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Abstract:In the underground coal gasification (UCG) process, cavity growth with crack extension inside the coal seam is an important phenomenon that directly influences gasification efficiency. An efficient and environmentally friendly UCG system also relies upon the precise control and evaluation of the gasification zone. This paper presents details of laboratory studies undertaken to evaluate structural changes that occur inside the coal under thermal stress and to evaluate underground coal-oxygen gasification simulated in an ex-situ reactor. The effects of feed temperature, the direction of the stratified plane, and the inherent microcracks on the coal fracture and crack extension were investigated using some heating experiments performed using plate-shaped and cylindrical coal specimens. To monitor the failure process and to measure the microcrack distribution inside the coal specimen before and after heating, acoustic emission (AE) analysis and X-ray CT were applied. We also introduce a laboratory-scale UCG model experiment conducted with set design and operating parameters. The temperature profiles, AE activities, product gas concentration as well as the gasifier weight lossess were measured successively during gasification. The product gas mainly comprised combustible components such as CO, CH 4 , OPEN ACCESSEnergies 2013, 6 2387 and H 2 (27.5, 5.5, and 17.2 vol% respectively), which produced a high average calorific value (9.1 MJ/m 3 ).

Coal, the most abundant fossil fuel resource in the world, has proven reserves that are expected to be primary energy source for the 21st century. From the perspectives of safety and efficient resource utilization, coal is the subject of great expectations to satisfy the rapidly increasing demand for energy. As a clean coal technology, Underground Coal Gasification (UCG) is used to create a combustion reactor in an underground coal seam, thereby enabling the collection of heat energy and gases (hydrogen, methane, etc.) through the same chemical reactions that are used in surface gasifiers. As early as 1912, the first plan for UCG experiments was proposed by Sir William Ramsay. They were conducted on a small-scale in Durham, UK 1). A f ield study of UCG technology was done in the 1930s in the Union of Soviet Socialist Republics (USSR) 1). The technology was developed to a limited degree in the US, Europe, China, and Japan later during the 1960s and 1970s 2-10). However, many countries have recently shown increased interest in this method: modern sensing and control techniques can reduce UCG environmental effects by curtailing greenhouse gas emissions to the air and by leaving no ash aboveground. The relevant literature describes experimental tests and modeling experiences of UCG that have been pursued in recent decades. Theoretical and experimental studies have increased year-by-year in many countries since the 1930s 1, 11-14). A typical UCG system is presented in Fig. 1. It includes a coal seam with two boreholes drilled down into it: one for injecting reaction gas for in-situ burning of coal and the other for extracting the product gas. Actually, UCG minimizes health hazards and improves miners' safety because it requires no underground work, eliminates environmental hazards, and

Due to the additional abutment stress, interactional hard roof structures (IHRS) affect the normal operation of the coal production system in underground mining. The movement of IHRS may result in security problems, such as the failure of supporting body, large deformation, and even roof caving for nearby openings. According to the physical configuration and loading conditions of IHRS in a simple two-dimensional physical model under the plane stress condition, mining-induced failure criteria were proposed and validated by the mechanical behavior of IHRS in a mechanical analysis model. The results indicate that IHRS, consisting of three interactional parts-the lower key structure, the middle soft interlayer, and the upper key structure-are governed by the additional abutment stress induced by the longwall mining working face. The fracture of the upper key structure in IHRS can be explained as follows: Due to the crushing failure, lower key structure, and middle soft interlayer yield, the action force between the upper and lower key structures vanishes, resulting in fracture of the upper key structure in IHRS. In a field case, when additional abutment stress reaches 7.37 MPa, the energy of 2.35 × 10 5 J is generated by the fracture of the upper key structure in IHRS. Under the same geological and engineering conditions, the energy generated by IHRS is much larger than that generated by a single hard roof. The mining-induced failure criteria are successfully applied in a field case. The in-situ mechanical behavior of the openings nearby IHRS under the mining abutment stress can be clearly explained by the proposed criteria.