Thèse Catalyseurs à Base de Ni-Fe - Biochar et de Déchet Inorganique Industriel pour la Production du Méthane Renouvelable à Partir du Syngas Issu de la Gazéification de Biomasse H/F Doctorat.Gouv.Fr

Établissement : IMT Mines Albi École doctorale : MEGEP - Mécanique, Energétique, Génie civil, Procédés Laboratoire de recherche : RAPSODEE - Centre de Recherche d'Albi en Génie des Procédés, des Solides Divisés, de l'Energie et de l'Environnement Direction de la thèse : Doan PHAM MINH ORCID 0000000177701487 Début de la thèse : 2027-10-01 Date limite de candidature : 2027-08-15T23:59:59 La production de méthane renouvelable à partir du gaz de synthèse issu de la gazéification de la biomasse solide permet de conjuguer la valorisation du carbone biogénique, le stockage d'énergie et la substitution du gaz naturel fossile. Toutefois, ce procédé repose principalement sur des catalyseurs au Ni et un syngas très épuré (cf. notamment pour l'élimination des goudrons), ce qui augmente le coût global. Cette thèse explorera une voie plus circulaire combinant le craquage catalytique des goudrons et la méthanation grâce à des catalyseurs à faible coût élaborés à partir de biochar et de résidus contenant du Ni et du Fe, issus de l'industrie minière et de secteurs connexes. L'hypothèse centrale est que la structure et la chimie de surface du biochar, associées à des sites interfaciaux Ni-Fe-C optimisés et à l'ajout d'un résidu inorganique, peuvent favoriser la méthanation du CO et du CO2 tout en convertissant une fraction résiduelle contrôlée de goudrons.
Le projet vise à établir les relations entre la composition du catalyseur, la structure du biochar, la configuration des sites actifs, les voies réactionnelles et la désactivation dans des conditions plus réalistes. Les catalyseurs seront formulés par pyrolyse à partir d'un mélange de biomasse et de résidus inorganiques riches en Ni et Fe. L'objectif est de produire un matériau composite constitué d'un biochar à porosité développée et de nanoparticules de Ni-Fe actives pour la méthanation catalytique, le tout associé à des oxydes inorganiques favorisant le craquage des goudrons. Les catalyseurs ainsi produits feront l'objet d'une caractérisation approfondie au centre RAPSODEE (France). La modélisation DFT permettra de modéliser des sites représentatifs de Ni, Fe et Ni-Fe supportés sur des structures carbonées. Les calculs d'adsorption et des étapes élémentaires de réaction impliquant H2, CO, CO2 et des molécules modèles de goudrons fourniront des descripteurs liés la performance des catalyseurs. Le criblage catalytique s'appuiera dans un premier temps sur des mélanges gazeux synthétiques pour évaluer, séparément et conjointement, la méthanation du CO et du CO2, l'influence de la vapeur d'eau et de la composition du gaz, ainsi que le comportement de composés modèles de goudrons sélectionnés. Les catalyseurs seront caractérisés avant et après utilisation afin d'analyser l'évolution de leur propriétés structurelles et de surface. Cette démarche permettra d'identifier les propriétés clés favorisant la méthanation du CO et du CO2 et d'optimiser la préparation des catalyseurs.
Les catalyseurs les plus prometteurs seront ensuite testés à l'UC3M (Espagne) en utilisant du gaz de synthèse réel à faible teneur en goudrons (En combinant la modélisation DFT, des techniques de caractérisation avancées, et des essais expérimentaux, le projet apportera des connaissances mécanistiques et des critères de conception pour des catalyseurs de méthanation issus de bioressources et de déchets. La complémentarité des expertises des deux équipes offrira au doctorant une formation interdisciplinaire couvrant la modélisation moléculaire, la catalyse, et l'expérimentation sur les procédés thermochimiques. À terme, le projet fournira les bases scientifiques du concept BtX plus simple et circulaire.
