Influence of gas velocity, bed dilution and particle size on limestone decomposition

https://doi.org/10.1016/j.cesx.2020.100071

“Fig. 2 shows the effect of these three parameters on the reaction rate. Increasing the GHSV from 22,500 to 45,000 h⁻¹ (Fig. 2a), has negligible effect on the decomposition of limestone at 885 °C and under 10.1 kPa of CO₂ partial pressure. Increasing the bed dilution by decreasing the mass ratio of sample/quartz from 1/15 up to 1/60 produced the same result regarding the rate (Fig. 2b). This implies that the sample density inside the reactor bed does not affect the rate and so the reaction does not substantially influence the bulk gas CO₂ concentration. Finally, testing the effect of limestone particles’ size on the rate it was concluded that the reaction was faster when decreasing the diameter from 500 down to 100 μm. Beyond that limit (d_p < 100 μm) the internal resistances are minimized and the reaction rate remains constant (Fig. 2c). Other authors have also reported that internal mass transfer is negligible for particles smaller than 90 μm (Escardino et al., 2013). The relative importance of the transfer phenomena compared to the reaction depends on the applied conditions, which determine the limiting stage. Hu and Scaroni (Hu and Scaroni, 1996) examined the calcination of 6–90 μm limestone in a drop tube furnace and found significant resistances due to mass and heat transfer. They observed a conversion gradient inside 63 μm particles after partial decomposition at 1200 °C. On the contrary, Borgwardt (Borgwardt, 1985) did not find any influence on the rate of 1–90 μm particles’ decomposition under pure nitrogen and up to the 1000 °C. Accordingly, the experimental design parameters for the investigation of the intrinsic calcination rate in the present work were chosen as follows: particle size range = 45–75 μm, GHSV = 22,500 h⁻¹ and sample/quartz mass ratio = 1/15.”

“Fig. 2. Effect of the GHSV (a), bed dilution (b) and particle size (c) on calcination rate of raw limestone.”