Study On Thermal Effect Of Methane Adsorption In Coal

The mechanism of methane adsorption and desorption in coal is an important theoretical basis for the prevention of methane disaster in coal mine and coalbed methane exploitation.The thermodynamic of methane adsorption is an important part of the mechanism of methane adsorption and desorption.The accurate depiction of adsorption heat is not only helpful to reveal the mechanism of methane adsorption and desorption,but also helpful to evaluate the methane adsorption capacity of coal.At the same time,the temperature effect caused by methane adsorption heat in coal has a guiding value for methane drainage and prediction of coal and gas outburst.At present,the research on the theory of methane adsorption and desorption is still not perfect,especially the understanding of the calculation theory and influencing factors of methane adsorption heat is insufficient.In this dissertation,a systematic study on the adsorption heat of methane in coal and its temperature effect was carried out,and the following results were obtained:（1）New heterogeneous adsorption models of methane adsorption in coal and theoretical models of adsorption heat are proposed.The simplified two-state model of adsorption heat was improved by statistical mechanics method.The analytical expressions of isosteric heat for Langmuir and its extended adsorption models was directly derived from Clausius-Clapeyron equation.Based on the non-uniform adsorption potential energy of coal,the new heterogeneous adsorption models of methane adsorption in coal were established,and these models can simultaneously quantify the adsorption amount and integral adsorption heat.The modified two-state model was adopted to estimate the adsorption heat of methane.The results show that the adsorption heat of methane on coal surface is greater than the condensation heat of methane,and the isosteric heat is about 24 kJ/mol.（2）The effects of temperature,and gas type on adsorption heat and adsorption isotherms were revealed.The adsorption isotherm and integral adsorption heat of methane with different temperature,gas type and coal rank were measured by adsorption calorimetry experiment.The results show that the isosteric heat is less affected by the temperature,and the isosteric heat can be used to predict the adsorption isotherms at different temperatures.The adsorption amount,integral heat and isosteric heat of carbon dioxide are the largest,followed by methane and nitrogen.Different adsorption models were used for fitting the experimental data,and the results show that the heterogeneous adsorption models have better fitting quality of adsorption isotherms and integral heat than Langmuir equation.By comparing the isosteric heat of different adsorption models,it is found that the isosteric heat of Langmuir model is constant,while the the isosteric heat of the heterogeneous adsorption models decreases with the increase of adsorption amount.The heterogeneous adsorption models can well reflect the non-uniform adsorption energy of coal surface.（3）The variation law of methane adsorption heat with coal microscopic pore structure was explained.The pore structure characteristics of different metamorphic grade coal samples were obtained by mercury injection,low-temperature nitrogen adsorption and carbon dioxide adsorption experiments,and the relationship between pore structure parameters and adsorption heat was established.The results show that the isosteric heat has a strong positive correlation with the specific surface area and proportion of the super-micropore（0.38²nm）of different coal samples.The more super-micropore,the larger the isosteric heat.The super-micropore plays a leading role in methane adsorption.When the super-micropore proportion increased by 10%,the isosteric heat increased by about 0.7 kJ/mol.（4）The influence mechanism of coal molecular structure on adsorption heat was studied.Structural parameters of coal macromolecule with different metamorphism degrees were obtained by ¹³C nuclear magnetic resonance,fourier transform infrared spectroscopy and X-ray diffraction.The results show that with the increase of metamorphic degree,the aromatic carbon rate increases,while the length of aliphatic chain,the number of oxygen-containing functional groups and the interplanar spacing of coal microcrystalline structure decrease.Finally,the coal is close to graphite.On the basis of coal molecular structure parameters,simplified coal molecular models with different number of benzene ring,length of side chain and oxygen-containing functional groups were established.The adsorption energy of different coal molecular models interacting with methane was calculated.The results show that the adsorption heat increases with the increase of the benzene ring number in the basic structural unit of coal.The presence of alkyl side chains and oxygen-containing functional groups can enhance the interaction between coal and methane molecules.In particular,the hydroxyl group has the greatest influence on the adsorption energy,and the adsorption heat can increase by about 2.4 times.In addition,the adsorption energy is also related to the distance between methane and coal molecules.There is an optimal adsorption distance,at which the adsorption energy is maximum.（5）The influence law of dynamic adsorption thermal effect on coal temperature is revealed.The dynamic heat release experiment of methane adsorption was carried out,and it was found that the temperature of coal particles showed a change law of slow decay after rapid rise during the methane adsorption process.The change of coal particle temperature was determined by the rate of heat production by adsorption and heat release.Besides,the multi-field coupling theoretical model of coal seam gas flow considering the thermal effect of adsorption and desorption is established,and the effects of the isosteric heat,adsorption volume constant of coal,initial temperature,initial methane pressure and initial permeability on coal temperature were studied by numerical simulation.Simulation results show that the variation of coal temperature is positively correlated with the isosteric heat,adsorption volume constant of coal,initial temperature,initial methane pressure and initial permeability.There are 63 figures,26 tables and 185 references in this dissertation.