The synergistic influence mechanism of the alkali metal sodium and CO2 on coal rapid pyrolysis soot: Experiments and DFT calculations

Fossil fuels are wasteful, and they pollute the environment when utilized as energy sources. Soot from the incomplete combustion of fossil fuels has received considerable attention due to its harmful effects on the environment and the human body, and its utilization value in terms of materials. In this study, experiments on the pyrolysis of coal mixed with sodium carbonate (NaNO₃) at different carbon dioxide (CO₂) concentrations were conducted on a drop tube furnace. Experimental results were explained using density functional theory (DFT) calculation by constructing structural models of the oxidation of naphthalene with and without sodium (Na) involvement. The results showed that the average particle size of soot decreased from 29.7 nm to 21.44 nm with an increase in CO₂ concentration before the addition of Na. After the addition of Na, particle size exhibited a tendency of decreasing and then increasing, indicating that CO₂ exerted oxidizing and adducting effects on soot. In an atmosphere with a high concentration of CO₂, Na promoted the adducting effect, and particle size increased to 34.86 nm. Moreover, the addition of Na significantly increased oxygen content on the surface of soot, indicating that the participation of Na enhanced the oxidation reaction of soot. The results of the DFT calculations also proved that Na facilitated the occurrence of oxidation reaction by decreasing the energy barrier required for oxidation reaction from 9.28 kcal/mol to −64.62 kcal/mol through the transfer of electrons and the formation of active centers.