The Calcium Looping process for energy storage: Insights from in situ XRD analysis

This work reports a novel in situ XRD analysis on the multicycle calcination/carbonation of natural limestone and dolomite at relevant conditions for thermochemical energy storage (TCES) in concentrated solar power (CSP) plants. The experiments allow analysing noninvasively the time evolution of the different phases involved in the Calcium Looping (CaL) process. Our work has revealed new key features to understand the progressive loss of multicyclic carbonation reactivity of the CaO derived from calcination. The CaCO3 structure formed in the first step of dolomite decomposition has smaller unit cell volume than the CaCO3 naturally present in limestone. The CaO that stems from decomposition of the CaCO3 derived from dolomite first decomposition shows a greater carbonation reactivity compared to the CaO derived from limestone. The smaller size of the CaO nascent crystals and their relatively higher reactivity for dolomite compared to limestone is related to the presence of inert MgO crystals, which prevent CaO sintering and crystallite growth. However, the size of the MgO crystals derived from dolomite decomposition increases monotonically with time, which progressively hampers their hindrance effect. Our work also shows a positive correlation between the growth of the CaO crystallite size and the decline of preferred orientation in the CaO (100) plane as the number of cycles is increased and CaO loses reactivity. The observed evolution with the cycles of CaO crystallite size and reactivity can be attributed to the incompletion of carbonation. The unreacted CaO that remains in the calcination step suffers severe sintering which hinders its reactivity. Thus, the fraction of fresh, reactive CaO derived from CaCO3 decomposition that nucleates on the old and less reactive CaO declines progressively with the cycles.