Comparative characterization of sorbents from Ca-looping investigations.

In this study, six limestones and one dolomite, coming from Germany, Greece, Italy, and Poland were tested for their CO2 uptake capacity during carbonation-calcination tests in a TGA apparatus and a lab-scale atmospheric bubbling FB reactor. The Ca-looping experiments were carried out both in the presence and absence of sulfur in gas phase, to study its likely inhibitory role to the CO2 penetration into the CaO particles. The mineralogy, microstructure, specific surface area (BET SSA, m¬2/g), and pore size distribution of the fresh, sulfated/carbonated, and non-sulfated/carbonated sorbents were comparatively evaluated by means of X-Ray Diffraction (XRD) Spectroscopy, Energy Dispersive-Scanning Electron Microscopy (EDS-SEM), and N2-porosimetry, respectively. All samples were examined after five cycles of carbonation-calcination. In most sulfated samples, a shell of anhydrite (CaSO4) was identified peripherally to the CaO particles, preventing part of their core from further carbonating. The meso-porosity (%) of the sulfated samples was increased, compared to the non-sulfated ones, suggesting that the former were sintered to a lesser extent, which is also supported by BET area measurements. Concerning carbonation, it was found that the formation of a porous anhydrite shell peripherally to CaO particle can most likely mitigate the undesirable consequences of sulfation. Thus, it is suggested that the partial pressure of CO2 may be deliberately increased in the process, so as to lead to direct sulfation of calcines, and to subsequently get as much porous anhydrite as it can be formed. Overall, this paper contains an integrated comparative characterization of the tested sorbents, accompanied with suggestions for their optimum utilization in Ca-looping.