Role of looping-calcination conditions on self-reactivation of thermally pretreated CO2 sorbents based on CaO

Abstract

The conversion of thermally pretreated CaO along successive carbonation/calcination cycles has been investigated, as affected by looping-calcination conditions, by means of Thermogravimetric Analysis (TGA). Sorbent samples have been subjected in situ to a thermal preheating program based on Constant Rate Thermal Analysis (CRTA) by virtue of which decarbonation is carried out at a low controlled rate, which is able to promote self-reactivation in the first carbonation/calcination cycles. Our observations support a pore-skeleton model according to which solid-state diffusion in the first carbonation stages, which is enhanced by thermal pretreatment, gives rise to a soft skeleton with increased surface area. Yet, the results show that self-reactivation is hindered as looping-calcination conditions are harshened. Increasing the looping-calcination temperature and/or the looping calcination time period favors sintering of the soft skeleton and eventually self-reactivation is precluded. A model is developed that retrieves the main features of multicyclic conversion of thermally pretreated sorbents in the first cycles based on the balance between surface area gain due to promoted solid-state diffusion carbonation and surface area loss due to sintering of the soft skeleton in the looping-calcination stage, which can be useful to investigate the critical looping-calcination conditions that nullify self-reactivation. The proposed model allows envisaging the behavior of the sorbent performance as a function of the pretreatment conditions.