Unraveling non-equilibrium kinetics in plasma-driven methanol ignition: A combined experimental and global modelling study

Methanol holds promise as a fuel for spark ignition engines in achieving carbon neutrality, though its cold-starting remains a key challenge. Non-equilibrium plasma excitation emerges as a transformative solution, where a mechanistic understanding of reaction kinetics, particularly electron-fuel interactions governing radical generation and chain branching, is pivotal to advancing the efficiency of plasma-assisted methanol ignition. This study investigates the kinetic interplay between non-equilibrium plasma excitation and methanol oxidation through combined experiments and modelling. A detailed kinetic mechanism for plasma-driven methanol ignition is developed, incorporating electron scattering cross-sections of CH₃OH calculated via the R-matrix method. For the first time, wall quenching of electronically excited species is integrated into the model. Experimental validation using GC confirms the modelling accuracy in predicting species evolution. Key findings reveal that non-thermal plasma reduces ignition delay time by 1–3 orders of magnitude compared to auto-ignition, with 89 % of discharge energy allocated to non-equilibrium excitation and <10 % to thermal effects. Path flux analysis reveals that there are distinct dissociation pathways for methanol under plasma conditions. Combustion reactions predominantly produce CH₂OH via H-atom abstraction, while plasma reactions primarily generate CH₃O through e + CH₃OH → e + CH₃O + H and CH₃OH + X^⁎(N(²D), O(¹D)) → CH₃O + XH. The difference is due to the fact that excited CH₃OH is dissociated into CH₃O and H, demonstrated via Quantum chemical calculations. Notably, electronically excited methanol undergoes two competing processes: the wall quenching reactions (CH₃OH(e) + Wall → CH₃OH + Wall) and the dehydrogenation reactions (CH₃OH(e) + OH/HO₂ → CH₂OH + H₂O/H₂O₂). The former quenches excited species, while the latter increases fuel radicals, thereby accelerating the chain reaction. These insights bridge plasma physics and combustion chemistry, offering a predictive framework for optimizing plasma-assisted methanol ignition under cold-starting conditions.