Effect Of Coal Vitrinite Macromolecular Structure Evolution On Methane Adsorption Capacity At Molecular Level

Coalbed methane is mainly stored in coal reservoir as absorbed gas.Study on methane adsorption in coal plays importance roles in coalbed methane exploration,coalbed methane exploitation and mine gas control.This thesis focuses on effect of coal vitrinite macromolecular structure evolution on methane adsorption capacity at molecular level.Different rank naturally-matured and artificially-matured coal vitrinite were chosen as the samples in this study.Experimental methods were employed to investigate the evolutions of vitrinite macromolecule,pore structure and methane adsorption capacity during coalification.Numerical methods were used to study methane adsorption mechanism in coal vitrinite.We also used the experimental results to validate numerical calculation and used the numerical calculation to investigate the microscopic mechanism of methane adsorption in coal vitrinite.The main findings and results in this thesis are as follows:（1）By employing the HRTEM,¹³C NMR and FT-IR experiments,aromatic clusters in different rank coal vitrinite was quantitatively characterized.It was found3×3 aromatic rings played connecting roles in the evolution of aromatic rings.Based on evolution of aromatic structures and aliphatic structures,new evolution model of vitrinite macromolecule was established.With maturity ranging from R_o,ran=0.5%to R_o,ran=4.2%,coalification process of vitrinite can be divided into five stages.The evolution characteristics of vitrinite macromolecule and its mechanism were further studied.（2）Pore structures of naturally-matured samples and artificially-matured samples were quantitatively characterized by using high pressure mercury intrusion,liquid nitrogen adsorption and carbon dioxide adsorption experiments.The results showed that the evolution of micropore during artificial coalification process is consistent with that in natural coalification process.By analyzing the micropore evolution characteristics and vitrinite macromolecular structure,it is found that the generation and evolution of micropores are controlled by the coal macromolecular vitrinite structure.In addition,the"three-stage"evolution mechanism of micropores during coalification is clarified.（3）Molecular simulation technology was used to theoretically investigate the methane adsorption in different types of pores.The results showed that methane adsorption in the micropores exhibited pore-filling characteristics and methane adsorption in mesopores and macropores exhibited two-layer adsorption characteristics.When the single methane molecule is freely adsorbed,the adsorption capacity of each chemical groups is ranked as:aromatic ring>methyl and methylene>methyl>carboxyl>hydroxyl.When forming the pore wall,the adsorption capacity of the chemical groups is ranked as:carboxyl group>hydroxyl group>aromatic ring>methyl group.Combined with the molecular structure and pore structure of the coal vitrinite,the calculation method of methane adsorption in the complex pore network under the control of the complex coal molecular structure was constructed.The influences of various factors on the methane adsorption in coal vitrinite were clarified.Micropores were identified as the main site of the interaction between macromolecules and methane and were major contributor to the total methane adsorption amount.（4）The macromolecular structure evolution of the vitrinite group dominates the evolution of micropore structure and the evolution of methane adsorption capacity.Additionally,by comprehensive analyzing of macromolecular structure evolution,pore structure evolution and methane adsorption mechanism in coal vitrinite,the evolution mechanism of methane adsorption capacity in coal vitrinite during coalification was further studied.It was found that evolution of vitrinite molecular structure controls the evolution of methane adsorption capacity.In different coalification stages,methane adsorption capacity was controlled by different part of vitrinite chemical structure.When R_o,ran is<1.4%,aliphatic structures provide the site and the interaction forces for methane adsorption.When R_o,ran is>1.4%,aromatic structures control methane adsorption in vitrinite.There are 116 figures,15 tables and 218 references in this thesis.