Thomas FONTAINE - Faculté des Sciences

Titre de la thèse: "Methane conversion by pulsed plasma discharges"

Résumé:
Hydrogen is a key molecule of the chemical industry, and a potential decarbonization tool for certain applications. However, its production is nowadays tied to greenhouse gas emissions, creating a need for alternative production routes. One of them is the direct splitting of methane to form hydrogen and solid carbon, or small hydrocarbons.
Methane dissociation can be achieved by thermal or catalytic processes, but plasma offer a direct means of activating stable molecules in a one-step process. Their complex chemistry relies on the activation of the molecule from electron impact reactions and thermal activation, which occur when a current flows through a gas.
In this work methane conversion was investigated in a dielectric barrier discharge (DBD) and in a “pin-to-pin” configuration, generating a spark. In both configurations, the objective of this work was to use different pulsed high-voltage sources to produce the plasma discharges. The plasmas were generated at atmospheric pressure. Methane conversion and product selectivity were studied at ULB, and analyses with high temporal resolution were performed at UMons to study the dynamics of the process at the nanosecond timescale. This work aims to improve our understanding of the phenomena involved in methane conversion and the influence of various parameters on this process.
In DBD, various sources of pulsed AC voltage were compared to generate the discharge. It was observed that micro- and nanosecond pulses reduced the energy cost of the conversion. Controlling the energy dissipated in each of the pulses made it possible to control the rotational temperature reached in the plasma (measured on the C2 band), thereby allowing the total power to be increased while lowering the discharge temperature.
In pin-to-pin experiments, the energy cost was reduced when higher temperatures are reached. Time-resolved analysis of the discharge revealed a transition from emissions from the CH and C2 bands, at the start of the pulse, to H line at the end of the pulse. This transition reveals the change from a filamentary to a spark regime.
The lowest energy cost for hydrogen production was achieved in the pin-to-pin configuration, with the longest pulse duration (500 ns).