Adsorption Characteristics And Their Thermodynamic Evolution In Variable Rank Coals

The adsorbed methane content in coal can account for more than 80-90%of the total adsorbed amount.Understanding the methane adsorption in coal is not only conducive to improving the recovery of coalbed methane,but also has an important guiding significance for the simultaneous mining of coal and gas.There are many factors that affect the adsorption properties of coal to methane,among which the degree of coal metamorphism has a great influence on the adsorption capacity of methane.In this paper,the isotherm adsorption experiments of coal samples with different coal rank are carried out,and the thermal dynamic changes of coal rocks with different degree of coal metamorphism are obtained by combining the adsorption heat theory and adsorption potential theory,and the corresponding prediction models of isotherm adsorption curve are established.Through maceral and industrial analysis of 8 coal samples with different coal rank,the maximum vitrinite reflectance(R_o,m)is 0.48-2.59%.It can be seen from the form of liquid nitrogen adsorption loop that the low rank coal mainly contains open pores,the middle rank coal mainly consists of impermeable pores closed at one end,and the high rank coal mainly consists of fine bottleneck pores and slit pores.With the increase of the degree of coal metamorphism,the length of fatty chain increases first and then decreases,the small fatty molecules and oxygen-containing functional groups fall off continuously,and the degree of aromatic ring polycondensation increases.Based on microporous filling theory?D-A and D-R models?,Freundlich adsorption and Langmuir monolayer adsorption theory,the adsorption process of methane on coal was evaluated.The correlation degree of adsorption models from high to low is as follows:D-A model>Langmuir model>D-R model>Freundlich model,which shows that the adsorption of methane on coal is not only single molecular layer adsorption,but also a lot of microporous filling and multi molecular layer adsorption.With the increase of coal metamorphism,the saturated adsorption of methane shows a U-shaped trend,reaching the lowest value at R_o,m,m of 1.4%.The adsorption experiments of five groups of typical coal samples at 298-318K show that the saturated adsorption capacity of methane decreased with the increase of temperature.Based on the theory of adsorption potential,under the same methane adsorption volume,the adsorption potential of high rank coal is significantly higher than that of low and medium rank coal,and the former has higher adsorption potential energy level.When the adsorption potential???is equal to zero,the adsorption space between the adsorption equipotential surface and the adsorbent surface is described as the limit adsorption capacity,which has lowest value in coking coal stage.The optimal value of k?k=7.19?in the saturated vapor pressure equation is calculated according to the adsorption potential theory,so as to improve the accuracy of predicting isotherm adsorption curve.According to the theory of adsorption heat,the isosteric heat of adsorption of coal samples with different coal rank increases linearly with the adsorption capacity.The ultimate heat of adsorption of coal samples fluctuates between 22.93-36.05 kJ/mol.The pore structure of coal and the distribution of characteristic functional groups?including oxygen-containing functional groups,aliphatic hydrocarbons and aromatic hydrocarbons?on coal surface have some influence on the adsorption heat.By establishing prediction models of adsorption curve including heat of adsorption and adsorption potential models,the relative error value of the adsorption potential model where k is equal to 7.19 is the lowest,which is 0.023-0.066.These results are of great significance to reveal the adsorption mechanism of coal on methane.