The kinetics of the thermal decomposition of CaCO3 is significantly influenced by atmospheric and self-generated CO2 due to the reversibility of the reaction. More detailed understanding of this well-known phenomenon is desired for establishing an effective Ca-looping in the CaO–CaCO3 system for energy storage and CO2 capture. This article shows the universal kinetics of the thermal decomposition of CaCO3 over different temperatures and partial pressures of CO2 (p(CO2)) with the aid of an accommodation function (AF) composed of p(CO2) and equilibrium pressure. An analytical form of AF with exponents (a, b) was derived based on the kinetic considerations for the consecutive elementary steps of the surface nucleation and interfacial reaction. The overall kinetics of the thermal decomposition of CaCO3 were described universally over different temperatures and p(CO2) values by introducing the AF, in views of the isoconversional and isothermal kinetic relationships using the extended Friedman and experimental master plots, respectively. The universal kinetic description was extended to the kinetic modeling based on the physico-geometrical consecutive process comprising an induction period (IP), a surface reaction (SR), and a phase boundary-controlled reaction (PBR). The proposed kinetic approach enables parameterizing the CO2 effect via the optimized (a, b) and tracking changes in the CO2 effect as the physico-geometrical reaction step advanced from IP to PBR via SR. Furthermore, using the established universal kinetic description across different temperatures and p(CO2) values, a challenge was set to quantify the contributions of atmospheric and self-generated CO2 on the kinetics.

In this study, we investigated the mechanisms of variations in the overall kinetic behavior of the physico-geometrical consecutive process of the surface reaction (SR) and phase boundary-controlled reaction (PBR) in solid−gas systems with varying particle size of the reactants. Thermal decomposition of potassium hydrogen carbonate (KHCO 3 ) was selected as a suitable model reaction owing to the significant changes in its kinetic behavior with particle size and less sensitivity to experimental conditions for recording kinetic data. The reaction was characterized by an induction period (IP) accompanied by the formation of a gelatinated surface layer. The subsequent mass-loss process was indicated by the consecutive SR and PBR, which was accompanied by the nucleation and growth of solid products in the gelatinated layer and inward advancement of the reaction interface, respectively. Formal kinetic analyses of systematically recorded kinetic data revealed variations in the overall kinetic behaviors with the sample particle size, including changes in the variation trend of isoconversional activation energy values as the reaction progressed and the shape of the experimental master plot. The kinetics of each reaction step in the physico-geometrical consecutive process was investigated using an advanced kinetic approach based on an IP−SR−PBR model. The results revealed variations in the overall kinetic behaviors of the thermal decomposition of KHCO 3 with particle size, owing to changes in the reactivity of the reactant surface in IP, overlapping degree of SR and PBR, and total migration length of the reaction interface in PBR.

This study focuses on the relationship between the aragonite–calcite (A–C) transformation and the thermal dehydration of included water in the biomineralized aragonite construction using freshwater pearl. These thermally induced processes occur in the same temperature region. The thermal dehydration of included water was characterized through thermoanalytical investigations as an overlapping of three dehydration steps. Each dehydration step was separated through kinetic deconvolution analysis, and the kinetic parameters were determined. A single-step behavior of the A–C transformation was evidenced using high-temperature X-ray diffractometry and Fourier transform infrared spectrometry for the heat-treated samples. The kinetics of the A–C transformation was analyzed using the conversion curves under isothermal and linear nonisothermal conditions. The A–C transformation occurred in the corresponding temperature region of the thermal dehydration, ranging from the second half of the second dehydration step to the first half of the third dehydration step. Because the thermal dehydration process is constrained by the contracting geometry kinetics, the movement of the thermal dehydration reaction interface can be a trigger for the A–C transformation. In this scheme, the overall kinetics of the A–C transformation in the biomineralized aragonite construction is regulated by a contracting geometry.

The effect of water vapor on the thermal decomposition of five different CaCO 3 samples was investigated to reveal the origin of the acceleration effect caused by water vapor. Biomineralized CaCO 3 samples decomposed at a relatively low temperature and were limitedly sensitive to variations in atmospheric water vapor pressure (p(H 2 O)). During the thermal decomposition of the mineral and synthetic CaCO 3 samples, an acceleration effect of water vapor was observed; however, the degree of the effect differed among the samples. A sample with smaller specific surface area and larger particle size (mineral aragonite) decomposed at higher temperatures but exhibited more significant reaction temperature reduction with increasing p(H 2 O). The kinetic analysis of the thermal decomposition of CaCO 3 (mineral aragonite) under different p(H 2 O) values revealed a variation in the surface reaction (SR) kinetics with p(H 2 O), indicating the enhancement of the SR at greater p(H 2 O). The subsequent reaction proceeded through a contracting geometry scheme, during which the CaO crystal growth and pore formation in the surface product layer were enhanced by the effect of water vapor. A universal kinetic analysis for the thermal decomposition of CaCO 3 under different temperatures and p(H 2 O) values was demonstrated by introducing an accommodation function (AF) of p(H 2 O), obtaining a single set of kinetic triplets and the exponent in the AF. The effect of water vapor on the kinetics was parameterized by the exponent in the AF, which can be a potential tool for evaluating the thermal decomposition of CaCO 3 in the Ca-looping system for CO 2 absorption and energy storage.

While heating a seawater spiral shell (Euplica scripta), thermally induced aragonite–calcite (A–C) transformation occurred within the temperature region of multistep thermal dehydration. Here, the kinetic interplay between the A–C transformation and thermal dehydration was studied as a possible cause of the reduction in the A–C transformation temperatures. The kinetics of the A–C transformation was systematically investigated under isothermal conditions by powder X-ray diffractometry and under linear nonisothermal conditions by Fourier transform infrared spectroscopy. The thermal dehydration was characterized as a partially overlapping, three-step process by thermogravimetry–differential thermal analysis coupled with mass spectroscopy for the evolved gases. The A–C transformation occurred in the temperature range of the final part of the second dehydration step and the initial part of the third dehydration step. The kinetics of A–C transformation and thermal dehydration were characterized by contracting geometry-type models, in which the respective transformations were regulated by a constant linear advancement rate and diffusional removal of water vapor, respectively. Based on the kinetic results, the mutual interaction of those thermally induced processes is discussed as a possible cause of the reduction in the A–C transformation temperature.