(175e) Impact of Pyrolysis Conditions of CO2 Adsorption of Biochar

As of 2022, the atmospheric CO₂ level has reached an all-time peak of over 420 ppm. In addition to rapid decarbonization, several methods of CO₂ removal such as direct air capture (DAC), afforestation, ocean fertilization, and soil carbon sequestration are needed to mitigate the harmful effects of climate change and decrease the level of CO₂ in the atmosphere. Biochar, made from pyrolyzing biowaste, has been cited for its potential in carbon sequestration due to its resistance to microbial degradation. Biochar has also been considered as an adsorbent for DAC. Previous work has found that biochar had CO₂ adsorption capacities up to 73.55 mg CO₂/g biochar, compared to industrial DAC sorbents. This study investigated how pyrolysis conditions affect the CO₂ adsorption capacity of biochar. Sawdust, coconut shell, corncob and walnut shell feedstocks were tested to see if nitrogen flow rate during pyrolysis impacted adsorption capacity. It was found that for walnut shell, maintaining high flow rate was important to capacity; when flow rate was dropped below 150 mL/min, a decrease of up to 16% was measured whereas above this flow rate threshold, no significant increase in adsorption capacity was observed. Biochar pyrolyzed from sawdust and coconut feedstock did not show such a strong correlation between nitrogen flow rate and adsorption capacity, indicating that this dependence is feedstock specific. In addition to flow conditions, the impact of pre-drying was studied. Adsorption capacity of the pre-dried walnut was approximately 40% less than the non-dried batch for pyrolysis temperatures between 500-800°C. However, this impact was found to be feedstock dependent as well, as sawdust and coconut biochar resulted in smaller differences. Our results indicate that changes in process conditions can greatly impact the chemical and physical properties of biochar and thus CO₂ adsorption.