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 powder 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 sample composition have been carried out using X-ray powder diffractometry. These analyses have revealed that the CaCO3 content after CO2 capture originates from 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 is significantly reduced, indicating that most of CO2 capture might take place through physical adsorption.