Parametric study on the CO2 capture capacity of CaO-based sorbents in looping cycles

An experimental parametric study on the CO2 capture activity of four limestone-derived CaO-based sorbents has been performed. Experiments were done in a thermogravimetric analyzer (TGA) at temperatures ranging from 650 to 850 degrees C. Three particle-size fractions of Kelly Rock limestone and powders obtained by their grinding were also tested, while the influence of carbonation and calcination durations was examined at 750 and 850 degrees C. Calcination is typically performed in an atmosphere of N-2 and carbonation in 50% CO2 (N-2 balance), and the influence of the effective CO2 concentration surrounding reacting particles was examined by changing the sample mass in some experiments. The results indicated that increasing the calcination/carbonation temperature had a negative influence on the sorbent activity, while the influence of particle size was small, although larger particles have higher activity. This was unexpected, but it can be explained by the higher content of impurities in the smaller particles. Grinding enhances sorbent activity, and this appears to be more than simply due to increased external surface area of the sorbent particles in the powdered samples. Prolonged carbonation time has a negative effect on the sorbent performance. The formation and decomposition of CaCO3 as well as its presence on the sorbent surface at higher temperatures appear to be key factors in the loss of surface area (i.e., decrease in sorbent activity). However, it is shown that the prolonged exposure to calcination conditions employed in this work (inert atmosphere) has a slightly beneficial effect on sorbent behavior as a function of the number of calcination/carbonation cycles. Experiments with larger sample masses typically resulted in better conversions. Analysis of scanning electron microscope (SEM) images of spent sorbent particles obtained from different reactor types indicated that thermal stresses are the main cause for sorbent particle fracture and attrition.