Analysis of the kinetic slowing down during carbonation of CaO by CO2

In order to improve the capture capacity of CaO-based sorbents, it appears important to understand the mechanism of calcium oxide carbonation and to get details on kinetic law controlling the reaction, which has not been really studied up to now. To investigate this mechanism, CaO carbonation kinetics was followed by means of thermogravimetric analysis (TG) on divided materials, of textural and morphological characterisations and of an original kinetic approach devoted to look for the rate-determining step controlling the reaction rate. In order to better describe the reaction mechanism, the influence of intensive variables such as carbonation temperature and CO2 partial pressure were investigated. TG curves obtained under isothermal (450-650 A degrees C) and isobaric conditions (2-30 kPa) show a strong slowing down of the conversion leading to incomplete reaction. This slowing down and the fractional conversion at which it appears depend on carbonation temperature and CO2 partial pressure. To explain these results, particular attention has been paid to the evolution of textural properties of the solid during processing. The solid powder consists of porous aggregates in which the porosity changes along the reaction due to the difference in the molar volumes of CaO and CaCO3. Temperature jumps during TG experiments have put in evidence a complex kinetic behaviour since three distinct domains must be distinguished, over all the conversion range, whatever the temperature and CO2 pressure could be. The discussion of the results emphasises the role of the porosity on the kinetic non-Arrhenius behaviour observed in the second domain.