₂ conversion technologies into valuable chemicals or fuels. In a study published in Frontiers of Chemical Science and Engineering, researchers present a comprehensive numerical simulation of atmospheric pressure non‑equilibrium plasma discharge in a needle‑plate electrode configuration, focusing on how the CO₂/H₂O concentration ratio and quenching pressure affect streamer dynamics and reaction pathways.

The two‑dimensional fluid model, implemented using the PASSKEY solver, includes 26 plasma species and 61 plasma chemical reactions, with electron transport coefficients calculated by BOLSIG⁺ under the local mean energy approximation. Photoionization is described by a three‑term Helmholtz model with experimentally‑informed quenching pressure parameters.

A critical reduced electric field of approximately 200 Td (corresponding to a mean electron energy of about 5.5 eV, the dissociation threshold of CO₂) was identified. When the reduced electric field is below 200 Td, increasing the initial water vapor content from 0.1 % to 10 % leads to strong dissociative adsorption reactions between electrons and water molecules, causing a sharp decline in electron density and energy. This suppresses radial expansion and contracts the discharge channel near the symmetry axis. Above 200 Td, electron‑impact dissociation and ionization dominate, and water vapor content has minimal effect on primary electron transport parameters.

Quenching pressure influences photoionization efficiency. As quenching pressure increases from 0 to +∞ torr, the spatially averaged photoionization rate rises significantly, and the ratio of direct ionization rate to photoionization rate decreases from about 426 to 11, yet plasma discharge remains primarily sustained by direct electron‑impact ionization. Both increasing quenching pressure and decreasing water vapor content promote streamer propagation.

The dominant reaction pathways for key products are revealed. CO is mainly generated by electron‑impact dissociation of CO₂ (e + CO₂ → CO + O + e) and by electron recombination with CO₂⁺ ions, while its primary loss is through electron‑impact ionization (e + CO → 2e + CO⁺). OH radicals originate predominantly from electron‑impact dissociation of H₂O, with three‑body recombination (H + OH + M) being the main consumption pathway. Notably, even with a maximum water vapor content of only 10 %, dissociative attachment reactions of water molecules contribute to electron loss comparably to recombination reactions with CO₂⁺ ions.

This work provides a theoretical foundation for optimizing atmospheric pressure CO₂–H₂O plasma discharge systems, guiding the design of plasma reactors for efficient CO₂ conversion and sustainable fuel production.