Multi-Scale Structural Characteristics And Methanes Adsorption Response Mechanism Of High Inner Moisture Low-Rank Bituminous Coal

Coal seam gas is not only a major source of disaster in coal mine production,but also a strategic and clean resource with rich reserves.The comprehensive utilization of coal seam gas is a powerful support to achieve the carbon peaking and carbon neutrality goals.At present,there are more mature studies on the structural evolution and physical and chemical properties of middle and high rank coals,but the low-rank coals have obvious differences from them,such as high inner moisture content and many macropore structures,which lead to different fluid-solid interactions and gas resource endowment of low-rank coals from middle and high rank coals.In this thesis,based on the characteristics of high inner moisture,a multidisciplinary cross-fertilization research method,such as surface physical chemistry,spectroscopy,fractal geometry and molecular dynamics,was used to combine modern testing techniques,molecular simulation and theoretical analysis to construct a macromolecular structure model of low-rank bituminous coal,and to conduct a study on the fugitive form of high inner moisture in low-rank bituminous coal and the response mechanism of methane adsorption under its influence.The following conclusions were mainly obtained:（1）Quantitative characterization of low-rank bituminous coal multi-scale pore structure by segmental association using Mercury intrusion porosimetry,Low-temperature N₂ adsorption experiment and Low-pressure CO₂ adsorption experiment.The results show that the overall connectivity of pores is poor,which is easy to form the accumulation of free methane in coal seams.The three-dimensional microstructure of low-rank bituminous coal including minerals,inert mass group,mirror mass group,and fractures was reconstructed by combining the observations of low rank bituminous coal from multidimensional by environmental scanning electron microscopy and computed tomography,in which the fractures were mainly distributed around the minerals and generated by the contraction effect and stress release of minerals.（2）Based on modern testing techniques of microscopic spectroscopy,the molecular structures of the 2 low-rank bituminous coals were characterized,and the results showed that the higher content of oxygen-containing functional groups and hydroxyl groups is the fundamental reason for their high inner moisture;the aromatic carbon content follows the dominance,but the higher content of aliphatic carbon chains makes the aromatic layer arrangement loose and disorderly,which has a large gap with graphite.Quantitative analysis by ¹³C solid-state NMR test yielded peri-bridge carbon ratios of 0.197 and 0.25 for ZX and GJH coals,respectively,and then combined with elemental analysis tests to tentatively determine the aromatic structure,fatty structure and heteroatomic morphology of the 2 low-rank bituminous coals.The"sideband effect"of the carbonyl carbon peak was eliminated by the quantitative analysis of the oxygen-containing functional group region in the diffuse reflectance infrared spectroscopy test.The modified molecular formulae of ZX coal and GJH coal were finally obtained as C₁₀₅H₉₅N₁O₁₃ and C₁₄₇H₁₃₅N₁O₁₇,respectively,and the basic structural unit model of the low rank bituminous coal was drawn accordingly,and the optimized model density was basically consistent with the data of Mercury intrusion porosimetry.（3）The coal samples maintained under multi-level humidity environment were measured by low-field NMR,and the results showed that:the T₂ spectra of the multi-level inner moisture samples of the 2 low-rank bituminous coals had a double-peak structure,and most of them were in the main peak of 0.05-3ms;the adsorbed state moisture increased with the increase of environmental humidity,and at atmospheric pressure,the gas-phase moisture could only be endowed in mesopores and below,and the liquid-phase moisture would actively or passively migrate to larger pores.For low-rank bituminous coal in Yonglong mining area,at low RH,moisture molecules are adsorbed in micropores in the form of filling;at medium RH,moisture molecules form adsorption in larger micropores in the form of multilayer adsorption and moisture molecule clusters,and gradually start to produce capillary coagulation phenomenon;at high RH,moisture after capillary coagulation and partial liquefaction migrates to mesopores and above or even hydrophobic pores.This migration of liquid-phase moisture from the inside out does not form pore blocking or moisture-lock effect as in the case of applied moisture.（4）Methane isothermal adsorption tests were carried out on multi-level inner moisture coal samples,and the results showed that the increase of inner moisture of low-rank bituminous coal effectively reduces the methane adsorption in coal,showing a nonlinear decay.As for the methane adsorption rate,since the equilibrium moisture-based preparation of low-rank bituminous coal does not produce a moisture-lock effect,it is considered that there will not be a significant regular effect.After combining the isothermal adsorption simulation and pore fracture characterization results of the macromolecular model of low-rank bituminous coal,it is concluded that the methane adsorption form in low-rank bituminous coal with high internal moisture is dominated by micro-pore filling below 1.5 nm,supplemented by monolayer adsorption,and the 2adsorption forms are synchronized.In the case of atmospheric pressure adsorption of inner moisture,it is mainly the occupation of adsorption sites in micropores that causes the change in methane adsorption,and the effect of inner moisture on methane monolayer adsorption is minimal,while the reason that methane always has adsorption sites in micropores is presumed to be due to the inside-out migration behavior of inner moisture after reaching the capillary coalescence stage,and this migration behavior continues or even intensifies with the increase of gas pressure,providing methane this migration behavior continues or even intensifies as the gas pressure increases,providing more micropores for methane molecules to fill the adsorption sites until equilibrium.This thesis contains 77 figures,27 tables,and 164 references.