Characterization of calcium looping sorbents with a novel twin bed reactor

Abstract

The experimental characterization of sorbents for Calcium Looping (Ca-L) processes is generally accomplished by thermogravimetric analysis or by using single fluidized bed bench scale reactors. These methods may present limitations, the most significant being the poor ability to reproduce the thermal history that sorbent particles experience in a real Ca-L plant. This limitation may severely affect the correct evaluation of the sorbent behavior in terms of CO2 capture capacity and attrition. In this study, a purposely developed experimental apparatus is applied for Ca-based sorbent testing under conditions that simulate a realistic thermal history of the sorbent. The apparatus consists of two identical lab-scale bubbling beds (Twin Beds - TB) operated batchwise as the calciner and the carbonator, respectively. The reactors are connected to each other via a rapid solids pneumatic transfer line, designed so as to enable rapid and selective transfer of the sorbent from one reactor to the other at the end of each calcination/carbonation stage. The two beds consist mostly of coarse-grained silica sand, acting as a thermal ballast, to which sorbent samples to be characterized are added.The TB apparatus has been used to investigate the effect of the thermal history on the performance of two limestones, in terms of CO2 capture capacity and attrition tendency, upon multiple calcination/carbonation cycles under the typical operating conditions of the Ca-L process. The results have been compared with those obtained for the same sorbents carried out in a single lab-scale fluidized bed reactor (SB) and under the same operating conditions. The comparison showed that the CO2 capture capacity in the TB system is larger than that found in SB tests for both sorbents. On the contrary, the absence of strong thermal shocks in the TB experiments leads in general to a decrease of the attrition tendency, with a reversal in terms of generated fines between calcination and carbonation stages. Indeed, the generation rate of fines measured in the TB tests during carbonation is smaller than that measured in the corresponding calcination stage, which is opposite to what was found during the tests in the SB device. These findings indicate that the sorbent thermal history plays a non-negligible role on its CO2 capture performance in Ca-L.