| In the process of mining and utilization of coal mines,once the coal body is in contact with oxygen,different degrees of primary oxidation will occur.And secondary oxidation will occur in the contact between coal and oxygen when the coal in the goaf is re-extracted and the fire zone is unsealed.Moreover,the spontaneous combustion characteristics of coal secondary oxidation are greatly affected by the degree of primary oxidation.This paper taked different termination temperatures as one of the main influencing factors of coal primary oxidation degree.The effects of different primary oxidation degrees on the secondary oxidation and spontaneous combustion characteristics of coal were analyzed from the aspects of characteristic temperature,oxidation kinetics,thermal effect and microstructure of secondary oxidation of coal by theoretical analysing and experimental testing.The mass and heat change characteristics of the secondary oxidation process of coal were analyzed by a synchronous thermal analyzer when the initial oxidation termination temperatures were 75 ℃,115 ℃,155 ℃ and 195 ℃ respectively.The primary oxidation reduced the dry cracking temperature T2 and the ignition temperature T6 during the secondary oxidation of coal,and the burnout temperature T8 increased slightly,but for the critical temperature T1,the thermal decomposition temperature T5 and the maximum weight loss rate temperature T7 had little effect.According to TG experimental data,the activation energy of each coal sample in the two stages of water volatilization and desorption and oxygen gain and weight gain was calculated by multi-heating rate method.Among them,the 155 ℃ oxidized coal had the lowest activation energy in these two stages compared with other coal samples,that is,the initial energy that causes the coal-oxygen composite reaction to occur was the lowest.The DSC and its integral curve in the secondary oxidation process of oxidized coal at different temperatures were analyzd,it were concluded that the thermal effect in the secondary oxidation process of coal was divided into two stages.The first stage was the volatilization of water in the coal body and the desorption of the stored material to absorb heat from the outside and the second stage was that the coal body absorbed oxygen and increaseed its weight,and decomposed with the increase of temperature.After reaching the ignition temperature,a severe oxidation reaction occurred to release heat to the outside.Primary oxidation reduced the total heat absorption during the secondary oxidation of coal and increase the total heat release Compared to other coal samples,In the secondary oxidation process of 155 0C oxidized coal the total heat absorption was the smallest,and the total heat release was the largest.From the perspective of heat,the heat release effect of the coal sample was stronger.The Fourier in situ FTIR spectroscopy and TG-DSC experimental data were used to analyze the functional group change characteristics and migration laws of various characteristic stages in the oxidation of coal at different temperatures.The characteristic functional groups of 155 0C oxidized coal in the secondary oxidation process are more intense than other coal samples,and the functional group content changes more.In the early stage of the reaction,the coal sample reacts more fully with oxygen,and the amount of oxygen adsorbed is larger.The coal-oxygen composite reaction accelerates into the stage of thermal decomposition,and the temperature is first reached at the ignition point.The coal body undergoes intense oxidative combustion reaction and releases a large amount of heat.The risk of spontaneous combustion of secondary oxidation is higher.The research results provide theoretical basis and guidance for accurate prediction and targeted prevention of spontaneous combustion in close-range coal seam group,thick seam slicing mining and re-mining face in China. |
