## 3 Results and Discussion
### Pyrolysis Gas and Ratio  
Pyrolysis gasses are alkanes, olefins, and alkynes produced by the decomposition of organic components during the spontaneous combustion of coal, mainly including CH\({}_{4}\), C\({}_{2}\)H\({}_{6}\), C\({}_{3}\)H\({}_{8}\), C\({}_{2}\)H\({}_{4}\), and C\({}_{2}\)H\({}_{2}\). CH\({}_{4}\) and C\({}_{2}\)H\({}_{6}\) are native endogenous gases in coal seams and are usually not selected as indicator gases alone. The presence of C\({}_{2}\)H\({}_{2}\) indicates that coal is in the combustion stage, so it is usually not selected as an indicator gas in the low-temperature oxidation stage [36]. The appearance of C\({}_{2}\)H\({}_{4}\) indicates that the spontaneous combustion of coal has entered the accelerated reaction stage, so C\({}_{2}\)H\({}_{4}\) is an important indicator gas [37]. The alkane ratio and olefin-alkyl ratio can effectively eliminate the influence of wind flow on the indicator gas and has a good linear relationship with temperature [38]. Therefore, C\({}_{2}\)H\({}_{4}\), the alkane ratio, and the olefin-alkyl ratio were selected as gas indicators for analysis in this section. Figure 4: CO concentration at the outlet of the sample reactor: (**a**) GZ; (**b**) LN; (**c**) SX. Figure 5: CO/\(\Delta\)O\({}_{2}\) behaviors of different coal types: (**a**) GZ; (**b**) LN; (**c**) SX. Figure 6 shows the same behavior of C\({}_{2}\)H\({}_{4}\) production for the different coal samples during the programmed temperature rise. C\({}_{2}\)H\({}_{4}\) is not detected at 30 \({}^{\circ}\)C, indicating that C\({}_{2}\)H\({}_{4}\) is not a gas that is originally present in these three types of coals. With the increase in temperature, C\({}_{2}\)H\({}_{4}\) began to appear, first with slow growth and then rapidly. The C\({}_{2}\)H\({}_{4}\) occurrence temperature of the oxidized coal was lower than in the raw coal, and the occurrence temperature of C\({}_{2}\)H\({}_{4}\) decreased with the pre-oxidation temperature increase. This means that oxidation causes the coal to enter the accelerated oxidation stage much earlier. For the LN and SX coal samples, at the later stages of oxidation, the amount of C\({}_{2}\)H\({}_{4}\) produced by the raw coal is greater than that produced by the pre-oxidized coal. Pre-oxidation will reduce the activation energy of the reaction and form some intermediate products. Therefore, C\({}_{2}\)H\({}_{4}\) can be produced at a lower temperature during the secondary oxidation process. Since high-temperature pre-oxidation consumes a portion of the group, the amount of C\({}_{2}\)H\({}_{4}\) produced is less than in primary oxidation. Once the gas generated by secondary oxidation enters the goaf, the use of the time when C\({}_{2}\)H\({}_{4}\) occurs to determine the beginning of the accelerating stage will yield a time earlier than of the actual situation. The alkane ratio is the concentration of alkane to the concentration of methane or ethane. Extreme values or abrupt increases in the alkane ratio during oxidation are a sign that the oxidation process has reached an intense stage. C\({}_{2}\)H\({}_{6}\)/CH\({}_{4}\) was selected as the index in this section. As can be seen from Figure 7, the LN and GZ coal samples increased first and then decreased with the temperature increase. Whereas SX increased first, and then decreased and increased again. The alkane chain ratio value in the secondary oxidation process is lower than in primary oxidation. The peak of the chain alkane ratio during the secondary oxidation of oxidized coal occurs at a higher temperature than in raw coal, especially after high-temperature oxidation. The GZ-raw coal's maximum value occurs at 90 \({}^{\circ}\)C, while the extremes value of GZ-150 occurs at 130 \({}^{\circ}\)C. LN-150's maximum value occurs 20 \({}^{\circ}\)C higher than in raw coal. Following a similar pattern, the difference between SX-raw and SX-150 is 10 \({}^{\circ}\)C.  
Scholars often regard the point of the occurrence of the extreme value of the chain alkane ratio as the beginning of the intense oxidation of coal.