Sintering and formation of a nonporous carbonate shell at the surface of CaO-based sorbent particles during CO₂-capture cycles

The existence and formation of a carbonate shell at the surface of the particles of CaO-based sorbents is investigated in this paper. Two sorbents were tested: natural Kelly Rock (KR) limestone and synthetic pellets (KR-CA-14) prepared from the same limestone and calcium aluminate cement (CA-14). Various different series of calcination/carbonation cycles were carried out in a thermogravimetric analyzer (TGA) apparatus, and the sorbent samples produced after those cycles were analyzed with a scanning electron microscope (SEM). It is shown that sintering during cycles is more pronounced at the surface of sorbent particles, which results in the formation of nonporous areas or even a totally nonporous shell that surrounds a partially reacted CaO core. However, the dependency of shell formation upon cycle number is difficult to elucidate by SEM because increasing cycle numbers achieve lower conversion levels, which reduce the chance of shell formation. Prolonged carbonation after a series of cycles showed that there is a limit in maximum conversion levels, which cannot be solely explained by product layer formation at the interior sorbent surface area. The SEM images of samples after prolonged carbonation periods clearly show the presence of more sintered areas at the outer particle surface and/or carbonate shell/partially reacted particle pattern. This is explained by the phenomenon of more pronounced sintering at the sorbent particle surface than seen in the particle interior because of surface tension and more pronounced loss of pore volume near the exterior of the particle. The formation of a carbonate shell at the particle surface is a different phenomenon from that of the formation of a product layer at the pore surface area and also limits diffusion during carbonation.