## 4 Results and Discussion
### Methane Desorption  
Coal is a medium with a high specific surface area and serves as the primary storage site for gas adsorption. The more developed the pores, the greater the gas adsorption capacity. The adsorption and desorption gas in coal are in a dynamic equilibrium stage under specified circumstances, and the equilibrium state will be disrupted if any condition changes, such as the temperature [38]. A greater temperature enhances methane desorption and diffusion since the thermal motion of methane molecules is enhanced with temperature and as a result, energy is supplied for methane desorption, making the adsorption effect between the methane molecules and molecules at the coal surface weaker [39]. Based on the above, the collected lignite samples were exposed to Ar gas using four different temperatures, and the synthesis of the produced gas stream was analyzed in order to quantitatively evaluate the amount of CH\({}_{4}\) that could be extracted. Figure 6 shows the results from the methane desorption process at 25 \({}^{\circ}\)C, 50 \({}^{\circ}\)C, 100 \({}^{\circ}\)C, and 150 \({}^{\circ}\)C for the three tested samples. It was observed that the highest amount of CH\({}_{4}\) (in ppm of the produced gas stream) that was extracted from a 3 g sample was reached at desorption conditions of 50 \({}^{\circ}\)C and 1 atm. Adsorption between the CBM and lignite is a physical process; thus, the CBM needs to absorb energy to break away from the surface and to overcome the intermolecular gravitational force between molecules [38]. Therefore, more energy is given as the temperature rises from 25 to 50 \({}^{\circ}\)C.  
However, further temperature increases (from 50 to 100 \({}^{\circ}\)C and up to 150 \({}^{\circ}\)C) had negative effects on the amount of CH\({}_{4}\) produced. A high temperature negative effect on desorption has been previously reported in other research studies on coal samples, which have shown that CH\({}_{4}\) production from coal decreases with the increase in temperature under the same conditions of pressure. This could be attributed to changes in lignite layer permeability, due to the possible lignite matrix shrinkage occurring with temperature reductions, leading to increased gas desorption [40]. As it was mentioned in the Section 1, the lignite samples used are mainly composed of huminite maccrals (69.0-91.9%). It is also noteworthy that in such elastic adsorptive solids, like lignite coal, the desorption process could be affected by factors such as the specific surface area, pore size distribution, moisture content, coal type (rank, maceral composition, ash, and mineral matter content), etc. [41]. Moreover, swelling and shrinking, which change the original volume of the coal developing linear and volumetric strains, may affect the desorption process. [PERSON] et al. [42] reported \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Sample** & **South 53** & **South T5** & **South Submerged** \\ \hline Total moisture (\% by weight) & 55.5 & 55.7 & 46.7 \\ \hline Dry ash (\% by weight) & 23.2 & 23.5 & 23 \\ \hline Net Calorific Value (Kcal/kg) & 1350 & 1352 & 1351 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Results of chemical analysis of three southern field lignite samples (% by weight). that hysteresis of methane desorption first decreases and then increases with an increase in temperature, due to the decrease of moisture and volatile content which makes the pores more unblocked. The thermal expansion of the coal caused by heat treatment leads to a change in the pore structure (increase in pores and the narrowing of the pore throat) and diffusivity. Therefore, further investigation is needed to reveal the exact mechanism responsible for low desorption at elevated temperatures. The total amount of CH\({}_{4}\) desorbed during the whole desorption process at 50 \({}^{\circ}\)C was 6 ppm/g of lignite from the South T5, 4.2 ppm/g of lignite from the South Submerged, and 2.1 ppm/g of lignite from the South 53 (Figure 6b).