Effects of Pyrolysis Temperatures on CO2 Adsorption Rates and Capacities of Biochar As an Alternative Solid Adsorbent for Carbon Capture

Due to rising atmospheric carbon dioxide (CO₂) levels, there is growing interest in removing CO₂ from ambient air through Direct Air Capture (DAC). DAC technologies typically capture harmful atmospheric CO₂ using liquid amines or solid adsorbents. Solid adsorbents for DAC can be costly to produce and maintain and can be environmentally harmful at the end of their lifetimes. Biochar is an inexpensive form of activated carbon made from biowaste with the ability to adsorb CO₂; therefore, it is a promising alternative solid adsorbent for DAC. Properties of biochars have been well-studied but not optimized for DAC. We investigated the CO₂ adsorption capacity of biochars between 25 and 50 degrees Celsius. We tested biochars made from various biowaste, including coconut shells, corncobs, sunflower seeds, walnut shells, pistachio shells, coffee grinds, and pumpkin seed shells. We tested the effects of pyrolysis temperature, CO₂ concentration during pyrolysis, moisture capacity, ash content, and lignin content on CO₂ adsorption rates and capacities. Walnut shells and sawdust biochars had a strong positive correlation between pyrolysis temperature and CO₂ adsorption capacity. As pyrolysis temperatures were increased from 500-800 degrees Celsius, walnut shell biochars demonstrated over a twofold increase in CO₂ adsorption capacity from 0.390 mmol/g to 0.815 mmol/g. Sawdust biochar pyrolyzed under pure N₂ at 800 degrees Celsius achieved the highest overall CO₂ adsorption capacity at 50 degrees Celsius (0.840 mmol/g). This value is highly competitive with current solid adsorbents used in industry for DAC. Our results indicate that CO₂ adsorption capacity might be independent of feedstock type when pyrolyzed at high temperatures (e.g. 800 degrees Celsius).