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 effect on the penetration of CaO particles by CO2. The mineralogy and microstructure of fresh, sulfated/carbonated, and non-sulfated/carbonated sorbents have been comparatively evaluated by means of X-ray diffraction (XRD) and Energy Dispersive X-ray Spectroscopy-Scanning Electron Microscopy (EDS-SEM), respectively. Their specific surface area and pore size distribution have been determined by means of N2-porosimetry. 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 was increased, compared to the non-sulfated ones, suggesting less sintering in the former, a fact also supported by the BET area measurements. On the other hand, micro-porosity showed no clear tendency with sulfation. The loss in microporosity, observed in particular cases of sulfated samples, was attributed to a drop in the associated conversion during carbonation. Overall, this work contains an integrated, comparative characterization study of the tested sorbents, accompanied by suggestions on their utilization in Ca-looping processes.