Method of gas drainage in a coal mine, general method of methane gas drainage and variable flow control valve

Abstract

The present invention relates to a method of gas drainage in a coal mine, general method of methane gas drainage and variable flow control valve. More specifically, the invention relates to methods and valves for methane gas drainage in which the valve progressively opens and closes with corresponding increases and decreases in methane concentration.

Description

[0001] METHOD OF GAS DRAINAGE IN A COAL MINE, GENERAL METHOD OF METHANE GAS DRAINAGE AND VARIABLE FLOW CONTROL VALVE FIELD OF INVENTION

[0002] The present invention relates to a method of gas drainage in a coal mine, general method of methane gas drainage and variable flow control valve. More specifically, the invention relates to methods and valves for methane gas drainage in which the valve progressively opens and closes with corresponding increases and decreases in methane concentration.

[0003] BACKGROUND ART

[0004] Coal seams contain Methane and other gases which are liberated during mining Some coal seams are very gassy, others are not.

[0005] Gas content in coal tends to increase with depth from the surface and with increases in coal rank. There is generally more gas in coking coal than in thermal coal, but the higher realisable cost of the coking coal generally covers the extra cost involved in managing the extra gas.

[0006] Each underground mine has a ventilation system in which large fans suck and / or force air through the mine. Clean air is sucked or pushed through intake airways from the surface to the face and then through return airways to the surface.

[0007] The intake and return airways are kept separate so that they can’t intermix. The ventilation is used primarily to dilute Methane gas emissions and dust and carry them away from the face. It also provides cooling air to offset the geothermal gradient (the ground temperature increases with depth), and to provide fresh air for the mine workers to breathe.

[0008] Methane presents three problems for underground coal mining. These are:• Explosion risk. Methane is explosive when mixed with air when the Methane concentration is between 5% and 15%. Hundreds of thousands of miners have died around the world over hundreds of years, from Methane explosions in coal mines. A Methane explosion can trigger a coal dust explosion, which can be even more powerful. The shockwave from the Methane explosion can lift the coal dust in the roadway and then compress it against the dynamic resistance of the compressed air through the roadway. The compressed coal dust / air mixture is then ignited by the flame front from the Methane explosion, like a spark plug does in a petrol engine. Minimising Methane explosions will therefore also minimise the risk of coal dust explosions.

[0009] • Outburst. An outburst can occur when Methane and other gases are present in large quantities in an absorbed state within the coal. When a fresh face of coal is exposed in the mining process and the gas pressure in the coal exceeds the strength of the coal holding the gas, this can in some circumstances result in rapid desorption. When this occurs, a significant amount of gas is released in a short period of time and the coal can explode from the face due to the internal gas pressure generated exceeding the strength of the coal. Thousands of miners around the world have been killed by outbursts over the last century. Reducing the gas pressure in the coal by pre-mining gas drainage reduces the risk of outbursts.

[0010] • Greenhouse Gas (GHG). Methane is 21 times more effective as a Greenhouse Gas than Carbon Dioxide. Methane is liberated into the mines ventilation as coal is mined. The mines ventilation system carries the gas out of the mine where it is released to the atmosphere. This is classed as a fugitive emission. The significantly higher effectiveness as a GHG combined with the large quantities of mine ventilation, make Methane Gas from coal mining a significant contributor to anthropogenic GHG increases. Pre-mining gas drainage means that a higher proportionof the gas is captured for use and lower proportions escape as fugitive emissions.

[0011] Methane is released during mining because the mining process breaks the coal into smaller pieces. The smaller pieces together have a much larger surface area per unit of weight. The rate of Methane desorption from the coal is proportional to the surface area, so the increased surface area means more gas is released.

[0012] The explosion risk is well known, and many systems have been devised to protect mine workers and the mine itself from Methane explosions. These systems stem back to using canaries to detect high concentrations of Methane after it is released by mining, to long hole directional drilling for pre-mining gas drainage that removes some of the gas before the coal is mined.

[0013] There are a whole raft of regulations and legislation in every coal mining country that set operating standards limiting gas concentrations in the mine ventilation, seeking to prevent the gas explosion risk.

[0014] As coal production increases, the rate of gas generation increases in the same proportion.

[0015] Companies have responded to the increased risk associated with increased production rates by developing technology that drains some of the gas from the coal seam and surrounding strata prior to mining. This means less gas is released into the ventilation and ultimately to the atmosphere.

