Segmented Electrical Characteristic Of Coal Spontaneous Combustion And Transient Electromagnetic Detection Technology On Localization Of Combustion Area In Coal Mine

Owing to our country's complex geological conditions of coal seams,the large number of them are easy to spontaneous combustion,the increasing promotion of the high yield and high efficiency intensive sub-level caving mining technology and the increasing mining depth as well as the oxidation characteristics of coal,spontaneous combustion fire accidents still happen in the process of coal storage,mining,transportation and preservation.The coal spontaneous combustion has severely affected our country's sustained and healthy development of coal industry,and brought serious damage to the country and people's property.In addition,there are more than 90% coal mines using underground mining technology.Once underground coal spontaneous combustion occurred,a large amount of toxic and harmful gas and high temperature smoke are very likely to enter into the normal underground ventilation system and spread to the workplace,causing workers poisoned,even accident of suffocation death with group character.Furthermore as mining depth continuously extended,some mines in Eastern and north-eastern area have entered a mining level of km deep mine,showing the characteristic of low gas mine turning into high gas mine,then high gas mine turning into outburst mine.As a result,the degree of danger of coal gas burning or explosion and other secondary disasters caused by coal spontaneous combustion increased year by year.However,whether the spontaneous combustion is in coalfield outcrop,in ground coal storage or in the goaf or the spontaneous combustion of coal pillar,their high temperature fire sources all have the concealment feature in common.Therefore,if we can quickly,effectively and accurately locate the area of high temperature fire source of the coal spontaneous combustion,it will bring important social and economic significance to our country's coal mine safety production and resources and environmental protection.The available fire exploration technologies in our country mainly include thermometry,physical exploration method,gas detection method and remote sensing method.Thermometry is a traditional fire detection method,based on the relationship of coal temperature and occurrence-development degree,location and range of coal spontaneous combustion,using the contact and non-contact detection instrument to locate the area of concealed high temperature fire source.The defects of this method are that the high temperature fire source location caused by coal spontaneous combustion often appears at a long distance of the coal's surface,besides its specific location is more likely affected by the surrounding air flow,thermal dispersion and heat convection between gas and solid,so the high temperature fire source location changes over time and is not easy to be found.Moreover because of the poor thermal conductivity of coal,when the heat is transferred from the inside out to the detecting area of the temperature measuring instrument by means of heat conduction,the internal fire has actually developed a large-scale fire,the hysteretic nature in time has brought great difficulties to the detection and identification of concealed high temperature fire source.The gas detection method has the advantages of simple operation and wide detecting range,but it is seriously influenced by air leakage.The remote sensing method is mainly applied to large area fire detection;meanwhile it has the disadvantages of high cost and complicated operation.Compared with the above methods,physical exploration method is still at exploring stage.It has the advantages of high efficiency,low cost,large detecting area,high resolution and strong empirical comparison of exploration and mining.Its advantages not only have gained general attention from many scholars at home and abroad,but also the State Administration for Safety Management and Control has pointed in the newly proposed draft of coal mine fire prevention technology development countermeasures(2015)that the next step of safety science and technology work will be multi-information detection technology that based on magnetic method and electromagnetic method in concealed coal spontaneous combustion fire area.At present,physical exploration methods which have certain application cases in high temperature coal spontaneous combustion fire detection aspect mainly include: high density resistivity detection technology,high precision magnetic method exploration technology,transient electromagnetic method detection technology,micro-seism method detection technology.The above physical exploration methods are mostly applied in coalfield.Due to more interference factors,limited working space and intrinsic safety requirements that the laws and regulations proposed on mineral equipment in the underground coal mine environment,it puts forward new requirements to physical exploration technology and detection methods in high temperature burning area caused by coal mine spontaneous combustion.In addition,the research on electromagnetic property parameters of coal is mainly concentrated in the last century before the 90 s,and there are few studies on the determination of electromagnetic property parameters in the process of different stages of spontaneous combustion and pyrolysis,as well the understanding of coal electromagnetic changing characteristics after high temperature oxidation and carbonization is not deep,these will undoubtedly increase the difficulty of physical exploration data processing and interpretation.After the analysis of the existing fire source positioning technology and compared a number of physical exploration methods,the author with his research team present that applying transient electromagnetic detection technology to detection of high temperature fire source in mine fire,and conduct experiments on coal sample that passed through spontaneous combustion and pyrolysis,which contain experimental study on medium and low frequency electromagnetic parameters.The research refers to digital and signal theory,electromagnetic field and electromagnetic wave theory,dielectric physics theory,coal spontaneous combustion theory and some relevant experimental schemes and field detection methods,and the research studies the detection method's key issues and key technologies from two aspects of theory and practice.This dissertation includes the following contents:i.)Theoretical analysis of transient electromagnetic method under full space effect.On the basis of reading a large number of domestic and foreign literatures,the detection principle of transient electromagnetic method(artificial controllable source)was studied.Aiming at the mine transient electromagnetic method,an emerging electromagnetic exploration method,this part studies