Response Mechanism And Molecular Dynamics Process Of Different Coal-based Media To CO₂ Hydration

CO₂ capture and storage based on the hydrate method is an emerging technology to effectively mitigate climate change.Coal-based medium is one of the solid materials suitable for storing CO₂ hydrates.Studying the response mechanism and molecular dynamics process of CO₂ hydrates with different coal-based media is of great significance for improving the gas storage capacity and evaluating the stability of CO₂geological storage.In this thesis,the nucleation and growth characteristics of CO₂hydrates are studied in different coal-based media.The purpose is to clarify the influence of the molecular structure and pore structure difference of coal-based media with different metamorphic degrees on the synthesis of CO₂ hydrate.To reveal the response mechanism of CO₂ hydrate within different coal-based media at the molecular level.Based on the four selected coal samples with different metamorphic degrees,the molecular structure and pore structure difference mechanisms of coal were characterized,combined with experimental tests and molecular structure models.Through physical simulation experiments of the CO₂ hydrate synthesis process in different coal-based media,the influence of molecular structure-dominated adsorption and pore characteristics was clarified.With the help of molecular dynamics simulation,the mechanism of solid surface spacing and surface hydrophilicity on CO₂ hydrate synthesis was revealed.The main achievements of this thesis are as follows:1)The structural characteristics of the aromatic structure,aliphatic structure,and oxygen-containing functional groups of coal with different metamorphic degrees were quantitatively characterized by Fourier transform infrared spectroscopy,carbon 13nuclear magnetic resonance experiment and peak fitting technique.The differential evolution law of the molecular structure of coal with different metamorphic degrees was revealed.The results show that with increasing coal metamorphism,aromatization and polycondensation reactions lead to the cleavage and removal of oxidized,hydrogenated aromatic rings and alkyl side chains from aromatic rings.The benzene-like structure and its related aliphatic side chain carbon condensed to form a polycyclic aromatic system,the content of aliphatic structure decreased,and the content of aromatic structure increased.The oxygen-containing functional groups were removed from the aromatic ring by deoxygenation and dihydroxylation.Based on the above results,combined with elemental analysis experiments and X-ray photoelectron spectroscopy experiments,a set of high-fidelity two-dimensional macromolecular structure model construction methods for coal was proposed,and four macromolecular structure models of coal with different metamorphic degrees were constructed（XZ-02:C174H128O8N2;YT-09:C167H114O5N2S2;YT-11:C151H96O7N2;ZC-15:C146H80O4N2）.2)Combined with high-pressure mercury intrusion experiments,low-temperature liquid nitrogen adsorption experiments,and low-temperature CO₂ adsorption experiments,fractal theory was used to quantitatively characterize the macropore（50-10,000 nm）,mesopore（2-50 nm）,and micropore（<2 nm）structural characteristics of different coal-based media.The three-dimensional macromolecular pore model of different coal-based media was constructed by CO₂ molecular probe technology,and the difference control mechanism of the molecular structure of different coal-based media on pore structure was revealed.Studies have shown that the removal of aliphatic long-chain functional groups increases the pore throat size;with increasing coal metamorphism,the pore wall of micropores gradually transforms from a relatively independent,small-scale connected morphology dominated by aliphatic functional groups to a large-scale connected morphology dominated by aromatic structures.The conversion node is located near the maximum reflectance of vitrinite 1.4%.At the same time,the effects of nitrogen,sulfur,and oxygen atom functional groups on the microporous wall were clarified.3)Based on the different characteristics of the molecular structure and pore structure of different coal-based media,the response mechanism of CO₂ hydrate to different coal-based media particles under 40%,70%,and 100%water saturation was studied based on the excess gas method.The adsorption and hydrophobicity characteristics controlled by molecular structure are beneficial to the synthesis of CO₂hydrates.The high concentration of CO₂ adsorption on the coal surface increases the gas pore pressure,shortens the induction time,and promotes the formation of hydrates.The pore size hinders the synthesis of CO₂ hydrate by reducing the mass transfer efficiency.In addition,the existence of the capillary effect in small pore sizes reduces the activity of water and indirectly inhibits the synthesis of CO₂ hydrate.Based on the synthesis morphology of CO₂ hydrate,a synthesis process model of CO₂ hydrate in water-rich and gas-rich environments is proposed.Based on the response mechanism of CO₂ hydrate to different coal-based media,the characteristics of coal-based media suitable for storage materials are clarified.Coal-based media with low apparent density and large macropores are more suitable for CO₂ hydrate storage.4)Based on the response mechanism of CO₂ hydrate to different coal-based media,the size effect caused by different surface spacing and the influence mechanism of different hydrophilicity on the nucleation and growth of CO₂ hydrate were revealed by constructing the two-phase system structure model of different solid surface spacing and different solid surface hydrophilicity.The results show that under the action of temperature?pressure coupling,the relatively low potential energy environment reduces the violent movement of molecules in the system,and CO₂ bubbles dissolve in the water phase to form a local low potential energy region,which induces the regular arrangement of water molecules to form the initial cage structure.The insertion of water molecules leads to the dissociation of the cage structure,which in turn forms hydrate clusters.After the hydrate clusters grow to a critical size,they begin to grow rapidly on a large scale.When the simulation domain is less than 5 lattice sizes,the higher system potential energy hinders the dissolution of CO₂ bubbles,thereby inhibiting the synthesis of hydrates.The introduction of hydroxyl groups leads to enhancement of the hydrophilicity of the graphene surface.The hydrogen atoms in hydroxyl groups combine with the oxygen atoms in water molecules,which limits the movement of free water molecules and reduces the dissolution of CO₂ molecules,thereby inhibiting the synthesis of CO₂ hydrate.There are 64 figures,29 tables and 263 references in this thesis.