Experimental Study On The Influence Mechanism Of Moisture On The Methane Time-varying Diffusion Dynamics Of Low-rank Coal

In order to study the influence mechanism of moisture on the time-varying gas dynamic characteristics of low-rank coal systematically,coal samples were taken from three typical low-rank coal mines including Tiefa Daxing Coal Mine,Shenbei Puhe Coal Mine,and Tiefa Xiaoqing Coal Mine,named as TFDX,SBPH and TFXQ.The middle-rank coal sample of Yima Xinyi Coal Mine was selected as the comparison,named as YMXY.The vitrinite reflectance?R_o?of the three low-rank coal samples ranged from 0.57 to 0.70%,and that of YMXY was 1.09%.The chemical structure of the four coal samples was analyzed using proximate analysis,ultimate analysis,and three spectroscopic techniques.The pore structure characteristics of the four coal samples were analyzed by means of gas adsorption,low-field nuclear magnetic resonance?NMR?and small-angle X-ray scattering?SAXS?.The steam-absorbing method was used to make uniformly wetting coal samples.The isothermal gas adsorption/desorption kinetic test device was used to test and analyze the gas adsorption and desorption characteristics of low-rank coal.Based on the three diffusion mathematical models,the gas diffusion coefficients of coal samples with different water contents were calculated,and the mechanism of the influence of moisture on the dynamic of low-rank coal time-varying diffusion was analyzed based on the time-varying diffusion coefficients.The main conclusions of this study are as follows:?1?The order degree of the four coal samples microcrystalline units is low,and the degree of graphitization and aromatization are generally low.Among them,the aromatization degree of three low-rank coal samples is lower than that of the coal sample YMXY,but the content of oxygen-containing functional groups is higher.The molecular structure of coal affects the development of pores in coal to a certain extent.The aromatic structure in the molecular structure inhibits the development of micropores and mesopores to a certain extent,but the accumulation and condensation of the aromatic ring layer promotes the development of mesopores.?2?According to the analysis by traditional physical adsorption methods,the specific surface area of the micropores of the three low-rank coal samples is in the range of 187.736?192.932 m²/g;the pore volume of the mesopores is 0.017?0.041cm³/g;the micropore specific surface area of the comparative coal sample is 182.813 m²/g,and the pore volume of the mesopores is 0.009 cm³/g.NMR experimental results show that the adsorption pores in the three low-rank coal samples are well developed and well connected,while the seepage pores in the comparative coal samples are well developed and well connected.SAXS shows that the mesopores of the three low-rank coals are more developed,and there are some closed pores with a pore diameter>16nm in the coal that cannot be detected by traditional N₂ adsorption methods.?3?The range of V_L of the three low-rank coal samples is 31.14?35.14m³/t,which is obviously larger than that of YMXY,and its value is 28.21m³/t.This is mainly because the adsorption capacity of coal to gas is largely affected by the pore structure and molecular structure.Micropores and mesopores play an important role in promoting the adsorption capacity of low-rank coal gas.The content of aliphatic structure in the molecular structure enhances the gas adsorption capacity,while the aromatic structure plays a role in suppression.?4?The order of limit analytical gas of four dry base coal samples from large to small is TFDX>SBPH>TFXQ>YMXY.The three low-rank coal samples have good connectivity of the adsorption pores,which can make the gas in the coal body desorb quickly in a short time.The increase of moisture in the coal body will reduce the limit desorption gas quantity of the coal body,but when it is increased to a certain extent,its influence on the limit desorption gas quantity is no longer obvious.?5?The time-varying diffusion coefficient calculated by the dynamic diffusion model can accurately characterize the full-time diffusion amount and diffusion law of low-order granular coal methane.The initial diffusion coefficients D₀ of the four dry-base coal samples are SBPH>TFDX>TFXQ>YMXY from large to small.As the moisture in the coal increases,the initial diffusion coefficients D₀ of each coal sample decrease.The smaller the pore size of the coal sample,the greater the decrease in D₀due to moisture.In the process of gas diffusion,as the pore size gradually decreases and the complexity of the pore network increases,the time-varying diffusion coefficient continues to decay.The better the coal sample connectivity,the shorter the time for the time-varying diffusion coefficient of gas to decay to zero.The increase in moisture in the coal reduces the time-varying diffusion coefficient,slows the rate of decay of the time-varying diffusion coefficient,and shortens the time to decay to 0.This article has 45 graphes,16 tables,and 134 references.