Kinetic Study of Low-temperature Methane Steam Reforming in Plasma/Catalyst Hybrid Reaction

Abstract

This paper presents a mechanistic study of low-temperature (300–600°C) steam reforming of poor biogas using dielectric barrier discharge and catalyst-bed hybrid reactor. Although excited species produced by high-energy electron impact are responsible for the synergistic effect between non-thermal plasma and catalyst-bed, heat generated by non-thermal plasma also accelerates catalytic methane reforming at low temperatures: when discharge power was excess in supply, catalyst-bed temperature significantly increased and synergistic effect due to radical production was no longer anticipated. In order to distinguish their respective contributions to the synergistic effect, we analyzed the overall methane-reforming rate based on power law kinetics (r=k [CH₄]^α [H₂O]^β). The forward reaction rate constant (k) was analyzed by means of Arrhenius plots with respect to catalyst-bed temperature measured by infrared camera. Reaction orders for methane (α) and water-vapor (β) were experimentally determined from the initial conversion rate of methane, showing both α and β were clearly increased in the presence of non-thermal plasma. Activation energy in the reaction-dominant regime (<450°C) was approximately 100 kJ·mol⁻¹ regardless of non-thermal plasma: reaction pathways seem to be essentially unchanged by the non-thermal plasma. On the other hand, pre-exponential factors increased by a factor of ten by applying non-thermal plasma. The sticking probability of excited molecules to the catalyst surface is expected to increase, which in turn accelerates the dissociative chemisorption of excited methane on the catalyst. The result also implies that excited water-vapor may have an ability to prohibit solid carbon deposition. Although methane dehydrogenation is the rate-determining step, activation of water-vapor is beneficial to maintain catalyst activity with minimum steam/carbon ratio.