Limestone Calcination Nearby Equilibrium: Kinetics, CaO Crystal Structure, Sintering and Reactivity

In this work, we analyze limestone calcination kinetics at environmental conditions involving a CO2 partial pressure P close to the equilibrium pressure P-eq by means of in situ X-ray diffraction (XRD) and thermogravimetric (TG) analyses. In contrast with previous empirical observations carried out mostly at conditions far from equilibrium (P/P-eq << 1), our results show that the decarbonation rate decreases as the temperature in increased while P/P-eq is kept constant, which is explained from a reaction mechanism including desorption of CO2 and the exothermic structural transformation from metastable CaO* nanocrystals to the stable CaO form. The crystal structure and sintering of nascent CaO during calcination has been investigated from in situ XRD analysis, physisorption analysis, and scanning electron microscopy (SEM), which shows that the ratio of the size of polycrystalline CaO grains to crystallite size increases linearly with the CO2 partial pressure in the calcination atmosphere. For high CO2 partial pressures, the size of CaO grains reaches a maximum value of around 1 mu m, which leads to a residual surface area of about 1 m(2)/g, whereas in the limit P -> 0 grain size and crystallite size (of the order of 10 nm) would coincide. Accordingly, sintering in the presence of CO2 would be triggered by the agglomeration of CaO crystals enhanced by CO2 adsorption, which increases the surface energy. The carbonation reactivity of CaO resulting from calcination scales proportionally to its surface area and is not determined by a growth of the CaO exposed surface along a preferred crystallographic direction wherein carbonation would be unfavorable as suggested in recent works.