One-step synthesis of spherical CaO pellets via novel graphite-casting method for cyclic CO2 capture

Granulation of CaO sorbents is the prerequisite for the practical application of the calcium looping system for CO2 capture at high temperatures. This work, for the first time, proposed a simple and facile graphite-casting granulation technique to achieve the one-step pelletization of CaO-based sorbent pellets. The as-prepared pellets exhibited good sphericity and a uniform diameter distribution of 2.5-3 mm. It was found that the granulation process caused the densification of the microstructures for the sorbent, thus decreasing the surface areas and CO2 sorption performance. Hence, a porosity promoter was employed to modify the pellets, endowing the granulated sorbents with enhanced porous structures. As a result, the CO2 sorption capacity of the modified pellets was increased by 48% compared to that of the unmodified pellets over 20 sorption/desorption cycles. In addition, the cyclic performance of the pellets under 50% CO2 desorption atmospheres was also evaluated. The results showed that the presence of CO2 at the desorption stage aggravated the sintering and, thus, decreased the sorption capacity of the pellets. Nevertheless, the pellets modified by the porosity promoter still performed much better than the unmodified ones. The graphite-casting CaO pellets exhibited good mechanical properties, including compression strengths and attrition resistance, which were quite important for the practical application of CaO sorbents in circulating fluidized bed. Good chemicophysical performance demonstrates that the graphite-casting granulation technique is applicable to the preparation of CaO pellets.