[0016] As mines get deeper, more gas, at higher pressures, is being encountered and consequently, outburst risk is increasing. The increased gas quantities and pressures encountered in the coal and the increased overburden pressure from the greater depth combine to increase the risk of outbursts. Most coal mining countries have regulations and legislation to reduce the risk of outbursts.Draining gas ahead of mining to reduce explosion risk also reduces the risk of outbursts.

[0017] Coal Mine Ventilation Methane is included in the register for anthropogenic Greenhouse Gas generation in most countries where Carbon Taxes apply. Coal mines within these jurisdictions pay Carbon Tax on gas released to the atmosphere. Even with low concentrations of say 0.5%, the amount of ventilation at a typical level of 250 m3 / s results in significant tax payments.

[0018] To put the above in context, each underground mine with 0.5% CH4 and 250 m3 / s ventilation would in a year produce:

[0019] 0.005 * 250 * 60 * 60 * 24 * 365.25 = 39.5 Mm3per annum

[0020] With a weight of 0.714 kgs / m3, this equates to 28,165 tonnes of Methane per annum.

[0021] With each tonne providing 21 times the Greenhouse effect of CO2 and Australian Carbon Credit Units trading at around $31 per tonne (1stQtr. 2024), if an Australian mine was required to pay for fugitive emissions, under the above scenario, it would have to pay:

[0022] 28,165 * 21 * 31 = $A18.3M per annum

[0023] Reducing the amount of gas released to the atmosphere has therefore a very clear financial benefit in countries where a Carbon Tax applies, in addition to the safety benefits.

[0024] Gas captured by the ventilation system is too dilute to be used as a fuel, although, if a power station is nearby, the mine ventilation can be used to provide the combustion air, which would result in this gas being burnt and the energy content recovered. As the Methane would be converted into Carbon Dioxide, the Carbon Tax under such circumstances would reduce to $18.3M / 21 = $0.87M per annum. This would appear to justify a small coal and or gas fired power station at every coal mine, as a means of reducing anthropogenic fugitive emissions.

[0025] The present invention aims to utilise ventilation air as the combustion air to burn gas recovered by pre-mining gas drainage, to consume all gas emissions created during mining, in order to minimise anthropogenic emissions from coal mining. It is envisaged that the extra efficiency gained by burning combustion air containing fugitive Methane emissions, coupled with the reduced transportation requirement to utilise the gas elsewhere would provide a more efficient form of energy generation.

[0026] If the methane recovered by pre-mining gas drainage is stored, it can be used as a source of firming supply to supplement renewable energy when the sun is not shining, or the wind is not blowing.

[0027] In order to facilitate burning of all of the ventilation air, the gas turbine station could be supplemented by a small coal fired power station providing some base load, so that none of the available energy is wasted. The extra generation cost arising from operating a smaller power station is very likely to be offset by having no transportation cost for fuel, and the added combustion efficiency from using the Methane enriched ventilation air.

[0028] It is considered that the avoidance of having to pay a Carbon Tax for the fugitive emissions would make this a very profitable operation in jurisdictions where a Carbon Tax applies.

[0029] This concept is quite different to “Ventilation Air Methane” (VAM) combustors where very low concentrations of Methane are burnt without any other fuels.

[0030] The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those describedabove. Rather, this background is only provided to illustrate exemplary technology areas where some embodiments described herein may be practiced

[0031] Various aspects and embodiments of the invention will now be described.

[0032] SUMMARY OF INVENTION

[0033] As mentioned above, the present invention relates generally to a method of gas drainage in a coal mine, general method of methane gas drainage and variable flow control valve. More specifically, the invention relates to methods and valves for methane gas drainage in which the valve progressively opens and closes with corresponding increases and decreases in methane concentration.

[0034] According to one aspect of the invention there is provided a method of gas drainage in a coal mine, the method comprising:

[0035] drilling a plurality of drainage holes into an area that will ultimately fall within a relaxation zone in a coal seam to be mined;

[0036] attaching a variable flow control valve to a collar of at least a first of the drainage holes, in a mining direction;

[0037] monitoring methane concentration within the first drainage hole at predetermined time intervals;

[0038] progressively opening the variable flow control valve with corresponding increases in the methane concentration;

[0039] fully opening the variable flow control valve if the methane concentration reaches a predetermined upper value;

[0040] progressively closing the variable flow control valve with corresponding decreases in the methane concentration;

[0041] closing the variable flow control valve if the methane concentration reaches a predetermined lower value; and

[0042] removing the variable flow control valve from the drainage hole.

