Combined experimental and grand canonical Monte Carlo simulation study of CO2 capture in nitrogen and sulfur co-doped biochar derived from biowaste: Cost analysis, kinetics, and equilibrium

Nitrogen (N) and sulfur (S) co-doped biochar was produced from an agricultural waste. Heat treatment was combined with the use of dopants including thiourea, CO2, and air. The results show that the modified N and S co-doped biochar with the use of CO2 gas agent at 1123 K for 2 hr in the modification process, had the highest CO2 adsorption capacity of 4.36 and 3.05 mmol/g at 273 and 298 K at 1 bar among others in literature even it had lower surface area or pore volume. This is due to the containing of N and S on the solid surface and superior pore size of 1.0 nm. Moreover, the modified biochar had superior kinetics and CO2/N-2 selectivity and was more economical than its unmodified biochar. To support experimental observation and better understand individual characteristics of functionalized carbons, a grand canonical Monte Carlo simulation was carried out to systematically illustrate the effect of N and S functional groups, pore size and pore volume on CO2 adsorption. The surface chemistry was the minor factor and played the key role in CO2 adsorption capacity at low pressures until it became saturated by adsorbate molecules at 0.3 bar. S was stronger than N for enhanced CO2 adsorption capacity. Whereas the pore size was the major factor in both low and moderate pressures (< 23 bar). The optimum pore size was found in a range of 0.66-4.0 nm which was pressure-dependent. The pore volume became the main factor at higher pressures (> 23 bar).