The adiabatic spontaneous combustion period of coal is an important index for the macroscopic characterization of coal spontaneous combustion, and it is affected by many internal and external factors. There are several methods to study it, but there are various shortcomings to these methods. Some require too much time, while others have too many interfering factors. To quickly obtain the accurate adiabatic spontaneous combustion period of coal, a rapid contrastive experimental method was designed. In this method, the coal samples of the experimental and control groups were the same, and air and nitrogen were used as control atmospheres. A theoretical calculation method for the adiabatic spontaneous combustion period based on this method is proposed. The experimental results showed that during the temperature-programmed coal spontaneous combustion experiment, the increase in the coal temperature was due to physical and chemical heating. Physical heating is the heating effect of the temperature-programmed furnace body and the heated gas on the coal sample. Chemical heating includes oxidative exothermic heating promoted by physical and adiabatic oxidation heating. The adiabatic oxidative heat release can be determined by the oxidation heat release in the air atmosphere minus the oxidation heat release corresponding to the coal sample temperature in the nitrogen environment at the same period. The adiabatic spontaneous combustion period of coal can be calculated from the adiabatic oxidation heat release. Our results provide a rapid contrastive experimental method to quickly obtain the accurate adiabatic spontaneous combustion period of coal.

To study the influence of different confining pressures on the pore structure and connectivity of high-rank coal, the high-rank raw coal of the Shanxi Xinjing Mine No. 9 coal seam was studied. A low-field nuclear magnetic resonance (LNMR) test system and a vacuum pressurized water saturation system were used to analyze the samples. The T 2 spectra of samples, saturated with water under different confining pressures and containing residual water after centrifugation, were tested. The coal sample pore size distributions, permeabilities, free fluid values, bound fluid values, and other parameters were obtained, and a calculation model of the coal pore connectivity ratio was established. The results were as follows. When the saturated pressures were 5, 10, 15, 20, 25, and 30 MPa, the pore diameters of the coal samples were mainly concentrated in the ranges of 0.00023–0.069 and 1.29–24.09 μm. Among them, micropores (<10 nm) and small pores (10 < 100 nm) account for the main part, mesopores (100 < 1000 nm) were underdeveloped, and relatively few macropores (>1000 nm) and fissures developed. As the confining pressure increased, the coal porosity and connectivity showed a trend of decreasing, then increasing, and finally remaining basically unchanged. The total pore connectivity rates of the coal samples were 37.0–62.6%. The interconnection rates of the micropores, small holes, mesopores, and macropores are 2.90–34.55, 89.09–99.03, 97.09–100, and 100%, respectively. The total pore connectivity followed an exponential functional relationship with permeability, and the critical confining pressure of high-rank coal was 25 MPa. These results provide a scientific basis for the high-pressure water injection of high-rank coal seams.

To study the influence of water immersion on the evolution of the groups and spontaneous combustion characteristics of coal samples with different sizes, raw coal from the Fengshuigou Coal Mine operated by Pingzhuang Coal Company in Inner Mongolia was studied. The infrared structural parameters, combustion characteristic parameters, and oxidation reaction kinetics parameters of D1−D5 water immersion coal samples were tested, and the mechanism of spontaneous combustion during the oxidation of submerged crushed coal was investigated. The results were as follows. The water immersion process promoted the re-development of coal pore structure, and the micropore volume and average pore diameter were 1.87−2.58 and 1.02−1.13 times those of raw coal, respectively. The smaller the coal sample sizes, the more significant the change. At the same time, the water immersion process increased the contact point between the active group and oxygen in the coal, and the C� O, C−O, and −CH 3 /−CH 2 − groups in coal were further promoted to react with oxygen to generate −OH functional groups and improve the reactivity of coal. The characteristic temperature of water immersion coal was affected by the temperature rise rate, coal sample size, coal voidage, and other factors. Compared with the raw coal, the average activation energy of the water immersion coal with different sizes decreased by 12.4−19.7%, and the apparent activation energy of the coal sample with a size of 60−120 mesh was the lowest on the whole. In addition, the apparent activation energy in the low-temperature oxidation stage was significantly different.