## 3 Results  
The pore structure of each group of coal samples was tested using mercury intrusion porosimetry, liquid nitrogen adsorption, and low-temperature carbon dioxide adsorption methods according to the aforementioned methods. The test results are shown in Figures 1-3. Figure 1: Mercury intrusion curves of samples from different coal ranks. (a) XJE, (b) PML, (c) HYS, (d) LXK. According to Figure 1, the mercury injection curves of the four coal samples have similar shapes, and the changes in the mercury injection curves exhibit three distinct conditions. When the mercury injection pressure reaches approximately 0.06 MPa, the amount of mercury injection increases sharply, and it first enters the macropores. When the pressure increases from 0.06 MPa to approximately 34.50 MPa, the curve changes relatively smoothly, and the coal continuously absorbs the kinetic energy of the injected mercury, whereas the liquid mercury further occupies relatively small pores. When the pressure exceeds 34.50 MPa, the curve changes significantly again, the pore volume is further compressed, and liquid mercury enters smaller capillary pores. The mercury injection and removal curves do not overlap, and hysteresis is prominent due to the large and complex pore volume in coal. During the mercury injection stage, high-pressure liquid mercury can enter the open, semi-open, and partially closed pores, whereas, during the mercury removal stage, the liquid mercury enters the open and semi-open pores, with partially closed pores retaining some mercury. Figure 1a shows the mercury injection curve with a larger opening, faster changes in the mercury removal stage, better pore development, and the best connectivity. Figure 2: Adsorption–desorption curve of liquid nitrogen method. (**a**) XJE, (**b**) PML, (**c**) HYS, (**d**) LXX. There are certain differences in the morphology of the adsorption and desorption curves among the groups in Figure 2, but all exhibit hysteresis. According to the IUPAC classification of adsorption hysteresis loop standard types [34], (a) the figure belongs to the H3 type, resembling sheet-like stacked narrow slit pores; (b), (c), and (d) belong to the H4 type; and the hysteresis loop exhibits no adsorption limitation in the region, with narrow slit-like pores and high relative pressure. From the analysis of maximum adsorption capacity, the relationship of the strength of the liquid nitrogen adsorption capacity of each sample is XJE\(\rightarrow\)LX\(\rightarrow\)PML\(\rightarrow\)HYS. HYS (lean coal) with the highest degree of metamorphism has the weakest liquid nitrogen adsorption capacity, whereas XJE (gas coal) with the lowest degree of metamorphism has the strongest. The relationship of the low-temperature carbon dioxide adsorption capacity of each sample in Figure 3 is XJE\(\leftarrow\)LXK\(\leftarrow\)PML\(\leftarrow\)HYS, which is diametrically opposite to that in Figure 2. HYS, with the highest degree of metamorphism, has the strongest low-temperature carbon dioxide adsorption capacity, whereas XJE, with the lowest degree of metamorphism, has the weakest. To facilitate quantitative analysis of pore size distribution, according to the classification standards of the International Union of Pure and Applied Chemistry [35], pores smaller than 2 nm are classified as micropores, pores between 2 nm and 50 nm are classified as mesopores, and pores larger than 50 nm are classified as macropores. The pore size  
Figure 3: Low-temperature carbon dioxide adsorption–desorption curve. (a) XJE, (b) PML, (c) HYS, (d) LXK. porous segment (accounting for 62.4060-67.9862%), followed by the mesoporous segment (27.4864-31.7487%), and the microporous segment is the least (3.2258-7.8195%). The pore volume curve distribution during the micropore (d \(\leq\) 2 nm) stage, as shown in Figure 4a, is relatively flat, with only two peaks appearing, with the maximum peak being 8.845 \(\times\) 10\({}^{-4}\) mL/g.