EXPERIMENTAL ANALYSIS OF Ca(OH)2 AS THERMOCHEMICAL ENERGY STORAGE IN A MOVING BED REACTOR

Abstract

The use of the gas-solid reaction of calcium hydroxide (hydrated lime) to calcium oxide (quicklime) and water vapour as a thermochemical storage system is promising due to its reversibility, low cost, abundance, non-toxicity and high energy density. Additionally, its applications can increase the reuse of waste heat in industrial processes, improve the cost efficiency in solar power plants or bridge the gaps between renewable electricity supply and demand by using power to heat concepts. Even though the bulk material is mechanically stable over reactive cycles, the flowability of the unmodified raw material is poor. This is of particular concern for the aspired development of gravity assisted moving bed reactors. In order to enhance flowability of the storage material, different modification approaches have been investigated. Among these modifications, Ca(OH)2 granules encapsulated in a ceramic shell or coated with nano particles were manufactured for testing. The flowability, mechanical stability and reactivity of the manufactured granules of the storage material have been investigated in this study. For this purpose, a lab-scale test bench with an indirectly heated moving bed reactor was designed, build up and set into operation. This reactor was used to test the materials over a number of cycles under temperatures ranging from 450°C to 550°C and pressures ranging 10kPa – 100 kPa for hydration and dehydration respectively. The experiments showed that for the ceramic shell encapsulated material the flowability through the reactor was achieved and the ceramic shell was stable over 3.5 reaction cycles. However the total conversion of this modified storage material is reduced due to the large mass of the non-reactive shell. For the material coated with nano particles, the conversion rate was close to the theoretical maximum of pure Ca(OH)2 and the formed outer layer was mechanically stable over 10 reaction cycles. Nevertheless, agglomeration of the granulated storage material occurred in the reactor especially after the hydration process. The experimental results indicate that further research is necessary to adapt the reactor design in order to prevent agglomeration of the granules and ensure a continuous flow of the material.