Study On The Electromagnetic Radiation Effects Of Coal And Rock Heating And Combustion Process

Coal is very easy to be threatened by spontaneous combustion in the process of mining,transportation and storage.China is the most dangerous country of coal spontaneous combustion in the world.Spontaneous combustion of coal causes a large amount of coal to burn out and pollute the ecological environment,causing threats to the life and property of the mine and its surrounding people.The coal field,the broken coal pillar,the left coal in the goaf and the coal heap(gangue hill)are difficult to be found at the beginning of the formation of the concealed spontaneous combustion fire.Although some progresses have been made in the detection technology of high temperature fire source of coal spontaneous combustion,the detection of high temperature fire source of coal spontaneous combustion is still a worldwide problem because of the formation and development of concealed fire as well as the complexity of factors.It is very necessary to detect the spontaneous combustion of coal and locate the location of the fire source efficiently and quickly.Aiming at the problem of hidden fire source detection of coal spontaneous combustion,through laboratory experiments,theoretical analysis and field tests,this paper mainly studies the time-frequency variation characteristics of electromagnetic radiation(EMR)signals in the heating and combustion process of coal.The change laws and mechanisms of EMR of coal heating and combustion process are revealed.A new EMR method for detecting the dangerous of concealed fire in coal spontaneous combustion is proposed.Finally,the EMR technology is applied to the detection and early warning of concealed fire in coal spontaneous combustion.The main results and conclusions of the paper are as follows:The time-frequency characteristics of EMR signals in the heating and combustion process of coal are studied.The test system of EMR during coal heating and combustion is established respectively.First,it is found that significant EMR signals can be produced in the process of coal combustion,and further fine tests have found that the EMR signals can be produced during the heating process of different metamorphic coal,and the frequency of the signal is covered by low frequency to high frequency range.EMR signals have a positive correlation with the temperature,which has a long range correlation,that is,the EMR signal is increasing with the increase of time and temperature.The growth and variation characteristics of EMR signals and CO are compared and analyzed.According to the multifractal theory,the morphology and fractal dimension of the fractal dimension of the EMR and the dynamic change of the fractal parameters ?? and ?f are used to characterize the combustion state and the deformation and rupture process of the coal.With the increase of temperature,the discreteness of EMR time series decreases,and the degree of thermal damage increases.The characteristics of EMR frequency domain are analyzed by fast Fourier transform.During the process of coal heating,the spectrum of EMR has the characteristics of broadening the main frequency band,showing the characteristics of low frequency and high amplitude.The internal damage and failure state of coal during heating up is analyzed based on the temporal variation of main frequency and amplitude of EMR.The time-frequency variation characteristics and differences of the EMR signals generated by the thermal damage of coal body and the loading fracture of coal and rock are compared and analyzed.Based on the experimental system of coal and rock combined damage and loading,the EMR signals of coal and rock under unrestrained conditions of heating up,under the condition of deformation and failure of coal and rock,under the condition of heating and loading are measured.The deformation and strength degradation characteristics of coal and rock under heating and loading are analyzed.The characteristics and differences of EMR of the coal and rock under different damage conditions are characterized by the change characteristics of the measurement value,change of the EMR signals and the change of the main frequency-amplitude.Because of the complex damage of coal and rock,the measurement and change trends of EMR signals are obviously higher than the changes under single loading or heating condition.The frequency of EMR signals is migrated,which is mainly high frequency signal,and the amplitude is gradually increasing.The generation mechanisms of EMR during coal heating and combustion process are revealed,the coupling relationship between EMR and temperature of coal is quantitatively analyzed.Scanning electron microscopy(SEM)and acoustic emission(AE)technology are used to characterize the evolution process of macro and micro fractures in coal and rock.When the coal is heated,the relaxation action produces free charge to accumulate to radiate electromagnetic waves,the transient change of the dual pole in the coal body and the thermal electron transition cause the free electron transmission to generate EMR.The charged ions in the combustion flame and chain reaction of charged ions of coal can both form the electromagnetic wave.In addition,the free electron migration caused by oxidative bonding of oxygen semiconductor compounds in coal heating,and the chain reaction of charged ions free radicals of coal spontaneous combustion can produce EMR.Besides,the thermal expansion coefficient and thermal stress in the heating process of coal and rock are analyzed,and the thermal stress of coal and rock under the action of temperature is calculated.The thermal damage degree of coal and rock mass is characterized by the change of elastic modulus.Based on the theory of damage mechanics,a thermo-electric coupling model of coal oxidation is established.The EMR detection method of coal field fire is put forward and its field application is carried out.The EMR spectrum and propagation characteristics of coal oxidation heating and combustion as well as coal rock loading are analyzed.The criterion of high temperature anomalous region located by EMR is obtained.Selected directional low frequency EMR antenna(range: 0~100kHz)combined with broadband antenna(0~500kHz)is used for field detection.Select the Xinjiang Urumqi Big Spring Lake typical coal fire,which test the EMR signals of high temperature region and outside the high temperature anomaly area.The EMR signals in high temperature abnormal area has good correspondence with temperature.The spatial characteristics of EMR detected in the coal fire are analyzed.EMR is used to orientate the abnormal high temperature region,and used the temperature of drilling to verifiy.The research results provide a basis for the application of EMR to detect hidden fire sources of coal spontaneous combustion,on-line monitoring and early warning of coal fires,coal pile fires and spontaneous combustion risk in mined out areas.