The rise in awareness of the risks of excessive CO2 emissions has triggered a global response aimed at plummeting emission rates. The key to realizing this has been through the transition from fossil fuels towards renewable energies as well as improving the efficiencies of industrial processes. The latter is particularly pivotal in processes where CO2 emission is unavoidable; e.g. cement production. In such scenario, carbon capture technologies come in handy. Popular capture technologies mainly utilize liquid solvents which, unfortunately, require large amount of energy for regeneration. Alternatively, solid sorbents can be designed to work at different temperatures. In particular, high temperature sorbents have good CO2 capture capacity and can be used to treat hot flue gas streams immediately after the combustion process, reducing the operating costs. There are many alternative HTS that can be considered, including CaO, Li2ZrO3, Na2ZrO3 and Li4SiO4. Unfortunately, CaO, Li2ZrO3 and Li4SiO4 suffer from critical drawbacks that hinder their commercial applications. Na2ZrO3 on the other hand presents a good and viable alternative solid sorbent for CO2 capture. However, researchers before have shown that the preparation method of solid Na2ZrO3 greatly affects its CO2 uptake and uptake kinetic.
So far, the solid-state synthesis which has the advantages of high yield, simple operation process and low costs is highly co-opted. Nonetheless, despite the solid-state method being well established and cost-effective, there is a lack of studies on its optimization. As such, further studies are needed to systematically investigate the effect of various solid-state synthesis conditions on the CO2 capture performance of Na2ZrO3. On this account, Heriot-Watt University researchers: Mr. S. Munro and Dr. Aimaro Sanna, in collaboration with Dr. M. Åhlén and Dr. O. Cheung at the Uppsala University in Sweden established a correlation between the parameters used in the solid-state synthesis and the Na2ZrO3 CO2 absorption properties under industrially relevant conditions (i.e. 20 vol% CO2). Their work is currently published in the Chemical Engineering Journal.
Their goal was to explore the effect of heating rate, holding time, reactants molar ratio and mixing mode. To realize this, the solid-state synthesized Na2ZrO3 was characterized by XRD, SEM-EDS, XPS and TGA. In fact, a structural, chemical, microstructural and kinetic analysis of the Na2ZrO3-CO2 system over one cycle was performed to identify the correlation with the sorbent performance.
The authors reported that the heating rate, the molar ratio of the Na2CO3 and ZrO2 used in the synthesis of Na2ZrO3, as well as additional powder processing steps of the reactants, all had a major impact on the sorbent’s CO2 capture performance. Additionally, the results obtained also suggested a linear relation between the crystal size and the two performance parameters, with both CO2 absorption rate and uptake capacity increasing according to the decrease of the crystal size.
In summary, the study demonstrated that the solid-state synthesis conditions used when preparing the Na2ZrO3 have a great impact of the CO2 uptake capacity and CO2 absorption rate. This was drawn from the fact that the Na2ZrO3 synthesized by the authors were seen to favor the ionic solid-state diffusion of Na and O from the core to the surface of the material to readily react with CO2. In a statement to Advances in Engineering, Dr Aimaro Sanna pointed out that their work demonstrated a sorbent with an excellent cyclic stability over 70 sorption/desorption cycles after an initial decay when the CO2 cycles were shortened to 5 min.
S. Munro, M. Åhlén, O. Cheung, A. Sanna. Tuning Na2ZrO3 for fast and stable CO2 adsorption by solid state synthesis. Chemical Engineering Journal, volume 388 (2020) 124284.