[0043] The variable flow control valve is generally removed from the drainage hole when the mining equipment breaches the hole or when cracks from the miningface to the hole allow ventilation air to be sucked into the hole to the point where the gas from the hole is diluted below the predetermined lower value.

[0044] The predetermined time intervals may vary from site to site, but are generally expected to be from 10-60 seconds, and typically about 20 seconds.

[0045] Similarly, the variable flow control valve may incrementally open in any suitable amount. In a preferred embodiment, the variable flow control valve progressively opens in 5% increments on monitoring predetermined increases in methane concentration. Preferably the variable flow control valve fully opens if the methane concentration reaches a predetermined upper value generally expected to be from 60-80%, and typically about 70%.

[0046] Likewise, in certain embodiments, the variable flow control valve progressively closes in 5% increments on monitoring predetermined decreases in methane concentration. The variable flow control valve preferably fully closes if the methane concentration reaches a predetermined lower value generally expected to be from 10-30%, and typically about 20%.

[0047] In preferred embodiments, each step is repeated in successive drainage holes in the relaxation zone along the mining direction. Several variable flow control valves may also be attached to adjacent drainage holes at the same time.

[0048] In certain embodiments, the method may further comprise calculating the flow rate of methane being desorbed from the drainage hole and adjusting the flow rate through the variable flow control valve correspondingly. This advantageously maximises the concentration of the methane in the gas flow.

[0049] Generally, the method further comprises recovering methane desorbed from the drainage holes, optionally combining the recovered methane with ventilation air, to provide a fuel source.According to another aspect of the invention there is provided a variable flow control valve comprising:

[0050] a gas inlet and gas outlet;

[0051] pipework extending between said gas inlet and gas outlet;

[0052] a variable flow control valve disposed between the gas inlet and gas outlet;

[0053] a Programmable Logic Controller (PLC) that controls the variable flow control valve; and

[0054] a mass flow meter programmed to monitor gas flow rate, pressure, temperature and methane concentration of gas entering the gas inlet at predetermined time intervals and transmit methane concentration to the PLC, wherein the PLC progressively opens the variable flow control valve with corresponding increases in the methane concentration, fully opens the variable flow control valve if the methane concentration reaches a predetermined upper value, progressively closes the variable flow control valve with corresponding decreases in the methane concentration, and closes the variable flow control valve if the methane concentration reaches a predetermined lower value.

[0055] The variable flow control valve may comprise an air, hydraulic, or electrically actuated flow control valve, or any other variable flow rate valve system.

[0056] The PLC, variable flow control valve, mass flow meter and any other associated devices will be suitably rated and certified for use in an underground application, based on the countries specific regulations where they are to be installed.

[0057] As with the previous aspect of the invention, the predetermined time intervals are generally expected to be from 10-60 seconds, and typically about 20 seconds. The time intervals may be varied according to the specific requirements of the application.

[0058] The PLC may be programmed to progressively open the variable flow control valve in 5% increments on monitoring predetermined increases in methane concentration. The PLC is preferably programmed to fully open the variable flowcontrol valve if the methane concentration reaches a predetermined upper value generally expected to be from 60-80%, and typically about 70%. The predetermined upper value may be varied according to the specific requirements of the application.

[0059] The PLC may be programmed to progressively close the variable flow control valve in 5% increments on monitoring predetermined decreases in methane concentration. The PLC is preferably programmed to fully close the variable flow control valve if the methane concentration reaches a predetermined lower value generally expected to be from 10-30%, and typically about 20%. The predetermined lower value may be varied according to the specific requirements of the application.

[0060] According to a further aspect of the invention there is provided a method of methane gas drainage, the method comprising:

[0061] drilling at least one drainage hole into strata containing methane to be drained;

[0062] attaching a variable flow control valve to a collar of the drainage hole; monitoring methane concentration within the drainage hole at predetermined time intervals;

[0063] progressively opening the variable flow control valve with corresponding increases in the methane concentration;

[0064] fully opening the variable flow control valve if the methane concentration reaches a predetermined upper value;

[0065] progressively closing the variable flow control valve with corresponding decreases in the methane concentration;

[0066] closing the variable flow control valve if the methane concentration reaches a predetermined lower value; and

[0067] removing the variable flow control valve from the drainage hole.

