Steam activation of biochar for CO2 capture

https://doi.org/10.1016/j.ccst.2022.100039

“Generally, the major reaction steps in H₂O activation include chemisorption, oxidation, gasification and water‒gas shift reaction (Lussier et al., 1998). Different from the homogeneous micropores formed under CO₂ activation due to the sluggish reaction with carbon species (Singh et al., 2019a), mesopores and macropores are dominant after steam activation due to the rapid oxidation of carbon matrix, effective penetration into the internal structure of carbon and transformation of micropores (Halder et al., 2016). Under 900°C, steam-activated carbon adsorbent from coconut shells enjoyed a higher surface area than that treated in CO₂ (1888 vs. 1425 m²/g) probably owing to the abundant meso- and micropores (Yang et al., 2010). Notably, for certain biomass precursor, the impact of steam and CO₂ might be different from previous examples. When Jatropha hull was activated in both CO₂ and steam, opposite results were obtained that CO₂-activated carbon possessed a surface area of 1207 m²/g while the steam-treated one only exhibited 748 m²/g (Duan et al, 2011). This indicates the strong relationship between the applicability of the activation method and the type of precursor. Similar with the CO₂/NH₃ and CO₂/N₂ atmospheres, a combined activation by CO₂/steam enables a better pore structure and higher surface area. The CO₂/steam co-activated olive stones possessed a higher surface area of 1187 m²/g than that of CO₂ (572 m²/g) and steam (1074 m²/g) alone (Román et al., 2008). Despite the abundant pores and high surface area obtained under steam activation, the loss of polarity and aromaticity may be an issue for CO₂ adsorption, which might be resolved by a careful control of operation parameters or combination with other activation methods (Ahmed et al., 2016).”