Underground coal gasification

Underground coal gasification (UCG) is the in-situ gasification process carried on in non-mined coal seams using injection and production wells drilled from the surface, which enables the coal to be converted into product gas. The process is flexible in operation and is capable of producing commercial quantities of gas to be used as a chemical feedstock or as fuel for power generation. The technique is able to be applied to resources which are otherwise not commercially viable to extract and also offers an alternative to conventional coal mining methods for some resources.

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History

In the year 1868, Sir William Siemens was, apparently, the first to suggest the underground gasification of waste and slack coal, in the mine. The first experimental work started in Co. Durham (UK), in the year 1912, under the leadership of William Ramsey. However, he was unable to complete this work before the beginning of World War I and all efforts to UCG development in Western Europe were discontinued to the end of the second world war. In the USSR the Skochinsky Institute of Mining research and development program during the 1930s. It led to the operation of industrial scale UCG in the 1950-1960s at several coal sites. However, activity subsequently declined due to the discovery of extensive natural gas resources and only one site is still in operation today at Angren in the territory of Uzbekistan.^[1]

Between the years 1944 to 1959, the shortage in energy and the diffusion of the results of the UCG experiments in USSR during the period 1934-1940 induced new interest for UCG in Western European coal mining countries. The first research work was directed to the development of UCG in thin seams, at shallow depth. The stream method was tested in Belgium, on the site of Bois-la Dame (1948) and in Morocco, on the site of Djerada (1949). The boreholes method was tested in the United Kingdom, on the sites of Newman Spinney and Bayton (1949-1950). A few years later, a first attempt was made to develop a commercial pilot plant: the P5 Trial in Newman Spinney (1958-1959). During the Sixties, all European work was stopped, due to abundance of energy and to the low oil prices. In the USA, a UCG program was initiated in 1972, which built upon Russian experience, as well as the implementation of an extensive field testing program; the latter being supported by a number of research institutes and universities.^[1]

In 1989, the European Working Group on UCG recommended that a series of trials should be undertaken to evaluate the commercial feasibility of UCG. The trial was undertaken by the Spain, the UK and Belgium, and was supported by the European Commission. The largest on-going program is conducted by China, which includes 16 UCG trials.^[2]

The successful demonstration conducted by Ergo Exergy Technologies Inc (Canada) near the town of Chinchilla, some 350 km west of Brisbane, in Queensland, Australia has resulted in a surge of interest in the technology. The Chinchilla demonstration began in December 1999, and is owned by Linc Energy Limited (Australia). Up until the end of the controlled shutdown program, which was completed in April 2003, the demonstration involved the gasification of 35,000 tonnes of coal, and resulted in successful environmental performance as per independent audit reports. Linc Energy plans to commence production in May 2007.^{[citation needed]}

Criteria for underground coal gasification

Underground coal gasification requires special properties of coal seam:

• Coal seam lays underground between 100 and 600 metres (preferably more than 300 metres)
• Coal seam thickness is more than 5 metres
• Ash content is less than 60%
• Minimal discontinuities in seam
• No good water aquifers^[3]

Technology

The basic underground coal gasification process has one wells drilled into the coal for injection of the oxidants, and another well to bring the product gas to surface. As coal varies considerably in its resistance to flow, depending on its age, composition and geological history, the natural permeability of the coal to transport the gas is generally not satisfactory. For high pressure break-up of the coal, a hydrofraccing, an electric-linkage, and a reverse combustion may be used with varying degrees.^[4]

There are two different commercially available underground coal gasification methods. One of methods uses vertical wells and a method of reverse combustion to open up the internal pathways in the coal. The process was used in the Soviet Union and later it was tested in Chinchilla by using air and water as the injected gases. Another method creates dedicated inseam boreholes, using drilling and completion technology adapted from oil and gas production. It has a moveable injection point known as CRIP (controlled retraction injection point) and generally uses oxygen or enriched air for gasification.^[4]

Economics

Underground coal gasification allow access to more coal resources than economically recoverable by traditional technologies. By some estimates it will increase economically recoverable reserves by 600 million tonnes.^[5] According to Linc Energy, the capital and operating costs of the underground coal gasification are lower than in traditional mining.^[1]

A number of new projects are currently in planning and development stages across the globe.

Underground coal gasification product gas can also be used for:

• Synthesis of liquid fuels at a predicted cost equivalent to US$17/bbl
• Manufacture of chemicals such as ammonia and fertilizers
• In these roles it replaces the use of natural gas and can provide substantial cost savings

References