Introduction of chemically bonded zirconium oxide in CaO-based high-temperature CO₂ sorbents for enhanced cyclic sorption

In this study, ZrO₂ is introduced in a new form to reduce sintering and enhance the cyclic stability of CaO-based sorbents. Such a material has potential for high-temperature CO₂ capture applications. Two Zr-modified CaO materials having a Ca/Zr molar ratio of 30 are prepared using the solid-state and citrate sol-gel methods. The solid-state method yields a physical mixture of CaO and ZrO₂, while the citrate sol-gel method induces chemical bonding between ZrO₂ and the CaO surface to form CaZrO₃. The CO₂ sorption uptake was significantly increased in both unmodified CaO and ZrO₂-containing CaO [77.3 wt% (17.6 mol kg⁻¹) and 73.2 wt% (16.6 mol kg⁻¹), respectively, at 650 °C and 1 bar] when the citrate sol-gel method was used. The CaO having the chemically bonded ZrO₂ reveals significantly enhanced cyclic stabilities, with an extremely high CO₂ sorption uptake of 70.5 wt% (16.0 mol kg⁻¹) on average during 10 cycles. On the other hand, the CaO containing the physically mixed ZrO₂ reveals an average cyclic CO₂ sorption uptake of only 37.2 wt% (8.5 mol kg⁻¹). The chemically bonded ZrO₂ is expected to be well scattered on the CaO surface and effectively cover the sorbent, resulting in the reduction of thermal sintering. In addition to cyclic stability, CO₂ sorption kinetics of CaO-based sorbents can be enhanced through the citrate sol-gel method, which is resulted from the significantly reduced size of CaO particles.