## 3 Results and Discussion
### Pyrolysis Gas and Ratio
Therefore, a certain percentage of secondary oxidation gas is simulated as being mixed into the primary oxidation gas to verify if there is an effect on the occurrence temperature of the extreme value. LN was taken as an example, for which different proportions of raw coal oxidation gas were mixed with secondary oxidation gas to calculate the extreme value of the alkane ratio. Figure 8 shows how the degree of influence is closely related to the oxidation temperature and the mixing ratio. At low oxidation temperatures, the difference in the chain alkane ratio between primary and secondary oxidation is small; therefore, the mixture of gases does not affect the occurrence time of the extreme value. When oxidation occurs at a high temperature, the occurrence time of the extreme value will only be affected when the secondary oxidation gas occupies a larger proportion. Therefore, the gas produced by secondary oxidation will  
Figure 6: C\({}_{2}\)H\({}_{4}\) concentration at the outlet of the coal sample reactor: (**a**) GZ; (**b**) LN; (**c**) SX. affect the determination of the alkane ratio when it is related to the oxidation temperature and the participation ratio of the secondary oxidation gas. The olefin-alkane ratio refers to the ratio of the presence of a long-chain olefin to methane or ethane. In this study, the C\({}_{2}\)H\({}_{4}\)/C\({}_{2}\)H\({}_{6}\) ratio was selected for analysis. It has strong regularity during temperature change, and it is often used as an auxiliary index to predict the spontaneous combustion of coal. Figure 9 shows that the C\({}_{2}\)H\({}_{4}\)/C\({}_{2}\)H\({}_{6}\) ratio increases with temperature. However, the olefin-alkane ratio and temperature are not linear in the secondary oxidation process of oxidized coal. The oxidized coal types show an increasing and then decreasing peak-like curve at low temperatures during secondary oxidation, where the peak becomes more obvious as the oxidation temperature increases. The temperature corresponding to the first appearance of C\({}_{2}\)H\({}_{4}\)/C\({}_{2}\)H\({}_{6}\) is also different from the primary oxidation one, which is due to the different C\({}_{2}\)H\({}_{4}\) occurrence times. The occurrence temperature behavior is the same as that of C\({}_{2}\)H\({}_{4}\). The extreme value (inflection point) of C\({}_{2}\)H\({}_{4}\)/C\({}_{2}\)H\({}_{6}\) can be used as a sign that the coal-oxygen reaction has entered the fission stage. At this time, C\({}_{2}\)H\({}_{4}\) and C\({}_{2}\)H\({}_{6}\) are produced in large quantities, but C\({}_{2}\)H\({}_{4}\) has a high formation rate, so the olefin ratio begins to decrease. In the case of the GZ samples, the minimum value did not appear at 170 \({}^{\circ}\)C, so it was impossible to judge whether the gas produced by secondary oxidation would affect the determination of the fission time, while the coal type at a higher temperature oxidation temperature made this point advance in time. The mixing of secondary oxidation gas will have an impact on its accuracy. However, the temperature in the fission stage is generally high (140-180 \({}^{\circ}\)C). At this time, spontaneous combustion is generally difficult to control. Therefore, it is less useful for early warning systems for fire, but it is still useful as an indicator for judging the degree of the spontaneous combustion of coal. Figure 8: The temperature at which the extreme value of the alkane ratio appears. Figure 7: Alkane ratio during temperature programming: (**a**) GZ; (**b**) LN; (**c**) SX.