Effect mechanism of nitrogen injection into fire-sealed-zone on residual-coal re-ignition under stress in goaf

Abstract

Coal is the one of foundations of energy and economic structure in China, while the unsealing of coal mine fires would cause a great risk of coal re-ignition. In order to explore the influence of pressure-bearing state on the re-ignition characteristics for residual coal, the uniaxial compression equipped with a temperature-programmed device was built. The scanning electron microscope, synchronous thermal analyzer and Fourier transform infrared absorption spectrometer was applied to investigate the microscopic structure and thermal effect of the coal samples. Moreover, the microscopic effect of uniaxial stress on coal re-ignition is revealed, and the re-ignition mechanism is also obtained. As the uniaxial stress increasing, the number, depth and length of the fractures of the pre-treated coal increases. The application of uniaxial stress causes the thermal conductivity to change periodically, enhances the inhibition of injecting nitrogen on heat transfer and prolonges the duration of oxidation exothermic. The content of oxygen-containing functional groups has a high correlation with apparent activation energy, and coal samples at 6 MPa is more probability to re-ignite while the fire zone is unsealed. Uniaxial stress could control the re-ignition mechanism by changing the structure of fractures and pores. The side chains and functional groups of coal structure are easier to be broken by thermal-stress coupling. The higher the ·OH content, the more difficult coal samples would be re-ignited. The research results would lay a solid theoretical foundation for the safe unsealing of closed fire-areas underground, tighten the common bond between the actual industry and the experimental theory in closed fire-areas underground, and provide the theoretical guidance for coal re-ignition preventing.