Impact of Pressure and Hydrogen Dilution on the Kinetics of Methane Decomposition in AC-Excited, High Pressure Plasmas

The kinetics of methane decomposition in low frequency (60 Hz) AC arc plasmas was investigated using on-line mass spectrometry and optical emission spectroscopy (OES) in a batch reactor configuration at pressures up to 3 bar absolute. Plasma conversion of CH₄ results largely from thermal dissociation and was seen to follow first-order kinetics up to high conversions (> 90%) without observing any rate impedance from reverse hydrocracking. H– and C-atom selectivities for H₂, C₂H₂, and C₂H₄ were 78% (1.56 mol H₂/mol CH₄ reacted), 36% (0.18 mol C₂H₂/mol CH₄), and 30% (0.15 mol C₂H₄/mol CH₄), respectively, at 3 bar. In other experiments, H₂ diluent concentration played an important role in CH₄ dissociation and final product distributions; H abstraction reactions increased the rate of CH₄ decomposition at low H₂ (y_H2 < 0.6) while high H₂ (y_H2 > 0.6) impeded CH₄ decomposition due to hydrocracking of C₂ products. The rate of CH₄ dissociation was seen to increase with pressure, up to 0.11 mol/m³/s, and the specific energy requirement (SER) decreased with pressure to 365 kJ/mol CH₄ at 3 bar. The latter suggests that even higher operating pressures may improve the efficiency of plasma conversion of CH₄, and ultimately that plasma pyrolysis may be a viable and energy efficient route to clean (turquoise) H₂ and further implementation of chemical process electrification.