Numerical simulation on particle density and reaction pathways in methane needle-plane discharge plasma at atmospheric pressure

Methane needle-plane discharge has practical application prospect and scientific research significance since methane conversion heavy oil hydrogenation is formed by coupling methane needle-plane discharge with heavy oil hydrogenation, which can achieve high-efficient heavy oil hydrogenation and increase the yields of high value-added light olefins. In this paper, a two-dimensional fluid model is built up for numerically simulating the methane needle-plane discharge plasma at atmospheric pressure. Spatial and axial distributions of electric intensity, electron temperature and particle densities are obtained. Reaction yields are summarized and crucial pathways to produce various kinds of charged and neutral particles are found out. Simulation results indicate that axial evolutions of CH₃ ⁺ and CH₄ ⁺ densities, electric intensity and electron temperature are similar and closely related. The CH₅ ⁺ and C₂H₅ ⁺ densities first increase and then decrease along the axial direction. The CH₃ and H densities have nearly identical spatial and axial distributions. Particle density distributions of CH₂, C₂H₄ and C₂H₅ are obviously different in the area near the cathode but comparatively resemblant in the positive column region. The CH₃ ⁺ and CH₄ ⁺ are produced by electron impact ionizations between electrons and CH₄. The CH₅ ⁺ and C₂H₅ ⁺ are respectively generated by molecular impact dissociations between CH₃ ⁺ and CH₄ and between CH₄ ⁺ and CH₄. Electron impact decomposition between electrons and CH₄ is a dominated reaction to produce CH₃, CH₂, CH and H. The reactions between CH₂ and CH₄ and between electrons and C₂H₄ are critical pathways to produce C₂H₄ and C₂H₂, respectively. In addition, the yields of electron impact decomposition reactions between electrons and CH₄ and reactions between CH₂ and CH₄ account for 52.15% and 47.85% of total yields of H₂ respectively.