Development of a moving bed reactor for thermochemical heat storage with Ca(OH)2

Abstract

The use of the reaction system Ca(OH)2/CaO offers several advantages as a heat storage system. For instance, as a thermochemical reaction it has a high energy density and offers the possibility to store the chemical potential energy for long periods of time without energy losses. This is of particular interest when seasonal storage applications are sought. Furthermore, its low cost, proven cyclability and generally worldwide availability as natural resource makes it economically and sustainably attractive. Nevertheless, the inherent properties of the base powder material e.g. low thermal conductivity and tendency to agglomerate present a major challenge when designing reactors. A cost-efficient solution is the detachment of the power and capacity i.e. the use of moving bed reactors. Nevertheless, due to the unfavourable characteristics of the material, it has to be subject of modifications to ensure efficient heat and mass transport. In this thesis, three reactors were developed and set into operation for the thermal cycling of modified Ca(OH)2 and demonstration of the moving bed concept. The first design corresponds to a reaction chamber designed for the rapid cycling and real-time tracking of the reacting material under technical scale. In addition, further material analysis (e.g. TGA, XRD and dynamometry) contributed to an extensive assessment of the granules. Two different samples were cycled 20 times in this setup: granules coated with Al2O3 nanostructured particles and Ca(OH)2/CaCO3 composites. The operation of the reactor was demonstrated as well as the full conversion of the granules. The positive effect on the particle stabilisation given by the Al2O3 coating and the CaCO3 share in the composites was confirmed and at the same time no evidence of agglomeration was found. The second design is an indirectly heated reactor with a tube bundle heat exchanger. After 6 thermochemical cycles conducted with two different samples, CaO granules encapsulated in a ceramic shell and Ca(OH)2 granules coated with Al2O3 nanostructured particles, it was proven that both modifications contribute to the particle stabilisation. Although the encapsulated granules proved the moving bed concept of the reactor, the energy density was significantly lower and their conversion incomplete. In contrast, the coated granules displayed a complete conversion with energy density higher than the powder storage material. However, the natural change in dimensions of the reactive material, as a result of the thermal cycling, could not be prevented and therefore the movement of the bed was hindered. After combining the experimental results of the indirect moving bed reactor and the determined characteristics of the storage granules, a novel directly heated reactor concept was developed. The lab scale reactor was designed taking into consideration two main concerns: to supply enough thermal energy to drive the conversion of the granules while avoiding the fluidisation of the bed. The latter condition seeks to minimise the mechanical impact on the particle stability of the granules. Due to the higher energy density, the Ca(OH)2 granules coated with Al2O3 were selected for a 10-fold thermochemical cycle in this reactor. Besides the full conversion of the storage granules, the movement of the material in this novel reactor configuration was demonstrated for the first time. Furthermore, it was discovered that the interaction between Ca(OH)2 and Al2O3 produces a layer that confers the enhanced stability of the granules. Therefore, the results of this work can be used as the starting point for the upscale of the reactor design towards a pilot facility that works with Ca(OH)2 modified following the particle stabilisation approach.