Characteristic Analysis Of Lignite During Low Temperature Oxidation And Numerical Simulation Of Coal Pile Spontaneous Combustion

The coal spontaneous combustion has been a dangerous factor affecting the safety of coal mines.Researchers also try to explore the mechanism of coal spontaneous combustion from macroscopic and microscopic aspects.The heat transfer characteristics is a significant feature in coal spontaneous combustion,and it is closely related to the coal samples properties.A deep understanding of the heat transfer mechanism can provide a theoretical basis for the coal spontaneous combustion control.This Research started from heat transfer characteristics.The analysis was based on the combination of coal sample characteristics and spontaneous combustion characteristic parameters.The results of kinetic calculation can be used as parameters to the coal pile model simulation.The simulation results can also provide a basis for the prevention and control of coal spontaneous combustion.First,the three kinds of coal samples had low degree of coalification,high volatile content and grade I spontaneous combustion tendency,all of which were easy to spontaneous combustion.The spontaneous combustion characteristic parameters(ignition temperature,flammability index,comprehensive combustion characteristic index and total heat release per unit)of three kinds of coal samples were studied with three particle sizes and six heating rates,and which compared with FTIR results.It was found that ZT lignite with the high moisture content and more functional groups was the most prone to spontaneous combustion.In addition,the particle size and heating rate also affected the spontaneous combustion characteristic parameters.Second,the low-temperature oxidation kinetics was analyzed.The lowtemperature oxidation was divided into three stages: slow oxidation,accelerated oxidation and rapid oxidation.The apparent activation energy was calculated by KAS method to MD lignite,which with the best spontaneous combustion characteristics.It was found that at the slow oxidation stage,the smaller the particle size,the greater the activation energy.This was because that the slow oxidation stage was mainly controlled by quality transfer.At the accelerated oxidation and intense oxidation stages,the larger the particle size,the greater the activation energy,which was because that these two stages were mainly controlled by heat transfer.The kinetic parameters of three kinds of lignite were calculated by Coats method.It was found that the activation energy at the slow oxidation stage was mainly related to the water content.Because the peroxide complex was formed by free water and it had catalytic effect on the reaction between coal and oxygen.ZT lignite with the highest water content,it also had the lowest activation energy in the slow oxidation stage.Besides,the activation energy of MD and XLT lignite was lower,which with a large number of oxygen functional groups.In addition,the reaction mechanism function was related to the reaction stage and had no direct relationship to the particle size.Finally,the coal pile model and the coal pillar model which at the chimney effect area were constructed,and the pre-exponential factor and activation energy which calculated by Coats method were plug into the simulation by FLUENT software.The results showed that with the wind speed increased,the porosity decreased and the time increased,the area affected by temperature were increased,and the highest temperature points move horizontally to the deep.At the same time,with the wind speed increased,the highest temperature points move vertically.The increase of wind speed and porosity accelerated the spontaneous combustion,and the seepage influenced by porosity was an important control step of spontaneous combustion time.Based on the experiment of coal pile covered with pulverized coal,it is found that the thicker the pulverized coal is,the better the flame retardant effect is.The simulation agreed well with experiment,which showed the accuracy of the model.Finally,the simulation method was used to predict the spontaneous combustion of the three lignite piles in the slow oxidation stage(< 90 ?),and the results showed that the temperature diffusion range of ZT lignite pile was largest because it with highest moisture content.Besides,ZT lignite pile was easier to spontaneous combustion that was consistent with the dynamic analysis results.That was to say,the prediction method of the model had important guiding significance for the coal spontaneous combustion prevention.