Study On The Cause Of Coal Oxidation Differences During The Processes Of Water Immersion And Dehydration

Coal spontaneous combustion is the main form of mine fires and a key focus of prevention and control.During the mining process,the mine is subject to the influence of mining.The water accumulated in the aquifer due to the development of overburden fissures is discharged to the goaf under the action of seepage and static pressure.The remaining coal is first soaked by water,and then soaked in water at a certain stage.When the mining conditions change,the environmental conditions where the remaining coal is located change.The loss of water in the coal body is a common phenomenon,which is manifested as water immersion and water loss in the project,However,the phenomenon of spontaneous combustion of residual coal after being soaked in water and air dried is more likely to occur.In order to explore the causes of the influence of water leaching dehydration physical process on coal spontaneous combustion,this paper uses research methods such as temperature programmed experiments,synchronous thermal analysis experiments,low-temperature nitrogen adsorption experiments,and infrared spectroscopy experiments to understand the effects and differences of water leaching dehydration physical process on coal spontaneous combustion.Based on the understanding of the effects and changes of water dispersion loss on the porosity,surface structure,pore structure,and functional groups of residual coal deposits,the role and changes of water dispersion loss are analyzed,Furthermore,from both macro and micro perspectives,the causes of changes in the spontaneous combustion characteristics of coal samples after water immersion and dehydration are revealed.The main research work is as follows:（1）Using temperature programmed experiments and synchronous thermal analysis experiments,coal samples with different particle sizes and residual moisture were tested and analyzed after water immersion and dehydration,as well as raw coal samples.The effects of the water immersion dehydration process on oxygen consumption,gas derivation,characteristic temperature points,heat absorption and release,and activation energy during coal low-temperature oxidation were studied,and the characteristics and differences of oxidation kinetics parameters and heat release effects were clarified.The research shows that in the temperature programmed experiment,when the coal temperature is between 30℃and 80℃,the oxygen consumption and oxygen consumption rate of the raw coal sample are lower than those of the coal sample after water immersion dehydration at the same temperature point,but the difference between the two is not significant.With the continuous reaction and the accumulation of heat,when the coal temperature reaches about 100℃,the oxygen consumption and oxygen consumption rate increase sharply.The concentration of gases such as CO,CO₂,and C₂H₄ increases with the increase of coal temperature.In the early stage,the generation of the three gases is not significant.When the coal temperature is about 100℃,the generation of CO and CO₂ also increases rapidly,consistent with the trend of oxygen consumption.As the initial water content in the coal sample increases after water immersion and dehydration,the concentrations of various gases show an increasing trend.The water immersion dehydration process enhances the tendency of coal spontaneous combustion and makes it more prone to coal oxygen composite reactions.According to the thermal analysis experiment,the thermogravimetric curve of the coal sample after water immersion and dehydration moves towards a lower temperature range compared to the original coal sample,and its critical temperature,drying temperature,ignition temperature,and burnout temperature all show a decreasing trend.As the particle size of the coal sample decreases and the initial water content increases after water immersion and dehydration,all characteristic temperatures show a decreasing trend.At the same time,the peak value of the heat flow curve of the coal sample after water immersion and dehydration is higher,and the temperature corresponding to the initial heat release temperature and maximum heat release power shows a decreasing trend,indicating that the temperature required for the coal sample to ignite after water immersion and dehydration is lower,and the risk of self ignition is increased.Through DSC curve calculation,it can be seen that after water immersion and dehydration,the coal sample releases more heat during the entire process of coal oxygen composite action,indicating that its oxidation combustion reaction is more intense.As the particle size of the coal sample decreases and the initial water content increases after water immersion and dehydration,the temperature corresponding to the initial heat release temperature and maximum heat release power shows a decreasing trend,while the maximum heat release power and total process heat release show an increasing trend.