Quantitative Study Of The Macromolecular Structures And Super Micropores Of Tectonically Deformed Coal Using HRTEM

In this study,the macromolecular structure and super micropores of macromolecular structure of six tectonically deformed coal samples with different metamorphism and deformation were analysed using high-resolution transmission electron microscopy(HRTEM).The high-resolution lattice fringe iamges were obtained,the macromolecular structure and super micropore were quantitatively analysed using the image analysis technique.Subsequently,the impact of metamorphism and deformation degree on the coal macromolecular structural lattice parameters and super micropores of macromolecular structure was investigated in depth.The results show that according to different metamorphism and deformation,the coal macromolecular structure and the characteristics of the super micropores were also different.(1)In the HRTEM lattice fringe images of low-rank coal,fringes mostly appear as single fringes with a disordered and sparse arrangement;two or more adjacent parallel fringes are rarely found,and they have a poor orientation.In the HRTEM images of mid-rank coal,although the fringes are mostly single and disordered,the arrangement is close,showing more localized fringe pairs,with significantly enhanced orientation and larger length.In the HRTEM images of the high-rank coal,the distribution of fringe pairs was increased significantly,and they had a closer arrangement and greater length.Additionally,the fringes mostly appeared in the thin edges,with a closer arrangement and longer length.The observation of internal structure showed significantly less fringe pairs,which were mostly isolated and disordered,in a sparse arrangement with a shorter length and poor orientation.(2)Coal rank has an important impact on the macromolecular structure and super micropores of coal,and at different metamorphic stages,the extent of the effect varies.(1)macromolecular structure: For weak tectonic deformations,the molecular structure of low-rank coal was disordered,with a large separation between the aromatic layers.With increase in coal rank,the macromolecular structure of coal showed larger aromatic layers by aromatization and condensation polymerization that presents an enhanced orientation,closer arrangement and smaller interlayer spacing.For the adjacent metamorphic stages of coal,changes in fringe length and fringe separation increased and then decreased,reaching maximum values during the evolution from mid-rank to high-rank coal.The change in fringe separation reached a minimum value at the high metamorphic evolution stage.The change in tortuosity gradually decreased.(2)Super micropores: For weak tectonic deformations,with increases in coal rank,super micropore width decreased and length increased,meanwhile,pore area increased.With increase in coal rank,the macromolecular structure of coal showed larger aromatic layers by aromatization and condensation polymerization that presents an enhanced orientation,closer arrangement and smaller interlayer spacing.Therefore,in the process of the evolution of coal,the rate of fringe length increases is higher than that of fringe separation decreases.For the adjacent metamorphic stages of coal,changes in pore length and pore area reached maximum values during the evolution from mid-rank to high-rank coal.The change in pore width reached a maximum value during the evolution from low-rank to mid-rank coal.(3)Tectonic deformations also had an important influence on the macromolecular structure and super micropores of coal,and at different stages of deformation,the extent of the effect varies.(1)macromolecular structure: With increases in tectonic deformation,the fringe length increased and fringe separation and fringe tortuosity decreased.The impact of tectonic deformation on the macromolecular structures of low-rank coal was stronger than that of mid-rank coal.The impacts of tectonic deformation on the nanoscale pores,coal methane adsorption capacity and coalbed methane contents of low-rank coal are higher than those of the mid-rank coal.(2)Super micropores: In the stage of low-rank coal,with increases in tectonic deformation,pore area decreased and pore width and length remain unchanged,it suggested that the rate of fringe separation decreases is higher than that of fringe length increases.In the stage of mid-rank coal,with increasing tectonic deformation,the pore width decreased and pore area and length increased,so the rate of fringe length increases is higher than that of fringe separation decreases.And the impact of tectonic deformation on the width and length of super micropores in coal macromolecular structure of mid-rank coal was stronger than that of low-rank coal.(4)The macromolecular structure of coal widely exists super micropores which width is bigger than diameter of the methane molecule(0.414nm).Nanoscale pores(<100 nm)in coal are the main spaces for CBM adsorption.Excess coalbed methane may be stored in super micropore of macromolecular structure,and stored in physical adsorption and chemical adsorption.Calculating adsorbed methane amount of the adjacent aromatic layers gap is helpful to truly estimate CBM content,and to improve the accuracy of the current coalbed methane resource evaluation.