CO2 activation of biochar for CO2 capture

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

“Under a CO₂ atmospheres, carbon matrix is partially gasified into CO, simultaneously generating high porosity and surface area (Prauchner et al., 2016). Via activation of a series of biomass molecules in CO₂, the specific surface area was significantly increased from 38‒92 to 400‒1000 m²/g (Zhang et al., 2004). In another scenario where starch was activated by CO₂, ultra-high surface area of 3350 m²/g was achieved, leading to a 21.2 mmol/g CO₂ uptake at 20 bar and 25°C (Li et al., 2016). Besides the surface area, the pore size distribution is sensitive to the CO₂ activation temperature. Compared with the AC derived from rubber sawdust under 740°C activation, a higher temperature of 760°C reduced the micropore volume percentage from 87.7% to 71.6%, indicating the transformation of micropores to mesopores and macropores at a higher temperature (Mazlan et al., 2016). In addition to the temperature, a longer residence time enables a higher surface area. With a prolonged activation duration (210 min vs. 1 h), the AC derived from coconut shell under 900°C and CO₂ atmosphere possessed a higher surface area from 1425 to 2288 m²/g (Yang et al., 2010). In another study, however, an opposite finding was demonstrated that severe reactions between CO₂ and carbon surface occurred during prolonged activation, leading to simultaneous pore opening and widening with a subsequent pore collapse. Furthermore, more ash residues originated during the intensified activation blocked the existing pores and inhibited the CO₂ capture (Rouzitalab et al., 2018). On the other hand, different from conventional heating, the volumetric heating effect caused by microwave quickens the activation process and lowers the energy input (Singh et al., 2019a). The hierarchical porous structure produced at a lower temperature rendered the AC with a twice of surface area as the AC heated in conventional way (Duan et al., 2011). Combined with other atmospheres, the activated adsorbent may possess a better physicochemical property. When cotton stalk was treated with CO₂/NH₃ mixture, both a larger surface area (627 vs. 224 m²/g) and higher nitrogen concentration (3.91 vs. 1.09 wt%) were obtained than those of unactivated one (Zhang et al., 2014). In another case, a successive activation by N₂ and CO₂ promoted the volatile generation and resulted in a higher surface area (1126 m²/g) than CO₂ alone (807 m²/g) (Jung and Kim, 2014).”