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 oC in a lab-scale
FB, water hydration for 3 h at 25 oC in a thermostatic bath, steam hydration for 3 h at 250 oC in a tubular
reactor, dehydration at 850 oC in the FB) and subjected to impact tests in a purposely designed impact test
rig, operated with particle impact velocities ranging from 4 to 45ms-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.