[0068] According to this aspect, the method may be applied to drainage of methane from:

[0069] flanking holes drilled next to development tunnels;goaf drainage holes drilled above a longwall from either the surface or underground;

[0070] seam drainage holes in adjacent seams;

[0071] sealed mines where the contained gas is planned for capture for electricity generation; and

[0072] landfill applications.

[0073] The method may further comprise recovering the methane.

[0074] BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

[0075] To further clarify various aspects of some embodiments of the present invention, a more particular description of the invention will be rendered by references to specific embodiments thereof, which are illustrated in the appended drawings. It should be appreciated that these drawings depict only typical embodiments of the invention and are therefore not to be considered limiting on its scope. The invention will be described and explained with additional specificity and detail through the accompanying drawings in which:

[0076] FIG. 1 illustrates a plan view of how the abutment stress is distributed.

[0077] FIG. 2 illustrates an isometric projection of stress distribution around a mining void.

[0078] FIG. 3 illustrates a detailed cross section of the stress distribution around a longwall face.

[0079] FIG. 4 illustrates long block holes drilled along a longwall, perpendicular to the mining face.

[0080] FIG. 5 illustrates cross block holes drilled parallel to the longwall face.FIG. 6 illustrates a line drawing of a variable flow control valve accordingly to an embodiment of the invention.

[0081] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0082] Hereinafter, this specification will describe the present invention according to the preferred embodiments. It is to be understood that limiting the description to the preferred embodiments of the invention is merely to facilitate discussion of the present invention and it is envisioned without departing from the scope of the appended claims.

[0083] PRE-MINING GAS DRAINAGE - PURPOSE

[0084] Gas drained prior to mining can be captured in concentrations high enough that the gas can be used as a fuel. The concentration can range from a typical low value of 30% Methane to concentrations exceeding 85%.

[0085] Capturing the gas prior to mining not only makes the mining process much safer, but it also reduces the amount of gas released to the atmosphere during mining.

[0086] Gas can be burnt more efficiently than coal. Gas turbine power stations with a steam “tail”, can operate at energy efficiencies over 60%, whereas black coal fired power stations are typically less than 38%, unless enhanced with gasification systems. This means that a coal fired power station, typically produces 50% more Carbon Dioxide than a Methane fuelled power station per unit of electricity produced.

[0087] The higher the proportion of gas that can be recovered ahead of mining, the less gas released to the atmosphere. Recovering gas prior to mining and using it as a fuel significantly reduces anthropogenic GHG generation. This results in a double benefit; there are less GHGs released by the mining, and electricity produced from the gas captured is produced with 50% less greenhouse gases generated than when produced from coal.Burning the Methane converts this highly effective Greenhouse Gas to the much less effective Carbon Dioxide, reducing the Greenhouse Gas effect by a factor of 21. This occurs as a result of the equation below.

[0088] CH4+ 202 = CO2+ 2H2O

[0089] If the quantities of gas drained are insufficient to justify the capital to burn the gas to produce electricity or heat, the gas can be flared, which almost achieves the same outcome, but the opportunity to obtain financial benefit from utilising the gas to produce electricity is lost. Flaring the gas however, reduces any Carbon Tax liability for fugitive emissions by a factor of 21.

[0090] The higher the proportion of gas extracted prior to mining:

[0091] • the less gas in the mine ventilation and therefore less explosion risk; • the less gas pressure within the coal and therefore the less outburst risk;

[0092] • the less gas in the mine ventilation and therefore the less GHGs released to the atmosphere.

[0093] All three problems benefit from programmes where the gas extracted prior to mining, as a proportion of the gas in the coal is maximised.

[0094] Some of the gas not drained ahead of mining but released during the mining process is captured by the mine’s ventilation and released to the atmosphere. Some of the gas remains absorbed within the coal and is either released more slowly during the time between mining and when the coal is used or contributes to the energy recovered when the coal is coked or burnt.

