Indirect heating thermochemical CO2 splitting: Fixed-Bed pellets and spheres vs. foam reactants in a tubular furnace

This study investigated thermochemical CO 2 splitting using ceria (CeO 2 ) reactive devices in a vertical tubular furnace simulating the reaction conditions of an indirectly heated solar thermochemical reactor. Five reactive configurations representing three geometry classes were tested: Foam 1, Foam 2, Pellet 1, Pellet 2, and Sphere. The device volumes ranged from V D = 0.0566 cm 3 to 55.1 cm 3 . Redox cycles were carried out with reduction at 1500 °C and oxidation at 1000 °C. Sharp and reproducible CO concentration peaks were observed across multiple consecutive cycles. The mass-specific CO volume ranged from 2.9 to 4.5 mL/gCeO 2 , and a common mean of 3.68 mL/gCeO 2 was determined by least-squares fitting. Although CO production was almost independent of the type of reactive device, the CO production rate varied significantly among device geometries. Pellet 1 ( ϕ = 0.589, V D = 0.912 cm 3 ) and Pellet 2 ( ϕ = 0.257, V D = 0.397 cm 3 ) demonstrated higher peaks in CO production rate compared to Sphere ( ϕ = 0.0890, V D = 0.0566 cm 3 ), Foam 1 ( ϕ = 0.884, V D = 49.1 cm 3 ), and Foam 2 ( ϕ = 0.641, V D = 55.1 cm 3 ). The relatively high reaction rates of the pellet devices may be associated with their fine internal pore structures, which could provide larger gas–solid interfacial areas than the millimeter-scale cells of the foam devices. Although the Sphere exhibited a lower reaction rate, it achieved the highest volumetric CO yield. This advantage was attributed to its low intrinsic porosity and high bed filling rate, which together enabled a high CeO 2 loading per unit bed volume. The mass-specific CO yields obtained under indirect heating were in the same order of magnitude as literature values reported for direct-irradiation systems, supporting the viability of the indirect-heating approach for solar thermochemical CO 2 splitting.