Optical and Mass Spectrometric Measurements of the CH4–CO2 Dry Reforming Process in a Low Pressure, Very High Density, and Purely Inductive Plasma

This paper presents a study of a CH4–CO2 plasma-reforming process carried out in a high power density (5–50 W/cm3), using toroidal transformer-coupled plasma, and operated at low pressure (0.2–0.7 Torr). Using the intermediate between a thermal and nonthermal plasma (electron density, n e ≈ 3 × 1012 cm–3 and a maximum gas temperature of ∼4000–6000 K along the center line), the low-pressure study provides a unique set of conditions to investigate reaction mechanisms, where three-body reactions can be ignored. Reactive species in the plasma were identified by optical emission spectroscopy. End products of the reforming process were measured by mass spectrometry. Quite high conversions of CO2 and CH4 were found (90%), with selectivities for CO and H2 of 80% at 300 sccm feed gas flow rate in a 0.5 Torr plasma, with a mole ratio CO2–CH4 of 1:1. A detailed reaction mechanism is presented, taking into account the combined detection of reactive intermediates in the plasma (H, O, CH, and C2) and stable product downstream.