###### Abstract
The maximum adsorption of coal improves with the increase of the total pore volume and total specific surface area, and the gas adsorption capacity of coal also increases gradually. [PERSON]'s research [6] showed that the higher the contents of vitrines are, the larger the adsorption capacity will be. [PERSON] [7] investigated the relationship between the adsorption capacity, coalification degree, and coal rock composition. Their research showed that there is a correlation between carbon content and adsorption. [PERSON] [8], a scientist from Australia, investigated the adsorption characteristics of coal from the Bowen Basin, Australia under different pressures. Their research showed that the higher the carbon content in the coal, the greater the adsorption capacity will be when the equilibrium pressure reaches 5 MPa. [PERSON] [9] found that there is no obvious correlation between adsorption and coal rank. [PERSON] [10] conducted an experiment studying the adsorption process on different ranks of coal. The rank was found to be positively correlated with adsorption capacity, but this rule is the same as for the transition process of coalification. Coalification is correlated with the porosity, as well as the physical and chemical properties of the material surface, and it has an influence on the occurrence space of coal bed methane and coal's affinity for methane. [PERSON] [11] found that the adsorption amounts of CH\({}_{4}\), N\({}_{2}\), and CO\({}_{2}\) increased with the rank of coal under the same conditions. Coal, a complex porous media with a strong adsorption capacity, is a natural adsorbent. The size of the pores in coal can vary. [PERSON] [12] found that the gas around coal forms a solid solution by the means of infiltration, replacement, and filling, using ray diffraction and electron microscopy techniques. The gas enters the micro pores of coal rock by adsorption [13]. [PERSON] [14], [PERSON] [15], [PERSON] [16], and [PERSON] [17] investigated the distribution of pore structures of coal and obtained the same results as those presented in Reference [12] by gas isothermal adsorption. [PERSON] [18] and [PERSON] [19] studied the pore characterization of different types of coal from coal. They found the main factor determined by the methane adsorption capacity is the micro pore structure. Through experiments, [PERSON] [7] proved that pore structure in coal determines the adsorption capacity of coal for CH\({}_{4}\). The more complex the pore structure, the larger the pore volume and specific surface area, and the stronger the adsorption capacity will be. Further studies in 2004 showed that the gas adsorption of coal is positively correlated with its pore surface area and micro pore surface area. The micro structure of coal includes the physical structure and chemical structure. The physical structure refers to the interrelationship and mode of action in the molecular structure of organic matter, including the crystal structure and the pore structure between molecules [20]. Common methods used to investigate the physical structure include the mercury injection method [21; 22], the gas adsorption-desorption method [23; 24], XRD (X-ray diffraction) [25], SEM (scanning electron microscopy) [26], HRTEM (high-resolution transmission electron microscopy) [27], SAXS (small-angle X-ray scattering) [28], X-CT (X-ray computed tomography) [29], and NMR (nuclear magnetic resonance) [30]. Among them, the mercury injection method, the gas adsorption-desorption method,SEM, and SAXS can obtain information about pore structure, so the gas adsorption-desorption method was selected to measure the specific surface area of coal in this study. The coal adsorption capacity is determined by its pore specific surface area, and it also affects the gas emission during mining. [PERSON] (1986) held that a coal reservoir is a double porosity structure composed of pore and fracture elements. [PERSON] (1988) thought that there are transitional types of pore and fracture that occur between the regular pores and fractures.