Direct CO2 Capture from Ambient Air for Synthesis of Renewable Methane

A composite sorbent was synthesized by impregnating cylindrical granules of mesoporous γ-Al2O3 with an aqueous solution of K2CO3. Three fractions of the composite with the characteristic grain size of 1-2 mm, 3-4 mm and 4-6 mm were tested for direct carbon dioxide capture from ambient air in a continuous-flow system. It was shown that dynamic CO2 absorption capacity of the composite sorbent increases significantly with decrease of the characteristic grain size, which indicates that the process is limited by mass transfer. The effect of temperature on thermal regeneration of the composite was also investigated. The change of regeneration temperature from 250 to 300°C enhanced CO2 absorption capacity for all the fractions studied. XRD in situ study showed that the reason of this enhancement is thermal decomposition of potassium dawsonite KAlCO3(OH)2 formed during synthesis of the composite material. It was demonstrated by SEM/EDX method that as a result of temperature-swing absorption cycling radial distribution of potassium in the granules of the composite becomes non-uniform with a distinct contrast between the center and the periphery of the grain. The change in potassium distribution is likely to be the result of the fluid-aided dissolution–reprecipitation of the supported alkali components. The composite sorbent demonstrated a great stability of its CO2 absorption characteristics, therefore it should be considered as a promising material for direct carbon dioxide capture from air.