*2.2. Coal Sample Low-Temperature Nitrogen Adsorption Experiment with Different Temperature Treatment*

During the experiment, after transporting the packaged fresh coal samples to the laboratory, the whole coal sample is knocked out with a geological hammer to take a small coal sample with a length of no more than 1 cm in width and height. The small coal samples were put into a programmed heating furnace and treated with a temperature constant temperature of 30 ◦C, 100 ◦C, 200 ◦C and 300 ◦C for 3 h and cooled freely. For 4 groups of experimental samples, the automatic specific surface area and porosity analyzer were selected for low-temperature nitrogen adsorption experiments. The coal samples treated at different temperatures were screened, and the pulverized coal with specifications 40–80 was obtained, and then the sieved pulverized coal was put into the dry sample tube for degassing because the process of thermal expansion and rupture of the coal rock mass was irreversible [31,32]. Thus, in order to avoid excessive deregulation temperature to change the coal pore fracture structure, the degassing temperature of the coal sample was set below the treatment temperature. The pore fracture structure of the coal body would not be affected at this time. Since degassing requires the removal of moisture and air, it is difficult to remove moisture at room temperature, so the samples treated at 30 ◦C are degassed in a vacuum for 48 h at 30 ◦C, while the samples treated at 100 ◦C, 200 ◦C and 300 ◦C are degassed at 100 ◦C vacuum for 12 h.

## **3. Study on the Thermal Damage Law of Coal Body during Spontaneous Combustion and Heating Process of Coal**
