Dynamic Dehydration Characteristics of Macerals in Lignite During Drying and Their Effects on Pore–Fracture Evolution and Physico-Mechanical Properties

Understanding the changes in physical and mechanical properties of lignite during dehydration is crucial for its sustainability in coal mining, exploitation of coalbed methane, and carbon dioxide sequestration. Through SEM and Computed Tomography (CT) scanning, combined with fractal theory, this study investigates dynamic dehydration characteristics of macerals in lignite during normal temperature drying (NTD), and their effects on pore–fracture development and physic–mechanical property evolution. The results show that the hard layers of lignite are mainly composed of ulminite (Ul), while the soft layers are primarily composed of fusinite (Fu), densinite (De), and Ul. Ul exhibits low dehydration efficiency but is prone to shrinkage and cracking heavily, whereas Fu has high dehydration efficiency and excellent thermal stability. The layered enrichment of macerals controls the development of the three-dimensional (3D) pore–fracture structures of lignite during NTD and leads to distinct cracking characteristics of fracture structures between hard and soft layers. Unlike soft layers, hard layers tend to form long, straight fracture structures with large apertures and exhibit extremely high fracture connectivity and fractal dimension (FD). In addition, the differential drying behavior of macerals causes the physical parameters of lignite such as moisture ratio (MR), drying rate (DR), and density (ρ) to show a dynamic evolution characteristic of “initial rapid decline (or increase) in the early stage–subsequent gradual decline (or increase) and stabilization in the later stage” during NTD. The unique pore–fracture structure controlled by macerals significantly alters the deformation resistance and failure mode of dehydrated lignite under uniaxial compression but has limited effect on its uniaxial compressive strength.