Assessment of limestone attrition in fluidized bed combustion

Limestone particles undergo different attrition/fragmentation phenomena when processed in a fluidized bed combustor: primary fragmentation (decrepitation), attrition by surface wear, attrition/fragmentation by impact loading. The present study addresses limestone attrition and fragmentation associated with impact loading, a process which may extensively occur in the jetting region of the bottom bed of fluidized bed combustors. An experimental protocol for the characterization of the susceptibility of limestones to undergo attrition/fragmentation by impact loading is reported. The application of the protocol is demonstrated with reference to an Italian limestone whose primary fragmentation and attrition by surface wear had already been characterized in previous studies. The experimental procedure is based on the characterization of the amount and particle size distribution of the debris generated upon impact of samples of sorbent particles against a target. The protocol has been applied to raw limestone as well as to calcined and spent (i.e. sulfated to exhaustion) sorbents. Measurement of particle voidage and pore size distribution by mercury intrusion was also accomplished to correlate fragmentation with the textural properties of the sorbent samples. Fragmentation by impact loading of the limestone is significant. Lime displays the largest propensity to undergo impact damage, followed by spent sorbent and raw limestone. Fragmentation of the raw limestone and of the sulphated lime follows pattern typical of the failure of brittle materials, with a clear-cut transition between chipping and fragmentation regimes. The fragmentation behaviour of lime better conforms to a ductile failure mode, with extensive generation of very fine fragments.