Evaluation of Microporous Biochars Produced by Single-step Oxidation for Postcombustion CO2 Capture under Humid Conditions

Water vapor is the third component of flue gases after N2 and CO2. The permanent dipole moment of the water molecule makes it strongly adsorbable on many adsorbents, which can negatively affect the adsorption capacity of carbon dioxide (even causing an irreversible loss in certain cases). Carbon materials have high stability in moist conditions and present a hydrophobic nature that makes these materials appealing adsorbents for post-combustion CO2 capture. Furthermore, these adsorbents present the added advantage that can be obtained from a globally available, cheap and renewable source of carbon: biomass. In the present work the effect of water vapor on the adsorption performance of CO2 using a microporous biochar developed from olive stones by single- step oxidation is evaluated. The equilibrium of adsorption of water vapor on the selected biochar was studied in a wide temperature range that is considered of interest for the post-combustion case (12.5-85 °C). This biochar presents a moderate water adsorption capacity and type V adsorption isotherms, which will facilitate the desorption of water vapor during cyclic operation. Breakthrough curves were obtained using a gas mixture which composition resembled flue gas in the presence and absence of water vapor. The breakthrough curves of CO2 obtained under dry and humid conditions overlap each other, which indicates that the presence of water vapor does not hinder CO2 adsorption in the short time scale. Moreover, the adsorbent recovered its full adsorption capacity after regeneration. These findings point out that this material could be used to separate CO2 from humid flue gas using cyclic adsorption processes.