Plasma-Driven Decomposition of Hydrocarbons in High Density Media: High-Pressure Gasses and Liquids

Decarbonizing the global energy and chemical infrastructure is a long-term challenge, owing in large part to the pervasive role of fossil fuels in the modern economy. Their vast availability and low cost have placed them in a central position to the operation of energy and chemical industries, making rapid and complete replacement difficult despite the urgency of reducing greenhouse gas emissions. As a result, near-term decarbonization will depend not only on the gradual replacement of fossil fuels, but also on the development of low-carbon methods to utilize them to help bridge the transition to a zero-carbon economy. This has motivated growing interest in finding efficient pathways for converting hydrocarbons into hydrogen and other value-added products without direct CO₂ emission. Plasma-driven processes are particularly attractive because they can be driven electrically, making them compatible with renewable energy sources, and can generate reactive species, such as radicals and ions, that are not readily available under thermal conditions, thereby enabling additional reaction pathways.

Despite this promise, the understanding of mechanisms and process design principles governing plasma-driven hydrocarbon processes remains insufficient. In particular, the relative importance of non-equilibrium plasma chemistry, thermal effects, and transport can vary widely with discharge conditions, pressure, and reaction environment, making product selectivity and energy efficiency difficult to predict. This lack of clarity continues to limit the design and optimization of plasma processes for practical applications.

This thesis investigates the mechanisms and efficacy of non-thermal plasma for methane and light hydrocarbon conversion into hydrogen and value-added hydrocarbon products. Methane conversion was examined using a custom pulsed plasma source to systematically determine the influence of plasma pulse energy and excitation timescale (nanoseconds to millisecond) on discharge characteristics, conversion and product selectivity. Quantitative mass spectrometry and high-resolution optical emission spectroscopy were used to characterize product distributions, gas temperature and electron density. This work demonstrates that methane dissociation proceeds through both electron-driven and thermal pathways in short discharge pulses; however, the overall product distribution is largely governed by thermal dissociation. These studies are complemented by high-pressure batch reactor experiments that measure methane dissociation rates under the influence of pressure and hydrogen dilution. Results indicate that increasing reactant density improves process energy efficiency and methane dissociation can be facilitated by H-abstraction reactions.

Building upon these gas-phase studies, the work is extended to direct liquid hydrocarbon conversion, where the electrical discharge is heavily influenced by the local hydrodynamic environment, introducing an additional layer of complexity. The product distribution, conversion and energy efficiency were strongly affected by plasma excitation frequency and fluid flow velocity. To better understand the physical basis of these effects, the breakdown mechanism of liquid hydrocarbon was further investigated using time-resolved shadowgraph imaging, and the influence of flow profile on bubble dynamics and discharge stability was examined in a liquid venturi jet reactor.

Overall, this work shows that non-thermal plasma offers a promising pathway for hydrocarbon reforming and identifies important operating parameters for future process optimization studies. Moreover, the demonstrated viability of direct liquid hydrocarbon conversion points towards the potential for direct reforming of liquefied natural gas (LNG). The development of such a process could make use of existing LNG infrastructure and facilities, making the transition towards sustainable energy sources more economically feasible.