## 3 Results and Discussion  
### Strength Characteristics of Coal Samples  
The inherent anisotropy and porous microstructure of coal necessitate stringent experimental protocols to take its structural heterogeneity into account. In this study, uniaxial compression tests were conducted on five sets of briquette coal specimens prepared under varying forming pressures and four sets of raw coal specimens to determine statistically robust uniaxial compressive strength (UCS) values. This approach mitigates the influence of intrinsic material variability, ensuring reproducibility across heterogeneous coal matrices. The results were shown in Figures 2 and 3. As illustrated in Figure 2a, the UCS of raw coal samples displayed significant variability, a direct consequence of their well-developed pore-fracture networks and abundant structural discontinuities (e.g., weak planes). The experimental results, summarized in Table 3, reported average values of UCS (5.63 MPa), elastic modulus (1.014 GPa), and Poisson's ratio (0.310) for the raw coal specimens. Acoustic emission (AE) monitoring could be employed to elucidate the instability, fracture propagation, and damage evolution mechanisms within coal-rock materials by tracking variations in AE event counts and energy release [58]. Figure 2b presents the AE characteristics of the RS-4 coal specimen under uniaxial compression. During the initial compaction and elastic deformation stages, the AE event count remained low, reflecting elastic energy accumulation within the coal matrix under low stress conditions. This phase was characterized by limited microcrack nucleation and localized propagation. As the specimen transitioned into the yield stage with increasing stress, the AE event count rose markedly, signaling crack propagation and internal damage--a precursor to structural instability. At macroscopic failure, both instantaneous and cumulative AE event counts surged abruptly, corresponding to the formation of an interconnected fracture network that established a continuous weak plane, along which the specimen ultimately failed. The sporadic occurrence of high-energy AE signals post-yield indicated that coal failure is not a continuous process but rather a progressive sequence involving microcrack nucleation, propagation, coalescence into macroracks, and eventual structural collapse. In contrast to raw coal, the briquette samples exhibited enhanced homogeneity due to the cementation of coal particles under forming pressure. Consequently, as shown in Figure 3, no distinct compaction stage was observed during uniaxial compression (Figure 3a). Acoustic emission (AE) monitoring revealed that the AE event count remained at a low level (Figure 3b), primarily originating from interparticle slippage and rearrangement within the briquette matrix. These signals were notably weaker than those generated by microfracture propagation in raw coal. The stable AE event count suggested uniform particle packing and consistent intergranular motion probabilities under axial loading. The cumulative AE event count exhibited a stepwise increase during compression, indicating significant static friction between coal particles. During static friction resistance phases, limited slippage occurred,  
Figure 3: UCS characteristics of briquette coal samples. (**a**) UCS of briquette coal samples with different sample forming pressure; (**b**) real-time acoustic emission characteristics of briquette coal samples under uniaxial compression. Figure 2: UCS characteristics of raw coal samples. (**a**) UCS of raw coal samples; (**b**) real-time acoustic emission characteristics of raw coal samples under uniaxial compression.