Nano-TiO2 promoted CaO-based high-temperature CO2 sorbent: influence of crystal level properties on the CO2 sorption efficiency

This work investigates the multi-cycle CO2 sorption and the kinetics of the carbonation reaction of nano-TiO2 promoted CaO synthesized from commercially available micron sized CaCO3. The morphology of CaCO3 coated with different wt% of nano-TiO2 (1 to 10 wt%) was investigated by field emission scanning electron microscopy (FESEM). The crystallite size, lattice parameters, and strain of the decomposed product for all the samples were estimated by Rietveld refinement of X-ray diffraction (XRD) data. The carbonation of nano-TiO2 promoted CaO sorbents was studied on a thermogravimetric analyzer (TG) under a CO2 atmosphere (0.02 MPa) at different temperatures (600, 650 and 700 °C) and the results were compared with the CaO obtained from pure micron sized CaCO3. The results show that the sorption capacity of nano-TiO2 promoted CaO sorbents is several times higher than that of the pure CaO. The improvement could be ascribed to the porous structure and smaller crystallite size of CaO in the presence of nano-TiO2, which is supported by FESEM and XRD. In fact, the sorption capacity of nano-TiO2 promoted CaO is found to be higher than that of the CaO obtained from nano-CaCO3. The shifting of the transition point between two-stage carbonation reactions is correlated with the wt% of nano-TiO2, the crystallite size, and the strain inside the CaO. The optimum wt% of nano-TiO2 and sorption temperature of the sorbents are explored to achieve the maximum sorption capacity. Finally, the experimental results are fitted with theoretical models based on the shrinking core model, and kinetic parameters are evaluated.