Parametric Study of Hydrocarbon Chain Growth from Methane via a Nonthermal Plasma Discharge Microreactor

Abstract

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.