338 (a) (b) Figure 8 – Results of the AE source location analysis; (a) 0~17.9 hours after ignition, (b) 17.9~53.4 hours after ignition 5. CONCLUSION A UCG field trial was successfully conducted in which coal was ignited within a 20 m-long co-axial well drilled from the coal seam outcrop along the seam dip. Following ignition, coal combustion and gasification were sustained by injecting an oxygen-enriched air into the well through an injection pipe. The main findings are as follows: Ignition of the coal required a considerable amount of time due to the influence of water within the coal seam and residual drilling water, highlighting the importance of sufficient pre-drainage and dewatering measures prior to in-situ UCG operations. The produced combustible gas was primarily composed of H2 and CO. Although its composition varied, combustible gas was continuously produced, suggesting that a stable gasification reaction zone had been formed. Compared to previous laboratory-scale experiments, the higher gasification efficiency suggests that gasification of underground coal seams can be achieved with reduced heat loss, enabling efficient gasification. It has been demonstrated that fracturing activity generated by the gasification of underground coal seams can be measured using geophones. This finding indicates that by improving measurement accuracy, it may be possible to monitor the movement of gasification zone in situ. ACKNOWLEDGEMENTS This study was supported by JSPS KAKENHI Grants JP 25K01716; Module Research Program of the Kyushu University Institute for Energy Research and Education (Q-PIT); and the Asia–Oceania Research and Education Organization (Q-AOS). The authors would like to express their sincere gratitude to Mikasa City, Hokkaido, and all personnel from the related companies and organizations for their valuable cooperation and assistance.
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