Titre de la dissertation: « Study of a gliding arc discharge for sustainable nitrogen fixation into NO_x ».

Promoteurs de thèse: Monsieur Rony SNYDERS et Mme Annemie BOGAERTS (Université d’Anvers)

 Résumé de la dissertation: With the growth of the world population, the agricultural sector is required to meet an increasing demand for nutrients. In order to achieve this, since decades the sector has relied on industrially produced fertilizers to increase its production yield. Among them, nitrogen-based fertilizers are the most common choice, although their production requires inert N₂ to be converted into more reactive molecules, such as nitric oxides NOx or ammonia NH₃, in a process called “nitrogen fixation”. Today, this is mainly performed in industry through the well-known Haber-Bosch process. Despite being the dominant choice for industrial nitrogen fixation for over a century because of an almost perfect optimization, the Haber-Bosch process is not ideal since it requires large-scale facilities to be economical and is associated with a high energy cost and high CO₂ emissions, thus, resulting in an environmental impact that is pushing for the study of different, greener, alternatives.

Among several options, plasma-based nitrogen fixation to NOx is considered a promising alternative to Haber-Bosch, thanks to (i) the possibility of implementing the technology at lower scales, (ii) its compatibility with intermittent, renewable energy sources, and (iii) the absence of direct CO₂ emissions. In this context, a type of plasma reactor that proved to be particularly suitable for nitrogen fixation is the gliding arc plasma, as it operates at atmospheric pressure and favors energy transfer toward vibrational excitation states, which is known to be beneficial for the NOx synthesis, although in practice, due to the high temperature in this “warm plasma” (order of 3000 K), vibrational-translation relaxation is quite efficient, reducing the populations of the vibrational excited levels, so they are in equilibrium with the gas temperature.

This thesis aims to study plasma-based nitrogen fixation focusing on a gliding arc plasma operating with N₂ and O₂ at atmospheric pressure. This is done both (i) on a fundamental level with the study of the physicochemical properties of the discharge aided by Fourier transform infrared spectroscopy and laser-induced fluorescence, (ii) on a more technological level by investigating the electrical stability of the process, and (iii) from a techno-economic perspective, benchmarking the plasma-based nitrogen fixation process.

More precisely, a new approach dealing with laser-based excitation of separate rotational lines has been successfully developed. Such an approach can be implemented on atmospheric discharges that produce rather high NOx densities where the high concentration of relevant absorbers, such as NO, imposes essential restrictions for the use of « classical » laser-induced fluorescence methods. The method is then implemented and used to provide a discussion on the two-dimensional distributions of both the gas temperature and the NO ground state density. Based on temperature map analysis, a clear correlation between the gas temperature and NO concentration is found and the effects of both the gas temperature and the plasma power on the concentrations of NO and NO2 produced are discussed revealing how the conversion into NO2 is already significant in the plasma afterglow region and the input gas flow rate is a crucial parameter affecting the temperature gradients in the afterglow region.

From a technological point of view, the conventional approach of introducing external resistors to stabilize the arc is challenged by studying both the performance and the stability of the plasma replacing the external resistor with an inductor. From this work, we conclude that similar stabilization results can be obtained while significantly lowering the overall energy cost, which decreased from up to a maximum of 7.9 MJ/mol N to 3 MJ/mol N.

Finally, this thesis studies whether a hypothetical small-scale fertilizer production facility based on a rotating gliding arc plasma for nitrogen fixation can be a local competitive alternative for the Haber-Bosch process. This is done by proposing a comparative model to understand how capital expenditures, gas price, CO₂ allowances, levelized cost of energy, and transport costs affect the fertilizer production costs. The model highlights how, with the current best available technology, plasma-based nitrogen fixation, while being an interesting alternative for the synthesis of NOx, still requires a more efficient use of H₂ for a direct production of NH3.