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.
The Ventilation part of the basic mining process/terminology section made mention of gas drainage, with a brief description of the way in which outbursts occur (where some change in seam conditions interferes with the usual gradual release of gas into the workings). Early gas drainage work in Australian mines was aimed at removing gas prior to mining (particularly secondary extraction) solely in order to reduce the demands on ventilation for gas dilution purposes. In fact in some cases gas drainage holes were intentionally stopped short of future roadway locations to avoid problems when the holes were intersected during development. At that time, outbursts were frequently not recognized for what they actually were at some mines, and terms such as "pressure bumps" or "slumps" were often applied to what were actually small outbursts. At other mines the true nature of the problem was recognized earlier and pioneering work related to removal of gas and devising methods of identifying when locations were safe to work were being developed. Such work gradually increased as the more widespread nature of the risk became apparent, sometimes through tragic incidents involving fatalities.
It is always risky to mention individuals involved in such work for fear of offending other deserving individuals by their omission, but in this instance work carried out by Allan Hargreaves, Ripu Lama and Ray Williams deserves particular recognition.
Early methods of mining in locations liable to outburst tended to involve giving the face a "shake" (by bumping it with the miner cutting head or using explosives) to initiate an outburst at a time when the crew was prepared for it, should one be imminent. While there is some validity in this, the degree of protection to personnel if a miner was used was minimal – the term "bomb squad" was sometimes used, with good reason. For a period efforts were concentrated on protecting personnel at the face in the event that an outburst did occur (utilising a fully enclosed cabin for the miner driver with protected air supplies, all other personnel having retreated outbye while cutting was in progress). Meanwhile work was progressing on ways to prevent an outburst occurring at any time.
The result of all the experimental and development work has led to the present methods of outburst control involving drainage of gas from the coal to reduce the gas content to below "threshold levels" at which outbursts will not occur, before normal mining is permitted. These threshold levels vary according to the gas composition. If it is found to be impossible (or impractical) to reduce the gas content sufficiently, then mining can only be carried out by means which do not require personnel to be at the face while coal is being cut. Such methods may involve remote operation of the equipment using CCTV to observe what is happening, or using "grunching" ( shotfiring the coal off a solid face i.e. no cut in the face to provide a second free face to shoot into). These methods are slow and cumbersome but do at least allow development to proceed safely, the slow rate usually being acceptable for a period.
Outbursts are rarer on longwall faces than on development, most probably because the extended face length and the high front abutment loads which cause the face coal to fracture, reduce the likelihood of any structure acting as an effective dam. Nevertheless outbursts have occurred on longwall faces and drainage to threshold levels should still be practiced, though the threshold values may be somewhat higher than those for development.
Threshold values appear to be consistent within a given seam in a given area, but it may be necessary to develop such values for other seams and locations. It may well be that threshold values are in fact far more variable, but those used are very conservative and cover such variations. If this is the case it is unlikely to change unless somebody can develop a valid system for calculating more accurate values. The consequences are too great for any risks to be taken in this area.
It is not safe to say that the outburst problem has been totally overcome, but it can be said that there have been no outburst incidents, certainly no outburst injuries where the present control systems have been used.
Gas drainage for the purpose of eliminating the outburst hazard consists of drilling holes within the seam being mined to allow gas to bleed off from the coal into the hole. In its virgin state, the gas is "adsorbed" onto the coal, a physical bonding largely governed by pressure. As the pressure is reduced (as will occur when the coal is excavated, including around a borehole drilled into the coal) the gas will "desorb" from the coal, the amount released increasing as the pressure becomes lower. Once desorbed, the gas can flow through microfractures or other structures in the coal and so be released into the excavation, or borehole. If the gas can be captured from the end of the borehole and led into a pipe range, it can be removed without polluting the mine ventilating air.
Purely drilling boreholes and allowing the gas to flow of its own accord will be effective in reducing the likelihood of outbursts and would normally be sufficient to reduce gas levels to below thresholds without the use of pipework. However most mines would find the gas in the ventilation system to be an additional hazard, difficult to handle, and therefore connect the boreholes to a pipe range to remove the gas to an area where it will not cause a problem to the mine environment, usually to the surface (ignoring at this stage any atmospheric pollution problems this may cause on the surface).
Most mines that carry out gas drainage for whatever purpose utilize a surface "gas drainage plant" which consists of vacuum pumps (with ancillary equipment) attached to an underground gas pipe reticulation system usually via boreholes. The vacuum has a minimal effect on the effectiveness of gas flow from the coal – the gas is frequently released at a pressure of the order of 1500 to 2000 kPa in the coal and few gas plants would provide a vacuum of more than 20 to 40 kPa at the boreholes, an additional very small amount in comparison. The main purpose of the vacuum is to take the gas away and deliver it where required. The vacuum may have some minor benefit in removing a small amount of extra gas after the natural gas pressure has reduced to a very small amount and most of the gas has gone. In order for gas to flow through the coal, any water in the coal has to be removed first. This flows into the boreholes and the vacuum does serve a purpose in removing the water from the boreholes, allowing gas flow to occur. The removal and handling of water from the boreholes and pipe system can be a major process in itself and has to be included in the design of a gas drainage system.
The number and size of drainage boreholes necessary to be effective will vary from seam to seam and mine to mine. The borehole density required for effective drainage will depend on the permeability of the coal which controls the rate of flow through the seam once the gas has been desorbed (the effect of water on porosity is the reason it has to be removed before gas flow is effective) and the time available before mining takes place in the location being drained.
