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

Abstract

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 directly 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 suggest that oxygen exchange occurs between CaO and H₂O under reaction conditions, but not between CaCO₃ and H₂O/CO₂, which may motivate further investigations.