## 3 Results
### SEM Characterization of Pore Structure in Coal with Different Volatile Matters  
Scanning Electron Microscopy (SEM) possesses high-resolution imaging capabilities and an advantage in surface morphology characterization, making it a crucial method for studying the pore structure of coal. As a complex porous medium, coal's pore system, including matrix pores and fractures, has morphological, size, and connectivity characteristics that directly impact the occurrence and migration mechanisms of coalbed methane [27]. SEM can provide high-definition two-dimensional images of pores ranging from micrometers to nanometers, intuitively revealing the geometric features and spatial distribution patterns of pores. For example, [PERSON] et al. [43] observed a honeycomb-like organic matter pore network developed in medium-rank coal using field emission SEM, clarifying the correlation between pore morphology and the degree of thermal evolution. Compared with indirect characterization methods such as mercury injection and gas adsorption, SEM has the advantage of allowing targeted observation of specific microscopic areas and identifying pore genesis types (e.g., primary pores, secondary dissolution pores, etc.) [44]. However, SEM technology also has certain limitations: Firstly, traditional SEM images provide two-dimensional cross-sectional information, making it difficult to quantitatively characterize the three-dimensional topological structure of pores [45]. Secondly, during sample preparation, drying or coating processes may lead to microfracture expansion or pore collapse, introducing artificial artifacts. In this study, SEM was used to investigate the pore structure characteristics of coal samples with different volatile matters.  
SEM images of the research samples reveal that when V\({}_{\text{daf}}\) (volatile matter on a dry ash-free basis) is high, as shown in Figure 5A,B, the coal structure appears loose with distinct bedding, accompanied by a small number of irregular pores of larger diameters on the surface alongside the loose bedding structure. Conversely, when V\({}_{\text{daf}}\) is low, as depicted in Figure 5C,D, the SEM images transition from a loose bedding structure to a dense structure, with the layered structure disappearing and numerous dense, regular small pores emerging on the surface. This suggests a clear correlation between the pore structure of coal samples and V\({}_{\text{daf}}\). Specifically, when V\({}_{\text{daf}}\) is high, pores in the coal samples mainly appear as irregular macropores, independently and in small quantities. As V\({}_{\text{daf}}\) decreases, the pore state in coal is completely opposite to that when V\({}_{\text{daf}}\) is high, with pores tending towards smooth, regular shapes and appearing dense, resulting in a decrease in coal sample surface roughness. It can be observed that changes in V\({}_{\text{daf}}\) have a revolutionary impact on coal pore structure and surface morphology, which aligns with previous studies using AFM to investigate coal pore structure, confirming the existence of the correlation between V\({}_{\text{daf}}\) and coal pore structure identified through AFM experiments.