Energy-Efficient Biochar Activation in a Fluidized Bed Reactor Using CO2–Air Mixed Atmospheres

Biochar activation is critical for producing high-performance adsorbents; however, conventional activation methods are energy-intensive and difficult to control, particularly when air is used as an activating agent. This study investigates CO2–air co-activation in a laboratory-scale fluidized bed reactor as an energy-efficient alternative. Experiments were conducted at 750–850 °C under varying gas flow rates with a constant CO2/O2 ratio. Optimal properties were achieved at 800 °C and 0.2–0.3 L/min CO2, yielding a maximum BET surface area of 479 m2/g, a micropore contribution of 42%, and controlled carbon conversion (~25–35% yield). Aspen Plus equilibrium simulations also confirm that CO2-only activation remains endothermic (heat duty up to +0.07 kW), air-only activation becomes strongly exothermic (down to −0.13 kW), while the CO2–air mixture exhibits near-thermoneutral to mildly exothermic behavior (+0.13 to −0.10 kW), thereby reducing external energy demand potentially by approximately 60–70% compared with CO2-only activation and significantly improving process stability. These results demonstrate that CO2–air co-activation offers a practical route to produce high-quality activated biochar with controlled porosity and improved energy efficiency.