(300a) Experiment Investigation of a Solar Reactor for Thermochemical Syngas Production Via the CeO2-CH4-CO2 Hybrid Redox Cycle in a Concentrating Solar Tower

Solar thermochemical processing offers an efficient pathway for producing sustainable fuels using concentrated solar energy as the source of high-temperature process heat. The targeted fuel is syngas − a specific mixture of H₂ and CO that can be further processed to drop-in transportation fuels. Within this context, a 2-step redox cyclic process for syngas production is considered using CeO₂ as the redox material. This cycle has been extensively studied for splitting H₂O and CO₂ [1] and its on-sun feasibility was demonstrated as part of the EU project SUN-to-LIQUID [2][3]. An alternative process is a hybrid redox cycle that uses CH₄ as a reducing agent, thus significantly lowering the temperature of the endothermic reduction step. Furthermore, it can operate isothermally in the range 800-1200°C and achieve higher non-stoichiometries (δ), indicating a higher proportion of O₂ exchange and therefore a higher specific fuel output. The hybrid redox cycle is represented by two steps [4]:

Endothermic reduction:

CeO₂ + δCH₄ ↔ CeO_2-δ + δCO + 2δH₂ (eq. 1)

Exothermic oxidation:

CeO_2-δ + δCO₂ ↔ CeO₂ + δCO (eq. 2a)

CeO_2-δ + δH₂O ↔ CeO₂ + δH₂ (eq. 2b)

Combining the two redox steps yields the net reforming reactions:

Dry reforming:

CH₄ + CO₂ ↔ 2CO + 2H₂ (eq. 3a)

Wet reforming:

CH₄ + H₂O ↔ CO + 3H₂ (eq. 3b)

We report on a joint collaboration of Synhelion, IMDEA Energía, and ETH Zurich, to test the dry (CO₂-based) hybrid redox cycle and reforming reaction at the concentrating solar tower facility of IMDEA Energía in Spain. The solar reactor consisted of a cavity-receiver with a windowed aperture and lined with a reticulated porous ceramic (RPC) structure made of CeO₂, totaling 21 kg, which featured dual-scale porosity for enhanced heat and mass transfer. With this arrangement, the RPC is directly exposed to concentrated solar irradiation entering through the aperture. The main operating conditions were: solar radiative power input of 10 kW at a mean solar concentration ratio of 500 suns, nominal cavity temperatures between 800-1200°C, mass flowrates of 100 L/min (L: normal liters), and up to 40% reactive gas concentrations diluted in Ar. The peak molar conversions obtained during hybrid redox cycling (eq. 1 and eq. 2a) were CH₄ = 70% and CO₂ = 40%, which increased with temperature as predicted by thermodynamics, resulting in higher non-stoichiometries. Reaction selectivity to CO = 50% and to H₂ = 30% was reached at 930°C and at a low reactive gas concentration of 3%. The dry reforming operation mode (eq. 3a) is dominated by an endothermic reaction, hence CH₄ conversions increased with temperature independent of feeding ratio (CO₂:CH₄) to a maximum of 50% at 955°C. Conversions and selectivity were improved by incorporating a second tubular (non-solar) reactor downstream of the solar cavity [5]. Overall, the project proved the feasibility of operating the hybrid redox cycle with a directly-irradiated solar reactor in a solar tower configuration.

References

[1] Marxer, D., et al., Solar thermochemical splitting of CO2 into separate streams of CO and O2 with high selectivity, stability, conversion, and efficiency. Energy & Environmental Science, 2017. 10(5): p. 1142-1149.

[2] Koepf, E., et al., Liquid fuels from concentrated sunlight: An overview on development and integration of a 50 kW solar thermochemical reactor and high concentration solar field for the SUN-to-LIQUID project, in SOLARPACES 2018: International Conference on Concentrating Solar Power and Chemical Energy Systems. 2019.

[3] Romero, M., et al., Solar-Driven Thermochemical Production of Sustainable Liquid Fuels from H2O and CO2 in a Heliostat Field, in Proceedings of the ISES Solar World Congress 2019. 2019. p. 1-12.

[4] Bulfin, B., et al., Thermodynamic comparison of solar methane reforming via catalytic and redox cycle routes. Solar Energy, 2021. 215: p. 169-178.

[5] Ackermann, S., et al., Process for the Production of Syngas, EP18195213.6, filed 18.9.2018.