Study On Rapid Forecast Of Coal Spontaneous Combustion Period

The period of coal spontaneous combustion is one of the major indicating parameters to denote the spontaneous combustion characters of coal, through which the fire accident in the mine sites can be better predicted and thus controlled. Based on the well-developed theoretical mechanisms of coal spontaneous combustion and the currently existent testing and analysis results, this thesis proposes to predict the period of coal combustion by considering the coal's self-heating, smoldering and combustion stage process. A dynamic chemical reaction, porous flow and unsteady heat transfer model was established based on the macroscopic analysis. An innovative testing device was developed and the rapid forecast of the coal spontaneous combustion period, both under the adiabatic boundary condition by using thermal compensation and constant heat flux by using an electric heater to accelerate coal oxidation, was realized. By comparing the results with those from the theoretical analysis, numerical solution, measurement, and the on-site data, it generally supports that the proposed rapid testing method and technical approach to predict the shortest period of coal spontaneous combustion is valid.Firstly, according to the theory of smoldering, the coal spontaneous combustion process was considered as starting from the self-heating and then to smoldering and finally it is ignited, based on which the mathematical model was established. The three reaction types during coal spontaneous combustion including physical adsorption, chemical adsorption and oxidation reaction were discussed. The process of coal spontaneous combustion was divided into self-heating stage, smoldering stage and igniting stages. The three stages proceed and are indicated by three characteristic temperatures that are known as critical temperatures. In the same time, the analytical solutions were obtained under the specified several typical conditions. By using the finite-control-volume method and so on, the distribution of seepage flow field, oxygen concentration field and temperature field in the loose coal were coupled for solution, under a two-dimensional, axis-symmetric coordinate. The validity of the mathematical model was confirmed by the corresponding data analysis.Secondly, based on the above self-heating, smoldering and ignition model, the experimental measurement system and the associated computational model which can be used to predict the period of coal spontaneous combustion was developed. The components and features of the test device was described, including the reactor body, wall structure, air supply device, temperature measurement, gas component analysis and so on. At the same time the testing and calculation method of the heat loss on the surface of the experimental device was presented. Through analysis of the influence factors such as coal particle size, porosity, coal thickness, and the airflow rate, the optimum initial testing environmental conditions were determined.Thirdly, the heat loss in the test system external was analyzed and the surface condition of constant heat flux was obtained. The oxygen consumption rate of the coal rapid oxidation was calculated by organizing the test data of the heating rate, oxygen consumption rate, carbon monoxide and carbon dioxide generation rate and other macroscopic parameters which characterize the coal's low-temperature oxidation in the various stages. The kinetic parameters such as the activation energy and the pre-exponential factor were obtained by using the Arrhenius equation. The system to rapidly predict the coal spontaneous combustion with a constant heat flux was established by solving the unsteady mathematical model of smoldering to coal spontaneous combustion. The rapid prediction of the period of coal spontaneous combustion was thus achieved in the laboratory.The model of coal spontaneous combustion with process from the self-heating to smoldering and finally ignition was verified by the experimental data for coal under the adiabatic oxidation and also under the accelerated oxidation test using different coal samples. By comparing the rate of oxygen consumption and temperature between the prediction and test, the obtained solutions have shown a good agreement with the measurement. The shortest period of coal spontaneous combustion in the adiabatic condition was obtained by switching the solution boundary conditions into the adiabatic type. The predicted spontaneous combustion period highly agrees with the on-site data, which shows the reasonability and accuracy of the proposed models.