Solar-driven calcium looping in fluidized beds for thermochemical energy storage

Integration between Concentrated Solar Power (CSP) and Calcium Looping (CaL) is gaining consideration in the perspective of large shares of renewable energy sources, to smooth the variability of non-dispatchable energy input. The scope of this study is to investigate the CaL process for ThermoChemical Energy Storage (TCES), by performing a dedicated experimental campaign in fluidized bed under realistic process conditions suitable for CaL-CSP integration. Chemical deactivation of the limestone-based sorbent has been assessed by measuring the extent of Ca carbonation along iterated calcination/carbonation cycles, correlated with physico-chemical characterization of the sorbent at selected stages of the conversion. Properties that have been scrutinized were particle size distribution, bulk density, and particle size, density, and porosity of bed solids. The attainable values of energy storage density were evaluated as well. A remarkable finding of the experimental campaign is the pronounced synergistic deactivation of limestone when it is co-processed with silica sand. Chemical interaction of CaO with the silica sand constituents at the process temperatures has been scrutinized as possible responsible for the loss of reactive CaO toward CO uptake. Post-process of particle density data, together with N-intrusion porosimetric analysis, and quantitative and qualitative XRD analyses, suggests that the sand/lime interaction induces a strong reduction of the total and reactive sorbent porosity and, in turn, of reactivity. Density-based classification to separate converted and unconverted limestone particles after the carbonation step has been evaluated with the goal of increasing process efficiency, by avoiding the circulation of streams with unreacted particles through the plant. For this purpose, the minimum fluidization velocity of calcined and carbonated particles has been measured after each reaction step at the relevant process temperature.