Study On Structure And Spontaneous Combustion Characteristics Of Non-caking Coal During Water Immersion

Inner Mongolia mining area is an important coal production base in China.Most of the coal seams in the mining area are close to each other and easy to spontaneous combustion.In the process of mining,under the influence of mining,fissure development forms water seepage channel,which leads to long-term immersion of coal,changes of physical and chemical properties of coal,and the phenomenon of"water and gas displacement"occurs in the process of water exploration and drainage,which increases air leakage and increases the risk of coal spontaneous combustion.Due to the unclear understanding of the mechanism of coal spontaneous combustion after water immersion,it is impossible to grasp the risk of coal spontaneous combustion after water immersion in goaf and to take measures to prevent and extinguish fire.This paper takes the non-caking coal of Hulusu and Menkeqing coal mines as the research object,explains the influence of water immersion on the characteristics of coal oxidation and spontaneous combustion from the perspective of the physical and chemical structure of coal,and formulates fire prevention and extinguishing design of water-immersed goaf based on the research conclusions,so as to provide some scientific basis for fire prevention and extinguishing related work of water-immersed goaf.Scanning electron microscopy(SEM)was used to compare the surface pore structure of soaked coal with that of raw coal.It was found that the surface structure of soaked coal was rougher.On the one hand,the minerals attached to the surface or pores dissolve in water,exposing more original pores and fissures,thus improving the connectivity of the coal surface.The connectivity of the hole is improved.On the other hand,after the matrix is soaked in water and crumbles,more secondary intergranular pores are formed,which increases the contact area between coal surface and air.Through the pore adsorption experiment,it was found that the micropore specific surface area and total pore volume of the soaked coal decreased in different degrees,while the mesoporous and macroporous specific surface area and total pore volume increased,and the proportion of macropore increased.The change of pore structure of coal by water immersion occurs in the form of"pore expansion"and"pore paralleling".30 days is the key time to change the pore structure of the soaked coal.When the immersion time is less than 30 days,the pore size distribution of coal has little change.After 30?60 days of immersion,the number of micropores decreased.The leaching caused the dissolution of minerals and the connection of some pores.After 90days of immersion,the number of micropores continued to decrease,while the number of mesopores and macropores above 40nm showed an increasing trend.This was mainly because the coal fully absorbed water and swelled,leading to the merger of pores in adjacent locations,and the number of pores decreased while the number of macropores increased.Functional group composition of raw coal and water-immersed coal were analyzed by Fourier transform infrared spectroscopy.It is found that the time of functional group change after immersion is related to the key time of pore structure,and 30 days is the key time of coal functional group structure change.When the immersion time is less than the critical time,the functional groups do not change significantly.After 30?60days of immersion,the content and proportion of oxygen-containing functional groups in the coal increased.After 90 days of immersion,the content of self-associating hydroxyl formed between the macromolecular structure of coal benzene ring and water molecules increased,and the proportion of corresponding phenolic hydroxyl ether bond increased,while the content and proportion of oxygen-containing functional groups decreased.The impregnation process leads to the increase of hydroxyl activity and the content of oxygen-containing functional groups,thus improving the reactivity of coal.The release heat and ignition activation energy of coal at different soaking time were measured by synchronous thermal analysis.Compared with the raw coal,the initial exothermic temperature and ignition point temperature of the soaked coal are lower,and the release heat of the soaked coal in the low temperature oxidation stage is higher than that of the raw coal,and the coal in the air atmosphere enters the exothermic stage and the oxidation acceleration stage in advance,and the reaction process is advanced.The ignition activation energy of waterlogged coal is less than that of raw coal,and the waterlogged coal is prone to spontaneous combustion.With the increase of soaking time,the spontaneous combustion tendency of coal increases.The spontaneous combustion characteristics of waterlogged coal were studied by programmed temperature experiment.It is found that the oxygen consumption and oxygen consumption rate of soaked coal are higher than that of raw coal.Before the programmed temperature rises to 100?,the oxygen consumption rate of coal soaked in water for 60 and 90 days is obviously higher than that of raw coal and other coal samples soaked in water,showing a strong oxygen consumption capacity.In the process of coal oxidation and heating,the production of CO,CO₂ and alkane gases is higher than that of raw coal.The C₂H₄ gas of raw coal and soaked coal appeared at 90?,and the C₃H₈ gas of raw coal appeared at 120?130?,and the formation temperature was advanced to different degrees with the increase of soaking time.After immersion,the temperature at the intersection of the three non-stick coals is lower than that of the raw coals,and the coal samples at 60 and 90 days of immersion are the least.The three kinds of non-stick coal raw coal are all spontaneous combustion,and the tendency of spontaneous combustion increases with the increase of soaking time,which turns into easy spontaneous combustion and the risk of spontaneous combustion increases.Combined with the above study on the influence of water soaking on coal structure,it can be explained that the"pore expanding"and"pore blending"functions of water soaking provide more places for coal to contact with oxygen and react with it.Meanwhile,the increase of the content of oxygen-containing functional groups and other active groups further improves the reactivity of coal,making it more prone to spontaneous combustion of coal soaking.Based on the variation characteristics of spontaneous combustion of submerged coal,and combined with the existing technology,the design of fire prevention and extinguishing technology for water-immersed goaf is proposed.Based on the CO,C₂H₄and C₃H₈ index gas,working face temperature and goaf water temperature,the natural combustion of coal in water-immersed goaf is predicted comprehensively,and the daily beam tube monitoring,crack sealing and reinforcement of small coal pillar,integrated prevention and fire extinguishing techniques of nitrogen injection and grouting are put forward to prevent and control the spontaneous combustion of coal in water-impregnation goaf.48 figures,20 tables,78 references...