Pore Structure and Surface Free Energy Difference of Inertized Coal Due to N 2 and CO 2 Injection in Gobs

N2 and CO2 injection in gobs results in the inertion of coal. However, the difference in pore structure and the surface free energy (SFE) remains unquantified for the coal inertized caused by different gases (N2 and CO2). To this end, lean-oxygen inertion, pore structure analysis, and isotherm adsorption experiment were conducted successively. The pore structure analysis reveals that all the inertized coal samples were mesopore-dominated. However, the CO2-inertized coal samples’ pore size demonstrated an average larger by 1.1739 nm, and specific surface area an average smaller by 3.3721 m2 /g, compared to those of N2-inertized coal samples. Moreover, the N2-inertized coal samples usually maintained higher fractal dimension values (2.62–2.79) than those (2.55–2.62) of CO2-inertized samples. Furthermore, the reduction of SFE (Δγ) of inertized coal adsorbing O2 results indicate that the Δγ increased with rising inertion temperature and decreased with rising adsorption temperature. A most striking result was found. The CO2-inertized coal’s Δγ values were consistently greater after adsorbing O2 than those of N2-inertized coal. Although CO2 indicates superior inhibitory effects on spontaneous combustion compared to that of N2, CO2-inertized coal possessed a higher risk of re-ignition after the cessation of inert gas injection. These findings will provide a theoretical basis for understanding the spontaneous combustion risk of inertized coal.