Effect of heating rate on the kinetics of limestone calcination

This paper reports on the effect of heating rates between 0.1 °C/s and 205.0 °C/s on the thermodynamics and kinetics of limestone calcination in air. These heating rates are orders of magnitude higher than those used in the thermogravimetric analysers that have been reported previously (0.1 °C/s to 0.8 °C/s). It was found that the activation energy, E, is reduced by 59%, from 205.0 kJmol−1 to 84.5 kJmol −1, when the heating rate is increased from a low heating rate regime (<1 °C/s) to a moderate heating rate regime (99.2––205.0 °C/s). Additionally, the average reaction rate constant K¯ increases with an increase in the heating rate. As an example, for 205 °C/s, K¯ is 18.7 times higher than that for 0.2 °C/s, when approaching the same asymptotic temperature. It is also found that a higher heating rate increases the specific surface area (SSA), for the same Tas. This is caused by microfractures which are associated with decreased activation energy, and in turn increases the surface-to-volume ratios and speeds up conversion. The results highlight the importance of accounting for both heating rate and calcination time in the modelling of the calcination of limestone. Specific surface area measurements also revealed that the prolongation of calcination under moderate heating rate reduces the SSA, typically by a factor of 1.6 from 15.8 m2/g to 9.6 m2/g when the residence time tr is increased from 120 s to 240 s. Therefore, to maximize the SSA, the residence time should not be longer than that required for complete conversion.