Gas-solid carbonation kinetics of Air Pollution Control residues for CO2 storage

Gas–solid carbonation of Air Pollution Control (APC) residues is a CO2 Capture and Storage (CCS) technology, where highly reactive Mg- or Ca-bearing materials adsorb CO2 and form stable Mg- or Ca-carbonates. The carbonation kinetics of this reaction have been studied at different temperatures, CO2 concentrations, and at atmospheric pressure in order to select the best operative conditions on the basis of CO2 stored and reaction time. The samples were initially heated up to the operative conditions under argon atmosphere and then carbonated under a CO2–argon atmosphere. All carbonation kinetics were characterized by a rapid chemically controlled reaction followed by a slower product layer diffusion-controlled process. Maximum conversions between 60% and 80% were achieved, depending on the operative temperature and CO2 concentration. Temperature did not notably affect the maximum conversion obtained in the experiments performed at temperatures equal or above 400 °C; the influence on the kinetics was masked by the change in initial composition due to dehydroxylation reaction and surface area while heating up to the operative temperature. A slight influence of CO2 concentration on the kinetics was observed, whereas no influence on the maximum conversion was noticed. The obtained results suggest that the flue gases with 10 vol.% CO2 concentration can be directly used to form stable carbonates, thus lumping capture and storage in a single step. The APC residues produced from the existing incineration plants would cover only 0.02–0.05% of the total CO2 European storage capacity required to comply with the Kyoto protocol objectives. Nevertheless, the proposed carbonation route could be applied to other residues, such as Cement Kiln Dust, Paper mill residues and Stainless Steel Desulphurization slags, characterized by a high content of free calcium oxides and hydroxides, thus increasing the impact of this process option.