## 3 Results and Discussion  
### Analysis of Microscopic Morphology Characteristics of Coal Samples  
SEM and EDS Results Analysis  
Bulleted lists look like this:  
Coal samples under four conditions--initial, post-humidification, post-heating, and after the coupled hygrothermal treatment--were observed using a Zeiss G300 scanning electron microscope (SEM). All sample surfaces were initially at the same baseline condition before their exposure to the respective environments. As shown in Figure 5, under the initial conditions, numerous debris-like particles are observed on the surface of the coal sample, possibly due to natural cracks within the coal matrix or the peeling of weakly bonded mineral surfaces, leading to a certain degree of surface roughness. The overall structure appears relatively dense, indicating that the microstructure of the coal body is largely intact before the exposure to hygrothermal environments. Although minor undulations, small pores, and a limited number of microcracks are present in localized regions, these defects are distributed in isolation, suggesting that the structural integrity of the coal body remains high in the initial state, reflecting a low level of structural damage. As illustrated in Figure 6, humidified conditions led to discernible changes in the coal's surface microstructure as a result of elevated moisture levels. Compared to the initial state, particle edges appeared blurred, likely resulting from water infiltration, which softened the intergranular boundaries. This blurring effect suggests a weakened interparticle cohesion and slight swelling of mineral phases. Additionally, microcracks developed along structurally weakened zones, while dissolution pits were observed in specific regions due to water-induced erosion and mineral leaching. Furthermore, the moisture exposure resulted in a partial disintegration of clay minerals, leading to localized pore expansion and more gradual transitions between particles and the surrounding matrix, collectively indicating a degradation of microstructural integrity. Figure 4: Measurement points layout. Figure 5: Initial state **(a)** Surface magnification 200\(\times\); **(b)** Surface magnification 1000\(\times\). As illustrated in Figure 7, compared to the humidification treatment, heating markedly decreases the inherent moisture content of the coal sample via a dehydration process that gradually advances from the exterior to the interior, leading to a notable increase in the localized surface roughness. Certain particle surfaces exhibited signs of exfoliation and spalling, attributable to the thermal stress-induced weakening of interparticle bonds and microstructural disruption. Moreover, prominent fragmented structures were observed in localized areas, suggesting the partial disintegration and collapse of weaker zones within the coal matrix. The microcrack density notably increased under heating, with multiple cracks propagating deeply within the sample, indicating the progressive evolution of thermally induced damage and mechanical degradation. As illustrated in Figure 8, the combined effects of humidification and heating resulted in pronounced alterations in the surface morphology of the coal samples, which differed substantially from those induced by humidification or heating alone. The surface roughness increased compared to the humidified condition, likely due to thermal stresses intensifying the weakening effects caused by moisture, but remained less pronounced than the roughness observed under heating alone. Following the coupled hygrothermal treatment, a significant increase in fine dissolution pits was observed on the coal surface, indicating enhanced mineral leaching driven by the synergistic action of the moisture and heat. This was accompanied by the continued development and eventual coalescence of microcracks, reflecting the progressive deterioration and cumulative damage of the microstructure under the coupled hygrothermal environment. Figure 6: After humidification.