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.

Development of catalysts for CO2 hydrogenation to light olefins

Light olefins, mainly ethylene and propylene, are amongst the organic compounds with the largest production volume. They are currently produced from fossil resources. The reduction of the carbon footprint of products synthesized from these intermediates necessitates the use of alternative feedstock, such as atmospheric CO2.
The objective of this phD is the development of catalyst for the direct hydrogenation of CO2 into light olefins. Fe based catalyst combining reverse water gas shift (RWGS) and Fischer-Tropsch polymerization (FT) capabilities will be developed. In order to have a better understanding of iron forms involved in the reaction, Fe nanoparticles of controlled composition and dsizes will be prepared and dispersed on different support (silica, alumina, carbon,…). The catalytic properties will then be evaluated on a dynamic reactor and finely characterized using numerous techniques (XRD, XPS, HRTEM, …).

The role of the signaling nucleotide ppGpp in plant resilience to climate change.

Amidst the growing challenges of climate change, crops face threats from rising temperatures and prolonged droughts, leading to reduced photosynthetic efficiency and the need for rapid stress acclimation. In this PhD project we will investigate the role of the nucleotide guanosine tetraphosphate (ppGpp) signalling pathway, a known regulator of plastid function and photosynthesis. Recent preliminary work from our and other labs indicate that ppGpp plays a pivotal role in plant stress acclimation, and we have indications that perturbation of ppGpp signalling affects plant responses to heat stress. This research aims to explore how ppGpp is involved in plant acclimation to heat and drought stress. Using a combination of physiological evaluations, biochemical techniques, transcriptomics, and biosensors this study will investigate the modulation of ppGpp levels under stress conditions, its impact on plastid genome expression, and its intersection with other signalling pathways. The ultimate goal is to enhance our understanding of ppGpp's role in plant acclimation, offering insights for improving crop resilience in a climate-challenged world.

Analysis and multi-scale thermal-hydraulic simulation of design transients of an innovating nuclear-to-heat reactor concept

The System optimization and pre-design Laboratory of CEA/IRESNE at Cadarache works on innovating nuclear reactor concepts in order to decarbonize all industry and urban sectors (flexible electricity, heat, cool, synthetic fuel, hydrogen). One of those innovating concept is the ARCHEOS passive water reactor dedicated to heat supply and designed to be intrinsically safe and simple to operate. The main challenge of this research is to understand and analyse the thermal-hydraulic behaviour of this reactor that fully operates in natural circulation, which is clearly an innovation in the domain. The PhD student will first identify normal and accidental scenarios and simulate them at the reactor scale. Thoughts for design improvements could emerge as a result of this research. Those simulations will be associated to a deep physical analysis of thermal-hydraulic phenomena that can play a role during the studied scenarios. An appropriate modeling (from 1D to porous 3D to 3D CFD) is to be found to capture the thermal-hydraulic phenomena of importance. This will be performed using CATHARE3 and Neptune_CFD tools. Working on such an innovating nuclear reactor concept represents a great opportunity for a PhD student. This experience will be provide the student with a solid background on various topics such as: nuclear safety, innovating reactor design, multi-scale thermal-hydraulic simulation, reactor physics in transient regimes as well as a solid knowledge on the CATHARE3 code which is widely used in French nuclear industry and research and the reference system code for many projects in the nuclear industry.

Synthesis and shaping of Metal-Organic Frameworks for the capture of noble gas (Xe, Kr)

The design of new nuclear reactors named MSR, for Molten Salt Reactor, is currently being studied at the CEA, but also internationally. During their operation, gaseous fission products are generated and must be extracted, in particular Xe and Kr. For this purpose, adsorption on solid support in fixed bed columns are considered, but such processes require the development of very selective materials with high sorption capacities. Recently, Metal-Organic Framework (MOF) materials have demonstrated exceptional selectivity for noble gas trapping. However, such materials are generally synthesized in a fine powder form, which is not compatible with an application in fixed bed processes.
This PD works aims to synthesize MOFs and develop a shaping technique, so that they can be used in columns for the trapping and separation of noble gases. Firstly, the most promising MOF structures will be identified in the literature and synthesized in laboratory. Then a process allowing their granular shaping will be developed. This shaping will optimize the application of MOFs in a fixed bed column process and their capture performances will be determined using a gas separation pilot.
The student must have a strong interest in experimentation. He/she will develop skills in synthesis and characterization of materials (SEM, XRD, nitrogen adsorption-desorption, etc.). More generally, the student will have the opportunity to address the complexities linked to a gas treatment process using fixed bed columns.