The relevance of limestone attrition during fluidized bed calcium looping cycles for CO2 capture

Attrition of six different limestones during calcium looping cycles for CO2 capture was studied in a lab-scale fluidized bed apparatus under realistic conditions representative of a process with calcination in an oxy-firing environment. Batch experiments under alternating calcination-carbonation conditions were carried out to investigate the effect of chemical reactions and temperature changes on the attrition propensity of the sorbent. Attrition processes were characterized by following the modifications of bed sorbent particle size distribution and the elutriation rates of fines throughout conversion over repeated cycles. The effect of the presence of SO2 in the flue gas was also tested in purposely designed experiments. Results show that relatively large attrition rates were experienced by the sorbent particles only during the first cycle. From the second cycle on the attrition rate progressively declines, also during the calcination stage when the softer CaO is produced. It is inferred that the combined chemical-thermal treatment affects the particle structure making it increasingly hard. At the same time the CO2 capture capacity rapidly decays toward an asymptotic level, possibly related to the very same structural modifications. The presence of SO2 in the flue gas significantly influences the sorbent CO2 capture capacity, but not the extent of attrition.