Microwave plasma methane pyrolysis: Scaling relations and energy cost assessment

Continuous plasma pyrolysis of methane to make hydrogen would be viable as a low carbon intensity technology if high-value solid carbon byproducts can be produced. We report on the design and operation of a 3 kW microwave plasma-based methane pyrolysis lab reactor with the ability to produce 43 kWh/kgH₂ enabled by a reverse vortex stabilized flow configuration. The high fluid velocity vortex enables core plasma temperatures of ∼4500 K, while confining the plasma within an annulus vortex flow such that the walls of the quartz reactor stay relatively cool. Analysis of the effluent composition on-stream using gas chromatography and a Raman-based analyzer noted that molecular H₂ selectivity exceeds 75% on an atom-basis while solid carbon is the second most abundant carbon-containing species after acetylene. The solid carbon product consists of an admixture of amorphous and crystalline domains. An investigation of methane conversion with varying input power and argon admixture ratios revealed that conversion scales linearly with the input power and square root of the methane fraction. These correlations show microwave plasma processes match efficiencies of electrolysis and suggest efficiencies of arc-based pilot plants (25 kWh/kgH₂) can be achieved or even exceeded by MW plasma pyrolysis by better heat management and reactor scale up.