Sintering mechanism of calcium oxide/calcium carbonate during thermochemical heat storage process

Abstract Thermochemical heat storage based on CaO/CaCO3 cycles suffers from an obvious decline in heat storage capacity attributed to the sintering of particles. Reactive molecular dynamic simulations are carried out to investigate the sintering mechanism of CaO and CaCO3 particles. Lower mass center distance of the CaO particles, larger degree of lattice disordering and higher diffusivities illustrate that the carbonation stage plays a dominating role in the sintering process. The calculated diffusion activation energy decreases to 49.9% and pre-exponential factor increases to 2.98 times of those in the absence of CO2, resulting in the accelerated agglomeration of particles. The activation energy is the dominating factor to enhance the diffusion at lower CO2 concentrations, whereas the pre-exponential factor dominates at higher CO2 concentrations. The lower CO2 concentrations raise the importance of CO2 molecules in bond breakage and formation. On the other hand, CaO particles have more opportunities to react with CO2 molecules at higher CO2 concentrations. High surface energy is the driving force towards sintering, and therefore, the decline of the surface potential energy and specific surface area through sintering are thermodynamically favored. The CO2 absorptivity of sintered CaO particles declines due to the reduced specific surface area. Finally, thermochemical heat storage experiments and microstructure characterizations show the increased degree of lattice disordering, which verifies the molecular dynamics simulation results before. The calculated quantitative descriptors and sintering mechanism will provide guidance for material modification, especially by increasing the atomic diffusion resistance and improving the initial skeleton structure of CaO materials.