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Coal Bed Methane (CBM): A natural resource base Orissa should Explore
"CBM as a commercially viable source of energy was developed for the first time in the late 1970s in USA, and in 1989 significant production was achieved. The principle of extracting CBM is simple, which is basically the lowering of the pressure within the coal beds, which releases the gas."
Dr. Nachiketa Das : February 27, 2008
Coal reserves of India are currently estimated at around 247 billion tons, 25% of which is in Orissa. Reserves of at least 61 billion tons make Orissa a coal rich state by any standards of the world. Moreover, the current estimate is a conservative one, and is bound to rise substantially with further exploration.
During the process of formation of coal, or coalification, where terrestrial plant materials are progressively converted into coal, large volumes of gases are produced. Methane (CH4) constitutes the bulk of these gases, and is a simple hydrocarbon that contains carbon and hydrogen. The carbon in methane when ignited burns and produces energy in the form of fire and heat. This methane stored in the internal surfaces of the coal is known as Coal Bed Methane (CBM). As coal contains large internal surface areas, it can play host to vast quantities of CBM. Coal is thus the source as well as the reservoir of CBM, composition of which varies from almost pure methane to up to 50% carbon dioxide. CBM may occasionally contain more than 10% ethane, but very rarely any other heavier hydrocarbons such as propane or butane, and never contains any natural gas condensates.
Reserves of CBM:
An accurate estimation of the reserve of CBM in India, at present, is not an easy task. Reserve estimation of CBM requires significant amounts of drilling of boreholes in the coal fields, which is obviously expensive, and makes the exploration very capital intensive. Estimates of the CBM potential of India have been proposed by various authors in the last ten years, which range from 800 to 1,500 billion cubic meters. One estimate is as high as 8,000 billion cubic meters. Let us accept a conservative and a round figure of 1,000 billion cubic meters. Since Orissa possesses about 25% of the coal reserves of the country, it is only logical (not strictly correct though) to assume that Orissa should contain about 25% of the CBM reserves of the country. And that makes CBM potential of Orissa stand at 250 billion cubic meters.
For the sake of argument if we accept the prices for LNG (liquefied natural gas) and CBM to be the same, at current market prices (of 40 cents per cubic meter in 2007), the CBM reserves of 250 billion cubic meters should fetch 100 billion US Dollars, which in Indian Rupees are 400 thousand crores. That is a lot of money by any standards of the world, and it could only be a conservative estimate, and the real figure could be substantially higher. Moreover, gas prices will continue to rise making the CBM reserves even more valuable.
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CBM and mine safety:
In coal mining operations, methane, which is a colourless, odourless and tasteless gas that is lighter than air, poses a major hazard. The most dangerously explosive mixture of methane is between 5 to 15% of the confined air above the mine floors. In poorly ventilated coal mines, the concentration of methane in the air builds up, which ignites easily, catches fire and causes serious explosions harming lives and property. Moreover, at times CBM within the coal beds combusts spontaneously setting fire to entire coal seams, as self heating temperatures for coals are quite low, could be as low as 30oC in low rank coals like lignite and sub-bituminous, and 60oC for bituminous coal. Spontaneous coal bed fires can occur in dry areas or when the water table is lowered significantly.
In the past, coal miners in Britain carried canary birds in cages to detect the presence of some of these coal gases. Canary is a chirpy little yellow bird that sings all day, and is extremely sensitive to the presence of carbon monoxide. The presence of even the slightest amounts of this lethal gas would make the canary stop singing and show signs of distress. An abrupt end to the canary songs was a warning of the impending doom, and the mine floors were evacuated in a hurry. The exact date of the first introduction of canary birds for the detection of carbon monoxide is uncertain, but their services in the British coalmines were institutionalised in 1911. Since then, the canaries were in continuous service for 75 years. Finally on the 30th of December 1986 the services of the canaries were phased out, and electronic gas detectors were introduced.
Canary birds, however, did not detect the dangerous build up of methane gas in the coal mines, where miners continued to perish in explosions. In 1815, British chemist Sir Humphry Davy invented a lamp widely known as Davy’s Safety Lamp, which came in handy to detect the presence of methane. This simple oil lamp contained a cylindrical rolled metal wire gauge of fine mesh around the flame enclosing it. Presences of methane made the flame change either its colour or size or even shape. Moreover, a pale blue flame around the main flame appeared in response to the presence of methane. Despite the Davy lamp, explosions in coal mines were a regular occurrence. In the past decades there have been many explosions in the poorly ventilated coal mines in China due to the accumulation of methane.
In order to dissipate methane to make the mining operations safer, often high capacity fans are installed which dilute its concentration and release the gas into the atmosphere. Although this method of ventilation averts the immediate dangers of explosion, it is quite wasteful. It wastes the precious resource and also contributes to global warming by releasing methane, which is a significant greenhouse gas, much more potent than carbon dioxide, into the atmosphere. The best way to remove this explosive gas, therefore, is by harvesting it.
Extraction of CBM:
CBM as a commercially viable source of energy was developed for the first time in the late 1970s in the USA, and in 1989 significant production was achieved. The principle of extracting CBM is simple, which is basically the lowering of the pressure within the coal beds, which releases the gas. The commercial extraction of the gas, however, is technologically challenging. Coal seams that are saturated by water could easily be depressurised simply by the removal of water. Subsequent hydraulic fracturing of these coal beds drains the reservoirs and allows commercial rates of gas production. Extraction of CBM requires the drilling of steel encased bore hole into the coal seam, often 100 to 1,500 meters below the ground level. As the borehole opens to the surface, the pressure within the coal seams decline, and both gas and water escape to the surface. CBM wells, however, often produce gas at much lower levels, the outflow being a function of cleat density, which simply means the amounts of cracks and fractures, within the coal beds.
The single biggest environmental concern of harnessing CBM is the release of water from the coal seams, which is generally saline, and enriched with chloride and bicarbonate. Such saline waters have to be treated properly and disposed carefully. In some places however, water from the coal seams can be potable ground water fit for human consumption. The benefits of harnessing CBM far outweigh the environmental damage it may cause.
(Author is the Director, NRI-Enviro-Geo-Tech - Australia, Sydney & presently based in Hiroshima, Japan)