Methane pyrolysis with N2/Ar/He diluents in a repetitively-pulsed nanosecond discharge: Kinetics development for plasma assisted combustion and fuel reforming

Abstract

Abstract Experimental and kinetic studies of the non-equilibrium plasma-assisted methane conversion into higher hydrocarbons and H2 with N2/Ar/He diluents are investigated. Experiments of CH4 pyrolysis with N2/Ar/He diluents in a repetitively-pulsed nanosecond discharge are conducted at different applied voltages. The measured results show that H2 and C2H6 are the major products of all mixtures. A plasma-assisted CH4/diluents pyrolysis kinetic model incorporating the key excited species is developed and validated. The evolutions of the excited species, radicals, ions, and electrons at different timescales are revealed by kinetic modeling. A pathway analysis shows that electronically excited N2(A), N2(B), N2(a′), and N2(C) significantly contribute to CH4 dissociation into CH3 and H. As small amounts of CH2 and CH are produced in CH4/N2 mixtures, little C2H4, C3H6, and C3H8, and less C2H2 and C3H4 are experimentally measured and numerically observed than mixtures of Ar or He. The quenching and transitions among N2(A, B, a′, C) and the relaxation of vibrationally excited N2(v) by hydrocarbons provide a thermal contribution to the CH4/N2 mixture, resulting in a higher gas temperature than that of CH4/Ar and CH4/He mixtures. In CH4/Ar mixtures, Ar* and Ar+ are the dominant species for CH4 pyrolysis. Comparatively, electron impact dissociation and ionization reactions are the primary pathways for CH4 consumption with He diluent. A pathway analysis of CH4 pyrolysis further reveals that dissociative recombination reactions between electrons and fuel ions are responsible for carbon formation. This work provides base CH4 pyrolysis mechanisms with different diluents for plasma-assisted CH4 combustion and reforming at low temperatures, as well as an insight into carbon formation pathways of plasma-assisted CH4 pyrolysis.