Oxidative Coupling of Methane with ac and dc Corona Discharges

The oxidative coupling of methane (OCM) is being actively studied for the production of higher hydrocarbons from natural gas. The present study concentrated on the oxidative conversion of methane in an atmospheric pressure, nonthermal plasma formed by ac or dc corona discharges. Methyl radicals are formed by reaction with negatively-charged oxygen species created in the corona discharge. The selectivity to products ethane and ethylene is affected by electrode polarity, frequency, and oxygen partial pressure in the feed. Higher C2 yields were obtained with the ac corona. All the ac corona discharges are initiated at room temperature (i.e., no oven or other heat source is used), and the temperature increases to 300−500 °C due to the exothermic reactions and the discharge itself. The largest C2 yield is 21% with 43.3% methane conversion and 48.3% C2 selectivity at a flowrate of 100 cm3/min when the ac corona is at 30 Hz, 5 kV (rms) input power was used. The methane conversion may be improved to more than 50% by increasing the residence time, but the C2 selectivity decreases. A reaction mechanism including the oxidative dehydrogenation (OXD) of ethane to ethylene is presented to explain the observed phenomena. The results suggest that ac and/or dc gas discharge techniques have significant promise for improving the economics of OCM processes.