Optimizing dense particles for efficient thermochemical fuel generation through a unified particle-level model

Abstract

Two-step thermochemical H₂O/CO₂ splitting offers a promising approach to convert intermittent solar energy into storable fuels. However, achieving efficient reaction kinetics in dense particles requires a comprehensive understanding of the bulk diffusion, surface reactions and concentration of local species. In this study, we present a comprehensive 1-D numerical model that accounts for gas–solid mass transfer, surface reactions, and bulk diffusion in reacting particles. The model was validated using previously reported experimental data for CeO₂ in the temperature range from 1173 to 1473 K. We used a resistance model to accurately quantify the rate-limiting steps. Our findings indicated that surface kinetics generally represent the primary limiting factor for small particle sizes, and the particles with a radius exceeding 60 μm, undergoing reduction at an oxygen partial pressure equal to 10⁻⁸ atm, experience rate limitations due to gas-phase mass transfer. In contrast, under extreme conditions, such as particle radius of 1 cm and diffusion coefficient of less than 10⁻⁶ cm² s⁻¹, bulk diffusion became one of the rate-limiting steps. This comprehensive modeling approach has potential to be applied to other candidate materials in thermochemical cycles, enabling fast material screening and structural designs.