Biochar adsorption of CO₂ from gas mixtures, a study of dry and wet conditions

Biochar, a green adsorbent produced from the thermochemical conversion of biomass, has proved its CO₂ capture potential in many research works. Nevertheless, when considering industrial-scale utilization, it becomes crucial to explore scenarios that closely resemble real-world emission points beyond the adsorption capacity of the adsorbent. In this work, we conducted a thorough review of the existing literature to assess whether all potential barriers and challenges related to the industrial-scale application of biochar as a CO₂ adsorbent have been investigated and addressed. In later parts, we conducted research focused on areas we identified as research gaps in our review of the literature. A careful scanning of the literature revealed that the majority of research studies on biochar CO₂ adsorption are concentrated on its adsorption of pure CO₂ or binary CO₂/N₂ mixtures. However, in actual emission scenarios, CO₂ is never discharged as a pure gas; instead, it is emitted as part of a gas mixture. Water vapor is a component that is always present in flue gas/gasification syngas and can adversely impact the adsorption of many common CO₂ adsorbents, such as zeolites, by strongly adsorbing to the adsorption sites. For this reason, a pre-treatment drying stage is necessary for many of the commercial adsorbents, adding to the capital and operational costs. The literature was obscure regarding whether biochar performance would be negatively impacted in the presence of water vapor. Our investigation on the water vapor impact on woody biochar CO₂ adsorption via computational and experimental methods revealed that the adsorption rate and capacity of the biochar were not negatively impacted by the water vapor. In fact, the biochar CO₂ adsorption capacity was improved at higher CO₂ partial pressures (>80 vol.%). The experiments were performed with binary CO₂/N₂ mixtures (20- 80 vol.% CO₂ balanced with N₂) at 20 °C and 1.2 bar under dry and humid conditions. For the wet experiments, N₂ was saturated with water at 20 °C before being mixed with CO₂ resulting in 20-80% relative humidity (0.5-1.8 vol% H₂O). The results showed that for 20-60 vol.% CO₂, the biochar CO₂ adsorption capacity was not impacted by the presence of water; however, at 80 vol.% CO₂, the adsorption capacity was improved by approximately 38% potentially due to carbonate formation as a result of CO₂ solution/reaction with water. Another component that is often co-released with CO₂ either due to incomplete combustion of fuel in the flue gas or as one of the main components in gasification syngas is CO. From our observation, no available literature experimentally challenged the biochar co-adsorption of CO and CO₂. In this work, the biochar CO/CO₂ coadsorption was examined both by computational and experimental methods. The binding energy calculations indicated that the adsorption of CO₂ on the biochar would release more energy than the adsorption of CO, demonstrating a stronger affinity of biochar for CO₂. Moreover, considering the adsorption capacities for pure CO and CO₂ on biochar (with a higher capacity observed for CO₂ (2.325 mmol/g) compared to CO (0.700 mmol/g)) led us to predict that if both gases were present in the feed gas, biochar would exhibit a preference for CO₂ over CO. However, biochar binary CO/CO₂ co-adsorption experiments were necessary for a reliable conclusion. The breakthrough curves of dynamic biochar CO/CO₂ co-adsorption experiments (10-90 vol.% CO₂ balanced with CO) at 20 °C and 1.2 bar showed that although CO and CO₂ were both adsorbed on biochar when the adsorption bed was fresh, some/all of the CO molecules were later removed by the more strongly adsorbed CO₂, resulting in 0 mmol/g biochar CO adsorption for experiments with lower partial pressures of CO (10-50 vol% CO). As the flue gas/gasification syngas are released at higher temperatures, examining the temperature impact on the biochar CO/CO₂ co-adsorption was necessary. The pure CO and CO₂ adsorption tests performed in the temperature range of 20-100 °C revealed that although biochar CO₂ adsorption capacity decreased as the temperature increased, biochar CO adsorption capacity was not impacted by temperature. Selected biochar binary CO/CO₂ co-adsorption tests repeated at 100 °C showed that the reduction of biochar CO₂ adsorption capacity at 100 °C compared to 20 °C resulted in lower removal of adsorbed CO molecules by CO₂, which consequently resulted in a higher CO adsorption from the binary mixtures at 100 °C compared to 20 °C.

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