(695d) Methanol from Sunlight and Air Using a Modular Solar Dish Reactor System

We report on the experimental pilot demonstration of a modular solar dish-reactor system for the production of methanol from ambient air using concentrated solar energy. We describe the complete process chain and its key components, and present representative on-sun runs with fully-automated consecutive CO₂-splitting and H₂O-splitting redox cycles.

The system integrates three thermochemical units operated in series, namely: 1) the capture of CO₂ and H₂O directly from ambient air via an adsorption-desorption cyclic process; 2) the co-splitting of CO₂ and H₂O to generate a specific mixture of CO and H₂ (syngas) via a reduction-oxidation cyclic process using concentrated solar energy; and, 3) the gas-to-liquid synthesis of methanol. The experimental validation of the entire process chain to solar methanol under realistic field conditions was accomplished by moving from a laboratory setup (Marxer et al. 2017) to a modular solar system, thereby advancing the technological readiness and its industrial implementation.

The 2^nd process in the aforementioned process chain is the 2-step thermochemical splitting of CO₂ and H₂O via ceria redox reactions, represented by:

1^st step) Reduction: CeO₂ → CeO_2-δ + δ/2 O₂ (1)

2^nd step)

Oxidation with H₂O: CeO_2-δ + δ H₂O → CeO₂ + δ H₂ (2a)

Oxidation with CO₂: CeO_2-δ + δ CO₂ → CeO₂ + δ CO (2b)

where the non-stoichiometry δ denotes the ceria reduction extent. Ceria is selected as the redox material because of its fast redox kinetics and high crystallographic stability. In the first, high-temperature solar-driven endothermic reduction step (eq. 1), ceria is partially reduced to a non-stoichiometric state. In the subsequent lower-temperature non-solar exothermic oxidation step, the reduced ceria is reacted with H₂O and/or CO₂ to generate H₂ and/or CO (eqs. 2a and 2b). The solar reactor for effecting this redox cycle consists of a cavity-receiver containing a reticulated porous ceramic (RPC) foam structure made of pure CeO₂. The RPC is directly exposed to the high-flux solar intensity and provides efficient radiant absorption and combined heat/mass transport within the reaction site. Two identical solar reactors are mounted side-by-side on a solar concentrator subsystem (Dähler et al. 2018). It encompasses a sun-tracking parabolic primary reflector and a planar secondary reflector. The later allows to rotate the focal point among 4 positions: the two solar reactors, a water calorimeter for solar radiative power input measurements, and a Lambertian target for solar flux distribution measurements. This optical arrangement enables the operation of the two solar reactors for performing both redox steps simultaneously by alternating the solar radiative input between them while making continuous and uninterrupted use of the incoming concentrated sunlight. Thus, while one solar reactor is performing the endothermic reduction step on sun, the second solar reactor is performing the exothermic oxidation step off sun. During the reduction step (eq. 1), the solar reactor is heated with concentrated sunlight up to 1500 °C and the total pressure is lowered to 4 mbar by the vacuum pump to evolve a pure stream of O₂ from CeO₂. During the oxidation step (eqs. 2a and 2b), CO₂ and H₂O are injected into the reactor’s cavity, react with the reduced ceria, and are transformed into syngas - a specific mixture of CO and H₂. Downstream of the two solar reactors, the syngas is compressed up to 200 bar and subsequently processed to methanol in a catalytic gas-to-liquid packed-bed reactor. The complete solar system is fully automatized to perform consecutive redox cycles.

Full day on-sun consecutive cycles demonstrate the stability and robustness of the system. Solar runs with varying H₂O/CO₂ mixtures further show the flexibility of the system to produce syngas in the desired quality and stoichiometry suitable for the downstream gas-to-liquid process, e.g. Fischer-Tropsch or methanol synthesis.

Acknowledgements: This work was funded in part by the Swiss Federal Office of Energy (Grant No. SI/501213-01), the Swiss National Science Foundation (Grant No. 200021-162435), and the European Research Council under the European Union’s ERC Advanced Grant (SUNFUELS – Grant No. 320541). We thank Patrick Basler, Thomas Cooper, Yago Gracia, Philipp Good, Andrea Pedretti, David Rast, Max Schmitz, Nikolas Tzouganatos, Alex Muroyama, and Michael Wild for their valuable contributions to the technology development.

References:

Marxer, D., Furler, P., Takacs, M., Steinfeld, A., 2017. Solar thermochemical splitting of CO2 into separate streams of CO and O2 with high selectivity, stability, conversion, and efficiency. Energy Environ. Sci. 10, 1142–1149.

Dähler, F., Wild, M., Schäppi, R., Haueter, P., Cooper, T., Good, P., Larrea, C., Schmitz, M., Furler, P., Steinfeld, A., 2018. Optical design and experimental characterization of a solar concentrating dish system for fuel production via thermochemical redox cycles. Solar Energy 170, 568–575.