Highly efficient conversion of methane using microsecond and nanosecond pulsed spark discharges

Plasma-assisted methane (CH4) activation is a promising way for a hydrogen (H-2) production. In this paper, we describe our studies of microsecond and nanosecond pulsed spark discharge plasmas use in a CH4 pyrolysis for a H-2 production. The dependence of CH4 conversion and gas discharge product composition on discharge power, discharge gap length and gas flow rate are studied. The electrical and optical characteristics of the discharges are also studied to reveal discharge plasma parameters and chemical reactions leading to CH4 pyrolysis. Experimental results show that H-2 and acetylene (C2H2) are the major gas discharge products accompanied by trace gas discharge products, such as ethane (C2H6), ethylene (C2H4) and carbon. The highest CH4 conversion and H-2 yield, 91.2% and 38.4%, respectively, are achieved with an energy conversion efficiency of 44.3% using the microsecond pulsed spark discharge at a gap length 6 mm and a gas flow rate 50 mL/min. The carbon balance under the studied operating parameters varies from 66.7% to 92.8%. The morphology of carbon deposition is presented by two crystal forms identified by SEM and Raman spectral analyses. Finally, comparatively low electron temperature and high vibrational molecular temperature are observed in our experiments, which suggests that V-V transition for CH4 excitation process and V-T transition for CH4 heating process play important roles in CH4 pyrolysis sustained by the pulsed spark discharge.