333 dioxide (CO2), hydrogen (H2), carbon monoxide (CO), methane (CH4), ethylene (C2H4), ethane (C2H6), propylene (C3H6), and propane (C3H8). Prior to the experiment, instrument calibration was conducted using certified standard gas mixtures containing known concentrations of each component. The micro-GC has two analytical columns, employing helium as the carrier gas for one column and argon for the other. Although helium is commonly used as the carrier gas, argon was also adopted in this study to enable quantification of hydrogen. At the end of the experiment, nitrogen was injected into the coaxial well to extinguish and stop reaction. Figure 4 – Injection conditions of oxygen-enriched air Figure 5 – Geophone arrangement 3.2 Ignition of Coal Seams Figure 6 shows the coal ignition system. Coal ignition was performed using a doublestructured injection pipe. LP gas was injected through the annular space between the inner and outer pipes, while oxygen-enriched air was supplied through the inner pipe. Ignition of the LP gas was achieved using a nichrome wire installed at the tip of the injection pipe. An ac transformer was used to pass a current of approximately 8 A through the nichrome wire, heating it to red-heat and thereby igniting the LP gas. During the ignition operation, temperature changes associated with LP gas ignition and subsequent coal ignition were monitored using a thermocouple installed near the tip of the injection pipe. In addition, a compact endoscopic camera installed approximately 3 m behind the pipe tip was used to visually confirm the red heating of the nichrome wire and the ignition behavior of the LP gas. Table 2 lists the gas injection conditions and ignition positions during the ignition operation. As shown in Table 2, four ignition attempts were conducted under different injection conditions and ignition positions. In the first to third attempts, LP gas combustion could not be maintained within the co-axial well. Furthermore, the temperature of the
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