Wood biochar as a point source CO₂ adsorbent-impact of humidity on performance

Adsorption of carbon using biobased materials is a sustainable approach to point source carbon capture and storage, particularly if the biomass is a "waste" and the adsorbent can then be re-used in soils. In this work, biochar derived from fast pyrolysis of softwood residues (sawdust) was investigated as a CO2 adsorbent under humid conditions to determine the effect of water on biochar adsorption performance. Dry and wet adsorption experiments were run at concentrations of CO2 between 20-80 vol.% (balanced with N-2) at 20 degrees C. Water concentrations were set by saturating the N-2 stream and resulted in 0.5-1.8 vol.% (relative humidity of 20%-80%). For the dry gas experiments, the adsorption capacities doubled (0.83-1.98 mmol/g) as the CO2 concentration increased from 20% to 80%. The capacity did not change between wet and dry experiments for 20, 40, and 60 vol.% CO2; however, at 80 vol.% CO2, the adsorption capacity increased by 38% for the "wet" gas from the dry gas. This is potentially due to carbonates formed due to CO2 dissolution/reaction in water. The adsorption time was not impacted by the range of water concentrations studied. The Avrami kinetic model best represented the rate of adsorption in both dry and wet conditions. The fact that neither the biochar adsorption capacity nor its adsorption rate was negatively impacted by water shows that biochar can be a promising option for stack gases, which contain water vapor. However, more experiments at higher temperatures are required.