Fluidized bed calcium looping cycles for CO2 capture under oxy-firing calcination conditions: Part 1. Assessment of six limestones

Six limestones were tested for CO2 capture during calcium looping cycles in a lab-scale fluidized bed apparatus. Batch tests were carried out under alternating calcination–carbonation conditions representative of a process with calcination in an oxy-firing environment (T = 940 °C, 70% CO2). The effect of the presence of SO2 was also studied at two concentration levels: 1500 ppm SO2 during both carbonation and calcination, simulating CO2 capture from uncontrolled flue gas and regeneration in an oxy-fired calciner burning high-sulfur coal; 75 ppm SO2 during carbonation and 750 ppm SO2 during calcination, simulating CO2 capture from already desulfurized flue gas and regeneration in an oxy-fired calciner burning medium-sulfur coal. Limestone attrition processes were characterized during the tests by measuring the changes of the sorbent particle size distribution and the fines elutriation rate along conversion over repeated cycles. Results showed that for all the limestones the CO2 capture capacity decreased with the number of cycles reaching a very low asymptotic value. The combination of high bed temperature and high CO2 concentration during the calcination stage significantly enhanced particle sintering. Moderate attrition rates were experienced by the sorbent particles mostly during the first cycle. In the subsequent cycles the attrition rate progressively declined, due to the concurrent chemical–thermal treatment making the sorbent surface increasingly hard. The presence of a high SO2 concentration significantly depressed the sorbent CO2 capture capacity, because of the buildup of a compact CaSO4 layer on the particle surface. Under a lower SO2 concentration level, the effect on CO2 capture capacity was less important. In both conditions, limestone attrition was only influenced to a limited extent by SO2. The slight variations of the sorbent particle size distribution indicated that very low particle fragmentation occurred over the repeated cycles.