Research On Formation And Recognition Of The Source Of Coalfield Fires According To Multi-field Coupling Method

There are more than 200 coalfield fires in China. The area of the fires only in 7 provinces in northern China amounts to 720km2. Fire in coalfield seriously pollutes the atmospheric environment. At the same time, fire causes a serious waste of resources and a significant loss of the precious coal resources. It is of vital significance to carry out the study on the mechanism and kinetics of combustion of coal fire area for the purpose of ensuring the safety of coal mining and of protecting the ecological environment.The formation and evolution of coalfield is developed from the shallow to the deep in the circulation dynamics of the process of temperature field,chemical field, stress field and flow field of multi field coupling effect. This paper focuses on the evolution of the fire by means of experimental studies, theoretical analysis, numerical simulation and mathematical statistics in order to reveal the dynamic mechanism of the fire expanding. The main contributions are summerized as followsThrough the experiment of coal spontaneous combustion, we studied the exothermic characteristics of coal fires in low-temperature oxidation stage, tested the parameters of the coal samples in different periods of oxidation, pyrolysis and combustion and calculated the heat release intensity, the temperature-rise rate, oxygen consumption rate, characteristic temperature based on the finite difference method, the minimum thickness of float coal, the minimum oxygen concentration, the maximum intensity of air leakage. According to the experimental results, the functions of maximum temperatures versus time, the heat release intensity versus temperature, the oxygen consumption rate versus temperature, CO and CO2 producing rates versus temperature were all fitted, in which the fitting results of the heat release intensity versus temperature follows exponential relationship, the correlation coefficient of the function of the oxygen consumption rate versus temperature is 0.9948. Using the TG, DTG and DSC experiments, the processes of thermal decomposition, combustion characteristics and development of coal fires were studied, during the heating and oxidation the function between the coal quality and temperature obeys the Boltzmann curve, the correlation coefficient reached 0.99876.MTS815.02 electro-hydraulic three axis rock mechanics experiment was conducted to study the mechanical characteristics from room temperature to 600℃. When the temperature exceeded 400℃, the peak of maximum stress is reduced from 29.95 MPa to 15.65 MPa, the peak strain increases more than 57%; When the temperature rised from 250℃ to 400℃,the elastic modulus decline even more than 90%. The permeability change of coal sample was studied under the condition of stress and strain. Fracture appeared in the rock sample which acted as the oxygen supply channel with the temperature from room temperature to 600℃. Two-dimensional sections of coal rock under different temperatures were obtained by CT scanning. Meanwhile, the characteristics of the images under actions of heat and stress were analyzed and the functions between physical parameters(stress, strain, elastic modulus, Poisson’s ratio) and the temperature of coal and rock were fitted.An experimental platform was built to simulate the dynamic evolution of coalfield fire. In addition, sensitivity analysis of the testing results was also conducted. In the A level test, the vertical displacement amounts to 0.46 m, the stress is up to 24 MPa, Strain changes obviously after the burning surface the maximum reached 0.15. In the B level test, the vertical displacement is up to 0. 5m, the stress of up to 19 MPa, the maximum strain 0.067. The stress, strain, displacement is up to the maximum of 32 MPa, 0.095 and 0.095 m in the vertical combustion depth of 0.8m and thickness of 0.3m respectively. With the increase of the thickness of coal seam, which shows the thickness of coal seam and combustion depth are significantly sensitive to the stress and vertical displacement.A two–dimensional simulation was also implemented to validate the simulating results through parametric scanning of the factors affecting coal fires. The simulation results were in good agreement with the testing results. On the basis of the prototype size, the evolution of the coalfield fire was simulated by direct coupling temperature field, heat transfer, mass transfer, chemical field, and the dynamic ODEs equations. When the simulation time up to 400 hours, the permeability of the maximum value increased from 2.3226×10-10m2 to 9.0579 ×10-10 m2; the minimum value from 1.1724 × 10-10m2 to 4.0959 × 10-10m2, the maximum porosity increased from 0.3667 to 0.4634, the minimum value was increased from 0.3581 to 0.3822.Three-dimensional model was built according to the images of cracks in the fire zones using CT scan. The dynamic evolution of the fire under multiple conditions of temperature, chemical reaction, heat transfer and mass transfer, and structure stress. The von Mises equivalent stress peak value reaches 60.392MPa; the first principal stress in the maximum tensile stress is 23.5MPa. The maximum displacement is-3.2429 × 10-7m, which indicates the subsidence area occurs.This is because the coal produces volume change obviously before and after combustion, the overlying strata forms caving fissure.The infrared imaging experiment of the fire source was conducted to study the propagation of the temperature field of the fire source towards the ground. When the center of the fire transfers to the surface for 4 hours, the highest surface temperature increased nearly 20℃, the highest surface temperature level began to decrease after 8 hours, the high temperature of the earth’s surface tends to be stable after 32 hours. Experiments show that: when the state of combustion center zone is linear, the surface temperature field with the highest temperature is distributed shape oval, a correlation exists between the center of the fire temperature and surface temperature field.The necessary conditions of the occurrence of the coalfield fires were analyzed according to the geological characteristics. “Four-zone” theory was posed to help divide the fire zones into four parts including the safe and stable region when 0≤Fc<1/243, the warning zone when 1/243≤Fc<1/32 and the dangerous zone when 1/32≤Fc<1, the combustion center zone when Fc≥1, distinguished by different colors. This theory can be used to judge the status of the coalfield fire. The probability function of the risk of spontaneous combustion with arbitrary numbers of factors was generalized.