Study On Microstructure And Spontaneous Combustion Characteristics Of Water-immersed Air-dried Coal In HuaiBei Mining Area

In recent years,with the increase of coal seam mining intensity and water consumption during coal seam mining,the influence of underground rock water,there are many large areas of water-filled goafs in our country’s mining areas.The coal seam in Huaibei mining area are buried deep,with small spacing,large amount of coal left in the goaf,and easy to spontaneous combustion.Because most working faces adopt grouting to prevent fire and are affected by the cracked rock water,the left coal in the goaf is soaked for a long time,resulting in changes in the physical and chemical properties of coal.In order to ensure the safe mining of adjacent coal seam,drainage work must be carried out first.The process increases the air leakage in the goaf,leading to the risk of coal spontaneous combustion increase.At present,there are many experimental studies on water-soaked coal,but there is a lack of on-site testing,and there is almost no research on the spontaneous combustion characteristics of water-soaked coal.Therefore,based on experimental research,the paper combined with field tests,compares the physical and chemical properties and spontaneous combustion characteristics of different soaked coal,and finally reveals the spontaneous combustion law of the remaining coal after the flooded goaf is drained.The results provide guidance for the prevention and control of spontaneous combustion in flooded goaf.The thesis measured the surface morphology change of raw coal and different soaking coal samples by scanning electron microscopy experiments（SEM）.At the same time,the low-temperature N₂ adsorption experiment was used to test the pore changes of coal.The results show that whether it is coal immersed in water on site or in the laboratory for different days,there are different degrees of the number of pores and cracks increases,the connectivity of cracks and pores becomes better,and the contact area between coal and oxygen increases.Finally,it is more conducive to the coal-oxygen reaction.Guobei and Xutuan have the largest specific surface area value of coal immersed in water on site,followed by laboratory immersion for 60 days.Qinan field immersion coal has the largest average pore size,followed by laboratory immersion for90 days,indicating that the water has a significant effect on the“hole expansion”of the coal at this time,and the pores are enlarged,which facilitates the circulation of oxygen in the pore channels and accelerates the coal-oxygen reaction.The XRD experiment was used to analyze the phase composition and unit cell structure changes of coal before and after water immersion qualitatively.Fourier transform infrared spectroscopy was used to compare functional group change characteristics of raw coal and different water-immersed coal samples.It can be concluded that:The mineral content of coal after water immersion and air drying has been reduced.Among them,the relative content of clay minerals and calcite of coal samples have been reduced.In particular,kaolinite and calcite are more dissolved,while the relative content of quartz is basically unchanged.The dissolution of minerals will affect the pore structure parameters,including specific surface area and pore volume.From the perspective of the structural characteristics of carbon microcrystals,there are also differences in the structural parameters of water-soaked coal and raw coal.The interlayer spacing of the water-soaked coal samples increases,while the average diameter of the aromatic layer and the effective stacking height of the aromatic layer are lower than that of the raw coal.At the same time,the coalification degree of the water-soaked coal is greatly reduced,indicating that the water immersion process increases the spacing of the fragrant layers,reduces the effective stacking degree and ductility of the fragrant layers,and reduces the degree of coalification of the original coal,especially the largest change in the on-site water-immersed coal.The water immersion process changes the percentage of active groups of coal.Compared with raw coal,the aliphatic hydrocarbon content and the percentage of the hydroxyl（-OH）content in the water-soaked coal have increased to a certain extent.The three coal samples have the highest content of active groups in the on-site water-soaked coal,followed by GB-S₆₀,XT-S₆₀ and QN-S₉₀.The content of main active groups of water-soaked coal is increased,and the percentage of aromatic hydrocarbon group content is reduced,which increases the oxidation reaction rate of water-soaked coal,resulting in greater spontaneous combustion risk.Through thermal analysis and temperature-programmed experiments,the differences in the thermodynamic characteristics,oxidation gas products,oxygen consumption rate,cross-point temperature,and spontaneous combustion tendency of raw coal and different water-soaked coal are studied.It is found that:compared with raw coal,the low-temperature oxidation stage characteristic temperature of water-soaked coal is lower,and the peak point of DSC curve heat flow value has no obvious change,indicating that the immersion air-drying process can mainly promote the low-temperature oxidation stage of the coal,especially the on-site immersion coal has the greatest advancement.Further study the thermal characteristic parameters of laboratories immersed coal for 0～90d,the T₀,T₁and H₁ of raw coal and water immersed coal have the same changes.The lowest characteristic temperature and the highest heat release of XT and GB coal samples were obtained when immersed in water for 60 days and 50 days,it is QN was immersed in the laboratory for 70d.At the same time,during the oxidation process,compared with raw coal,the gas product generation and rate of immersed coal are higher,especially in the rapid oxidation stage（>150°C）,and they are the strongest on-site immersion coal.In addition,the oxygen consumption rate of water-soaked coal is higher,the temperature at the intersection point is lower,and the spontaneous combustion tendency is reduced,indicating that the water-soaking process improves the low-temperature oxidation ability of coal,and it is more prone to spontaneous combustion.Using the method of combining on-site observation and experimental research,by observing the gas component concentration and temperature in the water accumulation area and the non-water area in the 3₂36 goaf of Xutuan coal mine,it was found that the CO concentration and temperature at each measurement point in the water accumulation area are higher,low-temperature oxidation capacity of immersed coal is stronger.At the same time,based on the difference in water quality,the damage mechanism of the mine water to the microstructure of the coal was analyzed,it was found that the mine water was weakly alkaline,and the ion exchange between the minerals in the coal and the solution was more active in the alkaline environment.So the microstructure and spontaneous combustion characteristics of on-site immersed coal have a greater change degree compared with laboratory immersed coal,and the spontaneous combustion characteristic of on-site immersed coal is higher.Finally,from the physical and chemical point of view,the mechanism of water immersion affecting the spontaneous combustion characteristics of coal was analyzed,it was found that water immersion increased the coal-oxygen contact area and the percentage of active groups,and resulted in the rate of gas product of water immersed coal during the oxidation process is faster and the risk of spontaneous combustion is higher.There are 48 figures,29 tables and 101 reference in this paper.