Study On Response Of Carboniferous Coal Nanostructure To Dike Intrusion In Tashan Coal Mine,Datong Coalfield

Different from the degree of metamorphism of deep-buried coal,the contact metamorphic coal formed by magmatic intrusion has the characteristics of rapid maturation,resulting in obvious nonlinear changes in its physical and chemical properties,which affects the mining and utilization of coal.Fine characterization of nano-structure（chemical structure and pore structure）of contact metamorphic coal is the basis for understanding the special properties of thermal metamorphic coal,which has important theoretical and practical significance for systematically understanding the mechanism of contact metamorphism and clean utilization of coal.The phenomenon of Carboniferous coal seam being invaded by magma is common in Datong coalfield,Shanxi Province,which is a natural place to analyze contact metamorphic coal.Therefore,this paper takes the contact metamorphic coal of Tashan coal mine in Datong coalfield as the research object,and combines proximate analysis,ultimate analysis,optical microscopy and scanning electron microscopy to analyze the differences in the metamorphic degree,elemental composition and microstructure distribution of coal at different distances from dike intrusion,and defines the microstructure characteristics of thermal metamorphic coal at different degrees of metamorphism.On this basis,the change characteristics of aliphatic groups and aromatic functional groups of coal were quantitatively characterized by advanced ¹³C-NMR technology,and the influence of dike intrusion on the chemical structure of coal was discussed.The pore distribution characteristics of thermal coal before and after deashing were studied by low temperature gas adsorption（N₂,CO₂）and small angle X-ray scattering（SAXS）,and the evolution mechanism of coal nano-pore under the co-evolution of organic chemical structure and inorganic mineral under dike intrusion mode was analyzed.Based on the above work,the response of coal nanostructure to dike intrusion is clarified.The main research achievements include:（1）According to the variation characteristics of the coal basic parameters,contact metamorphic coal can be divided into severe thermal coal(R_o,max>2%)and mild thermal coal(1%<R_o,max<2%).The content of moisture and ash in coal increases,while the content of volatile matter decreases in severe thermal coal.The porosity of devolatilization of natural coke develops,and it has obvious mosaic structure and flow structure.It contains a certain amount of pyrolytic carbon,and more carbonate minerals and clay minerals.The proportion of natural coke in slightly thermal coal decreases,the thermogenic structure weakens obviously,the mineral type is mainly kaolinite,and the brittle fracture tensile fissure develops.（2）For the effect of dike intrusions,The evolution of coal chemical structure shows the characteristics of"gradual change"and"drastic change",and the drastic change point is the stage from coking coal to anthracite.The evolution of chemical structure of contact metamorphic coal can be divided into three stages.（1）The removal of base side chain leads to the increase of aryl carbon ratio.The evolution mechanism is mainly aromatization,which is manifested as:（1）The CCH₂ groups removed faster than CCH₃ groups,suggesting that the major cleavage occur at the side chains instead of aryl rings.（2）The aromatic cluster size increases slowly,and the increase of aryl carbon and nonprotonated carbon is mainly caused by the increase of 2-3 aromatic rings.（2）The aromatic cluster size increased significantly,the alkyl side chain removal rate was still high,and the cyclic polycondensation controlled the increase of aryl carbon ratio.Perform as:The content of CH₃ and CH₂ groups decreased significantly,the number of carbon atoms in the aromatic cluster size increased sharply,the content of nonprotonated carbon increased rapidly,but the content of protonated carbon decreased slightly.（3）The protonated carbon content decreased sharply and the coal samples near the intrusive body retained a high content of alkyl and oxygen-containing groups.（3）For the effect of dike intrusions,Different from the normal metamorphic coal sample,the R_o,max of U-shaped inflection point is about 1.55%.The porosity,pore volume and specific surface area of micropores and macropores show a U-shaped trend with the increase of coal metamorphism.The newly formed minerals from contact metamorphism increase the macropore content but fill the pores at 0.4-0.7nm,thus reducing the micropore porosity.At the same time,when R_o,max<1.55%,the cleavage of aliphatic functional groups increases the pores in the macromolecular network structure,and thus reduces the micropore content.With the further increase of maturity,due to the influence of aromatization and condensation of thick ring,macromolecular structures gradually become tighter and micropore content increases.Contact metamorphism increases the content of closed pores,especially in coal samples near dike,and increases the heterogeneity of pore distribution and surface.