Ca-modified biochar by microwave irradiation for robust CO2 capture under realistic flue gas conditions

Porous carbon-based materials are promising candidates for CO 2 adsorption, but their performance under realistic flue gas conditions remains a major challenge. This study focuses on the synthesis of Ca-modified biochar, BC(Ca), using a microwave-assisted method for enhanced CO 2 capture under post-combustion conditions. BC(Ca) was thoroughly characterized and evaluated as a CO 2 adsorbent across a range of operating conditions, including changes in temperature, pressure and humidity. The macroporous texture of the biochar (BC) promoted uniform Ca dispersion without significant pore blockage, which is attributed to the microwave-assisted synthesis. BC(Ca), containing 4.6 at.% Ca, mainly as CaO and Ca(OH) 2 , and having a BET area of 445 m 2 g −1 , exhibited a CO 2 adsorption capacity of 2.42 mmol g −1 and a CO 2 /N 2 selectivity of 36 under 15 % vol. of CO 2 in N 2 , at 1 bar and 25 °C. This performance was maintained at elevated pressure, up to 25 bar, highlighting the potential of BC(Ca) for industrial applications. Under humid conditions, with pre-loaded relative humidity (RH) of up to 50 %, BC and BC(Ca) decreased their CO 2 uptake, whereas BC(Ca) showed superior stability. BC(Ca) retained 66 % of its CO 2 uptake, achieving 1.13 mmol g −1 and outperforming comparable carbon-based sorbents under particularly harsh conditions (50 °C, 30 % RH). The improved humidity tolerance was attributed to synergistic effects between physical adsorption, surface basicity enhancement, and water-assisted chemisorption, highlighting the robustness of BC(Ca) for CO 2 capture in humid flue gas streams. • Ca-modified biochar, BC(Ca), was produced by a microwave-assisted method. • BC(Ca) showed 17 % higher surface area than pristine biochar (BC). • BC(Ca) achieved 42 % higher CO 2 uptake than BC, i.e., 2.42 mmol g −1 . • BC(Ca) retained a 1.13 mmol g −1 CO 2 uptake at 50 °C and 30 % relative humidity. • Water-assisted chemisorption improved CO 2 capture.