In-situ synchrotron X-ray tomography and XRD for CaO-based carbon capture

https://doi.org/10.1002/cssc.202002078

“studying the morphological and structural changes using in-situ methods (i. e., during the reaction of CaO with CO₂ and during the release of CO₂ from CaCO₃) can provide time-resolved insight into the structural evolution of the sorbents.”

“Dunstan et al.³⁶ investigated the morphological changes in the macro-porous range during CaO carbonation by in-situ synchrotron X-ray tomography. Owing to the different densities of CaO and CaCO₃ a phase contrast is visible. The macro-porous network in CaO particles before and after carbonation is visualized in Figure 3b. This experiment clearly evidenced that also the volume in macro-pores reduces after 50 min of carbonation at 650 °C in 100 % CO₂. It has also been argued that apart from the porosity and the surface area, the CaO/CaCO₃ crystallite size can influence the carbonation kinetics.^37, 38 The effect of the crystallite size of CaO/CaCO₃ on their reactivity was investigated by Biasin et al. utilizing in-situ time resolved (0.25 s acquisition time) synchrotron XRD allowing to determine the rates of CaCO₃ formation.³⁸ Here, the carbonation kinetics of CaO-based sorbents of varying initial crystallite sizes were investigated and the degree of CaO conversion was determined by Rietveld refinement. The authors reported an inverse proportionality between the conversion of CaO in the first cycle and the initial crystallite size of CaO. It was hypothesized that smaller CaO crystallites increase the total length of crystallite boundaries when normalized by the surface area; and that the number of “reactive” sites for the CaO−CO₂ reaction depend not only on the surface area, but also on the length of crystallite boundaries. An interpretation of this observation is that grain boundary diffusion of CO₂ controls to some extent the rate of CaO conversion. However, this hypothesis needs further investigation.”

“Figure 3 (b) Visualization of the macro-pores in a CaO particle prior to and after carbonation. Adapted with permission from Ref. [36]; copyright Royal Society of Chemistry, 2016.”