[0095] The purity of the gas recovered is an important consideration. The higher the purity of the gas extracted, the more fuel usage options that can be considered. Low concentration gas has less value, as the possible applications to utilise the gas are fewer and the efficiency of the burning reduces. Low purity gastherefore generates a lower revenue stream, higher purity gas generates a higher revenue stream.

[0096] The best solution that addresses the mining problems and provides the greatest revenue stream to achieve that outcome is therefore to drain the highest proportion of the gas from the coal seam ahead of mining, at the highest possible concentrations.

[0097] PRE-MINING GAS DRAINAGE— CHALLENGES

[0098] Some coals are permeable, and gas can be drained easily by drilling holes into and along or through the seam. These holes are then connected to gas mains that are in turn connected to vacuum pumps that suck the gas out of the coal. If the permeability of the coal seam is low, more holes are needed to obtain the same amount of gas in the same amount of time from the same volume of coal.

[0099] Where high levels of gas are encountered and the coal seam permeability facilitates widely spaced holes, the sale of the gas can fully cover the cost of the gas extraction system. This was well proven at Baijigou in China where a gas drainage programme costing $14M recovered $25M worth of gas.

[0100] There is a diminishing gain from drilling more holes as permeability decreases. The cost of drilling the holes can increase at a faster rate than the additional gas recovered to the point where removing sufficient gas to comply with the applicable mining regulations is not cost effective. Failure to remove sufficient gas to comply with regulated limits can sterilise coal resources as the overriding requirement is to ensure that the coal is safe to mine.

[0101] It is possible to frac coal with a low permeability, by pumping fluids, sometimes with sand, into the coal seam under high pressure to create cracks that increase the permeability. The sand remains in the cracks, keeping them open, so that permeability is increased. This can also be expensive. This process is commonin coal bed Methane operations where the gas is extracted using boreholes drilled from the surface, but it is rare in coal mining.

[0102] Some mines are able to secure long lead times by drilling long directional gas drainage holes from the surface. This is called Surface in Seam or “SIS” drilling. Longer lead times can offset a low permeability by providing more time for the holes to drain. This is not possible at many mines due to surface access constraints.

[0103] If surface access for drilling is not possible, long hole directional drilling performed from underground locations is called Underground in Seam or “UIS” drilling.

[0104] Long directional holes are typically 1 ,000 metres. The record is apparently now over 2.5 kms for UIS and over 3 kms for SIS.

[0105] If the mine does not have the capital to utilise long hole directional drilling, rotary and or scroll drilling can be used.

[0106] Rotary holes using stabilisers can be drilled to longer than 300 metres in good conditions and remain within the coal seam, but lengths of half this amount are more typical.

[0107] Whilst holes longer than 300m holes can be drilled with the rotary equipment, without the directional control, the holes can droop and drop into the coal seam floor. The directional holes on the other hand can be controlled so the full length can remain within the coal seam.

[0108] Directional drilling also facilitates branching so that more hole length can be obtained from a single hole collar, and a greater coverage of the seam in height or in plan can be achieved.The advantages of long hole directional drilling when compared to rotary drilling are:

[0109] • The location where the holes can be placed due to the directional control, allows drilling around old workings or other obstacles;

[0110] • Extra lead time can be obtained by being able to drill further ahead of the mining area. Extra lead time facilitates a larger spacing between holes and hence fewer holes being required;

[0111] • Because of the directional control, which includes the ability to maintain a vertical alignment as well as horizontal, a higher proportion of the hole can be drilled within the seam; and

[0112] • Holes can be drilled into and along one seam from another or from stone drives above or below the seam.

[0113] The way the hole is drilled, and the length don’t affect the gas drained per metre of hole within the coal seam. This is a function of the coal seams gas parameters at that location.

[0114] ABUTMENT STRESS

[0115] Stress is the force applied, divided by the area over which it is applied.

[0116] Overburden stress is the force from the weight of the overlying strata applied over a given area of coal. The deeper the coal seam, the greater the weight of overlying strata, and hence the greater the overburden stress. There is of course an equal and opposite reaction force from below.

[0117] The abutment stress occurs in the zone where the overburden stress is concentrated as it is distributed around a mining void. Figure 1 illustrates a plan view of how the abutment stress is distributed.

