Simulation Of Spontaneous Combustion Of Coal And The Critical Conditions Leading To Ignition

Spontaneous combustion of coal is one of major disasters in mining, long distance transportation and storage, in terms of both safety and economics. The hazard level of the spontaneous combustion of coal is more than gas and coal dust explosion. The self-heating process of coal depends on many critical conditions, such as airflow rate, temperature, oxygen and the porosity of the coal pile. Therefore, studying the process of the spontaneous combustion of coal and the critical ignition condition has great academic and practical value. To investigate the occurrence and development process of the spontaneous combustion of coal, prediction of the shortest spontaneous combustion period and simulation analysis of the critical ignition conditions, a method is proposed through analysising experimental test and numerical simulation.For experimental studies, in order to simulate the low-temperature oxidation of coal, a test rig to accelerate the coal oxidation using auxiliary heat source is established. Based on the test results, the process of the low-temperature oxidation of coal can be divided into two phases with dual Arrhenius parameters. These are:(â…°) chemical adsorption at the slow oxidation stage, (â…±) chemical reaction (so-called direct burn-off) at the fast oxidation stage. On the basis, we curve-fit an oxygen consumption rate in different oxidative stages. Then, based on the oxygen consumption rate in different oxidative stages from experimental test and the theory of heat and mass transfer in porous media, an unsteady-state model for predicting the spontaneous combustion period of coal is established. The model gives explicit differential equations for conservation of airflow, mass (oxygen) and heat. In mass transport, both convection and diffusion are included, and in heat transport, convection and conduction. The effect of moisture on the spontaneous heating of the coal is neglected. Finally, the mathematical model is used to simulate the course of spontaneous combustion under multiple conditions. The course of spontaneous combustion of different porosity distribution, air flow velocity, air flow temperature can be evaluated, and the critical ignition conditions are determined.Two-stage model validations with both experimental data from test rigs and in-situ measurement are accomplished. Satisfying results prove the accuracy of foregoing set of the mathematical model. The predicted shortest spontaneous combustion period is found to be 38 days, which agrees with the actual 45 days recorded in the mine site. The results obtained in this study indicate that it is reasonable to divide the low-temperature oxidation of coal into two phases. It has been found that spontaneous ignition of coal in the test rig may occur if three primary conditions are satisfied:porosity distribution is between 0.3 and 0.4, air flow velocity is between 0.0001 m/s and 0.0003m/s.