Liquid-phase methane bubble plasma discharge for heavy oil processing: Insights into free radicals-induced hydrogenation

Hydrogenation, an alternative to conventional cracking, is poised to revolutionize heavy oil upgrading by raising the hydrogen-to-carbon ratio at lower process pressures and temperatures. The emerging low-temperature plasma-enabled hydrogenation is an effective, fast, and environment-friendly process; however, the conversion rate and energy efficiency are still insufficient. To address these limitations, here we propose an innovative bubble-assisted methane plasma-liquid process for heavy oil processing (using ethylbenzene as a model compound) and offer new insights into radical-assisted hydrogenation. Results from the plasma kinetics modeling and density functional theory calculations indicate that ·H and ·CH₃ radicals generated by CH₄ plasma are two main drivers for ethylbenzene hydrogenation, and their chemical reaction rates with ethylbenzene molecules strongly depend on their spatial distribution in bubbles, which further govern the reaction direction towards ethylbenzene hydrogenation or free radical recombination. Moreover, it is found that by controlling the bubble numbers and plasma parameters, the H density can increase by an order of magnitude from 1.4 × 10²⁰ to 3.88 × 10²¹ m⁻³, while the number of hydrogenated aromatic rings increases by ∼ 58%, which further confirms the excellences of the proposed plasma-bubble technology in chemical regulation and feasible processing. In addition, the feasibility of the plasma-enabled hydrogenation has been verified by the experiment, which highlighted the competition between the radical recombination and radical-aromatic ring interactions. Overall, the revealed interaction mechanisms between the plasma-generated radicals and ethylbenzene provide new insights and guiding principles for the future upgrading of heavy oils and other industrial hydrocarbon products.