## 4 Results and Discussion
### Effect of Minerals on Methane Adsorption of Coal-Rock Models  
The adsorption of methane in the coal-rock compound model can also be fitted by the Langmuir equation [49]. To express the adsorption results clearly, a temperature of 273.15 K was selected to simulate the methane adsorption of compound coal-mineral models. Figure 4 shows the methane isotherm adsorption curves and Langmuir fitting parameter, a, in anthracite-rock and coking coal-rock compound models at 273 K. When the adsorption temperature is constant, the adsorption capacity of methane molecules in the coal-rock compound molecular structure model rapidly increases and then stabilizes with  
Figure 3: Adsorption heat of methane at different temperatures in anthracite and coking coal models. (**a**) Adsorption heat of methane in anthracite. (**b**) Adsorption heat of methane in coking coal.  
Figure 2: Methane adsorption isotherm of anthracite (**a**) and coking coal (**b**) at temperatures in the range of 273.15–313.15 k, and Langmuir fitting constant, a, of anthracite (**c**) and coking coal (**d**).  
increasing adsorption pressure, which still conforms to the Langmuir adsorption model. When the adsorption pressure is constant, the methane adsorption capacity increases with increasing mineral content. In the anthracite-rock molecular structure model, the saturated adsorption capacity of methane reaches a maximum of 39.82 wt% calcite, which is 24.82 N/uc. In the coking coal-rock molecular structure model, the methane adsorption reaches a maximum of 40.11 wt% calcite, which is 15.18 N/uc. A probe with a diameter equal to the methane molecular diameter (0.38 nm) was selected to detect and count the micropores of the two coal-rock molecular structure models. In the molecular structure model of anthracite-rock, when the mineral content is 39.82 wt% calcite, the micropores volume reaches 0.70 nm\({}^{3}\)/uc, which shows the large amount of methane adsorption when minerals are present. The results indicate that the highly developed micropores in clay minerals in coal are the main places for methane adsorption in coal, which is also reported by [PERSON] et al. [50]. As can be seen in our previous paper [37; 49] and the simulation results from [PERSON] et al. [29], the surface of clay minerals like kaolinite also has the capacity to adsorb some methane. Besides, [PERSON] et al. reported that, from the Scanning Electron Microscopy (SEM) micrographs of the same coal sample with different densities determined by the X-ray CT scan, the mesostructures of cell cavity pores with non-compact packing of the clay minerals appear to be the primary sites of methane adsorption in coal. Therefore, it is more generally accepted by researchers that the microporesity and surface structure parameters in clay minerals jointly affect the adsorption capacity.  
Figure 4: Methane adsorption isotherm of the anthracite–mineral model (**a**) and coking coal–mineral model (**b**) at a temperature of 273.15 k, and Langmuir fitting constant, a, of the anthracite–mineral model (**c**) and coking coal–mineral model (**d**).  
The isothermal adsorption heat of methane in the coal-rock compound model can also be calculated by the Clausius-Clapeyron equation [49; 51]. [PERSON] et al. used the methane adsorption heat to characterize the energy release information of methane in kaolinite and determined that the reduction in the adsorption heat under the condition of high water content means that the interaction energy between methane molecules and kaolinite molecules was weakened [49]. Figure 5 shows the isothermal adsorption thermal simulation results of methane in the anthracite-rock model and coking coal-rock model at 273.15 K. When calcite and kaolinite minerals are added, the methane adsorption heat of anthracite is 28.13-23.09 Kj/mol, and that of coking coal is 26.23-23.28 Kj/mol. With an increase in the mineral content, the methane adsorption heat of the two coal rock compounds decreases, and the contribution of calcite is greater than that of kaolinite.