Methane Adsorption Influence Of Water Molecules On Coking Coal Molecules And Tensile Mechanical Characteristics

Gas is one of the main disaster sources of accident in coal mines,and is presented in free and adsorbed form in coal seams.Given the complex inhomogeneity of coal’s own structure and the accompanying presentation of water.the thesis investigates the influence of water molecules on the methane adsorption properties of coal molecules and their adsorption swelling and mechanical characteristics through indoor tests and Monte Carlo(MC)molecular simulation methods,providing a theoretical basis for the design of corresponding hydration measures for field engineering tests,which in turn effectively reduce gas content and pressure for coal seam.Based on these facts,coking coal samples from three mines,namely Chiyu(CY),Nanshan(NS)and Shaqu(SQ),were used for the study,and the elemental content,carbon skeleton form,heteroatom type and functional group attribution were clarified by means of elemental analysis,13 C NMR,XPS and FTIR,and the molecular structures were mapped and corrected;two types of morphological models,cluster water-coal molecules(CW-CM)and free water-coal molecules(FW-CM),with the same number of water molecules,were constructed to characterize their pore structure.Compare the methane adsorption patterns under different temperature and water-containing molecule conditions,and compare the simulation results with the pattern trends of laboratory tests to verify the validity of its model,analyse its adsorption influence characteristics,explore the coupling effect relationship of adsorption-diffusion-permeability and determine the ambient temperature;elaborate the variation pattern of adsorption potential and adsorption space and the reasons;reveal the intrinsic connection between the surface free energy of coking coal molecules and methane adsorption itself.To study the volumetric expansion and tensile mechanics characteristics of coking coal molecules,and discuss the relationship between the adsorption expansion and the maximum tensile strength.The effect of the relevant measures developed based on the methane adsorption law of water molecules on coking coal molecules and combined with the site geological conditions is examined.The main results and understanding of this study are as follows:(1)The pore structure distribution of CW-CM and FW-CM model mainly exists in the form of free and skeleton pore structure.As water molecules continue to increase,reducing the free volume of coking coal molecules,and the free pore structure is shifted towards a position at a smaller regional extent,and the skeletal small pore structure corresponds to a significant increase in the peak,the more water molecules occupy the pre-adsorption sites;(2)By analysing the adsorption of methane at different temperatures and water molecules,it is concluded that the absolute and saturation adsorption of methane decreases with increasing temperature without water molecules,while the increase in adsorption rate accelerates its thermal movement and produces some desorption.It is also demonstrated that the larger the free pore structure,pore volume,porosity,cell volume and aromatic structure,the greater the adsorption capacity,and conversely,the larger the H/C elemental analysis ratio,the smaller the adsorption capacity;at the same temperature,the absolute and saturation adsorption capacities decrease with increasing water molecules,while the decrease in the adsorption rate increases the hindrance of water molecules to the methane flow effect.Their simulation results verify the reliability of the laboratory test laws and confirm the validity of the model;(3)Heat of methane adsorption corresponding to Henry constant is greater than the average adsorption heat of methane under different pressures.It is proved that the greater the sulfur element content and S/C element analysis ratio,the higher the adsorption heat of both.The average heat of methane adsorption has little effect corresponding to different pressures when the water molecular content exceeds 4%.The coupling of adsorption,diffusion and permeability results in a significant decrease in the self-diffusion coefficient of methane with increasing water molecules,while an increase in temperature increases the self-diffusion coefficient,and a decrease in activation energy and an increase in temperature induces a lower permeability to diffuse the methane.T = 293.15 K was chosen as a reasonable reference basis by combining the underground and actual ambient temperature conditions in coal mines;(4)Based on the adsorption potential theory and combined with the change pattens of adsorption capacity,the relationship between the adsorption space and the adsorption potential was found to be a segmented linear and logistic “four-parameter” regression equation for water molecules and pressure,respectively,and it was determined that the adsorption potential does not change with water molecules.Based on the energy minimum principle,it is concluded that both the accumulated methane surface free energy and the overall surface free energy accumulated by adsorption on coking coal molecules diminish with the addition of water molecules,so that coking coal molecules occupy less methane and suppress their kinetic effect.At the same time the presence of water molecules releases the overall free energy of coking coal molecules and also weakens their specific surface area;(5)The relationship between methane adsorption and swelling is quadratic polynomial fitting function,exponential fitting function and linear function respectively.In SQ-FW-CM model,compression swelling occurs when the pressure is 9 MPa;a comparison of the energies of the methane and coal-water molecules after interaction yielded a lower maximum van der Waals negative energy for CW-CM model than for FW-CM model,but the adsorption capacity is just the opposite,which proves that the interaction energy cannot completely determine the change of adsorption swelling.The interaction of methane adsorption with carbon atoms in coking coal molecules and the effect of the expansion of the crystalline cell volume after the adsorption of methane has obtained that the larger the g(r)peak,the smaller the corresponding r(?)intercept,and the presence of water molecules has a repulsive effect on methane adsorption,which is not conducive to causing the expansion of the coking coal molecules’ crystalline cell volume;(6)Tensile strength of coking coal molecular cell is mainly determined by the pore size and maximum bond length after interaction.Whether water molecules exist or not,tensile strength of methane before adsorption is always greater than that after adsorption,which separates atoms(mainly sulfur atoms)from some coking coal molecular fragments.Tensile strength of FW-CM model is significantly lower than that of CW-CM model,and provides a reasonable relaxation space for the overall structure of coal molecules.(7)In response to the challenges of high gas content and difficult gas extraction in areas with prominent coal seams,and in combination with the combined measures of hydraulic punching + pre-pumping of boreholes effectively reduced the gas content and pressure in the coal seam,increased its extraction concentration and pure volume,and released some of the energy and stress in the coal seam.It also provides a reasonable basis for microscopic molecular simulation of methane adsorption reduction under water molecule conditions.A comparison of the SEM scanned pore structure with the free pore structure of the coal molecules reveals that although there are numerical differences between the simulated and experimental scans,the overall trend is essentially the same.