Renewable methane is an attractive complement to direct electrification because it can be stored, transported and used through much of the existing natural-gas infrastructure. Anaerobic digestion is currently the most established route for biomethane production, but it is constrained by the availability of suitable biodegradable feedstocks, such as manure, sewage sludge and wet agricultural or municipal residues. Even the full sustainable global potential for biogases is estimated to correspond to only approximately one quarter of current natural-gas demand [1]. Anaerobic digestion alone can therefore replace only a limited fraction of fossil natural gas, making complementary thermochemical routes necessary to valorise lignocellulosic biomass and dry biogenic residues which constitute the most abundant sources of bioenergy and are unsuitable for digestion and represent the more.
Biomass gasification extends the available feedstock base by converting solid biomass into a product gas or syngas mainly containing H2, CO, CO2 and CH4. Fluidised-bed gasifiers are particularly attractive because of their feedstock flexibility, effective gas-solid contact and potential for scale-up [2,3]. The resulting syngas can be converted into renewable methane through CO and CO2 methanation, although its H2/COx ratio may not fulfil the stoichiometric requirements for extensive methane production. The gas composition can be adjusted with water-gas shift, external renewable H2 or a combination of both.
The combination of biomass gasification and syngas methanation was already demonstrated at industrial scale in the GoBiGas project (2018), however it requires an extensive gas cleaning, including tar removal [4]. The main scientific challenge is therefore to transfer the high methanation activity observed with controlled synthetic mixtures to real biomass gasification gas. Most studies use highly purified feeds, whereas biomass-derived syngas contains steam, CO, CO2, CH4, residual hydrocarbons and tars, together with trace contaminants that can promote poisoning, carbon deposition and progressive catalyst deactivation [5,6]. Particles, sulphur and chlorine compounds will be removed upstream, while a real fluidised-bed gasification gas with a low but non-zero tar content will be supplied to the methanation reactor. Under these conditions, the catalyst must combine high activity and selectivity for CO and CO2 methanation with resistance to deactivation and the ability to tolerate or partially convert residual tar. Developing catalysts with these properties is essential to enable renewable methane production from lignocellulosic biomass and solid biogenic residues. Furthermore, this approach will allow to at least partially combine tar craking and methanation and thus reduce the number of operation units of the process. Consequently, the selection of active metals and support materials, the control of metal-support interactions and their effects on reaction and deactivation mechanisms are central scientific questions of this project.
Ni-based catalysts were largely used in CO2 methanation due to their high catalytic activity and CH selectivity. The high abundance of nickel justified the economic viability of its use in catalysis for large-scale industrial applications. A good dispersion of nickel nanoparticles in the support was shown to be required to enhance the catalytic activity. However, Ni particles sintering and carbon deposition during the exothermic methanation reaction lead to catalyst deactivation, limited selectivity, and stability under reaction conditions, especially for conventional alumina supported-Ni catalysts [7]. The use of new support materials, innovative synthesis techniques, as well as the addition of promotors, such as Ce, Ru or Pt was shown to optimize Ni-based catalysts performances [8]. However, these techniques usually imply the use of toxic or noble metals combined to commercial supports, such as zeolites and metal-organic frameworks (MOFs). In other related applications, such as Water Gas Shift (WGS), Fe was shown to be a promising substitute of nickel, showing comparable activity and selectivity with a lower coke deposit formation [9].
