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15.08.2018 Солнце в сеть




Производство оборудования и технологии
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MISCELLANEOUS PROCESSES

There are a number of gasification processes that for various reasons fall outside the categories that have been discussed here. These are in situ gasification of coal, also known as underground gasification, gasification in a molten iron bath, plasma gasi­fication, and hydrogasification.

5.8.1 In situ Gasification

Gasification of coal in situ has a number of obvious attractions. Such a process has the potential to tap resources not otherwise readily or economically accessible. It would also eliminate the safety hazards and costs associated with underground mining. The ash would be left underground.

The first recorded proposal for underground coal gasification (UCG) was by Siemens in 1868, followed by Mendeleyev 20 years later. Initial experiments in the United Kingdom were broken off by the advent of World War I. No further work was done until the 1930s when an experimental station was started in the Donetsk coalfield in the then-Soviet Union, to be followed by a commercial installation in 1940 (Weil and Lane 1949). Underground gasification continued at a number of locations in the Soviet Union until the late 1970s, with a gas production of some 25,000 million Nm3 of gas being produced from around 6.6 million tons of coal (Okten 1994). This production came from seams of 50 to 300 m depth.

In the 1980s a number of small experimental units were operated in the United States. In Europe, tests have been conducted in Belgium (1986-1987), and then in Spain. The Spanish trial was aimed at demonstrating the feasibility of UCG using modern directional drilling techniques at a depth of 600m. The trial operation lasted for a period of 300 hours, during which 290 tons of coal were gasified (Green and Armitage 2000).

The basic concept of UCG is to drill one or more wells into the coal seam where the blast is injected and others from which the fuel gas can be collected. There are a number of different methods that have been used to link injection and gas wells, some of which are depicted by Okten (1994). Simple though the basic idea may be, there are many practical difficulties still to be overcome, and it its already clear that the technology can only be applied to certain types of coal seam. The hydrogeology of the seam is important, since excessive ingress of water would render the process uneconomic, and leakage of gas into underground water supplies could represent an environmental hazard. Both air and oxygen gasification have been tried. With air, a very low Btu gas is produced, whereas with oxygen the cost of the blast and the losses make the process very costly. For these and other reasons, no commercial development has yet emerged.

There is currently one active project in Australia (Walker, Blinderman, and Brun 2001). Operations using air injection commenced in December 1999, and to date (October 2002) some 32,000 tons of coal have been gasified. The peak gas production capacity is 80,000 Nm3/h. Gas with a heating value of about 5 MJ/m3 is produced at a pressure of 10 bar and a temperature of 300°C. During this demonstration phase the gas has been flared. The operation has been shut down pending the proposed installation of a gas turbine.

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