Multi-cyclic evaluation of composite CaO-based structured bodies for thermochemical heat storage via the CaO/Ca(OH)2 reaction scheme

Experimental efforts have shown that thermo-chemical heat storage through cyclic hydration/dehydration of the CaO/Ca(OH)2 couple requires efficient CaO-based particles, in terms of both hydration capacity and structural robustness. Acknowledging the challenge caused by high fragmentation of pure CaO particles during multi-cyclic operation in bed reactors, the development of CaO-based materials with enhanced mechanical properties is essential. Promising results have been obtained for nearly spherical structured formulations using kaolinite as binder. The combination of natural limestone with 25 wt% kaolinite rendered mechanically strong materials with a hydration capacity of up to 50% compared to the maximum hydration capacity of pure CaO. These formulations remained intact and showed stable reactivity in the course of 10 hydration/dehydration cycles. In order to obtain valid conclusions for the suitability of these materials for the here suggested reaction scheme, examination upon multiple hydration/dehydration cycles was necessary. The current work is related to the assessment of CaO-kaolinite composite materials under three different evaluation protocols, as well as to the examination of their mechanical properties through crushing strength measurements and attrition tests. For most of the materials examined, more satisfactory results were obtained for hydration and dehydration reactions in vapor-rich atmosphere at 450 and 550 °C, respectively. Multi-cyclic evaluation (50-200 cycles) confirmed the initial findings of the 10-cycle tests. The final selected composition constituted a promising material for the suggested reaction scheme and was qualified as in-principle suitable for long-term operation in pilot scale units.