334 produced gas remained low (approximately 30℃) and only gas presumed to be mainly water vapor was observed, preventing the coal combustion. One of the reasons that coal combustion did not occur is the influence of water contained in the in-situ coal and borehole water. Although surface water did not flow into the well, the inner surface of the coaxial well was wet, and water had accumulated at the bottom. Considering such conditions, a significant portion of the supplied heat was likely consumed in heating the borehole water and converting it into steam. In the fourth ignition attempt, the injection conditions were set to 3 L/min of LP gas, 140 L/min of air, and 40 L/min of oxygen. To mitigate the influence of water, the ignition position was slightly shifted to 17.55 m from the inlet. Furthermore, to prevent oxygen deficiency, the oxygen injection rate was increased to 50 L/min 15 minutes after the start of ignition operation. As a result, the temperature of the generated steam increased rapidly, reaching approximately 50°C about three hours after ignition operation. After around 8 hours from the start of ignition, when the injection pipe was moved to 17.35 m position, subsequently, dark-colored product gas was observed. At this point, coal ignition was judged to have been successfully achieved. During the ignition operation, a total of approximately 3,900 L (about 7.8 kg) of LP gas was consumed. Assuming a calorific value of 50 MJ/kg for LP gas (Hashem, G et al., 2023), approximately 390 MJ of heat was supplied to the ignition zone prior to successful coal ignition. It is considered that most of this heat was consumed in heating and evaporating the in-situ water, as well as in heating the generated steam. The heat energy required to increase water temperature from 20℃ to 100℃ and vaporize it is approximately 2.6 MJ/kg (specific heat of water: 4.2 kJ/kg·K; latent heat of vaporization: 2,264 kJ/kg). Based on this estimation, roughly 150 kg of water would have been converted into steam before coal ignition was achieved (in practice, the combustion heat of LP gas is also considered to be used for heating water vapor). Assuming an in-situ coal water content of 10%, this corresponds to evaporating the water contained in approximately 1.5 tons of coal prior to ignition. If the heated section is assumed to be 1 m in length, 1.5 tons of coal corresponds approximately to the coal mass within a radial distance of about 0.6 m around the co-axial well. In previous laboratory-scale UCG model experiments using artificial coal seams (Hamanaka et al., 2021), ignition of the surrounding coal and generation of combustible gas were achieved within several minutes to several tens of minutes after ignition within the well. However, the field-scale experiment targeting an underground coal seam revealed that ignition is significantly prevented by the presence of inherent water and drilling-related water. For future operations, it is necessary to consider sufficient dewatering of the coal seam prior to ignition, as well as possible dehydration by pressurized air injection.
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