## 3 Results and Discussion
### Oxidation Gas Products and Ratios  
Coal has a complex chemical reaction with oxygen during spontaneous combustion, CO and CO\({}_{2}\) are the main oxidation gas products. Both gases have a good linear relationship with temperature. However, there is a CO\({}_{2}\) reserve in a coal seam, and the breathing of workers also produces CO\({}_{2}\), resulting in a non-unique source of CO\({}_{2}\). Therefore, CO\({}_{2}\) has not been selected as the only indicator gas, given that CO has a single source that can be used as a gas indicator [34]. CO/\(\Delta\)O\({}_{2}\) was selected as an auxiliary index that can avoid the influence of air volume on gas concentration [35]. CO is widely used as an indicator gas in coal mines in China. It can be seen from Figure 4 that all curves show the same behavior. The CO concentration increased slowly at the initial stage and then began to rise rapidly. In the early stage of coal oxidation, the CO produced by oxidized coal is greater than in raw coal. This is because the raw coal contains a high level of moisture, which will evaporate and absorb heat in the early stage of the spontaneous combustion of coal, thereby reducing the reaction rate of coal and oxygen, and thus the raw coal produces less CO. On the one hand, the evaporation of water increases the pore and specific surface area of coal, which increases the contact area between coal and oxygen, so more CO is produced in the secondary oxidation process. After 100 \({}^{\circ}\)C, the free water evaporates completely, and the raw coal also has larger pores and a greater specific surface area, so the coal oxygen reaction intensifies, along with the level of CO. Moreover, the CO-producing groups in the coal are not overconsumed because the raw coal is not pre-oxidized, making the amount of CO produced by raw coal greater than that produced by the pre-oxidized coal. This behavior is similar to the oxygen change law. CO is often used as a forecast gas for the early stage of coal oxidation. For LN, when the CO concentration is lower than 17.225 ppm, it is considered to be in the oxidation stage, but in the secondary oxidation process, when the temperature reaches 40 \({}^{\circ}\)C, the CO concentration exceeded 17.225 ppm; therefore, the mixing of secondary oxidation gases can cause the CO concentration to appear greater than the actual amount present, causing the forecast stage to occur after the fact. Figure 5 shows the variation of CO/\(\Delta\)O\({}_{2}\) during the programmed temperature rise for the different coal types. It can be seen from Figure 5 that the increase in the CO/\(\Delta\)O\({}_{2}\) ratio of the oxidized coal, during the secondary oxidation process, occurs linearly with temperature, like the raw coal sample. In the low-temperature oxidation stage (30-80 \({}^{\circ}\)C), the CO/\(\Delta\)O\({}_{2}\) ratio of oxidized coal rises very slowly, but the value is significantly higher than raw coal. Figure 3: Oxygen concentration at the outlet of different coals during temperature programming: (**a**) GZ; (**b**) LN; (**c**) SX. On the one hand, the decrease in moisture grants the pre-oxidized coal a larger specific surface area, thereby increasing the production rate of CO and the consumption rate of O\({}_{2}\). On the other hand, pre-oxidation causes some functional groups in coal to react with oxygen to form intermediate products, so less O\({}_{2}\) is needed in the early stage of the secondary oxidation process to generate more CO. In the case of the GZ samples, the difference between the CO/\(\Delta\)O\({}_{2}\) ratio of oxidized coal and raw coal after 80 \({}^{\circ}\)C is not significant.