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 CH 4 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 H 2 , C 2 H 2 , and C 2 H 4 were 78% (1.56 mol H 2 /mol CH 4 reacted), 36% (0.18 mol C 2 H 2 /mol CH 4 ), and 30% (0.15 mol C 2 H 4 /mol CH 4 ), respectively, at 3 bar. In other experiments, H 2 diluent concentration played an important role in CH 4 dissociation and final product distributions; H abstraction reactions increased the rate of CH 4 decomposition at low H 2 (y H2 < 0.6) while high H 2 (y H2 > 0.6) impeded CH 4 decomposition due to hydrocracking of C 2 products. The rate of CH 4 dissociation was seen to increase with pressure, up to 0.11 mol/m 3 /s, and the specific energy requirement (SER) decreased with pressure to 365 kJ/mol CH 4 at 3 bar. The latter suggests that even higher operating pressures may improve the efficiency of plasma conversion of CH 4 , and ultimately that plasma pyrolysis may be a viable and energy efficient route to clean (turquoise) H 2 and further implementation of chemical process electrification.