Impact of water contents on the chemical composition and structure of coals under supercritical CO₂ (ScCO₂)

In the process of CO₂ sequestration in deep coal seams, CO₂ usually exists in a supercritical state (ScCO₂) and undergoes complex physicochemical reactions with water and coal, resulting in changes of coal chemical composition and structure and thus affecting the CO₂ sequestration efficiency. To study the influence laws of ScCO₂ on coal chemical composition and structures under different water contents, ScCO₂-coal reaction experiments were conducted using coking coal samples. Then, the differences in coal composition and structure before and after the reaction were discussed. Finally, the influencing mechanism of water contents on the ScCO₂-coal reaction was revealed. The results show that: ScCO₂ dissolves carbonate minerals in coal more effectively than clay minerals, and the mass fraction of carbonate minerals in the residual coal increases and then decreases with the increase of water content; at higher water content, the reaction between ScCO₂ and carbonate minerals in coal is stronger than that of clay minerals, thus causing a more significant depletion of carbonate minerals. ScCO₂ and H₂O exert a significant influence on the oxygen-containing functional groups, but they give a weak impact on aromatic and aliphatic hydrocarbons; specifically, ScCO₂ reduces the content of oxygen-containing functional groups in coal. Furthermore, as the water content increases, the oxygen-rich index (I₂) increases significantly, while the hydrogen-rich index (I₁), aromaticity (I₃), and condensation degree (I₄) exhibit minimal variations. Under the swelling effect of ScCO₂, the aromatic layer spacing (d₀₀₂) shows an increase characteristic, while the ductility (L_a), the number of aromatic layers (N_c), aromatic carbon rate (f_a'), and aliphatic carbon rate (f_al) decrease. Furthermore, with increasing water content, d₀₀₂, L_a, and f_a' firstly increase and then decrease, while L_c, N_c and f_al exhibit an opposite pattern. Therefore, water can enhance the ScCO₂ extraction efficiency of low molecular weight compounds from coal, promoting the disordering of aromatic structure in coal. However, as the water content further increases, it will impede the permeation of CO₂ into the coal matrix. The carbonate minerals in the coal react with ScCO₂ preferentially, thereby weakening the extraction effect of ScCO₂ on the organic matter in coal. Consequently, ScCO₂ has more apparent effect on the dissolution of organic matter and the destruction of chemical structure in coal on air-dried basis, and more significant dissolution of minerals in coal on saturated basis.