Experimental Study On Oxidation Characteristics And Activation Energy Of Coal During Spontaneous Combustion

Coal is an important energy.Coal mining has expanded in the direction of high yield,high efficiency and intensive production with the rapid development of industrial technology,the gradual implementation of thick-coal-seam mining methods,and the increase in mining depth,which lead to coal spontaneous combustion in goaf that frequently causes mine fires.Therefore,it is of great significance to understand the characteristics of coal spontaneous combustion for guiding the development of fire prevention and control technologies in coal mine and coal-related industries.So three coal samples with different spontaneous combustion were selects in this paper,the product gas,oxygen consumption,the change rule of the endothermic and exothermic etc were measured,and combined with the active material and functional groups,the change rule of the activation energy and stage characteristics of coal spontaneous combustion during coal low-temperature oxidation.Oxygen is the key factor for coal spontaneous combustion.At low temperature oxidation process,the active substance and oxygen functional groups in the coal react with oxygen with releasing heat to make coal temperature increase and along with the generation of gas products.So,according to the oxygen consumption and the generation law of gases in coal oxidation experiences,the different stages characteristics of coal spontaneous combustion were analysed.The results show: with the increase of temperature,single gas generation amount,oxygen consumption and oxygen consumption rate increased as the enhancement of spontaneous combustion tendency,and the temperature inflections that CO generation rose sharply,oxygen consumption increased sharply and the C2H₄ emergence were same.The CO,CO₂ and C2H₄ generation rate had obvious stages,and the maximum production rate temperature of CO and CO₂ was same.At the same time,single gas can only predict the early spontaneous combustion of coal,and the prediction isn't accurate and not timely.However,the composite gas indexes aren't influenced by environmental factors,and the different composite gases can predict the oxidation process of coal with low,middle and high temperature to analyse the oxidation state of coal.The variation tendencies of composite gas as CO₂/CO,CH₄/C2H6,C2H₄/C2H6,C3H8/C2H6 was measured,and combined with the coal low-temperature oxidation characteristics,which could be used to forecast the oxidation process and coal temperature.Meanwhile,the heating and cooling experiment was done for three coal samples,the change rule of CO,O₂,C2H₄ different amount and rate was obtained during heating and cooling,which are useful for assessing and predicting coal oxidation.According to the low-temperature oxidation experiment of three coal samples with different spontaneous combustion tendency,activation energy and characteristic temperature were obtained by the computational model based on the functional relationship between CO volume fraction and coal temperature,and the change law of activation energy and oxidation characteristics of three coal samples were analyzed in terms of TG-DSC experiment.The results show: the change law of activation energy of coals with difference spontaneous combustion tendency are significantly different,when coals reached the active temperature,activation energy decreased rapidly,and the mutation points of activation energy were corresponding to the conversion characteristic temperatures in coal oxidation process.According to characteristic temperature and the change regulation of activation energy and of coal,the low temperature oxidation process can be divided into four stages,namely,surface oxidation of coal,thermal oxidation,accelerated oxidation and deep oxidation.The research mechanism of coal spontaneous combustion would be attributed to the prevention and control of spontaneous combustion of coal.To analyse the different inhibition effect of carbon dioxide and nitrogen on coal oxidation combustion,the mass and energy change of coals with different spontaneous combustion tendencies were measured in different reaction atmospheres using the TG-DSC system.And the effects of spontaneous combustion and heating rate were analysed.The results show that: small significant difference are observed among change curves for coal weight loss and characteristics temperatures T1,T2,and T3 in the first three oxidation stages under different air,carbon dioxide-air mixed gas and nitrogen-air mixed gas.When coal temperature reached the ignition temperature T4,a greater increase in the carbon dioxide and nitrogen concentration resulted in a greater difference in TG/DTG curves and a greater obvious delay phenomenon of coal combustion,and the maximum mass change rate temperature T5 and burnout temperature T6 increased also progressively.DSC curve moved to the direction of high temperature,the heat quantity gradually decreased,the exothermic peak decreased and the peak temperature elevated,and the oxidation combustion slowed.Carbon dioxide has a greater capacity to delay and inhibit coal oxidation combustion than nitrogen.Meanwhile,the oxidation combustion of coal was affected by the spontaneous combustion and heating rate.With increasing carbon dioxide and nitrogen concentration in mixed gas,the activation energy and pre-exponential factor of coal combustion decreased compared with those in air,but the decrease in the pre-exponential factor is approximately an order of magnitude larger than that in the activation energy,which leads to an overall decrease in the reaction rate.The phenomenon indicated that the coal spontaneous combustion could be significantly inhibited by CO₂ and N₂.