On the attrition/fragmentation of sorbent particles reactivated by steam hydration in Ca looping applications

The present study addresses the steam hydration as a reactivation technique of the CO2 capture potential of spent limestone-based sorbents from fluidized bed calcium looping systems. A reference, high-calcium, reactive limestone was deactivated by carrying out lab-scale fluidized bed calcium looping process tests (calcination at 940°C in a 70% CO2 atmosphere; carbonation at 650°C in a 15% CO2 atmosphere), and then steam hydrated (at 250°C in a 50% steam atmosphere) in the same fluidized bed, for times ranging from 10 to 60 min. On-line flue gas analysis, continuous capture of the elutriated fines and analysis of particle size distribution were performed during additional calcium looping process tests after sorbent reactivation. Thermogravimetric analysis, scanning electron microscopy and porosimetry were directed to characterize the microstructural features of the spent and the steam hydrated sorbents. Moreover, the different materials were subjected to ex situ impact fragmentation tests. In this way, it was possible to investigate the effect of the hydration time on: the changes in the physico–chemical and microstructural properties induced by the hydration treatment; the reactivation of the sorbent CO2 capture capacity; the attrition/fragmentation tendency of the reactivated materials. It was observed that the steam hydration followed by dehydration of the reactivated sorbent in the hot fluidized bed develops an increased porosity, hence improved rate and extent of CO2 uptake. At the same time also the attrition and fragmentation propensity of the reactivated sorbent is increased. For the given limestone, the optimal trade off between sorbent reactivity/uptake and mechanical strength is achieved after 60 min hydration of the spent sorbent, but it is expected that this result cannot be generalized, as it is critically dependent on the sorbent texture. The comparison between water and steam hydration of spent sorbents as reactivation means indicated that the steam hydration is more favourable. Albeit the liquid water hydration gives rise to larger water uptake, the prolonged soaking in liquid water makes the reactivated sorbent more susceptible to attrition and fragmentation.