Carbon-based materials, including activated carbon, biochar or structured carbon, were used as catalyst support in CO2 methanation, exhibiting promising results in terms of activity and selectivity. These materials promote CO2 adsorption and activation at the metal-support interfaces, which facilitates the formation of CO intermediates [10]. The presence of Alkaline and Alkaline Earth Metals (AAEM) on biochar can also enhance this mechanism, as well as act as catalyst promotors [9]. Biochar metallic content is generally enhanced by different techniques, among which the most frequent is direct mixing or impregnation of biomass before pyrolysis [11]. Natural materials, such as clay, red mud, mining waste or dolomites, can be used in composite catalyst formulation due to their abundance, cost-effectiveness, and intrinsic catalytic activity [12]. Some of them can at the same time bring active phases (Ni, Fe) or catalytic promotors (MgO, CaO) to the composite material [13], which avoids the use of commercial compounds. Red mud based-catalysts, rich on iron oxide, showed promising results in tar cracking [14]. The addition of inorganic oxides such as MgO was shown to reduce sintering and coke deposit on Ni-based catalysts [15]. As a result, the combination of highly dispersed Ni-nanoparticles on the porous structure of biochar and iron inorganic oxides may produce a biosourced catalyst suitable for catalytic methanation, which can simultaneously contribute to tar cracking. Furthermore, inherent and added metals on carbon supports can be converted on their reduced active form by well-established carbothermal reduction methods. However, the action mechanisms resulting from the interactions metal-support, very dependent on the synthesis technique of the composite material, still need to be explored into detail to optimize catalyst formulation and thus enhance its activity, cyclability and regeneration ability.
CO2 catalytic methanation can occur via CO2 dissociative or associative pathways, with the formation or not of CO intermediates, respectively [8]. Biochar properties, such as carbon structuration, surface functional groups and metallic phases, as well as their evolution during the chemical reaction, are crucial in determining the predominant reaction pathway. Up to now, most of the studies focused on producing, characterizing and testing biochar-based catalysts for CO2 methanation. Global kinetic models were proposed in some cases, without going into the details of the chemical reactions involved in the mechanism [16]. Furthermore, the link between biochar properties, reaction mechanisms and catalyst deactivation was not assessed into detail until the calculation of the energy barriers to identify the most probable reaction pathway [17]. Density Functional Theory (DFT) showed promising results in elucidating reaction mechanisms when combined to experimental results and catalysts characterization before/after use [18]. Nevertheless, to our best knowledge, DFT was not applied to model biochar-based catalysts behavior in CO2 methanation up to now. As a result, combining catalyst characterizations, DFT modelling and experimental results can contribute to optimize catalyst formulation, to investigate catalyst reaction and deactivation mechanisms and thus to propose a regeneration pathway to close the catalytic loop of the composite biochar catalyst.
The overall objective of the PhD is to establish the scientific basis for producing renewable methane as a C-based fuel from real biomass gasification syngas using low-cost Ni-Fe/biochar catalysts derived from biomass and metal-bearing industrial residues.
The specific objectives are:
- To formulate and to thoroughly characterise biochar-based catalysts containing Ni, Fe and Ni-Fe phases, determining how residue composition, biochar surface chemistry and preparation conditions control the formation and accessibility of the active sites.
- To elucidate CO and CO2 methanation pathways and the interactions of representative tar compounds with Ni-, Fe- and Ni-Fe-C interfacial sites by combining DFT calculations with experiments using controlled synthetic gas mixtures, thereby establishing structure-activity-selectivity relationships.
- To determine the effects of gas composition, steam and controlled concentrations of model tar compounds on catalytic activity, methane selectivity and stability, and to identify the main deactivation mechanisms through time-on-stream experiments and spent-catalyst characterisation.
- To validate the most promising catalysts using real syngas with a low residual tar content ( The methodology will follow an iterative approach in which catalyst formulation, physicochemical characterisation, DFT calculations and catalytic testing continuously inform each other. The work will be organised into five interconnected work packages (WP).
WP1- Litterature review (M1-6, M = month): A full literature review on the related topics, e.g. catalytic methanation of syngas, Ni-Fe/biochar catalysts, catalyst derived from inorganic waste in methanation reaction and DFT calculation of carbon-based catalysts will be performed at the beginning of the PhD.

