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Combined analysis of optical emission spectroscopy and infrared thermography revealing how liquid properties affect plasma ignition in a dielectric barrier discharge microfluidic system where methane-containing gas interacts with organic liquids. 

Reforming of methane (CH4) is a process to produce syngas (CO/H2) and other value-added chemicals including oxygenates such as methanol (CH3OH). Atmospheric pressure plasmas have the potential to be more energy efficient than traditional reforming methods as value-added chemicals can be synthesized directly in the plasma without requiring an additional step. In this paper, we discuss the results from a computational investigation of the formation of oxygenates by CH4 oxidation in the presence of Ar, including CH3OH and CH2O, in a nanosecond pulsed dielectric barrier discharge. The plasma is formed in a microfluidic channel whose small dimensions are ideal for plasma formation at atmospheric pressure. The production and consumption mechanisms of dominant radicals and long-lived species are discussed in detail for the base case conditions of Ar/CH4/O2 = 50/25/25. CH3OH is produced primarily by CH3O reacting with CH3O and CH3O2 reacting with OH, while CH2O formation relies on reactions involving CH3O and CH3. The most abundant oxygenate formed is CO (produced by H abstraction from CHO). However, the greenhouse gas CO2 is also formed as a by-product. The effects of gas mixture are examined to maximize the CH3OH and CH2O densities while decreasing the CO2 density. Increasing the Ar percentage from 0% to 95% decreased the CH3OH and CH2O densities. At low Ar percentages, this is due to an increase in consumption of CH3OH and CH2O, while at high Ar percentages (>40% Ar), the production of CH3OH and CH2O is decreased. However, both CO and CO2 reached peak densities at 70%–90% Ar. Changing the CH4/O2 ratio while keeping 50% Ar in the discharge led to increased CH3OH and CH2O production, reaching peak densities at 35%–40% CH4. The CO and CO2 densities decreased beyond 20% CH4, indicating that a CH4 rich discharge is ideal for forming the desired oxygenates. 

The conversion of methane, CH4, into higher value chemicals using low temperature plasmas is challenged by both improving efficiency and selectivity. One path towards selectivity is capturing plasma produced methyl radicals, CH3, in a solvent for aqueous processing. Due to the rapid reactions of methyl radicals in the gas phase, the transport distance from production of the CH3 to its solvation should be short, which then motivates the use of microplasmas. The generation of CH3 in Ar/CH4/H2O plasmas produced in nanosecond pulsed dielectric barrier discharge microplasmas is discussed using results from a computational investigation. The microplasma is sustained in the channel of a microfluidic chip in which the solvent flows along one wall or in droplets. CH3 is primarily produced by electron-impact of and dissociative excitation transfer to CH4, as well as CH2 reacting with CH4. CH3 is rapidly consumed to form C2H6 which, in spite of being subject to these same dissociative processes, accumulates over time, as do other stable products including C3H8 and CH¬3OH. The gas mixture and electrical properties were varied to assess their effects on CH3 production. CH3 production is largest with 5% CH4 in the Ar/CH4/H2O mixture due to an optimal balance of electron-impact dissociation, which increases with CH4 percentage, and dissociative excitation transfer and CH2 reacting with CH4, which decrease with CH4 percentage. Design parameters of the microchannels were also investigated. Increasing the permittivity of the dielectrics in contact with the plasma increased the ionization wave intensity which increased CH3 production. Increased energy deposition per pulse generally increased CH3 production as does lengthening pulse length up to a certain point. The arrangement of the solvent flow in the microchannel can also affect the CH3 density and fluence to the solvent. The fluence of CH3 to the liquid solvent is increased if the liquid is immersed in the plasma as a droplet or is a layer on the wall where the ionization wave terminates. The solvation dynamics of CH3 with varying numbers of droplets was also examined. The maximum density of solvated methyl radicals CH3aq occurs with a large number of droplets in the plasma. However, the solvated CH3aq density can rapidly decrease due to desolvation, emphasizing the need to quickly react the solvated species in the solvent.