Study On Gas Adsorption Characteristics Of Coal With Different Metamorphic Degree Under The Effect Of Water And Temperature

Under the background of"double carbon",coal seam gas,as a clean and efficient unconventional resource,has important development and utilization value,and accurate grasp of coal seam gas content is of great significance for reliable prediction of coal and gas outburst risk and accurate exploration of coal seam gas reserves.Water and temperature are important factors affecting the gas adsorption content of coal seam.It has become a long-term research topic to master the influence characteristics of the two factors on the gas adsorption performance of coal.Therefore,the research method combining theoretical analysis and numerical simulation is adopted to construct the spatial structure model of five kinds of coal with different degrees of metamorphism by molecular simulation method.The isothermal adsorption law of methane gas by coal with different degrees of metamorphism is simulated and analyzed under the condition of different moisture content and temperature.Furthermore,the quantitative characterization of coal adsorbed gas characteristics and coal adsorbed gas amount considering the interaction of water and temperature was carried out,and the main conclusions were drawn as follows:（1）The isothermal adsorption curves of five kinds of coal with different metamorphism are consistent with the characteristics of type I isotherm.The gas adsorption capacity of coal samples decreased with the increase of temperature and water content.Based on the results of different error statistical indexes,the fitting accuracy of the adsorption model is DA model,DR Model and Langmuir model in order from high to low.Therefore,the DA adsorption model has better applicability.It is analyzed that on the one hand,the coal has heterogeneous micropore surface and large specific surface area of micropore.Second,the structural heterogeneity parameter s in the DA equation of the microporous filling theoretical model is a non-fixed value different from that in the DR equation.（2）Water has an inhibitory effect on gas adsorption capacity of coal,which is closely related to the competitive adsorption of water molecules and methane molecules,and the water molecules occupy the adsorption space in coal pores.With the increase of temperature,the thermal motion of methane molecules is enhanced,which promotes the desorption of methane from coal surface,which leads to the decrease of coal adsorption capacity,and the net adsorption heatβE value increases with the increase of temperature.The saturated adsorption V₀of coal shows a U-shaped trend with the increase of volatile matter of coal,which is related to the differences of pore structure and chemical structure of different coal grades.There is a linear relationship between water content and temperature and the adsorption capacity of saturated gas V₀.The sensitivity coefficient of coal’s gas adsorption capacity to water content is much higher than that of temperature,indicating that the influence of water on coal adsorption capacity is stronger than that of temperature.It is also found that the influence degree of water and temperature on the adsorption capacity of different coal grades is also different.Water has an inhibitory effect on gas adsorption capacity of coal,which is closely related to the competitive adsorption of water molecules and methane molecules,and the water molecules occupy the adsorption space in coal pores.With the increase of temperature,the thermal motion of methane molecules is enhanced,which promotes the desorption of methane from coal surface,which leads to the decrease of coal adsorption capacity,and the net adsorption heatβE value increases with the increase of temperature.The saturated adsorption V₀of coal shows a U-shaped trend with the increase of volatile matter of coal,which is related to the differences of pore structure and chemical structure of different coal grades.There is a linear relationship between water content and temperature and the adsorption capacity of saturated gas V₀.The sensitivity coefficient of coal’s gas adsorption capacity to water content is much higher than that of temperature,indicating that the influence of water on coal adsorption capacity is stronger than that of temperature.It is also found that the influence degree of water and temperature on the adsorption capacity of different coal grades is also different.（3）The moisture correction coefficient of coal decreases with the increase of moisture content,and the exponential equationα=exp（-Bw）is more suitable to characterize the relationship between moisture correction coefficient and moisture content.At the same time,the influence coefficient of moisture and the volatilization of coal can be characterized by a quadratic polynomial of one variable,and a new expression of moisture correction coefficient is proposed.The exponential equationλ=exp[-n（T-T₀）]is used to characterize the relationship between temperature correction coefficient and temperature,and the temperature influence coefficient n decreases with the increase of gas pressure.The power equation n=i P^-jcan well characterize the relationship between temperature influence coefficient and gas pressure.Finally,the expression of temperature correction coefficient isλ=exp[-0.009P^-0.286（T-T₀）].（4）Combined with the DA adsorption model,moisture correction coefficient term and temperature correction coefficient term,the prediction equation of coal adsorption gas quantity considering the combined influence of moisture,temperature and volatile matter was established.Combined with the gas gas adsorption test data of32 groups of coal samples,the prediction equation of coal adsorption gas quantity was compared with the existing one.It is found that the newly established prediction equation of coal adsorption gas quantity has better indirect calculation effect,mainly because,on the one hand,DA adsorption equation of microporous filling theory is applied in the new prediction formula,on the other hand,the calculation result of the exponential equation of moisture correction coefficient is better than that of the traditional linear equation,and the exponential equation is used to represent the temperature correction coefficient.The expression of the influence coefficient of temperature and the power function of gas pressure is considered.