Influence of Fluidised Bed Inventory on the Performance of Limestone Sorbent in Calcium Looping for Thermochemical Energy Storage

This research work deals with the application of the calcium looping concept for thermochemical energy storage. Experiments were carried out in a lab-scale fluidised bed reactor, which was electrically heated. An Italian limestone (98.5% CaCO3, 420–590 μm) was present in the bed alone, or in combination with silica sand/silicon carbide (this last material was chosen as per its high absorption capacity in the solar spectrum). Calcium looping tests (20 calcination/carbonation cycles) were carried out under operating conditions resembling the “closed-loop” scheme (calcination at 950 °C, carbonation at 850 °C, fluidising atmosphere composed of pure CO2 in both cases). Carbonation degree, particle size distribution, and particle bulk density were measured as cycles progressed, together with the application of a model equation to relate carbonation degree to the number of cycles. Mutual relationships between the nature of the bed material and possible interactions, the degree of CaO carbonation, the generation of fragments, and changes in particle density and porosity are critically discussed. An investigation of the segregation behaviour of the bed material has been carried out through tests in a devoted fluidisation column, equipped with a needle-type capacitive probe (to measure solid concentration).