Underground coal gasification (UCG), a technique used to recover coal energy by the in-situ conversion of coal into gaseous products, enables recovery of coal energy from the potential coal resources abandoned under the ground for either technical or economic reasons. However, it might be difficult to adopt conventional UCG systems in Japan because of geological conditions that are complicated by the existence of faults and folds. Additionally, it is difficult to control the combustion/gasification area during UCG process because the process is invisible. Therefore, a co-axial UCG system that is compact, safe, and flexible to adopt is suggested with monitoring system by means of acoustic emission as an alternative UCG system. This UCG system has superiority in terms of applicability compared to the conventional one, but the recovered energy from the coal is relatively low because the gasification area in a co-axial system is limited around a well. In order to develop co-axial UCG system with high efficiency, the model UCG experiment with a large-scale simulated coal seam were conducted. It has been shown that 1) the gasification period can be extended by adopting proper oxygen inflow, 2) it is possible to control the combustion/gasification area and the product gas quality by controlling the position of oxygen inflow, 3) acoustic emission monitoring is an effective technique to evaluate the combustion/gasification area.
