Valorisation du CO2 par voie thermochimique solaire pour la production de combustible synthétique – CO2FUEL

In the long-term, the world's energy system may have to be based on non-fossil energy sources in order to reduce CO2 emissions. The decarbonisation of fossil fuels and the capture and storage of CO2 would help the transition to a future carbon-free energy system. The CO2FUEL project goes further and proposes to recycle and solar up-grade the CO2 emissions of existing industrial processes involving fossil fuels. The objective of the novel solar process is to chemically transform CO2 into carbon-neutral liquid fuels. The initial step intends to chemically reenergize CO2 into CO using concentrated solar power. The approach consists in breaking a carbon-oxygen bond in the carbon dioxide to form carbon monoxide and oxygen in two distinct steps. The cyclic process to be developed is based on metal oxide systems, which allows CO separation from CO2 at significantly lower temperatures compared to that necessary for straight CO2-splitting. Solar thermal energy and CO2 are the only process inputs, CO and O2 the only outputs. Then, CO can be used to make H2 from exothermal water gas shift reaction (CO + H2O --> CO2 + H2, DH° = -41 kJ/mol). This pathway could decrease the use of natural gas steam reforming to make H2, which could result in saving fossil fuels and decreasing CO2 emission. In addition, by using commercial processes (Fischer-Tropsch), H2 and CO produced by solar processes could be combined to make a synthetic hydrocarbon-based liquid fuel, such as methanol or even gasoline or diesel fuel, that is already compatible with existing infrastructures. While fossil fuel will become scarcer, synthesizing fuels for traditional gasoline and diesel engines may become an increasingly attractive alternative. This could help the transition from the today's fossil fuel economy to a future sustainable hydrogen economy. The overall process would result in the conversion of water and CO2 into fuel from sunlight, which is thus equivalent to a reverse combustion. The CO2FUEL project targets the production of carbon monoxide from carbon dioxide using novel solar thermochemical cyclic processes, thus contributing to reduce greenhouse gas emissions. The project is focused on the investigation of two-step reaction routes based on non-toxic metal oxide redox pairs as chemical intermediates. These species are recycled and can thus be considered as CO2-splitting catalysts since they are not consumed. Two kinds of thermochemical systems are proposed operating in the temperature range 1200-1700°C: the simple oxides (for example, Fe3O4/FeO, ZnO/Zn, SnO2/SnO, ') and the mixed-metal oxides such as ferrites, (Fe1-xMx)3O4 / (Fe1-xMx)1-yO. The latter is proposed because the maximal process temperature could be decreased below 1400°C. The objectives of the study are to identify and optimise the 2-step reaction routes that will be analysed by conducting thermodynamics (chemical equilibrium) and experimental studies (chemical reactivity testing and reaction kinetics, methods for materials synthesis and shaping, physico-chemical characterization), and to develop novel solar reactor concepts (design, testing, and modelling). Concerning simple metal oxide systems, the thermal reduction reactions of MxOy at high temperature and the CO2-splitting reactions will be studied in order to define optimal operating conditions (temperature, pressure, products quenching) and to compare the system's performances (chemical conversion and kinetics). Concerning the mixed-metal oxides, synthesis methods of active compounds for CO2-splitting will be developed. The reactivity of the mixed oxide powders will be investigated as a function of the nature and quantity of the doping metal M in the mixed compound. Both the O2-releasing reaction (activation step) and the CO generation reaction will be characterized. Then, the mixed compounds will be coated on stable ceramic structures (multi-channelled monolithic honeycombs or porous ceramic foams) allowing efficient solar radiation absorption and suitable for incorporation in a solar reactor. The implementation of these solar processes requires the thorough investigation of the reactive chemical systems at high temperature (thermochemical properties, fair kinetics, links between structure/composition and reactivity, stability upon cycling), and the development and performance evaluation of suitable solar chemical reactors. This includes the design of solar receiver/absorber compatible with the materials behaviour at high temperature, experimental testing, and modelling. Two solar reactor prototypes (2 kWth) will be developed and characterized. They will be based on different concepts depending on the thermochemical system considered. Specific models of thermal radiation will be developed and integrated in a global reactor model for operation simulation, optimisation, and scale-up.