IMPROVING THE PERFORMANCE OF CALCIUM LOOPING FOR SOLAR THERMOCHEMICAL ENERGY STORAGE WITH SIC/LIMESTONE MIXTURES

Calcium Looping (CaL) for thermochemical energy storage of Concentrated Solar Power (CSP) is widely investigated to increase the dispatchability of solar energy. High reaction heat, high operation temperatures and cheapness of natural sorbents are amongst the strengths of the CaL-CSP process. The strong decay of reactivity, further aggravated in directly irradiated reactors by possible overheating phenomena, and the poor absorption of solar energy, negatively affect its efficiency. The scope of this work is to investigate the use of physical mixtures of natural limestone and silicon carbide (SiC) to improve the absorption of solar energy and possibly shield limestone particles from overheating phenomena. An experimental campaign was performed in a directly irradiated fluidized bed reactor using 420–590 μm natural limestone particles. An operability map of SiC was created to select the most appropriate size cut of SiC to mix with lime, and tests with different SiC/CaO ratio were performed to optimize the SiC fraction. Finally, CaL tests with the optimal SiC/CaO mixture were performed in process conditions relevant for closed loop CO2 operation, namely 850/950 °C for carbonation/calcination and pure CO2 atmosphere. Results demonstrated that the use of a small fraction of SiC can boost the absorption of solar energy while preventing its chemical/physical interaction with CaO, which according to previous studies can be responsible of increased loss of reactivity. The obtained performance in terms of carbonation degree over cycling suggests that SiC was also able to shield lime particles, eliminating the decrease of reactivity induced by bed surface overheating. In terms of energy storage density, it was found that the CaL process outperforms the molten salts technology up to 10 reaction cycles. Altogether, the results of this work encourage the use of SiC/CaO physical mixtures to improve solar energy absorption for CaL processes in directly irradiated fluidized beds.