Experimental Study on the Oxidation Characteristics of Raw Coal Under Uniaxial Stress

To investigate the spontaneous combustion characteristics of coal pillars under geo-stress, a comparative study on the oxidation characteristics of raw coal under varying uniaxial stress levels was conducted using coupled temperature-stress loading, gas chromatography (GC), ultrasonic testing, and nuclear magnetic resonance (NMR) methods. Results indicate that, macroscopically, the CO concentration and oxygen consumption rate produced by coal oxidation first decrease and then increase with the increase of axial pressure. When the temperature is lower than 145°C, the inflection point pressure value of the CO concentration and oxygen consumption rate curve is 10MPa. When the temperature exceeds 145°C, the pressure value at the inflection point of the curve shifts to 5MPa. When uniaxial stress remains below the coal's real-time yield strength, the fractures are compressed, and higher uniaxial stress suppresses the coal-oxygen complex action. Once the uniaxial stress exceeds the real-time yield strength, the coal transitions from elastic to plastic deformation, triggering extensive new crack development. under these conditions, increased stress accelerates the coal-oxygen complex action. Microscopically, micropore structures under different stress levels exhibit polarization with rising temperature. pores with transverse relaxation times (T2) < 0.5 ms increase in number, while pores with 0.5 ms < T2 < 3.5 ms decrease and evolve into larger mesopores and macropores. The growth rates of mesopore and macropore porosity components in T2 spectra across all coal samples followed: A5 < A0 < A10 < A15. These findings suggest that coal pillars under geo-stress exhibit slow coal-oxygen complex action and minimal, difficult-to-detect CO release during elastic deformation. However, upon transitioning to plastic deformation, coal-oxygen complex action intensity surges rapidly, leading to hazardous CO concentrations.