Influence Mechanism Of Oxidation-reignition For Immersed Long-flame Coal Under Oxygen-lean Ambience

Coal spontaneous combustion is one of the main disasters threatening the safety of coal mining in our country.Large-scale coal fields in northwestern my country are shallowly buried and close to coal seams.The overlying coal seam is prone to water accumulation caused by the leakage of aquifers.When mining the lower coal seam,water needs to be drained from the upper goaf.After being immersed in water and air-dried,the surface structure and properties will change,which will aggravate the oxidation-resume process of coal under certain air leakage conditions.However,there are few studies on the oxidation-recombustion characteristics of pre-oxidized soaked coal in oxygen-poor environment,and the research on the physical and chemical properties and micro-reaction mechanism of pre-oxidized soaked air-dried coal is unclear.Therefore,a systematic study of the oxidation-recombustion characteristics and impact mechanisms of pre-oxidized soaked coal in an oxygen-poor environment is carried out to provide key theoretical basis and technical support for the prevention and control of spontaneous combustion of soaked coal in shallow buried coal seams.The influence of high-temperature pre-oxidation on the pore structure of water-soaked coal was found.SEM scanning method and low-temperature nitrogen adsorption method were used to test the surface and pore structure of coal samples in water soaking,pre-oxidation and pre-oxidation.The surface morphology,specific surface area,total micropore volume and average of coal samples treated with pre-oxidation soaking were analyzed.The pore size and other parameters and the pore size distribution law.The results show that:water immersion will increase the surface roughness of the coal body,increase the number of small and micro pores,and initially evolve into swelling pores,increase the specific surface area,and increase the water-locking capacity.With the increase of pre-oxidation temperature,the primary pores evolve into oxidized pores.The tiny pores gradually turn into medium and large pores,water can flow freely in the oxidized swelling pores,and the water-holding capacity is reduced;among them,the average pore diameter of the pre-oxidized 200℃immersion coal sample is the largest,the specific surface area is the smallest,and the proportion of large pores is higher than that of the 300℃oxidized coal sample.An increase of 37.4%.The oxygen-lean-recombustion characteristics of the pre-oxidized soaked coal body were studied.Using programmed temperature rise and TG-DSC experiments,the characteristics of index gas release,oxygen consumption change,low temperature oxidation and heating characteristics,stage evolution characteristics of characteristic temperature points,and oxygen-lean-recombustion heat effects of different processed coal samples were analyzed.The results show that:at the same temperature,the oxygen consumption of the water-soaked coal body is relatively large,and the amount of gas（CH₄,C₂H₄,CO and CO₂）generated by oxidation at a higher oxygen concentration（>15%）is relatively large.There is an intersection temperature between immersion and pre-oxidation immersion coal in an oxygen environment above 10%.The two treatments increase the oxidation and heating capacity of the coal.The intersection temperature of the pre-oxidation 200 immersion coal is the lowest（174.0℃,181.1℃,194.6℃,respectively）.The higher the oxygen concentration,the lower the ignition point temperature.The ignition point temperature of the pre-oxidized 200-soaked coal is the lowest（404.8℃,413.4℃,416.0℃respectively）,the activation energy is reduced,and the coal is easier to oxidize and reignite.The higher the oxygen concentration,the lower the maximum heat release temperature of each coal sample,and the maximum heat release reached 7.72 J/g.At 5%oxygen concentration,the total heat release of pre-oxidized 300 immersion coal is the smallest（3367J/g）,while at 3%oxygen concentration,the total heat release of pre-oxidation 200 immersion coal is the smallest（93.75 J/g）.The micro-reaction mechanism of the conversion of active sites and functional groups in the pre-oxidation leaching coal water is explained.FTIR was used to test the changes of functional groups in the pre-oxidized soaked coal,and analyze the changes of functional groups in different treated coal samples.The results show that water immersion and pre-oxidation treatment can increase the content of functional groups（-OH,C=O,COOH,-CH₃,-CH₂）.The higher the pre-oxidation temperature,the more obvious the increase,while the functional groups of pre-oxidized coal samples decrease after being immersed in water,200 pre-oxidation soaking in water will increase the value of-CH₃/-CH₂.Pre-oxidation activates the active sites in the coal.After oxidizing and immersing in water,the active groups of the coal sample will be pre-activated and lost.With the increase of pre-oxidation temperature,the functional groups will decrease sharply after immersion.The content of functional groups in the coal body is the lowest after pre-oxidation 300 immersion in water.