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

Abstract

The heat transfer and storage are most crucial for the concentrated solar power (CSP) plants, and yet limited heavily by the thermal deactivation of calcium-looping materials. Here, we introduce air or O2 to on-site regenerate Al/Mn/Ce-co-doped CaO-looping materials operated at 800 ℃, sharply reducing the loss of energy storage density from 29.5% to 9.6% after 1000 cycles of thermochemical energy release and storage and holding 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 CaO matrix, the coverage of coke deposits deactivates the carbonation reaction beyond 500 cycles. The 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 a practical guidance for on-site decoking of industrial Ca-looping process whether in the calcination step or the carbonation step, which is most imperative for reliably converting solar photon fluxes to dispatchable electricity in the next-generation CSP plants.