Coal is a complex porous medium with a highly anisotropic internal pore structure, shaped by factors, such as burial depth and formation processes. The pore size distribution within coal spans a wide range—from the nanometer to the micrometer scale. The pores in the coal matrix provide an extensive specific surface area, offering numerous adsorption sites for adsorbed gas. To simplify research problems, many scholars have proposed various classification schemes based on pore size, as shown in Table 2.1. Studies have demonstrated [135] that coal contains a large number of micropores and small pores, some mesopores, and a few macropores. Among these, micropores and small pores serve as the primary regions for gas adsorption, mesopores provide the main pathways for gas diffusion, and macropores function as the dominant channels for gas seepage. Together, micropores and mesopores form the principal storage space for gases in coal.