Low concentration CO2 capture in fluidized beds of Ca(OH)2 as affected by storage humidity

Carbon negative emission technologies (NETs) such as CO₂ direct air capture (DAC) are being already considered as necessary for climate change mitigation. This paper investigates CO₂ capture at room temperature and atmospheric pressure by a fluidized bed of Ca(OH)₂ powders as influenced by the relative humidity (RH) to which this sorbent was exposed during storage. Humidity is precisely controlled by means of several supersaturared salt solutions, and FTIR spectrometry is used to measure accurately the time evolution of CO₂ and H₂O concentrations in the effluent gas. Results show that CO₂ capture is promoted by an increase of the storage RH. The observed effect is particularly important for very high storage RH (~ 100%, near the dew point). After the capture tests, quantitative analyses of samples composition were carried out using X-ray powder diffractometry. These analyses revealed that the CaCO₃ content after CO₂ capture occurs via carbonation in samples that were stored under ~100% RH. On the other hand, in samples stored under low to moderate RH, the presence of CaCO₃ was significantly reduced, indicating that most of CO₂ capture takes place through physical adsorption.