Methane Adsorption Behavior Characteristics Of Multi-scale Pore Structure In Coal And Its Microscopic Influencing Mechanism

Energy endowment characteristics of"rich in coal,poor in oil and gas"determine that the development of new high-efficiency and clean energy has important strategic significance in China,the extraction and utilization of coalbed methane is not only conducive to the healthy development of China’s main energy industry and the targets of carbon peak and carbon neutrality,but also can effectively prevent mine gas disasters and reduce environmental pollution.Research on the interaction mechanism between CH₄ molecules and microscopic pores in coal,CH₄ adsorption characteristics,desorption kinetics and influence mechanism in pores with different sizes were carried out.It is helpful to systematically understand the CH₄ occurrence and migration characteristics in pore structures of different sizes and to guide mine gas drainage.This dissertation takes the rock mechanics,adsorption science,surface physical chemistry,quantum mechanics,fractal geometry and fluid mechanics theory as the guidance,combining the theoretical analysis,numerical simulation and experimental testing as the study methods,the quantitative relationship between the microscopic pore structure in coal and the CH₄ adsorption behavior is constructed.The correlation between the pore structure of different sizes and the initial CH₄ diffusion characteristics of granular coal is compared,and the CH₄ occurrence and migration characteristics in pore structures of different sizes in coal and its internal influence mechanism are revealed.The main conclusions of the dissertation are list as below:1)The limit adsorption capacity of CH₄ in coal mainly depends on the micropore structure below 1.5 nm.According to the quantitative characterization of pore structure in coal by mercury intrusion method,low-temperature N₂（77 K）adsorption method and low-pressure CO₂（273 K）adsorption method,the ratios of micropore volume and specific surface area（0.33-1.5 nm）to total pore volume and specific surface area in coal are over 68%and 97%,respectively.The correlation coefficients between the CH₄Langmuir volume（V_L）in coal and the micropore volume and specific surface area are0.9392 and 0.9116,respectively;it shows that the micropore structure below 1.5 nm is the main controlling factor to determine the CH₄ adsorption capacity in coal.2)CH₄ in coal is mainly adsorbed in the micropore structure（0.38-1.5 nm）in the form of micropore filling.By building the mathematical model of the CH₄ adsorption capacity in coal under extreme conditions,it is proved that the adsorption forms of CH₄in coal are micropore filling and monolayer adsorption.When the pressure is infinite,except for long flame coal,more than 93%of the adsorbed CH₄ in coal is stored in the micropore structure（0.38～1.5 nm）in the form of micropore filling,and less than 7%of the adsorbed CH₄ is adsorbed on the outer surface of pores（<1.5 nm）in the form of monolayer;with the decrease of pressure,the proportion of CH₄ adsorption capacity of small pores to the total adsorption capacity in coal gradually increased,indicating that the proportion of CH₄ adsorption capacity provided by the micropore structure（0.38～1.5 nm）in coal is much higher than 93%under low pressure conditions.3)The CH₄ diffusion kinetics in coal mainly depends on the pore structure below100 nm.According to the results of correlation comparison,it can be found that there is an obvious positive correlation between the initial effective diffusion coefficient of coal samples with different particle sizes and the pore volume in different ranges,and the pore size is about 50～100 nm.With the increase of CH₄ pressure,the pore range corresponding to the highest correlation shifts to the direction of larger pore size;and with the increase of desorption time,the pore range corresponding to the highest correlation shifts to the direction of smaller pore size.It shows that the pore structure below 100 nm is the main controlling factor to determine the CH₄ diffusion characteristics in coal.4)Pore structures in coal are divided into inaccessible pores,filled pores,diffusion pores and seepage pores based on the CH₄ occurrence and migration characteristics in the pore structure with different size.All pores（<0.38 nm）that CH₄ molecules cannot enter in coal are collectively referred to as inaccessible pores;CH₄ in the filled pores（0.38～1.5 nm）only exists in the adsorption state,and the adsorption form is micropore filling.In the pores above 1.5 nm,CH₄ coexists in the form of adsorbed and free states;the adsorbed CH₄ follows the monolayer adsorption form on the surface of the pores,and the free CH₄ occurs in the center of the pores according to Boyle’s law.The pores of 1.5～100 nm and above 100 nm in coal are divided into diffusion pores and seepage pores,respectively.The gas mass transfer behavior is driven by concentration gradient and pressure gradient,respectively.5)The DA-L_mi/mo adsorption model was constructed based on the adsorption form of CH₄ in coal.According to the CH₄ adsorption forms in different pore structures,a theoretical CH₄ adsorption model of complex pore structures in coal was constructed.Combined with the theoretical adsorption model was simplified to the DA-Lmi/mo adsorption model;the proportion of different adsorption forms under limit conditions,the theoretical CH₄ adsorption model is simplified to the DA-L_mi/mo adsorption model.Comparing the correlation coefficient between the Langmuir and DR adsorption models and measured CH₄ adsorption data in coal,it is found that the new DA-L_mi/moadsorption model can better characterize the CH₄ adsorption isotherm in coal,and can better reflect the CH₄ occurrence characteristics in different adsorption forms in coal.In addition,DA-L_mi/mo adsorption model can also overcome the limitations of Langmuir and DR adsorption models and reflect the occurrence characteristics of CH₄ in different adsorption forms in coal.6)A new pore characterization method was established based on the CH₄adsorption isotherm in coal.According to the CH₄ adsorption mechanism in pore structures with different sizes obtained by GCMC,a mathematical model between the microscopic pore structure and CH₄ adsorption isotherm in coal is constructed.The micropore size distribution of coal is inversely calculated by using the CH₄ adsorption isothermal data at 30℃;the model is verified by comparing the pore analysis results of low-pressure CO₂（273 K）adsorption method.The relative error of the two methods in analyzing the micropore structure（0.38～1.4 nm）of coal is small,and the maximum error is only 8.70%,which shows that it is feasible to analyze the micropore size distribution based on the CH₄ adsorption isotherm in coal.7)A new calculation method of absolute adsorption capacity is established based on the assumption of fixed adsorbed phase volume.According to the density distribution of CH₄ in pore structures obtained by GCMC,it is found that the limit adsorption phase density of CH₄ in the slit pore structures with different sizes is between0.253～0.396 g/cm³,indicating that the adsorption phase density of CH₄ in coal is much smaller than the liquid density（0.421 g/cm³）.Therefore,the calculated absolute adsorption amount of CH₄ based on the commonly used assumption that the adsorbed phase density is equal to the liquid density is not reliable.According to the stress state of CH₄ molecules in different regions,assuming that the volume of CH₄ adsorption phase in coal remains unchanged,the absolute adsorption amount of CH₄ in coal is obtained combined with the measured adsorption data.It is found that the limit density of CH₄ in the adsorption space in coal is between 0.304～0.358 g/cm³,which shows that the new calculation method of absolute adsorption capacity is feasible.The paper contains 124 figures,33 tables,and 303 references.