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
In this study, to better simulate underground coal gasification (UCG), an artificial coal seam was constructed to use as a simulated underground gasifier, which comprised coal blocks excavated from the coal seam. This study reports the process and results of three independently designed experiments using coaxial-hole and linking-hole UCG models: a) a coaxial model using a coaxial pipeline as a gasification channel, b) a coaxial model using the coaxial pipeline combined with a bottom cross-hole, and c) a linking-hole model using a horizontal V-shaped cross-hole. In the present work, the fracturing activities and cavity growth inside the reactor were monitored with acoustic emission (AE) technologies. During the process, the temperature profiles, gas production rate, and gas content were measured successively. The results show that AE activities monitored during UCG process are significantly affected by operational variables such as feed gas rate, feed gas content, and linking-hole types. Moreover, the amount of coal consumed during UCG process were estimated using both of the stoichiometric approach and balance computation of carbon (C) based on the product gas contents. A maximum error of less than 10% was observed in these methods, in which the gas leakage was also considered. This demonstrates that the estimated results using the proposed stoichiometric approach could be useful for evaluating energy recovery during UCG.
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