Stabilizers for CaO-based sorbents

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

” The currently dominating approach to mitigate sintering induced deactivation of CaO is the introduction of high Tammann-temperature metal oxide stabilizers.^9, 12, 55 It is believed that stabilizers act as physical barriers between otherwise adjacent CaO grains, hence reducing the rate of sintering and stabilizing the pore network and surface area of the sorbent.^23, 56, 57 Various metal oxides have been explored as stabilizers including Al₂O₃^{20, 23, 26, 58–60} (and the respective mixed calcium aluminates Ca_xAl_yO_z that form during calcination^23, 25, 61), MgO,^{17, 18, 24, 54, 62, 63} SiO₂,^64–67 TiO₂⁶⁸ or ZrO₂.^57, 69–72 Stabilizers are commonly added in quantities ranging between 5–20 wt %, with the optimal quantity being a trade-off between the degree of morphological stabilization and the quantity of CO₂ capture-inert material added. The addition of stabilizers has often resulted in significant improvements, in particular during the first few cycles (see Figure 6 e), achieving CO₂ uptakes that exceeded the values of the limestone-benchmark by 300–500 % after 30 carbonation-calcination cycles (calcination performed in CO₂ at 900 °C).^23, 24

According to sintering theory, a delay (or even prevention) of the coalescence of adjacent grains requires a deceleration of thermally induced diffusion processes (i. e., surface, grain boundary and volume diffusion).^73, 74 Therefore, there are at least three factors that influence the ability of a stabilizer to prevent sintering: (i) the morphology of the stabilizer phase, e. g. as a coating on top of grains or as (nano)particles between grains, (ii) the thickness of the coating/the size of the particles of the stabilizer phase and (iii) the diffusivity of Ca in the stabilizer phase and on its surface and grain boundaries.”