Methane conversion by repetitive nanosecond pulsed plasma

A detailed study of methane conversion by repetitive nanosecond pulsed plasma was accomplished. In this study, a conversion rate of about 60% was obtained at an energy conversion efficiency of more than 75%. The conversion performance reached the optimum value at an electrode gap of about 5 mm. To raise the temperature, the reactor was thermally isolated. At moderate frequencies, a non-isolated reactor with a lower temperature demonstrated a better conversion performance. This was attributed to the dominance of the vibrational dissociation channel. It was also demonstrated that the conversion process considerably improved at a pulse-to-pulse time interval of less than 100 µs, which is the lifetime of ion molecules at atmospheric pressure. A mathematical model based on two temperatures is developed in order to explain the dissociation mechanism. The model reveals that the greatest molecular dissociation occurs when there is a high vibrational non-equilibrium state in the molecule. This non-equilibrium state lasts less than a microsecond at the post-plasma stage. It explains the high efficiencies obtained in the conversion process and is specific to the pulsed plasmas.