To investigate the prevention and control of explosions in gas-oil-coal coexisting mines involving a hybrid system of gas and oil-soaked coal dust, the lower explosion limit (LEL), maximum explosion pressure, and explosion index of the hybrid system were experimentally measured under different concentrations of oil-soaked coal dust and gas volume fractions. The regulatory effects of concentration on explosion characteristics were revealed. The results indicate that as the concentration of oil-soaked coal dust increases, the LEL first increases and then decreases, reaching its lowest value in the concentration range of 4% to 8%. The maximum explosion pressure and explosion index initially rise and then decline, remaining at relatively high levels within the concentration range of 2% to 6%. Increasing the gas volume fraction reduces the LEL and enhances explosion intensity, with the explosion risk being most prominent at a gas concentration of 3%. By analyzing the evolution patterns of gaseous and solid explosion products, the study elucidates the explosion coupling mechanism: the initial ignition of gas promotes the pyrolysis of coal dust, releasing volatiles and triggering intense gas-phase combustion, followed by coke participating in heterogeneous combustion until oxygen is depleted. The synergistic effect of gas and volatiles significantly lowers the minimum ignition energy of the system, and the positive feedback energy mechanism further accelerates flame propagation and pressure rise. This study reveals the coupling mechanism of oxidative dehydrocarbonation and free radical chain reactions in gas-oil-soaked coal dust hybrid explosions, providing a theoretical basis for mine explosion prevention and control.
In-situ pyrolysis is widely regarded as a promising low-carbon pathway for the utilization of tar-rich coal resources. Tar-rich coal is characterized by a high tar yield during thermal decomposition; however, its large-scale underground application is constrained by fundamental challenges associated with inefficient coal-seam heating, limited heat transfer within deep coal seams, high external energy demand, and difficulties in process control and subsurface safety management. In this study, existing research on underground in-situ coal conversion and pyrolysis technologies is systematically synthesized, with a focus on their heating mechanisms, energy supply modes, and intrinsic technical limitations. Particular attention is given to the physicochemical behavior of tar-rich coal during pyrolysis and the oxidation and spontaneous combustion characteristics of the resulting pyrolytic semi-coke. The analysis indicates that the exothermic oxidation of semi-coke can be conceptually integrated as an internal heat source, forming a coupled pyrolysis–oxidation process that enhances thermal efficiency while reducing dependence on external energy input. From a coal-seam-scale perspective, key coupling mechanisms involving heat transfer in coal-bearing porous media, combustion stability, and thermal regulation are critically examined. Based on these insights, a conceptual technical framework for controlled in-situ pyrolysis of tar-rich coal is developed, emphasizing the integration of internal heat generation, enhanced heat transfer, system sealing, and real-time subsurface monitoring. The framework further incorporates CO2 geological sequestration as a complementary pathway for emission mitigation. Overall, this conceptual framework provides a unified perspective for advancing efficient, safe, and low-carbon in-situ conversion technologies for tar-rich coal and related unconventional solid carbon resources.
Coal biogasification is a biotechnological process in which microorganisms convert coal into methane. The dynamic characteristics of this system represent a critical gap in current understanding. This study aims to elucidate the temporal evolution patterns of the gas-solid-liquid (including microbial components) triphase system throughout the entire process of coal biogasification, using a temporal sampling approach. The research further seeks to uncover the underlying coordinated response mechanisms and construct a descriptive model of coal biodegradation. Results indicate that coal biogasification is a temporally dynamic, triphase evolutionary process driven by microbial community succession. Based on the flux dynamics of the triphase products and the sequential metabolic transitions of microbial populations, the process can be categorized into three consecutive stages characterized by progressively decreasing intensity, with each stage following an "acidification-methanogenesis" degradation pathway. Microbial activity leads to the accumulation and subsequent consumption of intermediate metabolites, thereby inducing multi-stage methane generation. Concurrently, significant transformations occur in the physicochemical structure of coal: pore connectivity is enhanced through repeated structural rearrangements, while the branching degree of aliphatic chains, oxygen-containing functional groups, and aromatic structures undergo continuous degradation. The organic matter exhibits a dynamic pattern of alternating dissolution and consumption. Drawing upon similarities with the established model of crude oil, this study proposes a generalized model for coal biodegradation, offering a crucial theoretical framework for understanding the formation mechanisms of biogenic coalbed methane.
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 similar to 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.
Metal hybrid hydrogen storage technology, with high density and excellent safety, is promising for H2 storage, transportation, and new energy. However, during material processing, H2–metal powder mixtures form, posing greater explosion hazards than single-phase media. This study systematically examines how H2 concentration affects explosion pressure, flame propagation, and their coupled evolution in different H2/metal powder hybrid mixtures. For the first time, based on pressure variation characteristics, explosion pressure evolution is divided into four distinct concentration regimes. Experimental results show that the coupling between explosion pressure and flame propagation is stage-dependent: flame speed accelerates continuously with increasing H2 concentration. In Regime I, explosion pressure decreases because H2 acts as a physical diluent that reduces oxygen concentration and weakens the metal powder combustion. In Regime II, pressure changes vary by metal—magnesium systems show continuous rise, while others first rise then fall. In Regimes III and IV, trends become more consistent: in Regime III, pressure first decreases then rises due to oxygen competition and fuel synergy; in Regime IV, under O2-deficient, high-H2 conditions, pressure rises then drops, driven by synergy and metal hydrogenation. This study clarifies the coupled explosion mechanisms in H2/metal powder systems, providing theoretical guidance for industrial risk assessment and safety management.
