Experimental Study On The Effect Of Water Immersion On Structures And Spontaneous Combustion Characteristics Of Coal

Coal spontaneous combustion is one of the main disasters threatening the safety of coal mining in China.In the process of mining shallow buried short-distance coal seams in Western China,it is found that spontaneous combustion occurs frequently after immersion and air-drying of residual coal in goaf of upper coal seam,and the risk of spontaneous combustion of immersed coal is higher than that of raw coal.However,there are few studies on the physical and chemical properties and spontaneous combustion characteristics of water immersed coal,which restricts the effective prevention and control of spontaneous combustion of coal in goaf under this condition.Therefore,it is of great scientific significance to study the physico-chemical structure and spontaneous combustion characteristics of coal by water immersion for the prevention and control of spontaneous combustion of coal in the process of mining shallow buried short-distance coal seams.The pore structure changes of coal after water immersion were studied by MIP and nitrogen adsorption method.The effects of water immersion on pore morphology,specific surface area,total pore volume,average pore size and pore size distribution of coal were analyzed.The results show that compared with raw coal,average pore size increases,total pore volume decreases,specific surface area decreases of immersed coal.After immersed,the pore absorbed water and swells,the pore enlarges,and the average pore size increases with the increase of soaking time.The pore size distribution maps of the two testing methods are analyzed comprehensively,and the pore size distribution curves of 0-10000 nm are obtained.There is a critical pore size value of 30-90 nm in the pore size distribution maps of lignite,long flame coal and anthracite,when the pore diameter value is less than the critical pore size,the distribution peak of raw coal is higher than that of immersed coal,and when the pore diameter value is greater than the critical pore size,the distribution peak of raw coal is lower than that of immersed coal.The proportion of micropore in immersed coal phase decreases and macropore increases compared with raw coal.The infrared spectra at room temperature and three-dimensional infrared spectra of lignite,longflame coal,anthracite raw coal and immersed coal were measured by in situ FTIR.The effects of soaking on the types of functional groups,the distribution characteristics of functional groups and the change law of main groups in the process of heating were analyzed.The results showed that the percentage of aromatic hydrocarbon groups in immersed coal was lower than that in raw coal.The proportion of aliphatic hydrocarbons and oxygen-containing functional groups in immersed coal is higher than that in raw coal.Compared with raw coal,the DOC value decreases,the CH₃/CH₂ value decreases,and the'C'value increases of immersed coal,which indicates that the rank decreases,the stability of aliphatic hydrocarbon chain decreases,and the maturity decreases of immersed coal.In the process of oxidation heating,the percentage of aliphatic hydrocarbons increases first and then decreases,and the aromatic hydrocarbon-CH,aromatic hydrocarbon-CH₂ and hydroxy-OH content decreased gradually,aromatic ring and methoxy C-O increased gradually.The carbonyl C=O content was low,which was an important group formed and participated in the oxidation process.The percentage of aliphatic hydrocarbon-CH₃,-CH₂-,-CH and hydroxyl-OH groups in the soaked coal is higher than that in the raw coal,while the aromatic ring C=C and methoxy C-O are lower than that in the raw coal.The soaking process changes the proportion of the main functional groups in the coal,and this effect can continue the whole stage of coal oxidation and spontaneous combustion.The oxidation and spontaneous combustion characteristics of raw coal and immersed coal were studied by coal spontaneous combustion experiment and TG-DSC simultaneous thermal analysis technology.The effects of water immersion on oxidation gas products,CPT,spontaneous combustion tendency and thermo dynamic characteristic parameters of coal were analyzed.The results showed that the absolute production of CO,CO₂ and C₂H₄ during low temperature oxidation of immersed coal was larger than that of raw coal.The initial formation temperature of C₂H₄ gas in lignite and long flame coal is lower than that of raw coal,indicating that the immersed coal can enter the accelerated oxidation stage ahead of time.The CPT of immersed coal is lower and the determination index I of spontaneous combustion tendency is lower,which indicates that immersed coal is easier to spontaneous combustion than raw coal.In addition,the law of gas generation in coal at different immersion time is consistent with the law of pore structure and main functional groups of coal.Compared with raw coal,the thermogravimetric curve of immersed coal shifts to the low temperature zone,the weight loss rate is higher,the peak value of heat flow curve is higher,and the exothermic process shifts to the low temperature zone.The initial exothermic temperature,critical temperature of low temperature oxidation stage,ignition point temperature and peak temperature of immersed coal are lower than that of raw coal,and the exothermic heat of low temperature oxidation stage and the whole exothermic process are higher than that of raw coal.The oxidation reaction of immersed coal is more intense.Based on the main functional groups in coal,the reaction modes of CO,CO₂ and alkanes were established,and the simplified reaction mode of coal oxidation was further established.Based on the experimental study on the physical structure,chemical structure and spontaneous combustion characteristics of coal,it is found that water immersion increases the content of active groups and the contact amount between coal and oxygen in the above reaction modes,and then increases the possibility of contact between oxygen and pore surface active sites,makes this reaction easier to occur,increases the reaction rate.Therefore,the production of CO,CO₂ and alkane gases in the reaction modes is larger then raw coal.The immersed coal has stronger oxidation and higher risk of spontaneous combustion then raw coal.