AC Plasmas Directly Excited in Liquid-Phase Hydrocarbons for H₂ and Unsaturated C₂ Hydrocarbon Production

AC plasmas directly excited within liquid hydrocarbons were investigated for the production of hydrogen and unsaturated C₂ hydrocarbon in a recirculating liquid "jet" flow configuration. Arc discharges were excited at two different frequencies (60 Hz and 17.3 kHz) in C₆-C₈ hydrocarbons (hexane, cyclohexane, benzene, toluene, and xylene) to produce H₂, C₂H₄, C₂H₂, and CH₄, along with liquid and solid carbon byproducts. AC frequency was seen to modify the plasma properties and gas bubble formation dynamics, significantly influencing the efficiency and reaction pathway. Higher discharge frequency increased energy efficiency more than 2-fold by minimizing thermal losses and favored the production of hydrogenated compounds due to shorter reactant-plasma contact times. Further optimization of hexane conversion was achieved by introducing fluid flow around the plasma electrodes, which led to competitively low specific energy requirements (SERs) of 3.2 kWh/kg C₂H₄, 4.9 kWh/kg C₂H₂, and 24.3 kWh/kg H₂. The effect of hydrocarbon feed chemistry was analyzed, showing that hexane and cyclohexane are preferable for C₂ hydrocarbon syntheses, whereas aromatic hydrocarbons produce more H₂. Gas bubble dynamics and liquid/solid products were analyzed using high-speed imaging, optical emission spectroscopy (OES), gas chromatography-mass spectrometry (GC-MS), scanning electron microscopy/transmission electron microscopy (SEM/TEM), and Raman spectroscopy. This work contributes to the understanding of plasma conversion mechanisms within liquids and demonstrates the potential for the energy-efficient transformation of hydrocarbons with plasma in unique reaction environments.