Study On Critical Transition Characteristics Of Coal Oxidation And Spontaneous Combustion

The fire caused by coal spontaneous combustion has the characteristics of oxidation spontaneous combustion,smoldering,holdover fire and re-ignition.The fire usually occurs in coal gob area with approximate oxidative and thermal accumulation conditions.The ignition sources are concealed and difficult to detect and locate.The coal fire also induces the compound disasters and secondary disasters,which are difficult to prevent and control.The coal seams in most mining areas in China are prone to spontaneous combustion or spontaneous combustion,and most coal mine fires are caused by coal oxidation and spontaneous combustion.Thus,it is greatly important to explore the transformation mechanisms of coal oxidation and spontaneous combustion,providing basis for formulating coal fire prevention and control measures and developing new technologies.The domestic and overseas researchers have carried out a lot of fruitful investigations on the coal oxidation and spontaneous combustion mechanisms from the perspective of macro,micro or a combination,which has effectively promoted the progress and development of coal spontaneous combustion theory.However,due to the complexity of transformation processes and mechanisms of coal oxidation spontaneous combustion and the limitations of research and experimental methods,the coal oxidation and spontaneous combustion mechanisms needs further exploration and development,and the research methods and experimental means also need innovation and improvement.This thesis proposes the research topic on the critical transformation characteristics of coal oxidation spontaneous combustion.Combined with coal quality analysis method,characterization method based on on-line infrared thermal image RGB value,probe in-situ infrared analysis method and high-temperature thermal radiation method,a comprehensive experimental study on the critical transformation characteristics of coal oxidation spontaneous combustion is carried out.The coal has the characteristics of heterogeneous composition,uncertain microstructure,spontaneous combustion complicated process and many influencing factors,which are the main obstacles to the study of coal oxidation spontaneous combustion mechanisms.Coal is different from the materials with certain molecular structure and molecular weight,which can be analyzed directly from the molecular level or micro perspective.When the accurate information such as coal molecular structure and molecular weight is lack,it is necessary to acquire the detailed coal quality and coal macro characteristics to carry out the experimental research on the critical characteristics of coal oxidation and spontaneous combustion.In strict accordance with the experimental requirements,the author has carried out the collection and inert preparation of coal samples.The industrial analysis,elemental analysis,ignition temperature and calorific value of coal are investigated,and the spontaneous combustion tendency of coal and coal dust explosion are experimentally identified,so as to providing basic data for the subsequent experimental investigations on the critical transformation characteristics of coal oxidation and spontaneous combustion.The critical transition characteristics of coal oxidation spontaneous combustion are studied for the first time by using the on-line infrared thermal image characterization method based on RGB value.Based on the open-type coal oxidation heating condition,the critical transformation process and characteristics of coal oxidation spontaneous combustion are characterized by comprehensively analyzing the variation law of RGB value with temperature or time.The experiment results indicate that there is a functional relationship between RGB value of coal oxidation spontaneous combustion and time or coal temperature,and the characteristics between the coals with different spontaneous combustion tendencies are very obvious.Accordingly,five critical characteristic temperature intervals based on the critical transformation characteristics of coal oxidation spontaneous combustion are divided,which are thermal storage period,thermal storage-self-heating critical period,self-heating period,self-heating-selfignition critical period and spontaneous combustion period,and the specific range and corresponding critical temperature of each characteristic temperature interval are determined.The critical transition characteristics of coal oxidation spontaneous combustion are characterized and analyzed from the perspective of RGB value and temperature.Furthermore,combined with the characteristics of RGB curve and the smoothness of curve,an evaluation method of typical coal spontaneous combustion tendency based on on-line infrared thermal image characterization method is explored.The changing rules of functional groups in the critical transformation process of coal oxidation spontaneous combustion are studied by probe in-situ infrared analysis method.Based on the open-type coal oxidation heating condition,the transformations of functional groups in different characteristic stages of coal oxidation spontaneous combustion,such as thermal storage period,thermal storage-self-heating critical period,self-heating period,self-heating-self-ignition critical period and spontaneous combustion period,are analyzed.The results indicate that lignite mainly occurs physical adsorption during the thermal storage period,and some oxygen-containing functional groups and aliphatic hydrocarbons are removed in the thermal storage-selfheating critical period.The aromatic hydrocarbons in lignite occur ring-opening fracture reactions in self-heating-self-ignition critical period and further oxidize in spontaneous combustion period to produce the aliphatic hydrocarbons and oxygenated functional groups.The critical temperature of oxygen-containing functional group removal and fatty hydrocarbon side group fracture reaction of bitumite is higher than those of lignite.The fracture of hydroxyl and ether groups of bitumite occurs in the thermal storage-self-heating critical period,producing quinone functional groups.The aromatization degree of anthracite is relatively high,and its oxygen-containing functional groups and aliphatic hydrocarbon side chains exhibit high thermal stability during heating and oxidation process.There are local high-temperature points in the coal gob area before the fire,under certain conditions,the high-temperature points radiate heat to the surrounding coal body through overheating radiation and ignite the coal body.The effect of hightemperature thermal radiation on the critical characteristics of coal oxidation spontaneous combustion is studied for the first time by using the method of high temperature thermal radiation and cone calorimeter.The experimental results indicate that the ignition time of coal decreases with the increase in thermal radiation value,and the ignition of bitumite is always earlier than that of lignite.Impressively,under the experimental conditions,anthracite cannot be ignited no matter how the thermal radiation value increases.The peak heat release rate and total heat release amount of coal increase with the increase in thermal radiation value.However,under the same thermal radiation value,the lower metamorphic degree of coal has higher peak heat release rate and total heat release values.The peak smoke emission rate and total smoke emission of coals with different metamorphic degrees increase with the increase in thermal radiation value.The peak value of carbon monoxide production rate and total carbon monoxide emission of lignite and bitumite decrease with the increase in thermal radiation value,while these values of anthracite increases with the increase in thermal radiation value.The peak value of carbon dioxide production rate of coal increases with the increase in thermal radiation value.The total carbon dioxide emission of lignite decreases with the increase in thermal radiation value,and this emission amount for bitumite and anthracite increases with the increase in thermal radiation value.Therefore,the high-temperature thermal radiation can obviously promote the coal oxidation and spontaneous combustion process.The research outputs of this thesis are of great significance to enrich and improve the theoretical system of coal oxidation spontaneous combustion process and mechanism,which effectively prevent and control coal mine fire accidents.The new technologies for coal mine fire accident prevention and control are developed,and the life safety of coal mine workers will be guaranteed.