Experimental And Modeling Study Of Gas Adsorption-Desorption Diffusion In Coal Particle Under The Effect Of Multiple Factors

"High ground stress,""high ground temperature,""high gas prominence,"and"low permeability"are typical characteristics of coal resources mining under the new normal.The adsorption-desorption-diffusion characteristics of pulverized coal bodies are closely related to coal and gas prominence risk.Therefore,studying the adsorption-desorption-diffusion law and mechanism of coal with different degrees of metamorphism under different conditions of temperature,stress,gas pressure,and particle size,establishing and improving the corresponding mathematical and physical models and validating them,can provide theoretical basis and technical support for the efficient and safe extraction of gas from deep coal seams and the prevention and control of gas disasters in mines,which is of great practical significance for the safe and efficient production of coal enterprises in China.However,the existing research describing the coal gas adsorption model and coal gas full-time desorption diffusion model under the action of multiple factors still needs to be improved,and the corresponding mechanism analysis needs to be deepened.Based on this,this thesis builds an experimental system that can realize the experiments of coal gas adsorption and desorption diffusion under the action of multiple factors,based on the experimental data combined with the nano-pore structure characteristics of coal and the surface structure and chemical composition of coal,we study the characteristics of coal gas adsorption and desorption diffusion under the action of multiple factors in a more systematic way,discuss the mechanism of coal gas adsorption and desorption diffusion under the action of multiple factors,and construct a model of coal gas adsorption including temperature,gas pressure,and axial pressure.A model of coal gas adsorption and full-time diffusion,including temperature,gas pressure,and axial pressure,was constructed to reference the systematic study of gas adsorption and desorption diffusion mechanism in deep coal seams.The main research results of this thesis are as follows.The nano-pore structure characteristics and surface structure,and chemical composition of coal were studied experimentally,the multi-scale pore size distribution of coal samples in the pore size range of 0.02-360000 nm was plotted,and the content of the three types of pores of coal samples was obtained by integrating the curves in the corresponding pore size range respectively.The carbon structure parameters of the three coal samples were obtained,and the contents and distributions of various functional groups in aromatic hydrocarbons,oxygen-containing functional groups,and aliphatic hydrocarbons of coal samples were calculated and analyzed.The occurrence forms of carbon,oxygen,nitrogen,and sulfur in the surface structure of coal were analyzed,and the contents of various functional groups were calculated.The influence of coal nano-pore structure and chemical composition on gas adsorption and desorption characteristics were analyzed.The following chemical compositions were found to promote adsorption:the content of tetrasubstituted benzene ring in aromatic hydrocarbons,the content of hypomethyl in aliphatic hydrocarbons,the content of aromatic units and their substituted alkanes(C-C,C-H)and carboxyl groups(COO-),the content of pyrrole-type nitrogen N-5 and the content of sulfoxide type sulfur,the content of the above functional groups and the microporous content of the coal samples were positively correlated with the adsorption performance of the coal.The following functional groups inhibit adsorption:benzene ring disubstitution,phenolic or ether carbon(C-O)and carbonyl(C=O),and pyridine nitrogen N-6.The gas adsorption law and desorption-diffusion law of coal particles under the effect of multiple factors were experimentally investigated.The results show that:the gas adsorption of coal decreases with the increase of temperature,which is approximately linear and negative and increases with the increase of gas pressure,which is generally non-linear.For the three coal samples studied in this thesis,the gas adsorption quantity decreases and then increases with increasing volatility;the microporous content of coal has an essential role in promoting coal adsorption;the adsorption quantity decreases with increasing particle size(40-60 mesh,60-80 mesh,and 80-100 mesh);the adsorption quantity does not vary monotonically with axial pressure at the same temperature and different gas pressures,but decreases and then increases with increasing axial pressure at higher gas pressures.When the gas pressure is high,the adsorption quantity decreases with the increase of axial pressure and then increases;when the gas pressure is low,it decreases with the increase of axial pressure.The cumulative desorption quantity of gas under different conditions without axial pressure loading is the same as that of adsorption;the cumulative desorption quantity of gas decreases with increasing axial pressure,which is different from the adsorption quantity with axial pressure.The magnitude of gas adsorption quantity on coal is related to the number of methane molecules n in the adsorption space and their residence timeτon the coal surface.The former is closely related to the temperature and gas pressure,and the latter is closely related to the degree of coal metamorphism,surface structure,and pore structure.The nature of gas adsorption by coal is due to induced and dispersive forces between coal molecules and methane molecules and dispersion forces between methane molecules.The effect of temperature and gas pressure on adsorption is due to changing the average kinetic energy and number of molecules of methane molecules.The pore structure of coal under the influence of gas and axial pressure will produce complex changes.In the presence of gas pressure,the pore structure of coal appears to change in three stages with the increase of axial pressure-pore compression stage,pore transformation stage,and pore compaction stage,which correspond to the three changing stages of gas adsorption amount.At a certain gas pressure,the adsorption amount decreases with the increase of axial pressure in the first stage,increases with the increase of axial pressure in the second stage,and decreases again with the increase of axial pressure in the third stage.The trend of gas adsorption with axial pressure was plotted,and the curve was found to shift to the upper left as the gas pressure increased.The cumulative desorption diffusion quantity of gas under the same conditions decreases with increasing axial pressure,different from the trend of adsorption quantity with axial pressure.Four different virtual saturation vapor pressure(P0)calculation methods(Reid method,Antoine method,Astakhov method,and Amankwah method)were applied to three adsorption models(two-parameter BET model,three-parameter BET model,and D-A equation)to select the applicable adsorption model based on the experimental adsorption data of six different volatile fraction coal samples.The D-A equation was a reliable adsorption model for calculating P0 using the Astakhov method,and the adsorption data of the six coal samples could be well fitted using this model.Based on the experimental data,adsorption mechanism,the preferred adsorption model,and the physical model of pore structure of coal samples,a model was constructed to calculate the gas adsorption of coal particles under the effect of multiple factors,and the gas adsorption of three coal samples with a total of 162 working points was calculated using this model.The average absolute errors were 0.5863 cm~3/g,1.0821 cm~3/g,and 0.6683cm~3/g for DLT,YW,and JG coal samples,and the average relative errors were 5.0131%,7.7593%,and 5.7422%,respectively,compared with the experimental data.The data matched the model fit.Based on the experimental data on desorption diffusion and previous research results,a model was constructed to calculate the gas desorption diffusion under the effect of multiple factors.The model can fit the data of cumulative desorption diffusion with time at different working points.Compared with the experimental data,the average absolute errors were 0.03498 cm~3/g,0.1899 cm~3/g,and 0.03549 cm~3/g,and the average relative errors were 1.4908%,4.2393%,and 2.0095%for the 54 working points of DLT,YW,and JG coal samples,respectively.The data matched the model fit.The surface potential(Ω)and Gibbs free energy change(ΔG)of the three coal samples were calculated using the constructed adsorption model,both of which were negative.The absolute value of surface potential increases with the increase of gas pressure,decreases with the increase of temperature and increases with the increase of micropore content of coal samples.The influence of axial pressure on surface potential is not monotonous.When the gas pressure is low,it decreases with the increase of axial pressure,and when the gas pressure is high,it first decreases and then increases with the increase of axial pressure.The absolute value of Gibbs free energy change increases with the increase of gas pressure,decreases with the increase of temperature,increases with the increase of micropore content of coal samples and decreases with the increase of axial pressure.The variation trend of the two thermodynamic functions with axial pressure is similar to that of coal samples’adsorption quantity and desorption quantity.