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basic mining process/terminology |
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gas content and type |
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Gas Content and Type
Most coals contain "gas" to some extent. The term "gas" is used rather loosely and in many cases is used to refer to methane whereas other gases are frequently present, in particular carbon dioxide and nitrogen. This "seam gas" is not present as free molecules within spaces in the coal material but is adsorbed into the coal itself, a physical attachment of gas molecules onto the carbonaceous material. The amount of gas which can be held in the coal in this way is dependent on the pressure at which the coal exists, the strata pressure, or stress, which results from the depth of cover and possibly other geological factors.
When the coal is mined, the pressure is relieved and the gas(es) are released into the mine atmosphere (or into a gas drainage system) where they have to be dealt with so that they do not become a hazard.
Generally methane is a higher risk gas to deal with because of its explosive properties (it is explosive within a range of concentrations between approx 5 and 15% in air). Methane in coal was formed as part of the process of coal formation from decaying plant material and trapped within the coal at that time. Other hydrocarbons (such as ethane, butane, pentane, etc) are often found associated with the methane but in very small amounts. The methane content varies from seam to seam and even within a seam, and it frequently increases with depth of cover, often being near zero close to outcrops (presumably some of the methane has escaped to atmosphere where the seams are near the surface). The gas in some instances has also migrated to strata above the seam and can be emitted into mine workings from such sources.
Methane is lighter than air and will tend to migrate to the top of cavities. If mine workings are down-dip, methane will tend to move away from the working area unless carried down by the ventilation flow.
While not explosive or poisonous, carbon dioxide will not support life and the concentration of carbon dioxide in areas where personnel work or travel must be maintained below safe (statutory) thresholds. While some of this gas may have been formed with the coal, its origin is more often volcanic. As with methane, while the amount of carbon dioxide contained within the seam, "the seam gas content", varies, it generally increases with depth of cover and also can be found in surrounding strata as well as in the coal.
Carbon dioxide is heavier than air and so tends to migrate down-dip, the reverse to methane.
Whatever gas is in the coal, the majority of it is released as, or soon after, the coal is cut. With the high rate of production in modern coal mines, there are many locations where the amount of gas released is too great for the mine ventilation system to keep gas concentrations at or below acceptable levels. In such cases a "gas drainage" system needs to be introduced where the gas is captured and removed to a location in the mine where it can be adequately diluted, or removed completely from the mine. The gas may be captured from the coal before it is mined by means of long boreholes drilled in the seam and connected to a gas drainage pipe range, or drawn off from goaf areas by either surface gas wells or underground gas drainage pipe ranges before it can reach the mine airways.
As mining is carried out, there is a stress increase ahead of the working face which tends to fracture the coal ahead of the workings. When this occurs gas in the coal can be released at a relatively slow rate and bleed off through microfractures into the workings. On occasions, some change in the seam (eg a dyke, or a localized mineralization of the coal seam, etc) can act as a dam to prevent the steady release of gas occurring. When this "dam" is breached or weakened as the mine workings approach, it can fail suddenly under the pressure of gas behind it. In seams where the gas content is high, this failure can be very violent and many tonnes of coal and large volumes of gas are ejected into the workplace. Such sudden releases are called "outbursts" and this phenomenon is a major hazard at some mines requiring particular safety measures to be employed. The hazard can be removed by removing a large part of the gas in advance of the workings utilizing an in-seam gas drainage system, usually attached to a vacuum system to remove the gas as it is released from the coal.
Slide presentation re gas in coal – little detailed explanation but indicates complexity.
COAL SEAM METHANE - Sigra – a commercial site for work carried out by Sigra, but briefly describes equipment and contacts.