CO2 diffusion through the CaCO3 layer during CaO carbonation

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

“In the diffusion-controlled regime, a number of diffusion processes, including grain boundary and bulk diffusion govern the rate of the carbonation reaction.^28, 45–47 Concerning bulk diffusion, which includes vacancy and interstitial diffusion^43, 48 and becomes dominating for longer reaction times, the most likely diffusing species are CO₂, Ca²⁺, CO₃²⁻ and O²⁻. Considering that charge balance must be maintained, three diffusion mechanisms can be envisioned, i. e. an inward diffusion of CO₂, a counter diffusion of O²⁻ and CO₃²⁻ or an outward diffusion of both Ca²⁺ and O²⁻ (Figure 5a).^46, 48, 49 To elucidate the prevailing diffusion mechanism, Sun et al.⁴⁹ performed an inert platinum marker experiment by placing a platinum marker on top of a sintered CaO pellet. The pellet was carbonated for 4 months at 650 °C in pure CO₂. SEM-EDX analysis of the reacted pellet shows that the platinum marker is on top of the partially carbonated CaO layer (Figure 5b). From this observation the authors concluded that there is an inward diffusion of CO₃²⁻ and an outward diffusion of O²⁻. However, the carbonation time of 4 months is unrealistic (a typical carbonation time in a TGA is 20 min, and even shorter in a fluidized bed) for the CaO/CaCO₃-system and the implications of this experiment for practical CaL applications are unclear. In addition, the diffusion of CO₂ by a sequential decomposition of neighboring carbonate ions in the product layer was not considered, which could be very effective as the carbonation is performed at temperatures above the T_T of CaCO₃ (T_T=553 °C).”

“Figure 5 (a) Possible outcomes of an inert marker experiment. (b) SEM image of a cross section of a carbonated CaO pellet after an inert marker experiment. Adapted with permission from Ref. [49]; copyright Elsevier, 2012.”

“Generally, it is very challenging to distinguish experimentally between the three diffusion mechanisms outlined in Figure 5a. However, ab initio, atomic-scale simulations of diffusion processes in calcite can provide valuable insights. Besson et al.⁴⁸ simulated the diffusion of oxygen and the relevant carbon containing species (including CO⁻, CO₂ and CO₃²⁻) in calcite at 527 °C by means of ab-initio calculations. Based on the calculated migration energies for different diffusion pathways of oxygen they found that the transport of oxygen ions in calcite occurs easily (migration energy ∼0.5 eV) and is mediated by either an interstitial or an oxygen vacancy mechanism, depending on the thermodynamic conditions. In contrast, the diffusion of carbon containing species requires complex point defects (CPDs; defects in the crystal structure that involve two or more sites). At the investigated temperature (527 °C), the diffusion of CO₂ can be ruled out due to the very high formation energy of the CO₂ CPD (>2.2 eV). Similarly, the diffusion of CO₃²⁻ via a CPD is associated with a migration energy barrier of 5 eV, which is too high to occur at the given temperature. The authors could not identify an energetically favorable diffusion pathway for carbon containing species in calcite. Note that the outcome of the calculations at the actual carbonation temperature in the CaL process (>600 °C) could be profoundly different. Thus far, no evidence for any of the three diffusion mechanisms sketched in Figure 5a has been reported. Therefore, further theoretical calculations in combination with diffusion experiments on (partially) carbonated CaO model systems under realistic conditions are needed.”