(570b) Low-Temperature Microwave Plasma Conversion of Methane to Higher Hydrocarbons

Technological developments in the unconventional oil and gas industry incited explosive growth in production of shale gas and oil known as the Shale Revolution. The resulting availability of the low-cost unconventional gas in the United States has enabled competitive use of ethane -- a relatively minor fraction of natural gas -- as a feedstock for large-scale ethylene production. However, the bulk of natural gas – methane – continues to be used only for low-value heat and electricity generation and, to a significantly lesser extent, as a source of industrial hydrogen through steam-methane reforming. Converting methane into higher value chemicals could significantly impact the chemical industry by adding new supplies of raw materials. However, methane activation remains a major challenge.

H Quest has demonstrated feasibility of applying a microwave continuous flow reactor to high-rate conversion of methane to hydrogen, acetylene, and ethylene. In the experimental set-up, the process gas mixture passes through a resonant cavity reactor, where a microwave discharge is created through an energy input of up to 4kW. The discharge creates a plasma environment and ruptures the C−H bond in methane. Resulting methyl, hydrogen, and other radicals participate in the rapid self-catalytic reaction, promoting activation of other methane molecules, as well as serving as precursors to C2+ chemicals, solid carbons, and free hydrogen. A slipstream exhaust gas with a nitrogen trace was analyzed with a 2-channel Inficon MicroGC to determine product distributions and conversion rates.

The product distribution and conversion rates were shown to be highly dependent not only on the energy density and process gas composition, but also on specific reactor configuration and geometry, as well as the gas flow patterns. Results (input energy requirements, methane conversion rates, and product selectivity) from the experimental sweep through process conditions and reactor configurations with and without a downstream catalyst bed will be discussed. A downward trend in energy requirements as methane concentration increased in the process gas mixture indicates a direction for process optimization.

Unlike other thermal decomposition approaches, this process does not rely on conventional (contact, convective, or dielectric) heating or use of thermal plasmas. Rather, it employs a microwave resonant cavity to create localized and high-energy reaction zones in the gas as it passes through the reactor’s active zone. Microwaves enable volumetric, non-contact energy transfer to the reactant flow, which is not achievable with radiative or conductive heating in furnaces, by accelerating free electrons in the partially ionized low-temperature plasma. Through electron-molecule collisions, these electrons both transfer the microwave energy to methane molecules and help overcome the high activation energy required by the rate limiting step of hydrocarbon (methane) pyrolysis – the endothermic cleavage of C-C and/or C-H bonds. This results in rapid, direct conversion of methane to chemically active species under atmospheric pressure and mild bulk temperatures: at least 500 lower than conventional decomposition methods.

Acknowledgements

H Quest Vanguard, Inc. is a privately held technology company, based in Pittsburgh, Pennsylvania, focused on the development and commercialization of novel hydrocarbon conversion technologies. This material is based on work supported by the Department of Energy, Office of Science under award DE-SC0017227.