Modifying Pyrolysis Conditions to Increase CO2 Adsorption of Biochar

Climate change is a pressing issue driven by excessive greenhouse emissions, particularly carbon dioxide. The atmospheric CO₂ level has significantly increased from pre-industrial levels of 280 ppm to a record high of 424 ppm in May 2023. While advancements in emissions reductions are the priority, carbon removal solutions such as direct air capture (DAC) should also be considered. Biochar, made from pyrolyzing biowaste, has been credited as a promising CO₂ adsorbent for DAC. The objective of this study is to examine if pyrolysis conditions impact biochar’s CO₂ adsorption. Walnut, sawdust, coconut husk, and corncob feedstocks were pyrolyzed at various temperatures to see if the pyrolysis temperature affects adsorption. As expected, increased pyrolysis temperature led to increased adsorption capacity. However, the extent of the increase depended on each feedstock, suggesting that the influence of temperature is feedstock-dependent. Pre-drying the feedstock before pyrolysis was also studied to determine if it impacted biochar performance. Pre-drying walnut resulted in 40% less CO₂ adsorption, and similar results were observed for all but the corncob feedstock where pre-drying was not shown to influence the resultant biochar’s capacity measurements. In addition to pre-drying, the effects of nitrogen flow rate during pyrolysis were tested to see if it influenced CO₂ adsorption capacity. Walnut had a threshold flow rate of 200mL/min, where, above this, there was no difference in capacity at higher flow rates but below it, a significant decrease of up to 24% was measured. Coconut followed a similar trend while there was no impact on sawdust or corncob. Not only does this show that higher nitrogen flow rates increase adsorption, but it also emphasizes the importance of mass transfer development during pyrolysis. These findings indicate that pyrolysis conditions, particularly pre-drying and nitrogen flow rate, significantly impact the adsorption of the subsequent biochar, and further, that these effects are feedstock dependent.