Multi-scale simulation for energy release performance of carbonation process in solar-driven calcium-looping: From grain to reactor

Calcium-looping energy storage technology presents a promising potential to address the instability issues associated with the renewable energy sources (e.g. solar power). In this work, a fluidized bed is comprehensively analyzed as an energy release device. A multiscale method is proposed to investigate the mechanisms involved in the reaction and heat release during the exothermic carbonation process, spanning from the grain scale to the reactor scale. The accuracy of the model in terms of reaction kinetics and heat transfer is rigorously validated against experimental results. It is revealed that the carbonation reaction exhibits three distinct stages within 40 s, characterized by changes in reaction rate: the rapid reaction stage, the transition stage, and the equilibrium stage. The rapid reaction stage experiences significant fluctuations in the reaction rate due to factors such as gas-solid temperature distribution, reactant gas concentration distribution, and the phenomenon of gas back-mixing. Observations in the reactor show distinct zones, including the bubble zone, dense phase zone, splash zone, and freeboard zone, where bubbles exhibit a cap-shaped morphology. The reaction rate near the bubbles is approximately 10 times higher than that in the dense phase zone, highlighting the critical role of dense and small bubbles in facilitating mass transfer during carbonation. Additionally, grain size significantly influences the carbonation process, with smaller grain sizes promoting the reaction. This study has established a fundamental mechanism of heat release during the carbonation reaction, providing a solid foundation for future investigations into the carbonation-calcination looping process.