Stranded natural gas and biogas comprise a vast energy resource that is mostly wasted via flaring. This represents an opportunity to investigate novel ways to harness this resource. Nonthermal plasmas consist of excited electrons, ions and neutral atoms/molecules that have the ability to disrupt chemical bonds at ambient temperatures and pressures. Methane conversion to value-added products is difficult in industry due to the stability of the C-H bond. The smaller scale of microscale technology offers many benefits including intensified momentum, heat and mass transport using reduced timescales and smaller spaces. Electrode spacing of less than 1 millimeter allows for lower plasma operating voltages (less than 1 kV). Direct current has historically been the type of electricity used to operate the plasma discharge using this particular electrode geometry and spacing. Any potential effects of using other types of electricity such as alternating current and pulsed direct current have not been explored. This work explores the effect of pulsing the current on the conversion, selectivity, yield and specific energy intensity of ethylene and synthesis gas production.
Frequencies of 0 Hz (straight DC), 60 Hz, 500 Hz, 1 kHz, 5 kHz, 10 kHz, 50 kHz and 250 kHz were chosen for this work. Operating parameters were fixed at 15.2 psia pressure, 20-25°C temperature (ambient), 40 sccm flow rate and a 3:1 CO2:CH4 molar feed ratio. Three replicates of each run were taken to enable statistical analysis. Chemical analysis was performed using gas chromatography and electrical analysis was performed using an oscilloscope.
Reactor performance was assessed at various frequencies at a duty ratio of 0.5 while keeping the average power at 3.1 W. Conversion was found to be constant when using a constant average power with all runs having around 19-20% average. The lowest and highest experimental values for selectivity ranged from 20.6% ± 4.04% at 500 Hz to 41.8% ± 9.63% at 250 kHz. The results of direct current and the results of the highest frequencies examined were not statistically discernable. Lower frequencies involved spurts of higher instantaneous current and power that averaged out to 3.1 W over time. Higher power density equates to a higher incidence of electron impact events at the molecular level. Higher-order hydrocarbons formed at these power levels will have a tendency to undergo more impact events and leave the reactor as lower-energy synthesis gas products.
The specific energy intensity of synthesis gas and ethylene formation was investigated. The lowest and highest experimental values for the specific energy intensity of synthesis gas ranged from 790. ± 56.0 kJ mol-1 at 10 kHz to 1,120 ± 105 kJ mol-1 at 250 kHz. Similarly, the lowest and highest experimental values for the specific energy intensity of ethylene ranged from 19,660 ± 4800. kJ mol-1 at 10 kHz to about 34,900 ± 9,990 kJ mol-1 at 500 Hz. Lower frequency experimental data showed a higher incidence of synthesis
gas production. These specific energy intensities were higher than values from different types of plasma work in literature. The focus of this work was to determine any discernable effects of using pulsed current with the same overall power input. Pulsing direct current promotes the formation of synthesis gas and is detrimental to selectivity towards higher order hydrocarbons.