Experimental investigation of CaO/Ca(OH)2 for thermochemical energy storage – commissioning of a 0.5 kWh experimental setup

Abstract

Thermochemical energy storage using the material system CaO/Ca(OH)2 and fluidized bed technology is currently becoming more interesting for applications at 400 °C - 600 °C. This work presents a new, unique setup for examining the material and process characteristics in a size relevant for a scale-up. It is specifically designed to enable time-efficient, high-throughput experiments into the long-term cyclisation of CaO/Ca(OH)2 and its production reaction from CaCO3 at precisely adjustable reaction conditions over the entire cyclisation process. The cylindrical reactor has a fluidized bed volume of 1.8 L (9.5 L total volume) with a height to diameter ratio of up to 4. This enables batch sizes of up to 1 kg Ca(OH)2, equal to a storage capacity of 0.5 kWh. Nitrogen or steam are used as fluidizing medium, enabling fluidization velocities of up to 0.3 m/s. Process conditions of up to 800 °C and 4 bar are possible in the reaction zone. To extend the range of analytics on the setup, a system is installed to allow measurements of the differential pressures between the windbox, reaction zone, freeboard and reactor exit and numerous temperatures in the reactor. A blowback system is installed to the off-gas filtration system. First experimental investigations using CaCO3 with a particle size of 250 µm - 400 µm as initial material show promising results in terms of performing the calcination reaction at 700 °C - 750 °C and successive cyclisation of the produced CaO in a pure steam atmosphere. Up to 40.5 storage cycles were achieved with no detectable loss in fluidization quality. The development of the particle size distribution throughout the storage cycles shows a discontinuous but steady increase of fines within the fluidized bed that is accompanied by entrainment of the fines.