Promoting effect of MgO on CaO-based CO2 capture

https://www.nature.com/articles/s41467-018-04794-5

” The cyclic CO₂ capture performance of the sorbents was evaluated in a TGA and compared to the benchmark sorbent, i.e., CaO derived from limestone. In order to comply with the practically relevant operation conditions, calcination was performed at 900 °C in a CO₂ atmosphere, and a rapid temperature ramp of 50 °C min⁻¹ was applied between the carbonation and calcination temperatures. For an initial assessment, ten cycles were performed. Already from these initial tests, it is clear that all of the hydrothermally synthesized CO₂ sorbents show a better performance than the benchmark limestone that underwent significant sintering during cyclic testing (Fig. 4b). In particular, the MgO-containing sorbents exhibit a stable CO₂ uptake, whereas CaO synthesized in the absence of a stabilizer (i.e., Ca100Mg0) reveals a characteristic decay in the CO₂ uptake with cycle number (Fig. 4c). This observation is a clear indication of the structural stabilizing effect of MgO. The fact that hydrothermally synthesized CaO without any stabilizer (Ca100Mg0), i.e., a material that initially features structures with favorable CO₂ capture characteristics, significantly outperforms the limestone-derived benchmark supports the underlying hypothesis that the particular structure of CaO, i.e., being composed of primary particles of <100 nm in diameter and containing sufficient void space to accommodate the volumetric expansion of the material, will yield high CO₂ uptakes.”

“The influence of MgO on the performance of the sorbents was examined further by comparing the temporally resolved carbonation profiles of the different CO₂ sorbents (Fig. 4d). Indeed, all of the MgO-stabilized sorbents maintained their characteristic of capturing a large fraction of CO₂ in the kinetically controlled regime, in which surface reactions largely prevail (the overlapping CO₂ uptake curves in the early stages of the carbonation reaction indicate that the reaction in the TGA is dominated by mass transport from the bulk reactive gas to the outer surface of the sorbents). Importantly, the quantity of CO₂ captured by the MgO-stabilized sorbents in the initial reaction stage was maintained throughout the ten carbonation/calcination cycles (~0.35 g_CO2/g_sorbent), whereas unstabilized CaO (Ca100Mg0) experienced an earlier transition to the diffusion-controlled CO₂ capture regime, decreasing the overall CO₂ uptake with increasing cycle number (from ~0.34 to 0.22 g_CO2/g_sorbent). In Fig. 4d it can also be seen that the transition to the diffusion-controlled reaction stage occurred more gradually for MgO-stabilized sorbents, indicating that a larger pore volume was accessible in these materials. In contrast, the transition from the kinetically controlled to the diffusion-controlled carbonation stage was relatively sharp for limestone and Ca100Mg0, most likely owing to the blockage of narrow pores such that product layer diffusion became rate-limiting at much lower conversions when compared to MgO-stabilized sorbents⁵³.”

“Structure–performance relationship. a Powder XRD patterns of the sorbents synthesized after the removal of the carbonaceous template. b SEM images of the limestone-derived sorbent in the freshly calcined state and following ten operation cycles (scale bars: 400 nm). c CO₂ uptake of the hydrothermally synthesized CO₂ sorbents compared to limestone-derived CaO under harsh operating conditions (carbonation at 650 °C, calcination at 900 °C) (dotted line represents the maximum theoretical CO₂ uptake of Ca100Mg0). d Temporally resolved CO₂ uptake profiles of the hydrothermally synthesized sorbents in the 1st (solid line) and 10th (dashed line) carbonation cycle”