A parametric study was conducted in a nonthermal plasma reactor with an emphasis on converting methane and carbon dioxide to longer chain hydrocarbons. In contrast to previous plasma literature for this process, the approach here was distinctly different in that the glow/corona regimes were used without catalysis. Also, a microscale gap distance (rather than milliscale) was employed. Finally, the role of elevated pressure on plasma performance was examined, as higher pressure would bring the proposed process closer to industrial relevance. A microscale reactor with an electrode gap distance of only 500 μm was used to help reduce the voltage requirement for creating a stable nonthermal plasma glow discharge. Factors such as pressure, residence time, power, and composition were studied to determine their effects on conversion, product distribution, and energy efficiency. While some factors followed expected trends such as increasing power and residence time leading to more conversion, pressure was found to have significant effects on selectivity and energy efficiency. Increasing the pressure from 110 to 220 kPa greatly reduced the selectivity to higher hydrocarbons (36.2–16.4%) and increased syngas production. Higher pressure did increase the efficiency of the process (7–15%). This increase in efficiency was largely due to the fact that fractional conversion was only slightly affected by pressure despite the increase in the mass flow rate that increasing pressure causes while keeping residence time constant. This implies that electrons had more than enough energy to cause reactions such that the same fraction of methane could be reacted even with an increase in the ratio of methane molecules to electrons.

The direct conversion of methane to longer hydrocarbons, including alkanes and alkenes (i.e., olefins), is achieved with a high energy and atom efficiency through the use of low-energy, nonthermal electric glow discharge plasmas acting on CH 4 / O 2 /N 2 and CH 4 /CO 2 /N 2 in a microstructured reactor. The process is neither dependent on limited lifetime catalysts nor consumable chemicals, enabling continuous operation over long periods. Similarly, it is carried out at atmospheric pressure and ambient temperature, thus simplifying process implementation. Because it does not require high temperatures, energy is not wasted producing sensible heat, allowing for high process energy efficiencies. Also, because it is designed in easy to number up modules, it can be readily scaled to the needs of the methane resource available. The effects of CH 4 /O 2 and CH 4 /CO 2 ratios, electric discharge current, flow rate, gap distance between electrodes, and the number of discharges on product distribution and energy efficiency were studied. Dependent on conditions, energy efficiencies in the order of 80%, methane conversion up to 75%, and carbon selectivity toward C 2+ products up to 90% can be achieved for these processes. The performance of this plasma-chemical microreaction system uses around two-thirds of the energy of that for the current commercial ethylene production process and is thus proposed as a promising alternative for industrial applications including valorization of stranded methane sources.

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Hydrocarbon processing using plasmas has tremendous potential, yet there still exist many uncertainties pertaining to practical operation over long durations. Previously, it has been demonstrated that a nonthermal plasma operating in a DC glow regime can transform methane into C2 species (acetylene, ethylene, ethane) in a microreactor. Using a DC glow regime in a microchannel reactor allows for lower power consumption, at the expense of greater consequence of fouling. Since biogas can be a source of methane, a longevity study was undertaken to understand how the microreactor system would change over time with a feed mixture of simulated biogas (CO2, CH4) and air. Two different biogas mixtures were used, one of which contained 300 ppm H2S, while the other had no H2S. Potential difficulties observed from previous experiments included carbon deposition on the electrodes, which could interfere with the electrical characteristics of the plasma discharge as well as material deposition in the microchannel, which could affect gas flow. It was found that raising the temperature of the system to 120 °C helped prevent hydrocarbon deposition in the reactor. Purging the reactor periodically with dry air was also found to have positive effects as it removed carbon buildup on the electrodes themselves. Successful operation over a 50 h time period without any significant deterioration was demonstrated.