A comparative characterization study of Ca-looping natural sorbents.

In this study, six high-Ca limestones and one dolomite from Germany, Greece, Italy, and Poland were tested for their CO2 uptake capacity during carbonation-calcination experiments in a TGA apparatus, as well as in a lab-scale atmospheric bubbling FB reactor. The calcium 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, 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) has been identified peripherally to the CaO particles, preventing part of their core from further carbonating. The macro-porosity (%) of sulfated samples is increased, compared to the non-sulfated ones, suggesting less sintering in the former, a fact also supported by their BET area findings. On the other hand, micro-porosity shows no clear tendency with sulfation. The loss of microporosity, that was in particular cases observed in the sulfated samples, is attributed to a drop in the associated conversion during carbonation. Overall, this work contains an integrated comparative characterization of the tested limestones, accompanied with suggestions for their optimum utilization in Ca-looping.