Carbon dioxide capture in biochar produced from pine sawdust and paper mill sludge: Effect of porous structure and surface chemistry

The CO₂ concentration in the atmosphere is increasing and threatening the earth's climate. Selective CO₂ capture at large point sources will help to reduce the CO₂ emissions to the atmosphere. Biochar with microporous structure could be a potential material to capture CO₂. The impact of feedstock type, pyrolysis temperature and steam activation of biochars were evaluated for CO₂ adsorption capacity. Pine sawdust biochars were produced at 550 °C, and steam activated for 45 min at the same temperature after completing the pyrolysis (PS550 and PSS550). Paper mill sludge biochars were produced at 300 and 600 °C (PMS300 and PMS600). The CO₂ adsorption capacity of biochars was tested at 25 °C using a volumetric sorption analyzer. Pine sawdust biochars showed significantly higher CO₂ adsorption capacity than paper mill sludge biochars due to high surface area and microporosity. Pine sawdust biochars were then evaluated for dynamic adsorption under representative post-combustion flue gas concentration conditions (15% CO₂, 85% N₂) using a breakthrough rig. Both materials showed selective CO₂ uptake over N₂ which is the major component along with CO₂ in flue gas. PSS550 had slightly higher CO₂ adsorption capacity (0.73 mmol g^^-1 vs 0.67 mmol g^^-1) and CO₂ over N₂ selectivity (26 vs 18) than PS550 possibly due to increase of microporosity, surface area, and oxygen containing basic functional groups through steam activation. Pine sawdust biochar is an environmentally friendly and low-cost material to capture CO₂.