Preparation and Application of the Sol–Gel Combustion Synthesis-Made CaO/CaZrO3 Sorbent for Cyclic CO2 Capture Through the Severe Calcination Condition

Abstract

Calcium looping method has been considered as one of the efficient options to capture CO2 in the combustion flue gas. CaO-based sorbent is the basis for application of calcium looping and should be subjected to the severe calcination condition so as to obtain the concentrated CO2 stream. In this research, CaO/CaZrO3 sorbents were synthesized using the sol–gel combustion synthesis (SGCS) method with urea as fuel. The cyclic reaction performance of the synthesized sorbents was evaluated on a lab-scaled reactor system through calcination at 950°C in a pure CO2 atmosphere and carbonation at 650°C in the 15% (by volume) CO2. The mass ratio of CaO to CaZrO3 as 8:2 (designated as Ca8Zr2) was screened as the best option among all the synthesized CaO sorbents for its high CO2 capture capacity and carbonation conversion at the initial cycle. And then a gradual decay in the CO2 capture capacity was observed at the following 10 successive cycles, but hereafter stabilized throughout the later cycles. Furthermore, structural evolution of the carbonated Ca8Zr2 over the looping cycles was investigated. With increasing looping cycles, the pore peak and mean grain size of the carbonated Ca8Zr2 sorbent shifted to the bigger direction but both the surface area (SA) ratio Φ and surface fractal dimension Ds decreased. Finally, morphological transformation of the carbonated Ca8Zr2 was observed. Agglomeration and edge rounding of the newly formed CaCO3 grains were found as aggravated at the cyclic carbonation stage. As a result, carbonation of Ca8Zr2 with CO2 was observed only confined to the external active CaO by the fast formation of the CaCO3 shell outside, which occluded the further carbonation of the unreacted CaO inside. Therefore, enough attention should be paid to the carbonation stage and more effective activation measures should be explored to ensure the unreacted active CaO fully carbonated over the extended looping cycles.