WP2 - Development of catalysts (M5-12): Candidate biochars and metal-bearing industrial residues will be selected and characterised. Residues containing Ni and/or Fe will be screened for their chemical and mineralogical composition, while monitoring additional elements that could influence catalytic behaviour or safe handling. Monometallic Ni- and Fe- and bimetallic Ni-Fe/biochar catalysts will be produced by the co-pyrolysis of biomass and inorganic residues. In a second step, biochar activation will be carried out to favor the development of biochar ultra-microporosity and ensure the dispersion of the metallic active particles on the biochar surface. The obtained biochar catalysts could be doped with Fe and/or Ni nanoparticles if their concentration would not be enough to reach the target values to ensure the catalyst activity (5-10wt%). The mixture biochar/inorganic residue will be adjusted to guarantee the presence of at least 15 to 20wt% of iron oxides in the final catalyst for favoring its tar cracking activity. Appropriate reference materials prepared from conventional metal precursors will be included to distinguish the effects of the residue matrix, the biochar support and the Ni-Fe interaction. Fresh catalysts will be characterised to determine their porosity, carbon structure, surface functional groups, elemental composition, metal speciation, reducibility and metal-particle distribution.

WP3 - Catalytic screening (M10-M20): The catalysts will be screened at RAPSODEE using controlled synthetic gas mixtures. CO and CO2 methanation will first be studied separately and subsequently under combined feeds representative of biomass-derived gas. The influence of temperature, H2/COx ratio, steam and controlled concentrations of model tar compounds will be evaluated. Catalyst performance will be quantified through carbon conversion, methane yield and selectivity, carbon balance and activity decay with time on stream. The effect of the two main components of the catalyst, namely biochar rich in Ni and Fe and remaining iron oxide, will be separately tested in methanation and tar cracking. Fresh and spent materials will be compared to identify changes caused by sintering, carbon deposition, poisoning or transformations of the Ni- and Fe-containing phases. The formulation and characterisation stages will then be iterated to optimise the most promising materials.
If the experimental work requires it, a 6-month internship student can be engaged to the project by using internal funding from RAPSODEE, so as to help the PhD student in catalysts formulation, characterization and methanation tests. This could be also the case for enhancing the collaboration with our partners on DFT modelling (e.g. WP 4 below) if the discussions require it.

WP4 - DFT modelling (M21-33): Experimentally informed DFT models will be developed for representative Ni-, Fe- and Ni-Fe-C interfacial sites supported on defective and functionalised carbon structures. Adsorption and activation of H2, CO and CO2 will be investigated, together with selected elementary steps leading to methane formation. The interaction of representative tar molecules with the active sites will also be assessed. Reaction energies, activation barriers and relevant electronic and structural descriptors will be compared with the experimental results. The models will be progressively refined as information on the actual catalyst phases and their evolution becomes available.

WP5 - Validation and PhD defense (M21-36): A limited number of catalysts will be selected for validation at UC3M. Biomass gasification will be performed in the existing bubbling fluidised-bed facility. Particles, sulphur and chlorine compounds will be removed upstream, whereas a controlled, low residual tar concentration will be retained. Where required, the H2/COx ratio will be adjusted through water-gas shift conditioning and/or external renewable hydrogen to provide a suitable feed for catalyst evaluation; comparison of hydrogen-management strategies will not constitute a separate research objective. The selected catalysts will be tested in downstream fixed-bed reactors at atmospheric pressure. Permanent gases and condensable organic compounds will be analysed to quantify CO and CO2 conversion, methane formation, residual-tar conversion and catalyst deactivation. Results obtained with real gas will be directly compared with the synthetic-gas baseline. Finally, the computational, characterisation and reactor data will be integrated to establish structure-activity-selectivity-stability relationships. These relationships will be used to identify the catalyst properties and operating conditions that control methane production, tar tolerance or cracking and lifetime. The observed deactivation rates will be used to define technically reasonable replacement criteria. The final assessment will establish the scientific feasibility of the proposed low-cost catalyst concept at laboratory scale.