Description : Underground Coal Gasification (UCG) is a promising environmentally acceptable clean-coal technology. A typical UCG system has two wells drilled from the surface into a coal seam with some separation of injection from production. The wells are connected underground by various linking techniques. After creating a combustion reactor in an underground coal seam, air and oxygen flow through the injection well. Heat energy and gases are collected from the production well. As gasification progresses, the combustion reactor is moved along the linking hole. Using this process, the fracturing activity inside the coal seam serves an important role for enlargement of the gasification zone because the surface area oxidization increases continuously by coal cracking. For effective coal gasification, fracturing activity must be controlled. Moreover, excess fractures inside the coal seam and surrounding rock can induce gas leakage, underground water contamination, subsidence, etc. Therefore, monitoring and control of fracturing activity in underground areas constitute key technologies for efficient and safe UCG. To monitor fracturing activity, we conducted laboratory and small-scale field experiments of UCG using acoustic emission (AE) measurements.
UCG model tests were conducted for linking and coaxial types UCG models. Coaxial UCG systems are compact and are anticipated for application in underground coal seams having a complex geological structure. The UCG model consists of a coal block molded in a steel canister (20 liter capacity) with heat-resistant mortar, with a linking hole or a coaxial-hole passing through the coal block. During coal combustion, coal temperatures, production gas contents, and AE activity were monitored under the control of injection-air and oxygen flow rates. Results show that the combustion propagated inside the specimen along the linking hole. Many AE events were generated during coal combustion. The AE activity was related closely to temperature changes inside the coal block. This AE generation apparently results from crack initiation and extension around the coal combustion area under the influence of thermal stress. However, comparing the results from UCG models of linking and coaxial types, the gasification of the linking type model showed an average calorific value as high as 11.3 MJ/m3. The coaxial type yielded an average calorific value of produced gas as only 5.79 MJ/m3. In the coaxial type of UCG model, the combustion region was limited to the area around the coaxial hole. The AE activity was lower than in the linking type model.
For small-scale UCG field experiments, linking and coaxial types were tested at an open pit coal mine in Hokkaido, Japan. Using a linking type UCG, 58.2 kg coal was processed in 39 h, with the calorific value estimated as 11 MJ/m3. However, the coaxial type UCG processed 7.0 kg coal in 9 h, yielding an estimated calorific value of 6.0 MJ/m3. These tendencies resemble those of results of laboratory UCG experiments.
Results of these experiments demonstrate that AE monitoring is a useful tool to evaluate combustion reactors for efficient and safe UCG systems. To develop a coaxial UCG system, results show that fracture control around combustion reactors is indispensable
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