The SOLSTICE Laboratory of Excellence (LABEX)

Title: Synthetic fuel production from solar-driven thermo-catalytic decomposition and reforming of natural gas into molten media
Keywords: Methane, solar reactor, hydrogen, syngas, catalyst, molten salts, molten metals. Localization: PROMES-CNRS laboratory (Odeillo Solar Furnace)

LABEX research topic: Solar fuels production, Innovative thermal energy storage integration into concentrated solar power plants and/or Open topics on solar concentrating science and technology

Details of the PhD Proposal:

Solar hybrid processes concern the different thermochemical transformation paths of a hydrocarbon using concentrated solar energy as a high temperature heat source. The considered reactions during the PhD work are thermo-catalytic cracking and reforming of natural gas. The use of molten media such as molten metals or molten salts will be investigated as heat transfer fluids. Cracking reaction produces hydrogen and solid carbon, while reforming produces syngas (H2/CO) usable for the synthesis of methanol or liquid fuels (via Fischer-Tropsch synthesis). These solar hybrid processes provide the following advantages: (1) fuel saving and chemical storage of solar energy, (2) elimination of greenhouse gas emissions (CO2, SO2, NOx) as compared to conventional processes, (3) non-contamination of products by combustion gases.

On the one hand, the research work will focus on the thermo-catalytic decomposition of methane by concentrated solar energy (CH4->C+2H2). This work will be done in close collaboration with IHI Corp. (Japan) and will follow previous works (ongoing since 2018) in the field of development and simulation of new solar reactors for the production of solar fuels [1]. This thesis aims to contribute to the development of new catalysts, to their implementation in an experimental solar reactor and to experimentally determine thermochemical and energy efficiencies. The different objectives of the study are:

(i) Kinetic analysis of the catalytic activity of different types of catalysts (metallic or carbonaceous) in a solar thermogravimetry system (effect of the nature and properties of the material on the kinetics of the CH4 dissociation reaction), microstructural characterization of the carbon produced (production of high added value carbon nanofibers/nanotubes).

(ii) Experimental study of a solar reactor based on a fixed bed of catalyst particles or a metallic porous structure also allowing volumetric absorption of radiation (determination of chemical conversions and of the composition of the gas leaving the reactor). The catalyst could be introduced into a receiver containing molten metal in order to improve heat exchanges and to study the possibility of hybrid systems capable of operating day and night (thermal storage in molten media). Also, in the case of cracking in molten media, carbon can be stripped at the bath surface limiting carbon deposition issues [2].

(iii) Study of catalyst regeneration (gasification reaction of carbon formed by H2O or CO2 according to C+H2O->CO+H2 or C+CO2->2CO).
(iv) Simulation of the reactor in its various configurations: fixed bed, porous volumetric receiver, liquid bath (coupled transfers, radiation and heterogeneous catalytic reaction).

On the other hand, natural gas reforming processes in molten salts will also be developed, with the aim of producing syngas, according to the reaction: CH4 +CO2-> 2CO + 2H2. The objectives

of the work relate to the study of this new emerging concept enabling enhanced heat transfer to the gas phase along with possible thermal storage and catalytic effects of the salt [3]. Both experimental and simulation aspects will be addressed. A solar reactor will be designed and tested for methane reforming with and without molten salt in order to highlight the expected positive impact of bubbling designs, especially in transient conditions. Output gas composition will be examined along with the salt stability. The use of molten salt is expected to homogenize the temperature inside the reactor, to increase heat transfers along with possible catalytic effects. In addition, it will increase thermal inertia of the reactor for a better response to transients that are intrinsically linked to solar energy. The investigation of dynamic response of such system is required and not yet reported. Moreover, thermal storage in molten salt can be envisioned for round the clock production, which has never been demonstrated. Dedicated CFD simulation will be carried out in steady and transient operation, including fluid flow and transfer phenomena (heat (including radiation) and mass).

In order to reach the objectives, this thesis will benefit from the CNRS-PROMES solar infrastructures (solar furnaces, solar reactors, gas analysis devices, ANSYS CFD Tool...) along with the long experience of the laboratory in materials and solar thermochemistry. The topic perfectly suits the scope of the LABEX ‘Solstice’ concerning the production of clean energy vectors (synthetic fuels) through the development of innovative solar processes. The topic also addresses energy storage for solar fuels processes through the investigation of cracking and reforming reaction directly into liquid heat transfer media that can also be used as heat storage materials.

Valorization of the research work:

The objective is to publish at least 3 papers in international journals (possibly in open access) and to participate to 2 congresses (SolarPACES, WHEC...). The PhD will also participate to the ED305 Doctoral schools.

Budget: Most of necessary equipment and experimental facilities are already available at the laboratory. The operating costs (fluids, chemicals, small laboratory equipment, consumables) and missions (conferences) will be funded by contracts with the industrial Japanese partner (IHI) associated to this PhD project (~8 k€ per year). Also the PhD will benefit from the young researcher grant of Sylvain RODAT, new CNRS researcher. This budget will be dedicated to the fabrication of a new hybrid solar reactor with the objective to demonstrate the possibility of continuous solar processes.

Contacts: sylvain.rodat_at_promes.cnrs.fr, stephane.abanades_at_promes.cnrs.fr

Bibliography:

[1] Abanades, S., Kimura, H., & Otsuka, H. (2015). A drop-tube particle-entrained flow solar reactor applied to thermal methane splitting for hydrogen production. Fuel, 153, 56-66.
[2] Geißler, T., Abánades, A., Heinzel, A., Mehravaran, K., Müller, G., Rathnam, R. K., ... & Weisenburger, A. (2016). Hydrogen production via methane pyrolysis in a liquid metal bubble column reactor with a packed bed. Chemical Engineering Journal, 299, 192-200

[3] Kodama, T., Koyanagi, T., Shimizu, T., & Kitayama, Y. (2001). CO2 reforming of methane in a molten carbonate salt bath for use in solar thermochemical processes. Energy & fuels, 15(1), 60-65.