Enhancing calcium looping for thermochemical energy storage of solar energy in fluidized beds by limestone/SiC 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 among the strengths of the CaL-CSP process. The decay of reactivity along CaL cycles, further aggravated in directly irradiated reactors by possible overheating phenomena, and the poor absorption of solar energy by Ca-based sorbents (e.g., limestone), negatively affect its efficiency. This study investigates the use of physical mixtures of natural limestone and SiC to improve solar energy absorption and possibly shield limestone particles from overheating phenomena. An experimental campaign was performed in a directly irradiated fluidized bed reactor using 420–590 μm limestone particles. A series of tests was carried out to optimize the size range of SiC and the SiC/CaO ratio. Then, CaL tests with the optimal mixture were performed under different process conditions. Results demonstrate that the use of a small fraction of SiC can enhance solar energy absorption while preventing chemical/physical interaction with CaO. The observed performance in terms of carbonation degree over cycling suggests that SiC was also able to shield lime particles, alleviating the decrease in reactivity induced by bed surface overheating. Energy storage density was computed with reference to either discontinuous (long-term storage) or continuous (short-term storage) CaL operation. Estimated values are high (530–1950 MJ/m3 depending on the conditions) and confirm the potential of this technology for both daily and seasonal energy storage. Altogether, the results of this study encourage the use of SiC/limestone physical mixtures to improve solar energy absorption for CaL processes in directly irradiated fluidized beds.