Nitrogen-doped activated carbons derived from microalgae pyrolysis by-products by microwave/KOH activation for CO2 adsorption

Abstract

Two types of nitrogen-doped activated carbons derived from microalgae pyrolysis by-products by microwave/KOH activation (thermal/KOH activation as comparison) were synthesized to separate CO2 from flue gas. The nitrogen-doped activated carbons were characterized, and CO2 adsorption performance, kinetics and thermodynamics and regeneration ability on nitrogen-doped activated carbons were studied. Results display that microwave/KOH activation contributes to developed pore structure and abundant active functional groups, resulting in high CO2 adsorption capacities. CO2 adsorption processes over nitrogen-doped activated carbons (C-NMK(2)/chlorella and S-NMK(2)/spirulina) are controlled via external mass transfer and are mainly physisorption. CO2 adsorption capacities of nitrogen-doped activated carbons by microwave/KOH activation are obviously better than thermal/KOH activation. The maximal CO2 adsorption capacities of C-NK(2) and S-NK(2) by thermal/KOH activation are 3.44 mmol/g and 3.09 mmol/g (25 °C), respectively, while the maximal CO2 adsorption capacities of C-NMK(2) and S-NMK(2) by microwave/KOH activation reach 4.21 mmol/g and 3.57 mmol/g (25 °C), respectively. The improvement of CO2 adsorption capacity (microwave vs. thermal) is attributed to more developed pore structure. CO2 adsorption capacities of C-NMK(2) and S-NMK(2) only reduce slightly (less than 10 %) after ten regeneration tests, showing excellent cyclic stability. The results offer beneficial reference for the application of low-cost microalgae nitrogen-doped activated carbons for CO2 separation, and effectively realize the utilization of microalgae pyrolysis by-products.