Modeling of a concentrated-solar, falling-particle receiver for ceria reduction

• A modeling tool for falling-particle solar receiver used to study ceria reduction.
• Ceria reduction and sensible heating increase greatly by reducing particle size.
• High reduction temperatures and low emissivity of ceria drive low efficiencies.

This paper presents a steady-state model coupling three-dimensional, spectral radiation heat-transfer to a quasi-one-dimensional particle flow model for assessing performance of a directly-irradiated, falling-particle solar receiver with a closed cavity for thermochemical reduction of ceria. A semi-empirical gas-phase flow entrainment model captures the heat and mass exchange between the surrounding gas and the reducing ceria particles. With bulk and surface thermochemistry for the oxide-ion transport and surface reduction kinetics, model results indicate that for particle diameters under 500 μm, surface chemistry controls the rate of ceria reduction in the receiver-reactor. For the range of particle inlet temperatures, flow rates, and diameters studied, the degree of ceria reduction correlates well with particle outlet temperature and increases to as high as 6% at maximum outlet temperatures of 1900 K. However, with the relatively low absorptivity of ceria, higher outlet temperatures lower the fraction of solar energy absorbed by the particles from approximately 30% at outlet temperatures of 1500 K to just above 10% at outlets of 1900 K. Most of the heat recovered in the ceria is due to sensible heating, and re-radiation losses can account for as much as 60–75% of the solar energy input due to the required high reduction temperatures and radiation properties of the ceria particles. The predicted low efficiencies show that ceria particle reduction must utilize significant heat recovery and alternative receiver optics other than simple direct radiation to improve the feasibility of solar-driven ceria redox cycles based on falling particle receivers.

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