Effects of eutectic alkali chloride salts on the carbonation reaction of CaO-based composites for potential application to a thermochemical energy storage system

Abstract

The reversible reaction of a CaO-CaCO3-based thermochemical energy storage (TCES) system is a suitable option for enhancing the efficiency of CSP system's energy storage due to its high working temperature and promising energy storage capacity. However, the CaO-based composite often suffers a reduction of reversibility during multicycle reactions, which steeply reduces the energy storage performance of the system. The carbonation is limited to the near-surface of CaO particles because the produced CaCO3 layer passivates the inner CaO from further reaction. In this study, we focused on adding eutectic alkali chloride salt (ECS) to CaO-based composites to enhance the carbonation reaction. The effects of the ECS additives were investigated in terms of carbonation temperature and melting point of the salts, and the resulting carbonation behaviors are discussed. For that, four kinds of ECS, which have different melting point, were added to CaO-based composites. It was observed that the temperature needed to induce the majority of the carbonation conversion was lowered when the ECS melting point was low. The predominant progress of carbonation conversion was followed by the production of molten ECS. The result indicates that molten ECS dissolves the CaO from a Ca-O grid, and provides an additional pathway for diffusion of free CaO ion pairs to CO2 adsorbed on the particle surface. Multicycle performances of ECS-added CaO-based composites were promoted compared to the CaO and CaO/MgO.