Study On Characteristics Of Coal Spontaneous Combustion And 3D Multi Field Coupling Of Loose Coal Spontaneous Combustion In Roadway

With the application of the fully mechanized top coal caving method, the roadway is excavated along the coal seam, with meters of coal seam at the top of the roadway. As the top coal has poor shear resistance, it is easy to get deformed and loose when the roadway is excavated. The oxygen in the roadway airflow can easily percolate into loose coal. The oxygen and the loose coal react with each other and emit heat. When the condition is suitable for heat accumulation, the continuously accumulated heat will lead to spontaneous combustion. With a fire point of only several square meters, the concealed location of spontaneous combustion of the loose top coal in roadway brings difficulties to the fire-prevention work. Thus, it’s of great importance to study the dynamic process of loose coal spontaneous combustion at the top of the roadway.Zaoquan Mine was taken as the research background. The loose coal spontaneous combustion at the top of the roadway was researched by using experimental research, theoretical analysis and numerical simulation. Through thermogravimetry analysis–differential thermal analysis–Fourier transform infrared spectroscopy(TGA-DTA-FTIR) experiment, oxidation experiment, mercury injection experiment and nitrogen adsorption experiment, the change laws of weight, heat, gas emission, pore structure in coal spontaneous combustion were obtained, and the relationship between rate of oxygen consumption, heat release intensity, specific heat, permeability, permeability coefficient and temperature were obtained too. Three dimensional mathematical physical model of coal spontaneous combustion contain loose top coal and roadway were built. The dynamic physical parameters obtained from experiments were used to do multi field coupling analysis of flow field, temperature field and mass transfer field. The change process of loose top coal spontaneous combustion were obtained. The displacement and stress distributions in the process of coal spontaneous combustion were calculated too. The experiments provided parameters for the numerical simulation and obtained important conclusions of coal spontaneous combustion influencing factors and pore structure change law.The main research contents are listed as follows:(1) The occurrence and development of coal spontaneous combustion-which includes a chemical kinetic process, heat transfer, weight change, and emission of various harmful gases, is a complicated physical chemical process. TGA–DTA–FTIR were conducted on several kinds of coals including Zaoquan coal to study the change rule of weight, heat, and generated gas during the coal spontaneous combustion. The change process of active structures on coal surface was obtained by analysis the experiment data. The experiment also provided values of activation energy and pre exponential factor, which are key parameters of coal spontaneous combustion.Research show that weight and heat are in mutual correspondence in coal oxidation process. The change in generated gas lags behind weight and heat changes in the last stage. The gas release process depends on the reaction characteristics of related active structures. The number of active structures in coal is the key factor in coal oxidation, and volatile matter can reflect the number of active structures. Experiment results show that, the volatile matter of coal has a significant impact on characteristic temperatures, concentrations of generated gases, heat release rate and activation energy. When volatile matter is high, the characteristic temperatures are low, the concentration of generated gas and rate of heat release are high and activation energy of low temperature is low.(2) The oxidation experiment was done on coal from No. 2 coal seam of Zaoquan Mine. Rate of oxygen consumption and heat release intensity were obtained through the experiment results. The specific heat value of coal at different temperature is measured by DSC. Rate of oxygen consumption, heat release intensity and specific heat are all key parameters of numerical simulation of coal spontaneous combustion.(3) The change of pore structure in the process of coal spontaneous combustion were researched by using SEM power spectrums analysis, mercury injection experiment, nitrogen adsorption experiment and fractal theory. Change laws of element around pores, the most probable aperture, porosity, specific surface area, permeability, permeability coefficient, pore fractal dimension and so on were obtained.SEM power spectrums analysis show that the content of carbon element in coal increases gradually with the increase of the distance from the pore. So coal spontaneous combustion begins from pores. Smaller pores oxides more quickly was abtained by comparing the values of carbon element content around pores. This conclusion is consistent with the result obtained from the fractal study.In order to improve the accuracy of the pore structure data, pore size distribution data measured by mercury injection experiment and nitrogen adsorption experiment were connected at 100 nm. The pore structure was studied by using combined pore size distribution map. Researches show that in the process of coal spontaneous combustion, the most probable aperture becomes larger and larger, porosity becomes bigger and bigger, specific area value increases first and then declines, density becomes smaller and smaller, permeability and permeability coefficient declines first, then increases rapidly, declines at last.Researches of pore fractal characteristics of coal spontaneous combustion process show that: the pore structure of coal has a certain self similarity within a certain range of scale. With the increase of temperature, the scale becomes larger and larger, fractal dimension becomes bigger and bigger. The pore structure of coal becomes more and more complicated, more and more heterogeneity with the increase of temperature. At the same time it also indirectly illustrates that in the process of coal spontaneous combustion, the expansion rate of smaller pores is larger. This conclusion is consistent with the results obtained by SEM energy spectrum analysis. So the fractal analysis is correct.