Structural integrity of calcium hydroxide granule bulks for thermochemical energy storage

•Experimental investigation of four granular calcium-based storage material bulks.•Analysis of mechanical stability and conversion of TGA-cycled samples.•Disintegration of shaped and encapsulated samples at high conversion.•Good mechanical stability at incomplete conversion.•Encapsulation inhibits fast hydration reaction. Thermochemical energy storage using the gas-solid reaction of calcium oxide with water vapour is a promising approach especially for high-capacity applications such as CSP. Current research is mainly concerned with lowering reactor cost and therefore concentrating on dynamic systems with granules instead of powder as solid storage material. As stresses affecting the structural integrity of the granules increase with increasing bulk size, small samples as used for thermogravimetric analysis (TGA) might underestimate their impact on granule stability. This work complements existing literature on granule stability by experiments at bulk scale: two shaped granule samples and two ceramic encapsulated granule samples as well as a reference powder were cycled in a lab-scale reactor as well as within a TGA apparatus. Even though preservation of the granule structure was observed after cycling within the TGA apparatus, crushing strength measurements already indicated a decrease of mechanical stability of 81–88% for the shaped granules and 6–11% for the encapsulated granules. As a consequence, structural integrity of the granule bulk was only preserved in case of partial hydration. Given high reactivity, the granule structure – shaped and encapsulated – was destroyed after the first hydration causing a bulk volume expansion of up to a factor of 2.5. Reaction behaviour of the reference powder was consistent despite formation of cm-sized agglomerates. This work confirms that reaction conditions within a bulk are more challenging regarding structural integrity of storage material granules due to greater stresses acting on the single granule. These stresses can be significantly reduced by partial hydration.