4.1.2. Univariate Analyses

The univariate analysis was conducted using the constructed GBDT model to further demonstrate how the adsorption isotherms are a ffected by the input features. The base value was set to be 20%, 1.0%, 88%, 80%, 2.5%, 15%, and 35°C for ash, inherent moisture, fixed carbon, vitrinite, vitrinite reflectance, equilibrium moisture, and temperature, respectively. These values were set as approximately the averaged ones that are shown in Table 1. Each input variable was tuned at four values (that are within the range of all the coal samples in this study) and the corresponding adsorption isotherms (at pressures of 1 to 8 MPa with a step of 1 Mpa) were sequentially calculated, which are shown in Figure 12.

## • Fixed carbon

Figure 12a depicts the adsorption isotherm with reference to varying fixed carbon. It is well demonstrated that the isotherm tends to move upwards as fixed carbon increases. Previous studies [16,56] observed that the methane adsorption capacity follows a first decreasing and then increasing trend with increasing fixed carbon, with the minimum occurring at ≈60–80%. This parabolic trend may be attributed to the variations in the micro-pore surface areas that are associated with the coalification jump that occurs approximately in the range of 75–85% fixed carbon [17]. More recently, Chattaraj et al. [1] showed that, for Indian coals with fixed carbon content of >75%, methane adsorption capacity is in positive linear correlation with fixed carbon. It is interesting to note that the coal samples used in this study have a generally high fix carbon contents of >77%, which suggests that our findings are in line with these previous studies.
