Underground coal gasification (UCG) is a process of producing combustible gases by the in-situ conversion of coal into gaseous products. Coal resources abandoned under the ground for either technical or economic reasons can be recovered with economically and less environmental impacts by UCG; therefore, this technology is regarded as a clean coal technology. UCG has several advantages of low investments, high efficiency, and high benefits compared to conventional coal gasification. However, some environmental risks such as gas leakage, surface subsidence, and underground water pollution are difficult to control because the process is invisible. The reactor in UCG is unstable and expands continuously due to fracturing activity caused by coal combustion. It is, therefore, considered that acoustic emission (AE) is an effective tool to monitor the fracturing activities and visualize the inner part of coal. For this study, UCG model experiments were conducted using coal blocks of 0.55 × 0.60 × 2.74 m to discuss the applicability of AE monitoring for the estimation of the crack generations during UCG process and the extent of the gasification area. Temperatures were also monitored to understand the inner part of coal blocks because the crack generations were strongly related to thermal stress occurred by coal combustion and heat transfer. The monitoring results of AE agree with the measured data of temperatures; the source location of AE was detected around the region temperature increased. AE monitoring are expected to provide a useful data to visualize the gasifier in the underground.
Underground coal gasification (UCG) is a technique to extract energy from coal in the form of heat energy and combustible gases through the chemical reactions in the underground gasifier. This technique enables to utilize coal resources that remain unrecoverable in underground due to either technological or economic reasons. Gasification reaction in UCG process is promoted by enlargement of the oxidation surface around the gasification channel with crack initiation and development inside the coal seam. Fracturing activities inside the coal seam are accelerated with an increase of thermal stress caused by exothermic reactions and heat transfer, as a result, gasification reaction and cavity growth is promoted. Cavity growth influences gasification efficiency because it is directly proportional to the coal consumption. At the same time, some of environmental issues have to be cared such as gas leakage, groundwater pollution, and surface subsidence associated with the cavity growth (Bhutto et al., 2013; Imran et al., 2014; Kapusta and Stańczyk, 2011; Kapusta et al., 2013, Shu-qin et al., 2007). Therefore, techniques to evaluate the fracture activity around the gasification area have to be developed for precise control of coal gasification in-situ and minimizing environmental impacts.