Time-resolved in situ measurements and predictions of plasma-assisted methane reforming in a nanosecond-pulsed discharge

The plasma properties and chemical kinetics of plasma-assisted methane reforming in a He diluted nanosecond-pulsed plane-to-plane dielectric barrier discharge (ns-DBD) were studied using the combination of time-resolved in situ laser diagnostics and a 1-D numerical model. Plasma-assisted fuel reforming kinetic mechanisms were mainly evaluated based on matching reactant conversion and syngas production to steady-state measurements, which cannot describe the full range of chemistry and physics necessary to validate the model. Adding 1% CH₄ to a pure He ns-DBD resulted in a faster breakdown along the rising edge of the applied voltage pulse, thereby lowering the reduced electric field (E/N), electron number density, and electron temperature. Further addition of CH₄ did not continue to alter the E/N in the model. Laser absorption spectroscopy was used to measure gas temperature, C₂H₂, H₂O, and CH₂O in a CH₄/CO₂/He discharge to serve as validation targets for the predicted reaction pathways. CH₂O was predicted within 25% of the measured value, while H₂O and C₂H₂ were under-predicted by a factor of two and three, respectively. Path flux analysis showed that the major pathway for CH₂O formation was through the reaction between CH₃ and O, while C₂H₂ formation had multi-step pathways that relied on ions like CH₃⁺ and C₂H₅⁺. It also revealed that CH₂ is a significant intermediate for production of both CH₂O and C₂H₂, and increased CH₂ concentration could improve model predictions. The results demonstrate that the use of reaction rate constants with lower uncertainties and inclusion of He₂⁺ are needed to improve the predictions. Varying the “equivalence ratio”, defined by the CH₄ dry reforming reaction to H₂ and CO, from 0.5 to 2 had a weak effect on measured product species and experimental trends were explained based on pathways extracted from the model.