Shortlisting of Composite CaO-Based Structured Bodies Suitable for Thermochemical Heat Storage with the CaO/Ca(OH)₂ Reaction Scheme

The CaO/Ca­(OH)₂ couple is a promising candidate for thermochemical heat storage applications based on the cyclic hydration/dehydration reaction scheme at temperatures between 400 and 550 °C. The fragmentation of CaO particles during multicyclic operation is an acknowledged phenomenon leading to significant challenges regarding particle reactor bed operation. With the aim to eliminate or significantly mitigate this phenomenon, the current work describes the development of composite CaO-based compositions with enhanced structural stability over multiple hydration/dehydration cycles. The preparation of composite materials was based on the utilization of kaolinite as binder, added at a weight percentage of 25% in natural limestone powder in order to ensure improved mechanical properties of active particles. Nearly spherical structured formulations were manufactured via a simple preparation method based on two solid mixing techniques. A parametric study examined the effect of different parameters (such as CaCO₃ particle size, calcination duration, and heating rate applied during the curing process) on materials hydration capacity and macrostructural integrity. The overall evaluation protocol involved studies on structural and morphological properties of fresh materials as well as measurement of particles mechanical properties before and after the cycles. The evaluation of hydration capacity was based on preliminary testing after 5 cycles and the most promising compositions regarding a combination of sufficient hydration capacity and mechanical stability were subjected to 20 cycles. Promising results were obtained for various compositions, which retained their structural integrity due to the formation of a ternary mixed Ca/Al/Si crystal phase. CaO consumption led to somewhat lower hydration capacities of the composite materials compared to pure CaO. A satisfactory combination of good hydration/dehydration performance with high structural stability upon cycling was achieved leading to promising materials that were qualified for future multicyclic experiments.