Real time visualisation and analysis of nano coated Ca(OH)2 granules for thermochemical storage
The use of the reversible gas-solid reaction CaO(s) + H2O(g) ⇋ Ca(OH)2(s)+ ∆H as a thermochemical energy storage system is promising due to its cycle stability, low cost, availability, non-toxicity and high storage density. The storage of thermal energy is particularly advantageous for the recovery of waste heat to improve the energy efficiency of industrial processes. At DLR, fixed bed storage reactors have been successfully demonstrated in pilot scale. However, for large storage capacities, the fixed bed concept becomes uneconomical since large heat exchange surfaces are required which in turn increases the investment cost. To overcome this challenge, one current research approach is to decouple the costly reactor (power) from the storage material (capacity) by moving the material through the reactor’s heat exchanger. The main drawback of moving bed reactors results from the poor flowability and low thermal conductivity of the bulk material. In order to improve the flowability, different modifications of the storage material have been developed. Among them, calcium hydroxide granules were coated with Al2O3 nanostructured particles and tested in a lab-scale moving bed reactor. However the reactor operation also revealed that the volume change of the granules which occurs during the reaction leads to clogging of the tubes which impedes the free flow of the material.
In order to understand the volume change and the impact on the flowability as well as the structural integrity of the granules a novel reactor device was developed. The main aim of the test set up was to enable the visualisation of the volume change in real time during the course of the reaction. The growth/shrink as well as the deterioration evolution of the granulated shape of the samples was analysed and compared to crushing strength (CS) values.
Overall, this contribution will present for the first time the visualisation of the changing state of the modified and unmodified Ca(OH)2 granules in real time during the hydration and dehydration. In general, a positive effect of the Al2O3 nanostructured particles on the mechanical stability of Ca(OH)2 granules was found. The derived results of this study serve as input for the manufacture of new modifications based on nanostructured particle coating and also to design a customised moving bed reactor that compensates the volume expansion of the granules and thus enables their flow.