Multiscale Pore-Fracture evolution and damage mechanisms of Tar-Rich coal under microwave irradiation

Efficient in-situ pyrolysis of tar-rich coal depends on the adequate development of fracture-pore networks. The formation of escape channels and pyrolysis products during the thermal decomposition of tar-rich coal constitutes a dynamically coupled evolution process. Current research lacks real-time dynamic understanding of the pyrolytic damage evolution and its underlying mechanisms within this coupled process. Such insight is particularly critical for the microwave in-situ pyrolysis of tar-rich coal, especially given the distinctive heating characteristics of microwave irradiation — owing to its volumetric and selective heating properties. In this study, a self-developed microwave pyrolysis-acoustic emission (AE) monitoring system was employed to systematically investigate the fracture development dynamics and damage behavior of tar-rich coal under different irradiation powers (0.3 ∼ 0.9 kW). The results show that: (1) increasing microwave power significantly accelerates the heating rate and promotes fracture propagation along pre-existing weaknesses, eventually forming a connected fracture network; (2) RA/AF (Rise Angle/Average Frequency) analysis indicates that shear failure dominates during microwave pyrolysis, whereas the proportion of tensile events increases at higher power levels; (3) low power induces an initial decrease in porosity, while high power leads to a pronounced porosity increase that exhibits a negative correlation with the NMR signal of coal tar. Signals associated with moisture and hydrogen-rich organic components decrease with increasing microwave energy and are negatively correlated with the degree of fracture development; (4) unlike the progressive damage mode at low power, high microwave power generates macroscopic fractures at an early stage, which then expand rapidly driven by internal gas pressure—constituting the key mechanism for enhancing pyrolysis efficiency. This study elucidates the effects of microwave selective heating and volumetric heating processes on the structural evolution of coal, and establishes distinct damage modes for high- and low-power microwave pyrolysis, providing a theoretical basis for optimizing in-situ microwave extraction of tar-rich coal.