| With the gradual transfer of coal mining to the deep part,the exploitation and utilization of CBM,an unconventional natural gas,is attracting more and more attention.The traditional view is that the adsorption and desorption of gas in coal is completely reversible.With the deep research on adsorption and desorption of scholars at home and abroad,the hysteresis of gas desorption in coal has gradually aroused concern.However,compared with other aspects of research,the study on adsorption and desorption of gas in coal is still in the initial stage.There is no clear conclusion on the microscopic mechanism,influencing factors and quantitative evaluation of desorption hysteresis.In this paper,samples of XJ,SL in Shuangliu coal mine,gas fat coal(JZ)in the coal mine of Jian Zhuang coal mine and the long flame coal(YZG)in the coal mine of the garden are carried out to experiment,and the pore characteristics of several kinds of coal samples are measured by the method of gas adsorption and desorption hysteresis in coal and its influencing factors are analyzed,and the mechanism of gas desorption hysteresis and the influence on gas content determination are put forward.The results obtained in this paper are as follows:(1)The total pore volume of the four coal samples is YZG>JZ>XJ>SL from large to small.The pores of YZG coal samples are mainly ink bottle type pores,and the pore connectivity is poor;the pore types of SL coal samples are mainly circular pores and V type pores,and the connectivity between pores is good;JZ coal samples large and medium pores are mainly inks,which are not favorable for mercury withdrawal,and small pores and micropores are cylindrical pores which are favorable for mercury withdrawal and connectivity better.The lag ring of XJ coal sample appears in the whole mercury withdrawal stage,which indicates that its pore connectivity is poor.Micropores and pores are the main pores in YZG coal samples,while mesoporous and large pores are basically undeveloped;the main pores in the pores of SL and JZ are mainly small pores,the pores are second,and the macropores are basically undeveloped;while the pores in the XJ coal samples are mainly micropores,and the pores,mesoporous and macropores are not developed.The pores of the four coal samples are mainly <6nm pores,accounting for more than 70% of the total pore volume of the pores.The fractal dimension of four groups of coal samples was analyzed by using the data of mercury pressure test and liquid nitrogen adsorption experiment respectively.The results showed that the fractal dimension obtained by the mercury pressure test and the liquid nitrogen experiment decreased first and then increased with the improvement of the metamorphism degree.It showed that the surface roughness of four groups of coal samples decreased first and then increased with the improvement of the coal rank.(2)The adsorption capacity of XJ,SL,JZ and YZG coal samples from large to small in turn is XJ> SL> YZG>JZ,and the gas adsorption speed is JZ,XJ,YZG,SL in order from fast to slow,and the residual gas content of coal samples is reduced from 30℃ to 45℃,and the gas adsorption capacity of coal samples and coal samples are all decreasing.The adsorption capacity of YZG coal sample decreased by 25.7%,and the adsorption capacity of SL coal sample decreased by 32.31%,indicating that the adsorption capacity of SL coal sample is more sensitive to temperature.With the increase of temperature,the residual gas amount of the two kinds of coal samples is decreasing,which indicates that heating is beneficial to gas desorption;the adsorption capacity of gas increases gradually and the adsorption speed increases gradually when the particle size of XJ coal samples is broken from 3mm to 0.074 mm.The amount of residual gas in the desorption process is reduced.This indicates that the process of coal crushing is the process of increasing surface area and shortening the adsorption desorption and diffusion path of coal samples.With the increase of moisture content,the adsorption capacity of YZG coal sample decreases and the adsorption rate slows down.(3)The main reason for the different degree hysteresis of coal in the process of desorption is the difference of pore structure characteristics,the worse the pore connectivity is,the more ink pore and more micropores less than 6nm,the more obvious the hysteresis phenomenon.YZG coal samples with <6nm micropore volume of dozens of times of other coal samples and a large number of ink bottle shaped pores are the most obvious causes of desorption hysteresis.The hysteresis degree of SL coal samples is less than that of YZG coal,the main reason is that the pores of SL coal are mainly open pores,and the connectivity is good,and the pore volume of the micropore is only 1/30 of the YZG coal sample.Four kinds of coal samples show the change law of table change with the increase of coal rank and the decrease of hysteresis first and then rise,which is in accordance with the micropore volume change of the four kinds of coal samples less than 6nm.(4)Through the adsorption desorption experiments under different conditions,the influence factors of gas adsorption and desorption hysteresis in coal are studied.With the increase of temperature,the hysteresis coefficient of YZG coal sample and SL coal sample decreases gradually.It shows that the higher the temperature,the higher the methane molecular activity is,the adsorption of methanes in coal is easier to overcome the drag reduction.With the decrease of coal sample size,the desorption hysteresis coefficient of XJ coal sample decreases,which indicates that the process of coal sample breakage reduces the desorption and diffusion of gas and simplifies the pore structure of coal.With the increase of water content,the degree of desorption hysteresis of YZG coal sample becomes larger,which indicates that the added moisture limits the desorption of gas.In addition to the main effect of pore structure on gas desorption hysteresis,the ash,moisture and temperature in coal are also factors affecting the degree of hysteresis.(5)Basis on analysis of the adsorption and desorption hysteresis experiments,the mechanism of the hysteresis of adsorption and desorption of coal seam gas is proposed,which is caused by the swelling and deformation of the coal matrix adsorbed gas and the decrease of the pore throat in the coal with the gas desorption flow,which leads to the difference of the gas amount in the pores.In addition,the water on the pore surface evaporates with the desorption of gas flow and provides new adsorption sites for desorption gas,resulting in the occurrence of adsorption and desorption hysteresis.Due to the existence of sorption desorption hysteresis effect,the Langmuir constant determined by high-pressure desorption method is small. |
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