The Influence Of Geological Structure Of No.3 Coal Seam In Xinjing Mine On Coal Pore Structure And Ad-desorption Characteristics

The coal is broken in the geological structure of coal seams.This fragmentation process changes the pore structure characteristics,which causes the gas ad-desorption performance changes.Therefore,it is necessary to study the influence of the geological structure of the coal seam on coal pore structure and gas ad-desorption characteristics.This paper selects coal samples of two different structural types in NanJiu roadway?NJY?and BeiJiu roadway?BJR?of Xinjing coal mine and classifies the coal sample into six different particle sizes?1-3mm,0.5-1mm,0.25-0.5mm,0.2-0.25 mm,0.074-0.2mm,and<0.074mm?under laboratory conditions,which imitates the geological structure of coal seams.Theoretical analysis,laboratory measurements,and other methods were used to systematically study the differences of pore structure and gas ad-desorption characteristics during the coal crushing process and quantitative analysis of the causes of these differences.Paper analyzes the role of coal particles with small particle size in coal and gas outbursts on this basis.The main findings of this paper are as follows:1)The process of geological structure has little effect on moisture,volatile matter,and fixed carbon content of the same type of coal sample.The ash content shows an increasing trend as the particle size decreases.The liquid nitrogen adsorption isotherms of coal samples all show continuous,no-jump rise characteristics.As the degree of fragmentation increases,coal samples with smaller particle size exhibit greater pore volume and adsorption capacity.The NJY coal sample contains both open pores and closed pores,the pores of the coal samples mainly consist of columnar pores.The BJR coal sample has large number of open pore,and the type of the coal sample pores is mainly ink bottle or the like.This shows that during the geological structure of the coal seam,the coal pore structure becomes more and more simple,and the pore type gradually transitions from the closed and semi-closed to the open pore morphology,which makes the path of gas entering or leaving the pore more shorter and the resistance more smaller.2)As the crushing degree of coal samples increases,the pore size distribution of NJY and BJR coal samples gradually transform from micropores and small pores to small pores and macropores.The pore surface area of coal samples is microporous.The pore volume and specific surface area of micropores,small pores and mesopores?<400mm?of small particle size coal samples are larger.Compared with NJY coal samples,the pore volume and specific surface area of microporous pores of BJR coal samples are larger.This shows that during the geological structure of the coal seam,the coal pore volume and specific surface area are getting larger and larger which means the coal pore volume and specific surface area with high fracture type are larger under the influence of geological structure and the ability of coal to adsorb and store gas is enhanced.The pore structure is more conducive to the migration and diffusion of gas.The fractal dimension of the coal pore structure is used as the fractal dimension of the liquid nitrogen adsorption method for coal samples.As the crushing degree of coal samples increases,the coal pore structure gradually becomes simpler,which shows that the coal pore structure gradually becomes simpler in the process of geological structure.3)The change of pore characteristics during the the process of geological structure has a significant effect on the ad-desorption properties.With the increase of coal sample crushing degree,the Langmuir pressure of NJY and BJR coal samples monotonically decreases,other indexes gradually increased,such as Langmuir volume,Q₁/Q₁₂₀ value,total desorption amount at 1min and 120min,limit gas desorption amount,initial gas desorption coefficient and initial gas desorption rate.Compared with NJY coal samples,the Langmuir pressure of the BJR coal sample is smaller and the other indexes are larger.This is due to the coal pore volume and the specific surface area increases during the geological structure of the coal seam,especially the pore volume and specific surface area of microporous have significantly increased,which provids more adsorption sites for the gas adsorbed on the coal surface.The coal pore structure becomes more and more simple,and the open pore type is developed,so that the path for the gas to enter the coal pore becomes shorter and the resistance becomes smaller,which accelerates the saturation of the adsorption process.The geological structure of the coal seam improves the desorption performance.As the degree of fragmentation increases,the amount of gas released by the desorption is greater,and the pore structure of the coal changes toward the release of coal desorption performance,especially the coal samples of small particles exhibit stronger initial desorption performance.4)Paper selects Langmuir volume and Langmuir pressure as adsorption parameters,and selects initial gas desorption coefficient,limit gas desorption amount and initial speed of gas diffusion as desorption parameters.The analysis established on these parameters and pore structure parameters?micropore volume,BET surface area,pore structure fractal dimension?find that in the process of geological structure of coal seams,as the degree of fragmentation increases,the coal pore volume and specific surface area become larger and the pore structure becomes more and more simple.This results in the larger ads-desorption capacity and the better initial gas desorption performance,also the initial speed of gas diffusion is greater,which means the coal has the higher outburst potential.5)When the outburst critical desorption speed sets to 0.05646m L/g·s,the average gas desorption speed at the first minute of the NJY and BJR coal samples is below the value when the particle size of the coal sample is larger than 0.25mm,which indicates that small particle size coal is a necessary condition for the occurrence of coal and gas outbursts.The gas expansion energy of the coal sample is mainly converted into coal transport work.With the decrease of particle size,the gas expansion energy of coal samples showed an increasing trend,indicating that the gas expansion of small-size coal samples was greater.The higher the gas pressure at the prominent point,the higher the expansion energy of the gas produced by the coal sample.The critical particle size corresponding to the outburst critical desorption speed of the BJR coal sample is larger than that of the NJY coal sample,and the gas expansion energy is also higher,indicating that the BJR coal sample has a higher outstanding potential.For NJY and BJR coal samples,the gas expansion energy required for the transport of outburst coal should reach 1000 kJ/t or more.The study shows the coal with small particle sizes provides an important energy basis throughout the coal and gas outburst process.