Titre de la dissertation: « Methane conversion by pulsed plasma discharges »

Promoteurs de thèse: Monsieur Thomas Fontaine et Monsieur François Reniers (ULB)

Résumé de la dissertation: 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 plasmas 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” discharge, 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. Initially, sinusoidal, triangular, and square-wave voltage waveforms were used. This variation in voltage waveform did not significantly affect methane conversion or selectivity. Next, different types of pulses were compared: “burst” pulses (sinusoidal voltage pulsed for a few milliseconds), 100–500 µs pulses, and 100–500 ns pulses. It was observed that the burst mode offered similar efficiency to the non-pulsed AC mode, but that the micro- and nanosecond pulses reduced the energy cost of the conversion. The nanosecond pulses also enabled the formation of acetylene, which was not observed in the other pulses. Controlling the energy dissipated in each of these 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.
The energy cost of converting one mole of methane in the DBD ranged from 4 to 8.4 MJ/mol, with the best results (between 4 and 5 MJ/mol) obtained using microsecond and nanosecond pulses. The reaction products are primarily hydrogen (>70%), ethane (20%), propane (5%), ethylene, and finally acetylene, which is produced only with the nanosecond pulses. The formation of a polymeric deposit with the empirical formula CH2.2±0.2 is also observed.
In pin-to-pin experiments, only nanosecond pulses were used. The impact of various parameters (applied voltage, electrode spacing, pulse frequency) was measured, but the most interesting results were observed when studying pulse duration. To this end, the conversion of methane using pulses ranging from 15 to 500 ns was investigated. When switching to 50 ns, a change in the discharge behavior was observed. The observed color shifted from blue to pink, and the filaments changed from relatively diffuse to a more linear appearance. Using optical emission spectroscopy, the emission of the CH and C2 bands (located in the blue-green region, at 430 and 516 nm respectively) is observed at the start of the pulse (during the first 40 ns), subsequently replaced by the emission of the H line (located in the red region, at 656 nm). The discharge diameter, estimated from images taken every 5 ns, also decreases sharply at the time of this emission. This transition is explained by the shift from a so-called filamentary regime (observed in other studies) to a spark regime.
The cost of converting methane in this configuration ranged from 1.5 to 2 MJ/mol, and the cost of producing one mole of hydrogen ranged from 1 to 1.5 MJ/mol. The main product is hydrogen (70–75%), followed by acetylene (10–15%), ethylene (5–10%), and ethane (5–10%). A deposit composed of solid carbon was also observed.

Lieu: de la défense publique ULB- le forum E sur le campus de la plaine de l’ULB