Determination of carbonation/calcination reaction kinetics of a limestone sorbent in low CO₂ partial pressures using TGA experiments

The carbonation/calcination reaction cycle of a limestone sorbent was used in a novel greenhouse calcium looping process to provide the heat and CO₂ requirement of greenhouses. The unique conditions of this process (low temperature and low CO₂ partial pressure) led to the determination of the carbonation/calcination reaction kinetic parameters in a temperature range of 400 to 800 °C and CO₂ partial pressures of 0.04 to 0.16% (400 to 1600 ppm) which were carried out experimentally via a thermo-gravimetric analyzer (TGA). Various gas-solid reaction mechanisms were considered to determine the best reaction mechanism for the carbonation reaction. The diffusion function or G(x) = x² had the best least-square linear fit, which resulted in a first order reaction for the carbonation reaction in the greenhouse calcium looping process. Moreover, the activation energy and pre-exponential factor of the carbonation reaction were established to be 19.7 kJ mol^-1 and 295.8 min^-1 kPa^-1. The same methodology for reaction mechanism determination was carried out for the calcination reaction and G(x)=1-(1-x)^1/3 was found to have the best linear fit. The activation energy and pre-exponential factor determined for the calcination reaction were 103.6 kJ mol^-1 and 6.9 × 10⁶ min^-1, respectively.