Enhancement of a CaO/Ca(OH)2 based material for thermochemical energy storage

The application of hydration/dehydration reactions of CaO/Ca(OH)2 to thermochemical energy storage systems can be facilitated by the development of Ca-based materials with improved mechanical properties when compared to the natural Ca-precursor, while maintaining good chemical activity. For this purpose, we are developing composite materials for fluidized bed or fixed bed applications synthesized using sodium silicate to bind fine CaO/Ca(OH)2 particles. In this work, the mechanism of decay in the mechanical properties of the resulting CaO/Ca-silicates composites over hundreds of hydration/dehydration cycles has been investigated. Based on the observed mechanism, improvements to the method of synthesis of the materials have been introduced, in order to retain the original carbonate grain volumes, which are larger than those of the equimolar quantity of the expanding Ca(OH)2 during hydration. A noticeable improvement in the mechanical stability of the resulting pellets was observed when the material was exposed to temperatures of around 880 °C in pure CO2 before calcination in order to avoid the decomposition of CaCO3 during the formation of the hard Ca-silicates that provide mechanical strength to the composite. Further gains in mechanical stability were achieved by avoiding the complete hydration of the CaO grains in the composites. These results confirm the primary role of Ca(OH)2 anisotropic expansion as the main cause of the reduction in crushing strength of the pellets. It can be concluded that the formation of calcium silicates as binders of CaO rich grains is a promising route for the development Ca-based composite materials. However, more effort is still needed to overcome the decay in performance observed after several hundreds of cycles.