Study On The Microscopic Influencing Factors Of Methane Hysteresis In Coal And Its Quantitative Evaluation Method

The adsorption and desorption properties of coal for gas are the fundamental properties of coal seams,and the study of their properties is of great significance in mine gas management and coalbed methane development.At present,scholars at home and abroad have found that the desorption isotherm of coal generally lags behind the adsorption isotherm due to the difference of methane adsorption/desorption behavior,i.e.,methane adsorption-desorption lag phenomenon.The microscopic pores of coal,as the site of methane adsorption/desorption and the main channel of diffusion,have an important influence on the characteristics of methane adsorption-desorption hysteresis.In this paper,six sets of coal samples from Huabei,Pingdingshan and Yaojie mining area were selected to obtain methane adsorption-desorption characteristics,and Fréchet distance hysteresis evaluation index(FDI),which is sensitive to pressure order,was proposed to accurately and quantitatively characterize the degree of methane adsorption-desorption hysteresis.Based on the quantified hysteresis degree,the effect of coal microstructure on methane adsorption-desorption hysteresis was systematically investigated using low-temperature nitrogen(77 K)and low-pressure carbon dioxide(273 K)physisorption methods.The main conclusions are:(1)The methane adsorption/desorption isotherms of all the six coal samples showed significant hysteresis.The fitting results of Langmuir equation showed that the Langmuir volume corresponding to the desorption isotherm was 67.41?87.61%of the adsorption isotherm,and the hysteresis phenomenon made the ultimate adsorption capacity of coal samples significantly underestimated.In addition,based on the analysis and summary of the existing hysteresis evaluation methods,the FDI hysteresis evaluation index was proposed,which has higher reliability and stability.(2)In the pore size distribution curves,the ratio of the maximum wave peak to the minimum wave peak of the mesoporous and macropore stages of the coal samples is5.94,which is larger than that of the micropore at 4.98;the difference between the mesopores and macropores is more significant.the specific surface area of the mesopores of the coal samples is 9.43-18.28 times larger than that of the macropores in the BJH specific surface area measurements,and there is a very high correlation between the BJH mesopore specific surface area and the BET specific surface area(R²=0.986).The HSW coal sample has the smallest fractal dimensions D_FHH2,D_NK,D_WLand D_SPS,and has the lowest pore surface roughness in the mesopore and micropore stages.In addition,the linear correlation coefficients R² between the mesopore fractal dimension and the micropore fractal dimension were 0.1187?0.2030,indicating that the surface roughness between the micropore and mesopore of the coal samples was unrelated.(3)The increase of BET specific surface area,BET constant and DR specific surface area corresponds to the increase of FDI,which is the hysteresis index of methane adsorption-desorption;the increase of DA most available pore size corresponds to the decrease of FDI,and the FDI of coal samples generally increases with the decrease of DA pore size distribution width.The correlation analysis of microporous,mesoporous and macropore volumes with FDI shows that the degree of methane adsorption-desorption hysteresis generally decreases with the decrease of microporous volume and total pore volume.The FDI index values of the coal samples with the largest number of peaks in the DFT micropore pore size distribution curve and the BJH mesopore pore size distribution curve are the largest,indicating that the discrete pore distribution enhances the adsorption-desorption hysteresis.The correlation analysis of micro-pore pore length and FDI showed that micro-pores of0.38-0.72 nm had the highest correlation with FDI(R²=0.9171).(4)The regression analysis of fractal dimension and hysteresis index FDI showed that the pore surface roughness was positively correlated with the size of methane adsorption-desorption hysteresis in general,and the increase of pore surface roughness led to more significant methane adsorption-desorption hysteresis.Meanwhile,the correlation between FDI and D_FHH2,which characterizes the mesopore surface roughness of coal samples,was the highest(R²=0.8078),indicating that the hysteresis was mainly controlled by the mesopore surface roughness.In addition,there is a positive correlation between the multifractal singularity spectral width?f and FDI(R²=0.8263),indicating that the coal samples with smaller pores dominate in the pore size distribution range of 4?24nm have smaller desorption hysteresis.(5)In this paper,we explain the reason for the hysteresis in terms of the difference in methane adsorption/desorption transport processes.That is,the difference in the distribution of methane in the pores makes the intermolecular forces promote the entry of methane molecules into the micropores during adsorption,while hindering the outward transport of methane molecules in the very micropores(<0.7nm)during desorption.This difference is manifested as the adsorption-desorption hysteresis phenomenon at the macroscopic level.The hysteresis study based on Monte Carlo simulation was also used to support the control mechanism of methane desorption and transport processes within the 0.38?0.72nm pore space during desorption.There are 46 figures,29 tables and 100 references in this paper.