| The occurrence conditions of coal seams in China are complex,especially when the coal production reaches to deep areas,the possibility of coal and gas outburst accidents will increase.In recent years,typical outburst accidents in China show that outburst accidents mostly occur near small-scale structural areas,and the coal bodies are mostly soft and broken tectonic coal.The structure of tectonic coal is different from intact coal,the physical structure and chemical structure of have been fundamentally changed,greatly improving the gas adsorption and desorption capacity.In this paper,coal mechanics,adsorption science,surface physical chemistry,diffusion dynamics,molecular dynamics and other theoretical knowledge were mainly used.Besides,a three-dimensional macromolecular structure model for simulation analysis of methane adsorption by coal was constructed by means of theoretical analysis,laboratory test and molecular simulation.Based on the pore structure parameters,the pore structure complexity model is established to analyze the evolution characteristics of gas desorption characteristics with the complexity of pore structure,and reveal the influence of structural function on the physical and chemical structure of coal.Finally,the necessity of the existence of pulverized tectonic coal in the process of outburst is clarified.The main conclusions are as follows:1)The tectonism will change the macromolecular structure of coal,and then affect the hardness of coal.Based on the analytical chemistry method of micro spectroscopy,the fat structure,aromatic structure and microcrystalline structure parameters of coal samples were analyzed.The results showed that the aromatic carbon ratio of tectonic coal increased by 3.5%?9.7%,the fatty carbon content decreased by 11.3%?17.0%,and the condensation degree of aromatic ring increased by 1.57 times.All of these indicate that the tectonism can promote the shedding of the side chain of fat,and increase the ability of dehydrogenation of non aromatic compounds to aromatic compounds,the content of aromatic carbon and the degree of aromatic ring polycondensation.In the microcrystalline structure of tectonic coal,the distance between one-sided network of aromatic layers decreases and the degree of stacking increases,which indicates that tectonism promotes the orderly development of aromatic structure and the increase of aromatization and condensation degree.The decrease of oxygen atom and oxygen bridge and the shortened side chain of fat in tectonic coal will weaken the binding force and cross coupling force between molecules and reduce the hardness of tectonic coal.The comprehensive analysis shows that tectonism can promote the formation of small-sized aromatic rings in some fat structures through aromatization.Some small aromatic rings form benzene ring and naphthalene ring through aromatizing.Naphthalene ring and 2×2 aromatic ring form 3×3 aromatic ring through polycondensation,which improves the aromatization and polycondensation degree of tectonic coal and affects the macromolecular structure of coal.2)After the complex geological structure,the pore structure and pore fracture morphology of tectonic coal have changed significantly.The surface of tectonic coal is rough and uneven,the fracture combination is complicated,and the directionality is poor.The mesopore specific surface area and volume of tectonic coal are 0.99?12.64times and 0.89?15.42 times of that of intact coal,respectively.The macropore specific surface area and volume of tectonic coal are 1.07?6.05 times and 1.19?4.69 times of that of intact coal,respectively.The micropore specific surface area and volume of tectonic coal are 094?2.88 times and 0.86?2.58 times of that of intact coal,indicating that tectonism will promote the development of full-scale pore structure.The pores in the micropores are mainly 0.45?0.65 nm,accounting for 53.61%and 52.6%of the total micropore specific surface area and volume,respectively.Besides,the pulverization process will also promote the development of pore structure,but amount of secondary pores produced is smaller than that of intact coal,indicating that tectonism has already produced a higher degree of change in the pore structure of tectonic coal.The developed pore structure of tectonic coal and small particle size coal make it have higher gas adsorption and flow capacity.3)The tectonism will affect the adsorption characteristics by changing the pore structure of coal.The limit gas adsorption volume of tectonic coal increased by7.6%?41.8%compared with that of intact coal.With the decrease of particle size,the maximum limit gas adsorption volume of tectonic coal and intact coal is 1.03?1.21times and 1.11?1.37 times of the minimum,respectively.The adsorption capacity is determined by micropore volume and specific surface area.Thus,the developed microporous structure of tectonic coal and small particle size coal is main reason for high adsorption capacity.The space fractal dimension,Hausdorff dimension and singularity index of tectonic coal is smaller in comparison to intact coal.On the contrary,the information dimension and correlation dimension is larger than that of intact coal,which indicate that tectonic coal has simple pore network,dense and uniform pore size distribution characteristics.As a result,tectonic coal has more limit gas adsorption volume compared with other types of coal.4)The molecular simulation characteristics of methane adsorption by coal before and after tectonism are discussed.The three-dimensional macromolecular structure model was constructed under periodic boundary condition.The measured micropore volume of tectonic coal(0.03898 cc/g)is higher than that of intact coal(0.03318 cc/g),which indicates that tectonism promoted the transformation of some unmeasurable pores into measurable ones.The simulated limit gas adsorption volume of tectonic and intact coal is 10.81 m3/t and 10.35 m3/t,respectively accounting for 60.3%and 70.5%of the limit gas adsorption volume in isothermal adsorption experiment.Based on the calculation of single-layer adsorption and micropore filling,the limit gas adsorption volume of tectonic and intact coal is 17.586 m3/t and 15.828 m3/t,and the limit gas adsorption volume in micropore filling accounts for 99%of the calculated limit gas adsorption volume,which proves that micropore is the main space for gas adsorption in coal,and gas mainly exists in the form of micropore filling.5)The change of pore structure of coal resulted by tectonism has a obvious effect on the capacity of methane desorption.Desorption volume of tectonic coal is obviously larger than that of intact coal with the same particle size,and the average gas desorption speed in the first minute is 1.43?8.83 times of that of intact coal.The range of initial effective diffusion coefficient and initial diffusion coefficient of coal is 10-6?10-4 m2/s and 10-13?10-11 s-1,respectively,which is several,or even dozens times of the value of intact coal.Due to tectonism,the average value of pore structure complexity evaluation index of tectonic coal is 16.8%?70.6%lower than that of intact coal.Tectonism and pulverization can simplify pore structure,facilitate the gas flow in coal and improve the gas desorption capacity.At the same scale,the matrix size of tectonic coal is smaller than that of intact coal.The decrease of pore channel path and the evolution of pore shape are the main reasons for the acceleration of gas adsorption equilibrium and the higher gas desorption capacity of tectonic coal.6)There is an an important role for tectonic coal in promoting the outburst process.On one the hand,in the initial desorption stage,a large amount of gas can easily transmit from interior of particle to its surface.On the other hand,gas expansion energy supplied by rapid gas desorption of tectonic coal is several times that of intact coal,and the ability to transport fractured coal rocks is stronger.According to the outburst test case in Zhongliangshan coal,it is demonstrated that the gas desorption rate under regular size is between 0.003087 m L/(g·s)to 0.061241 m L/(g·s),which is about one-tenth to one-half of the desorption rate required for transporting coal.Based on the relationship between particle size and gas desorption speed,it can be obtained that some particles should be broken to 0.116-0.406 mm or more powdered particles in order to transport outburst coal effectively.This dissertation has 101 figures,58 tables and 210 references. |
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