Porous biochars derived from microalgae pyrolysis for CO₂ adsorption

In order to slow down the greenhouse warming caused by excessive CO₂ emissions and effectively exploit the byproducts produced during the biofuel production process, N-doped porous biochars derived from the byproducts of microalgae (chlorella and spirulina) pyrolysis by combining urea and KOH modification were synthesized in this article to remove CO₂ in simulated flue gas. The physicochemical properties of the microalgae porous biochars were investigated via characterization tools, involving pH, Brunauer−Emmett−Teller (BET), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR). CO₂ adsorption kinetics, thermodynamics, and CO₂ adsorption performance were also studied. Results reveal that the functional groups and specific surface areas of microalgae biochars are substantially increased via urea and KOH modification, and nitrogen-containing functional groups (mainly involving N−H, C−N, etc.) are major adsorption sites for CO₂ adsorption. The key control step for CO₂ adsorption is external mass transfer, and the CO₂ adsorption process over the microalgae porous biochars is mainly physical adsorption. Moreover, the chlorella-based porous biochars CNK-2 and spirulina-based porous biochars SNK-2 have optimal CO₂ removal performances, and their maximum adsorption capacities, respectively, reach 3.44 and 3.09 mmol/g at 25 °C. The results of regeneration studies demonstrate that, after 10 regeneration experiments, CNK-2 and SNK-2 still possess high CO₂ sorption performances (reaching 3.09 and 2.78 mmol/g, respectively), exhibiting good regeneration potential.