(87b) Solar Fuel Production from CO2 and H2o Via the Hybrid CeO2-CH4 Redox Cyclic Process

Within the context of solar thermochemical fuel production, a redox process is discussed in which CeO₂ is chosen as the redox material due to its crystallographic stability and rapid kinetics. This material has been previously applied for the splitting of H₂O and CO₂ to produce syngas via a two-step thermochemical redox cycle using concentrated solar energy. In this solar cycle, temperature/pressure swings between the reduction and oxidation steps are thermodynamically favored for high fuel yield, but they impose severe thermal stresses on material structures and energy inefficiencies due to significant heat rejection. Alternatively, the CeO₂-CH₄ process is a hybrid solar-driven redox cycle that uses CH₄ to eliminate the temperature/pressure swings while further increasing the fuel output. This is because the addition of CH₄ as a reducing agent lowers the operating temperature by approximately 500°C, runs isothermally at about 1000°C, and achieves higher non-stoichiometry (δ) of CeO_2-δ thus indicating a higher proportion of oxygen exchange and, consequently, yielding a higher specific fuel output per unit mass of ceria. The hybrid CeO₂-CH₄ cyclic process is represented by two redox steps:

Solar endothermic reduction:

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

Non-solar exothermic oxidation:

CeO_2-δ + δCO₂ ↔ CeO₂ + δCO (2a)

CeO_2-δ + δH₂O ↔ CeO₂ + δH₂ (2b)

The two redox reactions (i.e. eq. 1 and eq. 2) have opposing thermodynamic favorability with respect to temperature and are also affected by the extent of the reaction. Insights into the thermodynamic nature of the redox reactions allow one to predictably and efficiently control a reactor system by avoiding side reactions and operating in regimes of high reaction favorability.

Modelling and experimental studies are performed on a thermochemical reactor concept for effecting this hybrid solar-CH₄ redox cycle. The reactor concept for effecting the hybrid solar-CH₄ redox cycle consists of an alumina tube containing a porous structure of ceria exposed to an upward flow of the reacting gas, namely CH₄ during the reduction step (eq. 1) and CO₂/H₂O during the oxidation step (eq. 2). Various porous structures are investigated, including reticulated porous ceramic (RPC) foam-type structures and packed bed of pellets, granules, and other morphologies with varying specific surface areas (SSA). The tubular reactor is heated externally by a heat transfer fluid which carries high-temperature heat from the solar receiver. A lab-scale prototype reactor was fabricated using a 19 mm-diameter 300 mm-length Al₂O₃ tube, heated externally by an electrical furnace. Preliminary experimentation above 900°C indicated that an improved CeO₂ pelleted morphology increased the CH₄ conversion over that of the baseline RPC. Detailed experimental results will be presented.

A numerical heat and mass transfer model is developed to simulate the thermochemical reactor. OpenFOAM is applied to formulate and solve numerically the governing equations for fluid flow across porous media and combined convection-conduction-radiation heat transfer coupled to the chemical reactions. The effective transport properties of the RPC are incorporated [1]. The model is validated against laboratory experiments and applied for design optimization and scale-up.

Acknowledgements – This work was funded by the Swiss Federal Office of Energy (Grant No. SI/501854-01).

References

[1] S. Ackermann, M. Takacs, J. Scheffe, and A. Steinfeld, "Reticulated porous ceria undergoing thermochemical reduction with high-flux irradiation," International Journal of Heat and Mass Transfer, vol. 107, pp. 439-449, 2017.