Assessment of limestone attrition by impact loading during fluidized bed combustion

An experimental protocol aiming at the assessment of the propensity of limestones to undergo attrition/fragmentation by impact loading in FB combustors has been set up. 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. Experiments were carried out in a range of particle impact velocities comprised between 7 and 42m/s, consistent with jet velocities corresponding to typical pressure drops across FB distributors. 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. The fragmentation index versus impact velocity profile follows trends similar to those reported for semi-brittle materials, with a transition between chipping and fragmentation patterns occurring at impact velocities of about 28m/s. The extent of fragmentation was well correlated with the particle voidage of the different samples.