Investigating the mechanism of CaCO3 sintering in calcium looping conditions

Abstract

A crucial factor determining the performance of calcium looping technology is sorbent stability and reactivity, both are significantly influenced by sintering. This study investigates CaCO₃ sintering under calcium looping-relevant conditions—between 500 and 900 °C and up to 60 h, in high CO₂ partial pressures—using commercial CaCO₃ powder and natural limestone as simulated sorbents. Rietveld refinement of synchrotron powder X-ray diffraction (SPXRD) data revealed a sharp decrease in the crystallite size initially, followed by a gradual increase with sintering temperature and duration. Analyses of the surface area models, including the German-Munir (GM) and generalised power law equation (GPLE), identified surface diffusion as the dominant material transport mechanism, with a low apparent activation energy (E_a) of 18 – 33 kJ/mol. The GPLE model demonstrated better surface area prediction, while the GM model provided insights into a possible acceleration of sintering above 800 °C. Modelling of the shrinkage of CaCO₃ powder pellets gives an activation energy (E_a) of 65 ± 13 kJ/mol for densification, potentially due to a synergistic effect between grain boundary diffusion and surface diffusion, as well as experimental factors such as high heating rates and particle size distribution. Surface area modelling was extended to limestone, revealing a similar, but more pronounced transition in sintering behaviour. This transition manifests as a bigger inflection in the GM correlation parameter with temperature at 770 °C, than that of the GM model for CaCO₃, which was partially linked to dolomite decomposition. The low activation energy obtained for CaCO₃ sintering suggests that facile CaCO₃ migration could be a major contributing factor driving the accelerated sintering of calcium looping sorbents in CO₂ atmospheres.