Direct conversion from methane to methanol is one of the key technologies for more efficient utilization of fossil fuels, because low-quality or low-temperature heat sources corresponding to 100°C can be used and regenerated in the reforming of methanol to hydrogen, whose exergy loss through the combustion process is by far the lowest among hydrocarbon fuels. In this study, direct conversion from methane to methanol has been successfully realized using a methane/oxygen gas mixture, with a newly developed nonequilibrium plasma of pulsed silent discharges under atmospheric pressure at 373 K. Major products are methanol, water, carbon monoxide, and a small amount of carbon dioxide and formaldehyde. Effects of initial oxygen concentration, residence time and strength of electric field on the methanol synthesis have been clarified. Methanol is significantly formed only when the applied electric field is raised above that for electrical breakdown of oxygen gas, about 21.3 kV/cm, and the high selectivity of 32% for methanol formation has been attained at low initial oxygen concentrations of about 5%. Furthermore, we have also confirmed the possibility of methanol formation using a methane/water-vapor mixture, by the same method. The time-dependent changes of emission intensity for CH radicals have shown that streamer-type discharges are essential for the effective formation of radical species and thus methanol formation.
