Hydrogen production by microwave plasma CH4 pyrolysis: Characterization via optical emission spectroscopy and response surface methodology

Microwave plasma pyrolysis of methane (CH 4 ) is a route for hydrogen (H 2 ) production with low CO 2 emissions. This study utilizes Optical Emission Spectroscopy and Design of Experiments to characterize plasma pyrolysis of CH 4 using a 2.45 GHz microwave-based plasma reactor operated at atmospheric pressure with N 2 as the plasmagen gas. Measured characteristic plasma temperatures include electron temperatures of 15,000 ± 1500 K, rotational temperatures of 7500 ± 1000 K, and vibrational temperatures of 6000 ± 1000 K, with little variation across microwave power (1–3 kW) and flowrate (15–50 L/min). Electron microscopy shows the pyrolysis carbon exhibits carbon black-like morphology. CH 4 conversion, H 2 selectivity, C 2 H 2 selectivity, HCN selectivity and specific energy consumption are modelled using Response Surface Methodology. Results indicate a trade-off between reactivity and energy efficiency. Complete CH 4 conversion is possible but requires high energy input, while the lowest energy use (60 kWh/kg H 2 ) occurs at ∼50 % CH 4 conversion. • Microwave plasma pyrolysis of CH 4 for H 2 and solid carbon production is shown. • Plasma temperatures were measured with Optical Emission Spectroscopy. • Response Surface Methodology was utilized to model process metrics. • A system trade-off in CH 4 conversion and energy efficiency was determined.