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.

Small-scale underground coal gasification (UCG) model tests with a linking hole were carried out using two types of specimens made of either block coal or crushed coal to clarify their characteristics of combustion and gasification. Many similar characteristics were found between both specimens in terms of temperature change and its spatial distribution as well as concentration and heating value of each gas product as long as the crushed coal specimen was sufficiently consolidated. The shape and dimension of the cavity formed in the both specimens were also similar. Texture of the both specimens was changed after the combustion and gasification. Initiation of radial cracks from the linking hole was found in the both specimens. Within the zone with the radial cracks initiation, grains in the crushed coal were bonded whereas cleats in the block coal were healed. The grain bonding and the cleavage healing can be explained by melting and expansion of coal due to temperature increase. The radial cracks are likely initiated after the grain bonding or the cleavage healing due to tensile thermal stress induced by temperature gradient in the coal specimens. It can be concluded that the characteristics of combustion and gasification of the crushed coal and the block coal are similar because both types of coals become similar in texture through combustion and gasification. These results indicate that characteristics of combustion and gasification of coal seam can be mostly estimated from a model test with artificial coal seam made of crushed coal.