（2）Using a self-designed experimental system to observe the water loss pattern of crushed coal,the characteristics of porosity changes during the water loss process of crushed coal are studied by controlling three factors:particle size,initial water content,and environmental temperature during the water loss process after water immersion.The mechanism of the influence of these three factors on porosity is analyzed.Research has shown that after water immersion and dehydration,the porosity of coal samples increases compared to the original coal sample,and decreases with the loss of water in the coal,but still exceeds the porosity of the original coal sample.At the same time,as the environmental temperature of the coal sample increases,the initial water content increases,and the particle size of the coal sample decreases during the dehydration process,the porosity shows an increasing trend.The apparent volume of residual coal changes during immersion and dehydration,leading to changes in porosity and permeability.After dehydration,the pore volume of the coal sample increases,increasing the contact area between coal and oxygen.This can promote the low-temperature oxidation process of coal after a certain dehydration stage.（3）By using scanning electron microscopy and low-temperature nitrogen adsorption experiments,the surface structure and pore structure characteristics of raw coal samples and coal samples after water immersion dehydration were compared and analyzed.The main effects of environmental temperature,coal particle size,and initial water content of coal samples on the microstructure of coal samples after water immersion dehydration were analyzed,and the micro pore modification characteristics of coal samples during the water immersion dehydration process were clarified.Research shows that compared to raw coal samples,the number of pores and pore volume of coal samples increase after water immersion and dehydration,resulting in rougher coal surfaces and more developed pores and fractures.With the decrease of coal sample particle size after water immersion and dehydration,the increase of initial water content in coal and the increase of ambient temperature during air drying,the active site on coal surface increase,the pores in coal connect,the number of large pores increases,and the coal oxygen contact area increases due to swelling,mineral dissolution,and water scouring of coal,which is conducive to the occurrence of coal spontaneous combustion.According to the adsorption/desorption curves of raw coal samples and coal samples after water immersion and dehydration,it can be seen that different types of coal samples contain a large number of mesopores and macropores,with narrow porous pores as the main pore morphology and some cylindrical,ink bottle,and wedge-shaped pores.As the particle size of the coal sample decreases,the initial water content and environmental temperature increase after water immersion and dehydration,the specific surface area,total pore volume,and average pore size in the coal show an increasing trend.From the cumulative pore volume,it can be seen that there are more micropores and small pores in the coal sample;From the pore size distribution map,it can be seen that as the pore size increases,the peak distribution of coal samples with smaller particle sizes increases,indicating that the water leaching dehydration process has an effect on increasing the pore size of coal samples.（4）Experimental testing and analysis were conducted on raw coal samples and coal samples with different initial water contents and particle sizes after water immersion and dehydration using an infrared spectrometer.The changes in the main functional groups of the coal samples before and after water immersion and dehydration were analyzed,and the evolution characteristics of the main functional groups were qualitatively and quantitatively analyzed;At the same time,based on group reactions,the effect of water leaching dehydration on the coal oxygen composite effect was explored.Research shows that as the degree of coal sample fragmentation increases,the content of active functional groups in the raw coal sample increases;The total amount of functional groups of coal samples after water immersion and dehydration is lower than that of raw coal samples.It is speculated that the slow coal oxygen composite reaction of middling coal samples during water immersion and dehydration consumes some active functional groups.At the same time,under the action of water immersion and dehydration,the relative proportion of relatively stable aromatic hydrocarbons in coal decreases,while the relative proportion of easily oxidized fatty hydrocarbons increases.From the structural parameters,it can be seen that the structure of coal becomes more loose and has poorer thermal stability;The side chains of aromatic hydrocarbons in coal exist in the form of long chains,which are more prone to fracture,making coal more prone to oxidation reactions and spontaneous combustion.Through the above research,it was found that the spontaneous combustion tendency of coal samples increases after the water immersion dehydration process.On the one hand,after water immersion and dehydration,the porosity of the coal increases,making the gas flow channel in the coal larger;After water immersion and dehydration,the surface morphology of the coal sample becomes rougher,and the pores and cracks in the coal are more developed.The increase in pore volume and number increases the contact area between coal and oxygen.On the other hand,it can be seen from the perspective of micro chemistry that the water leaching dehydration process makes the coal macromolecular structure more loose and the number of active functional groups easy to participate in the reaction increases,which makes the coal more active and more prone to oxidation reaction.Combining macro and micro scales,the causes of oxidation differences in water immersed and dehydrated coal samples were revealed.The paper has 77 figures,13 tables,and 106 references.