Hydration Kinetics of Calcium Oxide in a Solid–Gas System: A Critical Process for Thermochemical Energy Storage

Abstract The present study investigated the intrinsic kinetic behavior of the hydration reaction of CaO to form Ca(OH)2, which is the counterpart reaction to the thermal dehydration of Ca(OH)2 in the reversible reaction processes utilized in a thermochemical energy storage system. The objective of this study was to provide a comprehensive description of the kinetic behavior across a range of temperatures (T) and partial pressures of water vapor (p(H2O)), employing an extended kinetic equation founded on the unified kinetic theory that is applicable to both the thermal decomposition and solid–gas reactions within reversible reaction systems. Two distinct hydration reaction behavior modes were distinguished within the T and p(H2O) region investigated in this study. The hydration reaction in a lower T and p(H2O) region was characterized by a two-step mass gain process. Conversely, a single-step mass gain process was observed in a higher T and p(H2O) region, indicating a solid–gas reaction mode. A universal kinetic description of the solid–gas reaction mode across varying p(H2O) values was achieved under both linear cooling and isothermal conditions via formal kinetic analysis based on the extended kinetic equation, yielding the intrinsic kinetic parameters. Under isothermal conditions, an induction period (IP) was observed. The subsequent mass gain process was characterized by a sigmoidal mass gain curve attributed to the physico-geometrical consecutive process of the surface reaction (SR) and subsequent phase boundary-controlled reaction (PBR). The kinetic behavior under isothermal conditions was universally described across a range of p(H2O) values based on the IP–SR–PBR model, yielding the intrinsic Arrhenius parameters for the individual physico-geometrical reaction steps.