Most mines now use directional drilling (i.e. types of drill rigs where the direction of the borehole being drilled can be surveyed and controlled) to allow long holes to be drilled with accurate targeting of the hole for its full length. Typical holes are around 90 mm diameter and vary in length from 100m or even less up to 1km (possibly even longer but few underground drill rigs can successfully drill far beyond this distance). Typically holes are drilled in a fan pattern from relatively few locations (relocating drill rigs is a major operation), often curved from the starting point to then form a series of parallel holes over much of their length.
Ideally the location of a future set of roadways would be drained by holes drilled from a roadway parallel to the line of the future ones, this being the simplest drainage pattern and allowing drilling to be separated from the development process. Frequently however, such drilling sites are not available and drilling has to be carried out from the standing end of a set of roadways to cover the next section ahead. This can create problems with continuity as development has to stop and wait for drainage to occur.
At times when drainage has been unsuccessful, either because the gas will not flow effectively or there has been insufficient time for successful drainage, it may be possible to complete drainage to threshold levels by drilling a very dense hole pattern ahead of the face, usually for relatively short lengths of advance.
The increasing use of coal bed methane production, dealt with elsewhere, will have a major effect on gas drainage as the need for any further drilling and draining from the mine workings may have been removed or at least reduced. To check that an area has been successfully drained, further drilling is required to obtain coal samples in order to measure the remaining gas content and ensure threshold levels have been attained. Most commonly this entails relatively short holes drilled ahead of a development panel, often using purely rotary drilling (not directional drilling) and surveying the holes on completion. There are several variations to this process at various mines. Because of the high risk and consequences involved in seams liable to outbursts, the drainage and testing process is strictly controlled with procedures set out in Management Plans as one of the major hazards to be controlled.
The excavation of coal as part of the mining process causes gas to be released which passes into the mine ventilation air and is exhausted through the main fans on the surface. In some mines however, gas is captured by a gas drainage system either prior to mining (pre-drainage) or after mining (post drainage), with captured gas usually being piped to surface. Note that mines with gas drainage systems do not capture all the gas, and typically around 50% is still exhausted in the mine ventilation. The majority of mines vent the captured gas to atmosphere, in some cases after flaring, although a few mines have, since the 1970's utilized the captured gas to generate power for both mine and community consumption. There is now a growing move to utilize gas from mines, both because of a desire to reduce atmospheric pollution and because it is a wasted resource. In some cases the captured gas (at least a large proportion of it) is carbon dioxide and there is no means of utilizing this gas – it has limited commercial use, such as in soft drinks and fire extinguishers, which would be only a small fraction of the gas produced by mines, and it is easier to extract it from normal air. Many mines however produce methane which can be burnt to utilise the heat either directly or, more commonly, to generate electricity. This process actually converts the methane to carbon dioxide and water so actually increases the carbon dioxide output, but the effect of methane on the atmosphere is far worse than carbon dioxide, by a factor of around 8.
It would be possible to burn the methane in a boiler to produce hot water for whatever purpose, but there are no examples of this in Australia. Two methods are currently used to generate electricity in Australian mines at present:
· Burning the methane in a gas turbine which drives a generator.
· Burning the methane in a "gas engine", a diesel generator set designed to run on methane gas as a fuel instead of diesel.
Both methods have proven successful although gas engines can be designed to run on very lean mixes and so are suited to mines which produce a mix of gases. Either option is suited to very rich mixes. Another advantage of gas engines is that they are typically smaller capacity and are suited to building up capacity of a "power station" over time by the addition of other units, thereby allowing expenditure to be spread and for spare capacity to be provided to facilitate maintenance. In some cases it is beneficial to allow a connection to an alternative gas supply, if available, to allow generation to continue in the event the gas supply from the mine becomes too lean. It would not be normal to continue generating if the gas supply ceases as the main purpose of such a plant is to utilize mine gas; it would probably be uneconomic to run on alternative supplies. In most cases the power produced is utilized in the mine, feeding in with the external power supply. In the event that the power produced is greater than the mine usage, the excess if fed into the grid. Arrangements are made between the mine and the external supplier regarding payment for power exported. In some cases the gas utilization plant may be owned and operated by a company other than the mine on a toll basis.
In general, any utilization system should be designed to operate within the capacity of the gas capture system - any decision to change mining operations for the benefit of the utilization system would have to be based on financial or social considerations.
A process which is growing rapidly at present is referred to as "coal bed methane recovery". This is a process of draining methane from coal using surface boreholes, usually drilled at an angle other than vertical and curved to eventually follow a coal seam for some distance. This process is primarily aimed at recovering methane for commercial use in its own right and as such is not strictly part of coal mining. It is possible however, and in some cases is occurring, for a coal mining operation to carry out the same process primarily to remove the gas to assist later coal mining operations. The practicality of this process depends on the availability of surface drill sites and there being sufficiently long lead time available to drill and drain the gas before the coal is to be mined. Whatever the primary reason for the methane recovery, it would be utilized either as described above or would be fed into a gas supply grid. Gas mixtures with a high carbon dioxide content would not be suitable for the latter.
ACARP reports – various reports on research work related to use & collection of gases, in "Mine Site Greenhouse Gas section"
COAL SEAM METHANE - Sigra – a commercial site for work carried out by Sigra, but briefly describes equipment and contacts.