Objectives and Methods  The CO₂ adsorption phase density is a key factor determining the CO₂ adsorption sequestration capacity in coal seams. To deeply understand the characteristics of CO₂ density distribution in pores of the coal seam, taking the No.13 coal from Liuzhuang Mine of Huainan mining area and the No.7 coal in Huaibei mining area as the research objects, the CO₂ adsorption phase density of at different pressures and temperatures with different dominant pore diameters was simulated through the simplified local density theory (SLD). The CO₂ adsorption phase densities were obtained by different methods, the variation characteristics of CO₂ adsorption phase density in coal were analyzed, the variation characteristics of the CO₂ adsorption phase density in coal and the relationship between the phase state distribution of CO₂ in coal and pressure, temperature and pore diameter were discovered, to reveal the contrfol mechanism of CO₂ adsorption phase density and point out the further research direction of SLD simulation of CO₂ adsorption phase density in coal seams.

Results and Conclusions The results show that: (1) At the same temperature, the density of CO₂ in coal pores of the same size increases with pressure. When the pore diameter is ≤2.0 nm, the relationship between CO₂ density at the pore center and pressure follows Langmuir characteristics, and CO₂ exists mainly in the adsorbed phase; for pores>2.0 nm, Langmuir fitting does not converge, and free-phase CO₂ appears only when the pore center pressure reaches the supercritical level. (2) When the pressure is <8 MPa, the density of CO₂ in coal pores decreases as temperature rises; when the pressure is≥8 MPa and the pore diameter exceeds 3.0 nm, the CO₂ density below the critical temperature becomes lower than that above it, and this trend becomes more pronounced with increasing pore size. (3) At a pore diameter of 3.0 nm, free-phase CO₂ appears at the pore center, while at>4.0 nm the center is entirely dominated by the free phase. (4) The essence of the temperature–pressure–pore size synergy in controlling CO₂ adsorbed-phase density lies in regulating the thickness of the adsorbed layer, thereby affecting its density. (5) This study reveals the synergistic mechanism of temperature, pressure, and pore size on CO₂ adsorbed-phase density in coal, provides a theoretical basis for understanding effective CO₂ sequestration, and indicates that future SLD simulations should further consider the effects of heterogeneous pore structures.