Coating of Ca(OH)2/gamma-Al2O3 pellets with mesoporous Al2O3 and its application in thermochemical heat storage for CSP plants

Abstract

Thermochemical heat storage using the Ca(OH)2 dehydration/hydration reaction for CSP plants requires the development of advanced materials that avoids the breakage shown by conventional CaO pellets. In this work, spherical cores made up of pelletized 60 wt. % Ca(OH)2 and 40 % γ-Al2O3 powders were coated with different mesoporous alumina coverings by means of a dip-coating route, in order to enhance its structural integrity for dehydration/hydration cycles. Three strategies of preparation were devised using as coatings alumina gels containing cetyltrimethylammonium bromide as surfactant in neutral (A2 CS) and acid pH (A4 CS) and without surfactant in neutral medium (A3 CS). In both A2 CS and A4 CS materials, denser alumina coatings were attained originating higher crushing strengths, pointing out the positive role played by the surfactant in the gel makeup. SEM images indicated that A2 presented small alumina grains (0.3 – 3 µm) highly dispersed over their surface, while over A4 CS, long sheets were appreciated. All the materials exhibited high hydration capacities after 10 cycles (> 80%, > 800 kJ/kg) at 250 ºC, but only A2 CS hold its structural integrity that was preserved even after 20 cycles without loss of its hydration capacity.