various arrangements of transmitting and receiving coil devices for transient electromagnetic method,influence of multi-turn small loop device on observation signal and the characteristics of forward and inversion theory under the full space response theory.Besides waveform characteristics of Rectangular Current Wave,Trapezoidal Current Wave,Half Sinusoidal Current Wave and Triangular Current Wave in transient electromagnetic excitation field was also studied.This dissertation also studyed the noise suppression principle of bipolar wave,analyses the necessity of using bipolar wave signal circuit in transient electromagnetic detection system from the perspective of electronic circuit theory,and analyses the range of frequency values that selected in transient electromagnetic exploration.The spectral characteristics of each current waveform in the frequency domain were analysed by using of Fourier spectrum analysis theory.According to Biot-Savart-Laplace Law,the magnetic field distribution formula of single-turn rectangular coil was deduced.The magnetic field distribution of the circular coil can also be calculated by the reciprocal theorem transformation in electromagnetism.Numerical analysis of the characteristics of the primary field before the off current were studied,and it was concluded that the excitation range of the detection coil to the underground space can be effectively improved by increasing the number of turns of the transceiver coil.ii.)Experimental study on the critical temperature of coal spontaneous combustion at low temperature.Through the self-generated coal spontaneous combustion low-temperature program heating system,this part carries out the oxidation experiments on 3 different spontaneous combustion tendency and 5 kinds of different degree of deterioration of coal samples in the temperature range 30-250 ?,studies the relationship between the release rate of gas products and the heating temperature by gas chromatograph,and uses this result as one of the division bases of temperature characteristic of coal under low temperature oxidation.According to the theory of combustion dynamics analysis,the kinetic relationship between the oxidation reaction rate and the coal temperature was obtained.The linear fitting method was used to fit the oxygen consumption rate in different temperature sections,and the apparent activation energy change-point of different temperature sections was obtained.As a result the critical temperature of each coal sample under low temperature was determined.iii.)Experimental analysis of characteristic temperature of coal in high temperature stage.This part analyses the combustion characteristics and the variation law at high temperature spontaneous combustion stage by using of thermogravimetry.From this the dehydration temperature TO1,the oxygen uptake temperature TO2,the mass maximum temperature before the ignition temperature TO3,the ignition temperature TO4,the coal maximum weight loss temperature TO5 and the burnout temperature TO6 are determined.The pyrolysis characteristics and temperature characteristics of the pyrolysis stage are analysed by thermogravimetry as well.From this the dehydration degassing maximum weight loss rate temperature point TP0,the initial temperature TP1,the initial pyrolysis temperature TP2,the pyrolysis maximum weight loss rate temperature point TP3 and semi-coke completion temperature TP4 is obtained.By means of average error method,interpolating the above-mentioned characteristic temperature,nine groups of coal sample processing temperature points under oxygen conditions and 10 groups under nitrogen conditions are obtained at last.iv.)Experimental study of direct current at characteristic temperature.In fact,there are many present situations that make against to experimental development.For example,there is a lack of measurement standards for electromagnetic parameters of coal and rock(resistivity and dielectric constant)both at home and abroad,the existing measuring instruments can't be used under high temperature conditions and the samples are prone to deformation under high temperature oxidation and pyrolysis.Based on the consultant of the relevant literatures and the comparison of the test techniques,related characteristic point temperature coal samples that made under oxidation and pyrolysis conditions are obtained and the experimental scheme of DC and AC measurement of the electromagnetic parameters of each sample under the condition of room temperature was determined.Through the experiments and analysis of the related electrical characteristics to the 95 groups of samples,the change rule of high voltage DC resistivity under different characteristic temperature points is obtained.According to the common frequency range of AC current in the present engineering,the resistivity-temperature model in the process of coal oxidation and the temperature-resistance model of the pyrolysis process were obtained.Then,by introducing the Arrhenius formula,the physical environment temperature correction factor was added to the above two fitting formulas,which can semi-empirically show the change of resistivity in the actual temperature rise process;The complex relative permittivity from the experimental results with the change of law were fitted,and the relevant mathematical models were obtained.The above-mentioned characteristic laws of electrical changes provide the physical condition to the effectiveness of detecting high temperature fire zone of coal spontaneous combustion by means of alternating current method(such as transient electromagnetic method).v.)Similar physical simulation experiments.This part deduces a similar simulation criterion under the condition of transient electromagnetic method.A model holder,similar materials and a small antenna used for collecting and transmitting transient electromagnetic signals for the simulation experiment were designed.According to the different setting parameters of the anomalous body in the high temperature fire zone,the transient electromagnetic response characteristics are simulated.Experiments show that the depth,range and inclination of the abnormal body in the high temperature fire zone will produce different electromagnetic response effect.vi.)TEM field test.In this dissertation,the 4+9# coal seam in Beizu coal mine is selected as the actual detection case of transient electromagnetic method.The detection method based on advanced detection and profile detection is proposed to eliminate the inconvenience of detection results.On account of the geological data and infrared detection results,the validity of detecting high temperature fire source of coal spontaneous combustion by transient electromagnetic method was verified,which will provide a technical reference and theoretical basis for the future detection of coal spontaneous combustion area by using transient electromagnetic method.