Influence of porosity of CaO on CO2 capture

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

“The transition between the two regimes has been linked to a critical product layer thickness.²⁹ Most works report a calculated critical product layer thickness in the range of 20–50 nm, although these values are obtained through estimations using simple (bulk) morphological models, in which a product layer is assumed to be a homogenous, continuous and flat layer.^29–31 Furthermore, the total pore volume and surface area of the sorbent rapidly decrease in the kinetically-controlled regime due to the large difference in molar volume between CaO (16.7 cm³/g) and the product CaCO₃ (36.9 cm³/g). Hence, it has been argued that the initial pore volume critically influences the CO₂ capacity of the sorbent.^26, 32, 33 For a given ”family” of a CaO-based sorbent, a larger initial pore volume (in pores with d_pore<100 nm) often results in higher CO₂ uptakes (e. g., for CaO-based sorbents with an inverse opal type morphology,²⁶ see Figure 2b). Nevertheless, when plotting the initial pore volume (d_pore<100 nm) versus the CO₂ uptake in the first cycle for a range of different CaO-based sorbents no obvious correlation between the CO₂ uptake and the initial pore volume of the sorbent can be found (Figure 2b). The same finding can be made when plotting the same data over the BET surface area, determined before the first carbonation step, see Figure 2c. Therefore, recent studies have introduced advanced characterization techniques, including in-situ X-ray techniques and atomic force microscopy (AFM), in an attempt to identify the structural and morphological features that exhibit a direct relationship to the CO₂ uptake and, in general, to obtain a more fundamental understanding of the processes occurring on the atomic scale during the carbonation and calcination reaction.^34–37”

“Figure 2 (a) Schematic of CaCO₃ product formation during the carbonation of CaO in the kinetically- and diffusion-controlled carbonation regimes and plot of CO₂ uptake as a function of pore volume (b) and surface area (c) for a range of different pure (i. e., only CaO-containing) CaO-based CO₂ sorbent materials.”