Plasma-Assisted Non-Oxidative Coupling of Methane: Effects of Bead Size Distribution and Operating Pressure in a Co-axial DBD

A co-axial packed-bed DBD reactor was used to conduct plasma-assisted non-oxidative coupling of methane (NOCM) utilizing glass beads as packing material at a fixed plasma power of 30 W. The influence on NOCM of five different bead size distributions (2000–5000 µm, 900–1100 µm, 425–600 µm, 212–300 µm, 150–212 µm) and operating pressure (1.2 bar, 1.7 bar) was investigated. The observed products consist of a mixture of saturated and unsaturated C₂–C₅ hydrocarbons. The conversion of methane decreased from 8.5 to 3.7% with decreasing bead size, while the selectivity towards unsaturated C₂ compounds increased from 16 to 50% with decreasing bead size. These reactor performance variations are associated with the transitional plasma dynamics and degree of partial discharging, as determined by characterization of non-ideal charge–voltage plots for the five tested glass bead sizes.