Underground Coal Gasification (UCG) can exploit the energy stored in underground coal efficiently and with fewer environmental impacts. Valuable gas products can be obtained by gasifying coal in situ with a UCG operation [1
]. The general UCG system, comprising of an injection well and a production well at the surface, is shown in Figure 1
, in which both wells are connected by a linking hole within the coal seam [15
]. During the process of gasification, the gasification area is gradually enlarged along the linking hole. The product gases can be obtained, as they are useful in the creation of many products, such as chemical feed stocks, liquid fuels, hydrogen, synthetic gas, and the generation of electric power [17
]. Recently, UCG technology has attracted greater attention as an alternative to conventional mining methods, especially when exploiting coal resources located deep underground or exploiting low quality coal resources with high ash and high sulfur. Gasification provides the technological basis for theoretical and experimental research of pollution control, specifically, the emissions of sulfur, nitrous oxides, and mercury, which is useful for the elimination of ash after coal burning. An alternative UCG system must be developed in Japan because its geological conditions are complicated by the existence of faults and inclined coal seams. Given this background, we are developing a coaxial UCG system that is compact, safe, and highly efficient, as shown in Figure 2
. Only well drilling and a double pipe were used in the coaxial UCG system. Gasification agents are injected from the inner pipe to expand the combustion zone. The production gas is recovered from the outer pipe. The designated inner (injection) pipe can be slid up and down to adjust the gas outlet position.
However, associated environmental issues (such as gas leakage, groundwater pollution, and surface subsidence associated with cavity growth) [23
], improperly executed operations, and gasification processes can restrict the applicability of UCG. Therefore, in order to ensure effective combustion and efficient gasification, the evaluation of the coal gasification cavity growth and precise control of the reactor are important. It was suggested that the gasification efficiency is directly affected by the enlargement of the oxidation surface around the gasification channel following crack initiation and development inside the coal seam. Several research activities were carried out to evaluate cavity growth and the velocity of the gasification flame, based on a mathematical model [8
]. These research methods were effective in estimating the volume and progress of cavity growth and in creating the design of the UCG operation, however these methods were also needed to evaluate the cavity in real-time during the UCG operation, because it is sometimes difficult to predict the cavity in a coal seam precisely due to its heterogeneous characteristics. In order to evaluate fracturing activity around the combustion area, acoustic emission (AE) monitoring was applied in our previous research [30
]. It has been proven that the AE technique has great potential for the measurement of fracture extension around the combustion reactor. In typical UCG, a geophone, a type of transducer with low frequency microseismicity, which is functionally similar to the AE accelerometer employed in this work, could be used to monitor fracturing occurring inside the gasifier. This technique makes it clear when and where microfailure phenomena occur. Until now, various UCG model experiments have been carried out to develop the coaxial UCG system [31
]. However, the energy recovered from the coal is relatively low because the gasification area in a coaxial system is limited around a well. Therefore, an application of the coaxial UCG system with a horizontal well is discussed to improve the total efficiency of the gasification process in the study (Figure 3
). The coaxial UCG system is expected to be used as a local energy source in small communities, as the cost of constructing drill holes and purchasing ground equipment is lower than those for the traditional UCG system that has a linking hole.
This paper presents ex situ experiments conducted with the coaxial type UCG model. In order to simulate UCG conditions with the coaxial-hole model in an artificial coal seam, the research team designed and established larger-scale UCG systems, which were different from the previous laboratory-scale model experiments [31
]. We also estimated the energy recovery and gasification efficiency with a theoretical calculation, based on the measured product gases.
Results obtained from this experiment revealed that coal generated AEs with special AE activity patterns that were caused by thermal stress. The AE technique can visualize fracture extension around the combustion reactor. The results from gas energy recovery were evaluated with a stoichiometric method [36
], based on the measured product gas compositions. After approximately 72 h of UCG operation in this experiment, product gas with an average calorific value of approximately 6.85 MJ/m3
Our research explored the coaxial UCG model by using an artificial coal seam, and the effective coal gasification in the coaxial UCG model was also observed. The initiation and extension of the gasification zone inside the coal could be visually monitored by the AE source locations. AE activity is closely related to local temperature change. Movement of the AE cloud also reflected the gasification area size and cavity growth in the gasifier. It could be confirmed that AE monitoring is available for evaluation of coal damage and gasification zone propagation during the gasification process.
This study focuses on the coaxial UCG system with a horizontal well; the injection position can be moved in order to improve the total efficiency of the gasification process. Results show that it is possible to control the gasification area by changing the position of the injection pipe; gasification reactions are activated around an injection pipe because most of the oxidant is consumed near the injection pipe.
Additionally, the recovered coal energy from a coaxial UCG system with a horizontal well is comparable with that of a conventional UCG system in terms of gasification efficiency, due to the large improvement in product gas quality. According to the experimental results, the average gas calorific value yielded in this coaxial model was 6.85 MJ/Nm3, which improved by approximately 30–50% compared when compared with small-scale laboratory tests conducted using the coaxial-hole model in our previous work. Therefore, a coaxial UCG system may be a feasible option to utilize coal resources abandoned underground, by controlling the injection position and designing a coaxial well along the coal seam dip.