Two stage carbonation of CaO and critical thickness

DOI: 10.1038/s41467-018-04794-5

“As in the case of most non-catalytic gas-solid reactions, the carbonation of CaO can occur in two consecutive regimes, i.e., an initial, rapid, kinetically controlled reaction stage that is followed by a relatively sluggish conversion stage. The latter reaction stage is controlled by diffusive limitations, first owing to migration of CO₂ through the network of pores and later the growth of a CaCO₃ product layer that has significantly poorer diffusion characteristics for CO₂ when compared to CaO (D_CaO/D_CaCO3 ≈ 100)^23,26,27. Considering the relatively short residence times in practical fluidized bed reactors, it is therefore essential that CO₂ capture is performed largely in the kinetically controlled regime. This can be achieved by ensuring a nano-structured morphology of CaO, minimizing the diffusion lengths of CO₂ through the freshly formed CaCO₃ layer. Previous, albeit indirect, measurements have indicated a critical product layer thickness (i.e., the thickness of the product layer when the reaction becomes diffusion limited) of ~50 nm^28,29. Furthermore, the CO₂ sorbent should possess a certain level of porosity in order to be able to compensate for the large volumetric changes that occur during cyclic operation, as the molar volume of CaCO₃ is more than twice as high as that of CaO.”