Characteristics Of Coal Structural Jump And Its Effect On Methane Adsorption And Diffusion

The methane adsorption and diffusion characteristics of coal are the core of coal mine gas geology research,and periodic change of the physical and chemical properties of coal in the process of coalification is determined by its structural jump.In view of coal structure jump and its effect on methane adsorption and diffusion,based on the two-phase flow theory of coal structure,Modern advanced coal structure analysis techniques,such as spectrogram analysis technology,solvent extraction and group composition chromatography separation technology and fluid injection technology,are adopted in this paper to investigate the macromolecular network structure microcrystal growth jump characteristics,small molecular group composition structure filling jump characteristics,micropore structure and methane adsorption and diffusion capacity jump characteristics of coal.The jump characteristics of coal structure and methane adsorption and diffusion properties in the process of coalification have been clarified,and a quantitative method for simulating and characterizing coal micropore structure using high-resolution transmission electron microscopy（HRTEM）phase analysis technique has been preliminarily established.It is revealed that the growth of macromolecular microcrystals and the filling of small molecular compounds restrict the jump characteristics of methane adsorption and diffusion properties of coal.The main research results are as follows:（1）HRTEM imaging technology characterized the structural jump characteristics of microcrystalline macromolecular network during coal evolution,obtaining the sensitive parameter identification of structural morphology and spatial arrangement of macromolecular microcrystals of coal at different coalification stages.HRTEM found that in the coalification stageⅠ(R_o,max=0.5%～1.5%),the macro morphologies of the aromatic fringes of the samples on the whole were small,curved and irregularly arranged;Raman spectra indicated the existence of a large number of amorphous structure defects;combined with the results of HRTEM and XRD determination,the abrupt decrease of the aromatic fringe spacing(d₀₀₂)in this coalification stage was marked as the jump characteristics,showing graphite-like microcrystalline rudiment in coal gradually formed.In the coalification stageⅡ～Ⅲ(R_o,max=1.5%～3.5%),the microcrystalline aromatic fringe in coal continued to extend and increase,the curvature decreased sharply,the directional arrangement area increased obviously,and the amorphous structure defects decreased greatly,in this stage of coalification,the sharp decrease of curvature of aromatic fringe and the sharp increase of length became the jump characteristics,indicating that the graphitic microcrystalline structure in coal was continuously oriented growing.In the coalification stageⅣ(R_o,max>3.5%),the macroscopic aromatic fringe morphology of the sample had a significant orientation trend,and adjacent aromatic streaks began to coalesse to form larger local molecular orientation domains（LMO）,and the plane defects diminished to disappear;at this stage of coalification,the jump characteristics are characterized by rapidly increase of the vertical height of LMO formed by stacking.（2）By means of solvent extraction and column chromatography separation,the jump characteristics of composition structure of small molecular compound families in organic matter during coal evolution were investigated.The solvent extraction rate jump curve of coal evolution showed the content of soluble small molecules in coal is significantly different,and it mainly occurred in filling state in the samples of coalification stage I～Ⅱ(R_o,max=0.5%～2.1%),showing a general stage change trend of first increasing and then decreasing with the fat coal as the jump point（turning point）;at the same time,column chromatography and gas chromatography-mass spectrometry（GC-MS）detection showed that the relative abundance of detectable small molecular phases in the aromatic distillate of fatty coal became significantly stronger and more diverse.The joint characterization results of FT-IR and Boehm titration were all characterized by the highest content of hydroxyl,carboxyl and lactone functional groups in fatty coal as the jump characteristic,which revealed that the rank of fatty coal was the turning point of small molecular structure evolution of coal.（3）The mechanism of the effect of small molecular structure and composition jump characteristics on methane adsorption and diffusion in mid-rank coal（in the evolution stage from fat coal to coking coal）was revealed.Aiming at the the problem of low gas adsorption capacity of mid-rank coal but high coal and gas outburst tendency,by comparing the change characteristics of micropore（<2 nm）structure of medium rank coal before and after extraction,it was found that the dissolution of small molecules had the effect of"pore augment"and"pore enlargement"on its micropores.Considering that the variation characteristics of micropore specific surface area of mid-rank coal after extraction were consistent with the variation characteristics of macromolecular void area constructed by adjacent aromatic fringes of coal obtained by quantitative simulation analysis of HRTEM image,it was suggested that the"filling effect"of the occurrence of small molecules will block the ultra-micro pores in coal.The change characteristics of the average dynamic diffusion coefficient（（?））of coal and methane with different degrees of coalification under the power function model were investigated by using the booster adsorption method.It was found that with the increase of pressure,the （?） of middle rank coal showed an exponential jump trend,believing that the occurrence of small and medium-sized molecules in coal had a"filling effect"under the pressure of 0.5MPa,which made it difficult for methane molecules to enter and exit freely from the pores interwoven by the small molecule side chain structure in the filling state,leading to the weak adsorption and diffusion capacity of methane;the increase of pressure would cause the rearrangement of the small molecule side chain structure in the original filling state,leading to the gradual dredge of methane migration channels（micropores）in coal,resulting in the average dynamic diffusion coefficient of methane in the middle rank coal continued to increase faster than that in other ranks,and making the middle rank coal seam rich in small molecules had a higher coal and gas outburst tendency.（4）The controlling mechanism of orientation growth characteristics of macromolecular microcrystalline structure on methane adsorption performance jump was elucidate.According to the phenomenon that the gas adsorption performance of super anthracite decreased significantly,the structural jump marks of macromolecular microcrystalline network in coal at high coalification stageⅣ(R_o,max>3.5%)were mainly influenced by the macromolecular oriented domain（LMO）stacked by the spatial arrangement and combination of adjacent parallel aromatic fringes,the void area of LMO was simulated and quantitatively characterized by using HRTEM image analysis technique.It was found that the LMO structure occupying a large amount of pore area in coal grew rapidly during the process of coal evolution to graphite at the high coalification stage,revealing that the increasing area of LMO formed by macromolecular microcrystalline structure of coal is the reason for the constantly decrease of the ultra-micro pores of coal in the high coalification stage,which indicated that the methane adsorption characteristics of superanthracite coal was controlled by the structural jump of LMO formed by the orientation growth of macromolecular microcrystalline structure of coal.（5）The small molecular filling effect of coal structure and the jump characteristics of microcrystal growth during coalification were identified.In view of coal chemical jump phenomenon,and based on the scientific thought of structure-deterministic nature,the control of jump characteristics,such as the morphology and arrangement of microcrystalline macromolecules,the filling effect of small molecules and the surface properties,on coal ultrastructure pore and methane adsorption and diffusion was investigated from the perspective of molecular structure evolution of coal.It was found that the occurrence（filling）state of small molecular compounds in organic matter is a jump characteristic of coal structure in the low coalification stage（In the evolution stage from fat coal to coking coal）.The coal structure at the high coalification stage(R_o,max>3.5%)is characterized by the orientation growth of graphitized microcrystalline structure of macromolecules,which reveals the effect of the filling and occurrence of small and medium molecules and the growth law of macromolecular microcrystalline structure on the control of methane adsorption and diffusion.It enriched the theory of coalification jump and provided theoretical support for the study of methane adsorption and diffusion.There are 81 pictures,33 tables and 212 references in this paper.