Impact of Pyrolysis Conditions on CO2 Adsorption of Biochar

As of 2022, the atmospheric CO₂ level has reached an all-time peak of over 420 ppm. As CO₂ levels continue to rise, increased action is needed to mitigate the harmful effects of climate change and decrease the level of CO₂ in the atmosphere. In addition to rapid decarbonization, several methods of CO₂ removal such as direct air capture (DAC), reforestation, ocean fertilization, and soil carbon sequestration are also needed. Biochar, made from pyrolyzing bio-waste, 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 that had adsorption capacities up to 66.02 mg CO₂/g sorbent. This study investigated how pyrolysis conditions affect the CO₂ adsorption capacity of biochar. Sawdust and walnut feedstocks were tested to see if nitrogen flow rate during pyrolysis impacted eventual CO₂ adsorption capacity. It was found that for walnut, maintaining high flow rate was important to capacity; when flow rate was dropped below 150 mL/min, a decrease of up to 16.24% was measured whereas above this flow rate threshold, no significant increase in adsorption capacity was observed. Biochar pyrolyzed from sawdust feedstock did not show 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. It was found that moisture in the feedstock led to the development of porosity during pyrolysis, as the adsorption capacity of the pre-dried batch was on average 40% less than the standard batch. Our results indicate that changes in pyrolysis conditions can greatly impact the chemical and physical properties of biochar and thus CO₂ adsorption.