Study On The Effect Of Thermal Damage On Coal Structure And Spontaneous Combustion Oxidation Characteristics

With the coal mine gradually entering deep mining,the coal structure of mining coal seam is affected by more and more factors.Among them,thermal damage is one of the most important influencing factors,which mainly occurs in the mining process when the temperature of the surrounding rock rises,the magma intrudes into the coal seam and the coal field fire area causes the coal seam temperature to rise significantly.Under the action of thermal damage of coal seam,the physical structure of coal body such as pore and fissure as well as the chemical structure such as molecular group and free radical change,which has an important influence on the occurrence and development of coal spontaneous combustion.Therefore,this paper uses the research method combining theoretical analysis and experimental test to explore the change law of coal pores,free radicals and functional groups in the process of thermal damage,and then reveals the influence mechanism of thermal damage on the physical and chemical structure of coal and its spontaneous combustion oxidation characteristics.The main research contents are as follows:The effect of thermal damage on pore structure of coal body was studied based on low temperature liquid nitrogen adsorption technology.The results show that compared with raw coal,the pore volume of coal after thermal damage decreases,the average pore diameter increases,and the specific surface area decreases obviously when the thermal damage temperature is 225?.With the increase of thermal damage temperature,the specific surface area of coal increases slightly at first and then decreases obviously,and the average pore diameter increases gradually.The pore volume fluctuates,in which the micropore volume decreases gradually,the mesopore volume increases gradually,and the macropore volume increases in general.Thermal damage can promote the expansion and connectivity of coal pores,making micropores gradually develop into mesopores and macropores.At the same time,it is found that 150??225?is a key temperature period of thermal damage.The thermal damage effect at this stage can greatly promote the connectivity of coal pores,greatly expand the open space of coal pores,and contribute to the coal oxygen composite reaction.Based on in-situ electron spin resonance spectroscopy,the effects of thermal damage on free radicals in coal and the evolution of free radicals in raw coal and heat-damaged coal during the oxidation process were studied.The results show that the free radical concentration Ng and line width?H in the coal after thermal damage are obviously higher than those in the raw coal,while the g factor fluctuates.With the increase of thermal damage temperature,the g factor in coal decreases first,then increases and then decreases,the free radical concentration increases first and then decreases,and the line width gradually increases.In the process of oxidation and heating of coal,with the increase of oxidation temperature,g factor in coal is basically unchanged at first and then increases sharply,free radical concentration continues to rise,and line width generally increases.In the whole oxidation and heating stage,compared with the raw coal,the g factor,free radical concentration and line width of the coal body after thermal damage are larger and the rate of change is faster,indicating that thermal damage can improve the activity of free radicals,accelerate the coal oxygen reaction rate,and promote the coal oxygen complex reaction process.Moreover,when the thermal damage temperature is at 150?,the number of high active free radicals in coal is the largest,and the promotion effect on coal spontaneous combustion is the strongest.Based on Fourier transform infrared spectroscopy(FTIR),the effect of thermal damage on the molecular structure of coal and the evolution of the main functional groups in raw coal and heat-damaged coal during the oxidation process were studied.The results show that with the increase of thermal damage temperature,the OH-OR hydrogen bond content in hydroxy-OH has little change,and the OH-OH hydrogen bond content decreases gradually at first and then increases slightly.The change of-CH₂-content in aliphatic hydrocarbon was the most obvious,and the symmetric-CH₂-content increased gradually.The content of C-O remained unchanged,while the content of C=O decreased gradually.The content of aromatic hydrocarbons decreased first and then increased.In the process of oxidation and heating,the content of-OH in coal decreases rapidly at first and then slowly decreases,the content of C-O increases gradually,and the content of C=O increases.The aliphatic hydrocarbon content of-CH₂-,-CH₃ decreased rapidly at first and then slowly;The content of aromatic hydrocarbons changed little and only increased slightly in the later stage of coal oxidation.In the whole oxidation and heating stage,compared with the raw coal,the reaction rates of-OH,C-O,C=O,-CH₂-,-CH₃ and aromatic hydrocarbon in the coal after thermal damage are faster,indicating that thermal damage can reduce the degree of intermolecular crosslinking,increase the number of active sites on the coal surface,and make more active groups in the coal contact with oxygen.Through the contrast analysis of thermal damage to physical and chemical structure of coal before and after the change,and the microscopic characteristics of coal spontaneous combustion and thermal damage of coal oxidation,thermal damage effects to broaden the channels,improve the reaction activity and increase the reaction site to speed up the coal and oxygen reaction rate,promote the process of coal and oxygen reaction,and when the temperature of the thermal damage is at 150?,the promoting effect is strongest.There are 43 pictures,11 tables and 96 references in this paper.