CO₂ capture from ambient air using hydrated Na₂CO₃ supported on activated carbon honeycombs with application to CO₂ enrichment in greenhouses

CO₂ capture from ambient air is an interesting option for CO₂ enrichment in greenhouses. In this study, adsorbents comprised of hydrated Na₂CO₃ supported over activated carbon honeycombs were prepared, characterized and tested for CO₂ capture from air. The adsorption of H₂O and the formation of the hydrates were studied by means of FT-IR spectroscopy. The inlet CO₂ concentration showed to have a major influence on the conversion yield into NaHCO₃, and the results fitted well to the Toth model. A statistical model of the CO₂ capture capacity was obtained to get insight into the key parameters of the adsorption process. The air temperature and its moisture content showed to have the largest impact on the CO₂ capture, while the flow rate had a minor influence. The chemical reaction path during the CO₂ adsorption showed to be determined by the relative humidity conditions inside the reactor. Addition of more salt on the carrier showed to improve the CO₂ capture capacity, but this is limited by the strength of the honeycomb carrier. Finally, a preliminary desorption test via a mild temperature and moisture swing was run to assess the feasibility of the process for application in greenhouses. The results showed that the required volume of adsorbent would be roughly 1/1000 of the total volume of a closed greenhouse assuming a target CO₂ level of 1200 ppm.