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

The conversion of thermally pretreated CaO along successive carbonation/calcination cycles has been investigated, as a ected 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 rst carbonation/calcination cycles. Our observations support a pore-skeleton model according to which solid-state di usion in the rst 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 ∗To whom correspondence should be addressed †University of Seville ‡CSIC-University of Seville 1 is hindered as looping-calcination conditions are harshened. Increasing the loopingcalcination 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 rst cycles based on the balance between surface area gain due to promoted solid-state di usion 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. Introduction The Ca-looping (CaL) process, based on the carbonation reaction of CaO to capture CO2 and the subsequent decarbonation of CaCO3 to regenerate the sorbent, is at the basis of a promising postcombustion capture technology whose suitability has been demonstrated by sustained CO2 capture e ciencies over 90% in large pilot-scale plants. 3 In practice, carbonation/decarbonation of CaO is carried out in two interconnected uidized bed reactors through which the material is continuously circulated. In the carbonator, CaO particles become carbonated at contact with the postcombustion gas containing CO2 in a vol% of around 15%. The carbonated particles are driven to the calciner where they are decomposed by calcination at high temperature, which produces a concentrated stream of CO2 suitable to be compressed and transported for sequestration. By taking into account the tradeo between the reaction equilibrium driving force and the reaction kinetics, carbonation is carried out at optimal temperatures of around 650◦C. On the other hand, decarbonation in the CO2 rich atmosphere of the calciner requires application of temperatures above 900◦C, which are accomplished by burning coal with a stream of pure O2 (oxy red combustion). Thermogravimetric analysis (TGA) studies show that carbonation of CaO solid particles