1566 1. INTRODUCTION Coal, one of the world’s primary energy resources, remains unmined, with more than 50% of reserves left due to economic and technical limitations of conventional mining methods. According to a report by the International Energy Agency (IEA, 2024), the reserves-to-production (R/P) ratio at the end of 2023 is approximately 50 years for oil and natural gas, whereas coal is estimated to have an R/P ratio of about 130 years. This indicates that coal represents a comparatively stable and long-term energy resource relative to oil and natural gas. Sasaoka et al. investigated technologies to recover these unmined coal resources by stabilizing mining cavity using support systems and backfilling materials, thereby expanding the extractable region (Sasaoka et al., 2016). However, the utilization of coal in thermal power generation, steel production, and cement manufacturing results in substantial CO2 emissions, raising serious environmental concerns such as global warming and climate change. Therefore, to achieve a decarbonized society and carbon neutrality, sustainable and environmentally-friendly utilization of unmined coal resources is strongly required. In recent years, hydrogen energy has attracted considerable attention as a clean energy carrier. Technologies that utilize hydrogen in combustion or power generation processes are being actively researched and developed to reduce CO2 emissions and promote decarbonization (Stańczyk et al., 2010). Given these background, Underground Coal Gasification (UCG) has attracted attention as a technology that converts unmined coal resources in situ into combustible gases, primarily hydrogen (H2) and carbon monoxide (CO), through underground coal combustion and gasification. In UCG, a part of the coal seam is combusted to form a high-temperature environment exceeding 1000℃, where the steam gasification reaction (Equation (1)), the water-gas shift reactions (Equation (2)), and the boudouard reaction (Equation (3)) occur, generating H2 and CO. In UCG systems, the CO2 co-produced with hydrogen can potentially be separated, captured, and stored within the numerous voids and fractures formed in the coal seam after gasification. This integrated approach can contribute to greenhouse gas mitigation and the production of CO2-free (blue) hydrogen (Yang et al., 2016; Khadse et al., 2007). s+õ#Ω(æ)→õ# +sΩ (1) sΩ+õ#Ω(æ)↔õ# +sΩ# (2) s+sΩ# →2sΩ (3) From 2016 to 2021, a series of laboratory-scale UCG model experiments using an artificial coal seam were conducted based on a horizontal co-axial well method. In this method, a single horizontally drilled well within the coal seam is used both for oxidant injection and product gas recovery. The oxidant is injected through an injection pipe installed inside the co-axial well, while the UCG product gas is recovered through the annular space between the outer surface of the injection pipe and the well wall. Previous laboratory-scale model experiments demonstrated that this method enables control of UCG gas composition by adjusting the oxidant injection rate and achieves relatively stable combustion and gasification (Iriguchi et al., 2024). Furthermore, it was confirmed that periodically changing the tip position of the injection pipe effectively shifts the combustion/gasification zone to a new reactive coal face, thereby sustaining the gasification process (Su, F et al., 2024). It has also been demonstrated that the migration and expansion of the UCG reaction zone can be visualized by locating the sources of acoustic emission (AE) generated during combustion and gasification using uniaxial and triaxial acceleration transducers (Iriguchi et al., 2023). In addition, in order to promote upward expansion of the combustion/gasification zone around the ignition point, a composite co-axial UCG system has been attempted, in which an auxiliary production well was installed above the co-axial well in the artificial coal seam. In this arrangement, an auxiliary production well was installed approximately 40 cm above the co-axial well, and a vertical well near the ignition point was used to connect the co-axial well and the auxiliary production well. Oxidant injection was conducted using the same double-structured injection pipe inserted into the co-axial well as in conventional co-axial well method; however, UCG gas was produced from both the co-axial well and the upper auxiliary production well. As a result, enhanced expansion of the combustion/gasification zone toward the upper portion of the artificial coal seam was confirmed (Iriguchi et al., 2023). Although substantial knowledge has been accumulated from gasification experiments
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