On-site decoking of the Ca-looping process enables 1000 cycles of stable thermochemical energy storage

Abstract

Heat transfer and storage are crucial for concentrated solar power (CSP) plants, yet their performance is severely limited by the thermal deactivation of calcium-looping materials. Here, we introduce air or O₂ for on-site regeneration of Al/Mn/Ce-co-doped CaO-looping materials operating at 800 °C, sharply reducing the loss of energy storage density from 29.5% to 9.6% after 1000 cycles of thermochemical energy release and storage and maintaining a final energy storage density of ca. 1039 kJ kg⁻¹ at the 1000th cycle. Besides the slow phase separation of two formed heat-resistant compounds, Ca₃Al₂O₆ and Ca₂MnO₄, from the CaO matrix, the coverage of coke deposits deactivates the carbonation reaction beyond 500 cycles. Mechanistic studies reveal that the rich oxygen vacancies formed by Ce doping into CaO NPs and Ca₂MnO₄ NPs are mainly responsible for the formation of coke deposits on the surface of CaO NPs because they are thermodynamically favorable for directly splitting CO₂ to C and O₂. These findings offer practical guidance for on-site decoking of industrial Ca-looping processes in either the calcination step or the carbonation step, which is imperative for reliably converting solar photon fluxes to dispatchable electricity in next-generation CSP plants.