Study of the thermoconversion and de-polymerization mechanisms of plastic wastes in supercritical water conditions

The waste valorization is a hot topic that has attracted great interest in the Circular Carbon Economy. Substantial efforts have been devoted to strengthening sustainable processes in recent years. These are based on the development of systems to improve carbon circularity (material and energy recycling).Global production of plastics doubled from 230 million tons in 2000 to 460 million tons in 2019. This exponential production/consumption has significant consequences on the environment. Despite the existence of recycling methods, only 9% of global plastic production is currently recycled, and the remaining quantity (not valorized) represents a real source of pollution [1].
Mixtures of different types of plastics make sorting stages difficult, which represents the main disadvantage for material recycling systems. An interesting application recently reported in the literature is the use of the hydrothermal gasification process to treat waste (and mixtures of difficult-to-sort) plastics to produce a gas rich in CH4 and H2 [2]. Hydrothermal gasification (HTG) is a thermochemical process which employs the supercritical conditions of water (T > 374 ° C, P > 221 bar), in order to convert the organic carbon contained in the wet feedstock into a gaseous phase (which contains CH4, H2, CO and CO2, mainly). In addition, the flexibility of the process also allows the study of de-polymerization of these wastes in conditions close to the critical point of water, which facilitates the production of chemical intermediates (and their reuse) in the chemical industry.
Thus, the understanding of the conversion mechanisms of different types of plastics (and their mixtures) seems essential to valorize these wastes. However, the identification of reaction pathways is still a major scientific obstacle. The objective of the thesis is the study of the reaction mechanisms of transformation of model plastics (and their mixtures) in supercritical water conditions. Understanding the phenomena will lead to the optimization of the HTG process (with and without catalysts) to facilitate the production of a gas rich in CH4/H2 and the production of intermediates for the chemical industry. The focus of this PhD work will involve: i) the study of thermo-conversion and de-polymerization of plastics; ii) the study of the behavior of catalysts in the supercritical water environment (activation/deactivation); iii) the study of selectivity towards the production of a gas containing CH4/H2 and the production of chemical intermediates.

Understanding the Impact of Operating Conditions and Utilization Profiles on Solid Oxide Electrolysis Stacks Lifetime

The shift to a low-carbon European Union (EU) economy raises the challenges of integrating renewable energy sources (RES) and cutting the CO2 emissions of energy intensive industries (EII). In this context, hydrogen produced from RES will contribute to decarbonize those industries, as feedstock/fuel/energy storage. Among the different technologies for low carbon H2 production, high temperature electrolysis (HTE) enables the production of green hydrogen with extremely high efficiency. The solid oxide cells (SOC) are typically operated in the 650-to-850°C temperature range, and arranged in pile-ups or stacks to increase the overall power density and address (pre-) industrial markets.
The technology has recently entered a phase of aggressive industrialization. However, significant efforts are still required to turn the high efficiencies into a competitive levelized cost of H2. As long as such cost remains largely controlled by that of stack manufacturing, stack degradation and the relationship with operating conditions remain a crucial subject of research and development. Moreover, recent advances have shown that to properly evaluate stack lifetimes, actual testing beyond 5 kh is critical [1,2]. A better understanding of degradation over the 5-to-10 kh range [3–5] could thus enable the development of both accelerated stress tests (AST) to reduce the necessary test duration, as well as optimized operational strategies to extend stack lifetimes.

Deployment strategy for energy infrastructures on a regional scale: an economic and environmental optimisation approach

The general context is "Design and optimisation of multi-vector energy systems on a territorial scale".
More specifically, the aim is to develop new methods for studying trajectories for reducing the overall environmental impact (underlying LCA) of a territory while controlling costs in various applications, for example:
- Opportunity to develop infrastructures (e.g. H2 network, or heat network) to enhance decarbonisation, by expanding new uses of energy where these infrastructures exist or will exist, while reducing the overall environmental impact for given uses.
- Based on these studies, study the impact of centralising or decentralising production and consumption resources,
- Taking into account the long-term dynamic of investments, with the compromise of renovating/replacing installations at a given moment, in order to reduce the overall environmental impact for given uses.
Possible applications for hydrogen infrastructures have been identified or are being identified

A better understanding of diffusion welding in a+ß titanium alloy

As part of a short-term nuclear project, the CEA/LITEN is supporting the manufacturing activities of a titanium alloy steam generator by HIP (Hot Isostatic Pressing). Depending on its thermal and/or thermomechanical history, the alloy Ti64 presents phases in different proportions, chemical compositions and crystallographic structures.
How does diffusion welding take place between two different phases? Is there one that cross the interface preferentially and if yes, why? Which HIP parameters have a real influence? What starting microstructure allows optimal diffusion welding?
These are the questions that the thesis should answer.