Direct synthesis of methanol from methane and water-vapor mixture using ultra-dhort pulse plasma

Direct synthesis of methanol from methane/water-vapor mixture has a high possibility to realize a highly sophisticated energy utilization system with exergy regeneration. However, this process can never occur by conventional thermochemical methods due to the large increase of Gibbs free energy. In this study, the direct synthesis of methanol from methane/water-vapor mixture has been successfully realized by low-temperature and nonequilibrium plasma chemical reactions using a newly developed ultra-short pulse plasma, and effects of reaction time, peak voltage, pulse width and initial gas composition on the methanol formation characteristics have been clarified. In the case of spatially uniform pulse-glow plasma, maximum yield of 0.2% and maximum selectivity of 3% have been achieved. The time dependent change of emission intensity for CH radicals also showed the advantage of using ultra-short pulsed voltage for high energy input into the reaction field. Further, the methanol synthesis has been significantly improved by use of a newly found "filamentary" discharge mode which is realized by adjusting gas pressure and pulse parameters of peak voltage, pulse width and pulse frequency, and much higher values of 0.52% and 10% for methanol yield and selectivity have been attained, respectively. It has been also found that the mixing of rare gas largely enhances the methanol yield and its mechanism is discussed.