The influence of reactivation by hydration of spent SO2 sorbents on their impact fragmentation in fluidized bed combustors

The relationship between calcination/sulphation and attrition/fragmentation of calcium-based SO2 sorbents in fluidized bed (FB) combustors has long been recognized, but only recently did attrition by impact receive due consideration. There is limited available information in the literature on the propensity of exhausted calcium-based sorbents to undergo high-velocity impact fragmentation after they have been reactivated by steam or water hydration. The present study addresses the relationship between hydration-induced reactivation of spent Ca-based sorbents and attrition by impact loading. The sorbent used in this work (a high-calcium Italian limestone) was pre-processed (sulphation at 850 °C in a lab-scale FB, water hydration for 3 h at 25 °C in a thermostatic bath, steam hydration for 3 h at 250 °C in a tubular reactor, dehydration at 850 °C in the FB) and subjected to impact tests in a purposely designed impact test rig, operated with particle impact velocities ranging from 4 to 45 m s−1. The particle size distribution of the debris was worked out to define a fragmentation index and a probability density function of the size of generated fragments. The effect of hydration/reactivation of spent sorbent on propensity to undergo impact fragmentation was assessed, and results are discussed in the light of a mechanistic framework. It was observed that the prevailing particle breakage pattern was splitting/chipping for water-reactivated samples, disintegration for steam-reactivated samples. Characterization of sorbent microstructure by porosimetry and microscopic investigation on the reactivated samples highlighted a clear relationship between the extent of fragmentation and the cumulative specific volume of mesopores.