A model on the CaO multicyclic conversion in the Ca-looping process

Abstract

The CaO conversion of CO2 sorbents subjected to repeated series of carbonation/calcination cycles is investigated as affected by thermal pretreatment. The behavior of nonpretreated sorbents is characterized by the progressive sintering of the initial CaO skeleton derived from a quick preheating of its precursor. As a consequence, CaO conversion suffers a drastic decrease in the first cycles and gradually converges towards a residual value. The evolution of conversion in the first cycles for pretreated sorbents is determined by a diverse mechanism, which is mainly characterized by the growth of a renewed soft skeleton due to promoted solid-state diffusive carbonation, which leads to an increase of conversion with the cycle number up to a maximum value. A mathematical model is proposed based on the balance between the surface area loss due to sintering in the calcination stage of each cycle and the surface area gain due to regeneration of the skeleton. A key feature to describe self-reactivation is that renovation of the soft skeleton becomes progressively less efficient as the number of cycles builds up. An extensive series of data reported in the literature has been analyzed in the light of the proposed model, which has served to quantify the level of self-reactivation as affected by a wide variety of experimental conditions such as preheating temperature and duration, hydration, calcination under a CO2 enriched atmosphere, grinding, addition of thermally stable dopants, and presence of foreign ions in the limestone.