Investigating the enhancement of the rate of CO2 capture of CaO in the presence of steam through 18O Isotope labeling: Pitfalls and findings

The reaction of CO2 with CaO to form CaCO3 can be used to remove CO2 from gas streams in post-combustion CO2 capture schemes at high temperatures (> 600 °C). The rate of CO2 uptake is increased substantially in the presence of steam, but the underlying reasons have not yet been resolved, although several explanations have been proposed in the literature. In our study we generated steam from labeled water (H218O) to track 18O in the gas and solid products using mass spectrometry and Raman spectroscopy, aiming to understand whether oxygen (or OH-) contained in H2O participates in the formation of CaCO3. Unfortunately, it was not possible to investigate the interaction of H218O with CaO/CaCO3 isolated, because in the presence of CO2 oxygen was exchanged between H2O and CO2 in the high-temperature reaction chamber of the thermogravimetric analyzer before any interaction of the gaseous reactants with the sorbent. 18O was detected in the CaCO3 product, but it originated from 18O in CO2 rather than H2O. Yet, our measurements showed that oxygen exchange occurs between CaO and H2O under reaction conditions, but not between CaCO3 and H2O/CO2, which may motivate further investigations.