(4) Three dimensional mathematical physical model of coal spontaneous combustion contain loose top coal and roadway was built. Choose k-? turbulent model to simulate the field in the roadway. The calculation result of turbulent flow field was used as the boundary condition of the seepage in the loose coal. The numerical simulation method directly links the ventilation of roadway and the spontaneous combustion of loose coal. Those dynamic parameters obtained from above experiments were used to complete the coupling calculation of flow field, temperature field and mass transfer field and the change process of three physical fields were obtained. The numerical simulation results are basically in accordance with the actual situation in the field. It proves that the simulation method is feasible.In the process of coal self-heating, the high temperature zone extends inward from the wind inlet along the roadway continuously and the highest temperature point moves towards the wind inlet constantly. On cross sections of the roadway, the loose top coal near the roadway has the lowest temperature, top area second. Highest temperature zone is in the middle and low part of the top coal. The high temperature zone gradually moves from the middle and low part to the low part of the coal, namely the wind inlet side. The influence of ventilation and pore structure parameters on heating rate and spontaneous combustin period were discussed. The law of the spontaneous combustion of the loose top coal in roadway can provide basis for fire prevention work on site.The displacement and stress fields in the process of coal spontaneous combustion are solved for the first time. In the process of coal spontaneous combustion, loose coal at the top of the roadway moves toward the roadway. As the heating rate increases, the displacement increases faster and faster. Principal stresses in loose coal are mainly compressive stress. Tensile stress exists only at the lower part of loose coal. Values of principal stresses increase with temperature. Third principal stresses increase much slower than first principal stresses. The bottom part of loose coal is the most dangerous area. It is damaged first.In the process of completing the above research, there are several innovative points:(1) Change rules of weight, heat and generated gases during coal spontaneous combustion were obtained from TGA–DTA–FTIR experiment. Changes of active structures on coal surface were obtained, it is new progress on the mechanism of coal spontaneous combustion.(2) In the simulation of coal spontaneous combustion, many parameters of pore structure are usually set to be fixed values or use empirical formula. Aiming at this problem, the change of pore structure during coal spontaneous combustion was studied. Change laws of porosity, specific surface area, permeability, permeability coefficient, pore fractal dimension and so on with temperature were obtained.Researches of pore fractal characteristics of coal spontaneous combustion show that the expansion rate of smaller pores is larger. This conclusion is consistent with the result obtained from SEM energy spectrum analysis. It is proved that the fractal study of pore structure is correct.(3) Three dimensional model containing loose top coal and roadway was built to study the spontaneous combustion of loose top coal in roadway. The air leakage intensity in loose top coal in the roadway was obtained from the calculation of the turbulent flow in the roadway to escape the error caused by setting the flow velocity or flow pressure with free will. The numerical simulation method directly links the ventilation of roadway and the spontaneous combustion of loose coal. Use COMSOL to complete the three dimensional coupling calculation of flow field, temperature field and mass transfer field and obtain the transient process of three physical fields. The displacement and stress field of coal spontaneous combustion were solved for the first time.(4) Pore size of coal is not uniform. There are pores smaller than several nm. There are also pores whose pore size is bigger than 100000 nm. At present, there is no equipment can give accurate measurement for such a large pore size range. Considering the best measurement range of nitrogen adsorption and mercury injection and characteristics of experimental data, pore size distribution data measured by two experiments were connected at 100 nm. The method improves the accuracy of data.The research has made some progress and achievements, but there are still many deficiencies. There is still a lot of research work to be carried out in the future:(1) Heat release intensity-parameter of coal spontaneous combustion was estimated with bond energy balance method using rate of oxygen consumption, the generation rate of CO and CO2. In the future, we should study how to realize the direct measure of heat release intensity, to improve the reliability of data.(2) The spontaneous combustion model of loose coal contains a lot of physical parameters. These parameters are constantly changing in the process of spontaneous combustion. But in the present numerical simulation, there were still some parameters were set to be constant or expressed through empirical conclusion. Further researches of the change laws of physical parameters during coal spontaneous combustion should be done.(3) Because three dimensional multi field coupling of coal spontaneous combustion is very difficult, coal is taken as linear elastic material during the calculation of displacement and stress distribution. The practice should be revised in the future to reduce the simulation error.