Direct non-oxidative methane conversion in arc plasma reactor: Physical and chemical solutions to lower energy cost*

Methane conversion to higher hydrocarbons such as ethylene and aromatics has been referred as a great challenge in the petrochemical industry, which is gaining the increasing interest because of increasing cost of other fuel feedstock compared with methane cost (the least expensive feedstock). This work presents key solutions (i.e., related to both physics of plasma reactor and chemistry of the process) to increase the energy efficiency of methane conversion. The physics of plasma reactor herein mentioned to increase the energy transfer from arc to the gas against energy loss by modifying the plasma reactor design and increasing the length of arc column. The chemistry of reaction process herein referred to the effect of additional gases diluted with methane. The study indicated that modifying reactor design significantly increased the energy efficiency by virtue of focusing arc. Moreover, elongation of arc length is the key parameter to further reduce the energy cost. In the chemistry point of view, all of additional gases diluted with methane has a negative effect of energy cost of acetylene production. However, in the point of acetylene selectivity; hydrogen has a strong effect on the reaction kinetics of methane pyrolysis process in order to prevent methane dissociation and inhibit the carbon solid formation to increase acetylene selectivity. Finally, we successfully created the arc discharge in an optimized arc plasma reactor (called nozzle-type rotating arc reactor) using pure methane, which results in a very low energy cost (9.8 kW/kg-C₂H₂), reported the first time. This achievement is an important leap of methane-to-acetylene conversion process because of its extremely reduced energy cost and using the simplest discharge gas (pure methane); other additional gases like H₂, N₂, Ar, or He are removed from the discharge gas mixture, as a results, the burdens for experiment preparation and product separation are decreased.