[0118] The abutment stress is greater than the pre-mining stresses as the same amount of force from the weight of the overlying strata must be applied over the initial area minus the area of the mining void.As mining progresses along a longwall or a tunnel, the abutment stress occurs as a travelling wave form ahead of the face. This abutment stress induced by the travelling wave progressively crushes the coal as it advances.

[0119] Coal in the abutment zone is essentially crushed and fractures, but it remains compressed to the extent that the crushed coal is able to support the abutment stresses. The compression within the abutment zone keeps the cracks closed, so whilst the coal is broken and the available surface area for gas desorption increases, the permeability is not necessarily increased within this area.

[0120] Figure 2 illustrates an isometric projection of stress distribution around a mining void. When the longwall shearer, the machine that cuts the coal, cuts a metre off the face, the abutment stress advances a metre. It stays a uniform distance in front of the advancing face.

[0121] The zone where the abutment stress applies moves with the mining face staying a relatively uniform distance, specific to each coal seam in each mine, in front of and alongside it. This distance is dependent on the strength of the coal, the overburden stress applied and the size of the mining void.

[0122] The fractured coal is less able to support the full weight of the overlying strata, but because the fracturing reduces as distance ahead of the face increases, an increasing amount of force can be supported by the coal seam as distance increases ahead of the face.

[0123] When the crushing reaches a point where the coal is no longer strong enough to support the abutment stress, the coal in that area “relaxes” and the cracks open up, thus significantly increasing permeability. This “relaxation” zone is typically from the face itself for between 3 and 7 metres in front of the longwall face.Whilst it may be fractured, only some of the coal spalls from the face. The rest, due to its dead weight stays in place and provides some support to the coal behind it. The amount of support that can be provided by the crushed coal increases with the distance ahead of the face because the extent of crushing reduces.

[0124] The Abutment Zone extends from the end of the relaxation zone to a point 10 to 15 metres ahead of the face of the longwall.

[0125] A longwall is a large mining void, typically between 200 and 300 metres wide, with unsupported broken ground for >50 metres behind the face, whereas a tunnel is a smaller void, typically only 5.5 metres wide with smaller abutment stresses. In a tunnel, the crushed zone with increased permeability may be as little as 1 or 2 metres along both the sides of the tunnel and at the face.

[0126] A more detailed cross section of the stress distribution around the actual longwall face is shown in Figure 3. Much more overburden stress needs to be distributed around a longwall face than is required to be distributed around a tunnel.

[0127] Visible cracks can be seen in the mining face or rib of both longwalls and tunnels. These cracks diminish in width as the distance from the face or rib into the solid coal increases.

[0128] A camera inserted into a freshly drilled hole in a tunnel rib at a mine in Russia with strong coal, showed visible cracks to 600 mm. Microscopic cracks probably extended to about 1.2 metres, with solid coal thereafter.

[0129] The coal in the relaxation zone was originally compressed, has been fractured, and then, as the abutment stress moves forward, the reduced compression forces on the coal behind, allows the cracks in the fractured zone to “open up”. It is in this area, the relaxation zone, that permeability is markedly increased.The mining process results in the abutment stress effectively “fraccing” the coal, which creates increased surface area and permeability within the relaxation zone. The increased surface area allows more gas to be desorbed, and the increased permeability allows the gas to flow at an increased rate.

[0130] The mining machinery then crushes the coal even more, further increasing surface area and resulting in even more gas being released. This gas is then collected and carried away by the mine’s ventilation.

[0131] DRAINING GAS IN THE RELAXATION ZONE

[0132] Most gassy mines drill lots of pre-mining gas drainage holes both along the longwall and / or across it. Gas drainage from such holes in coal with low permeability will initially have low gas yields. The yield increases as water drains from the strata because water stops gas migrating along the cleats or cracks in the coal to the drainage holes. The gas yield peaks after the water is drained, and then progressively reduces as the “easy” gas is drained. Gas drainage holes are generally disconnected well before the face reaches each hole.

[0133] Long block holes are drilled along the longwall, perpendicular to the mining face as shown in Figure 4. These holes are disconnected as soon as the longwall commences as the longwall cuts these holes with each shear from the outset and they would otherwise mostly suck air. The same applies to holes drilled at an angle to the face. The current practice is to disconnect these holes when the longwall commences.

[0134] Cross block holes are drilled parallel to the longwall face as shown in Figure 5 and are cut when the longwall mines through them. They are typically disconnected 20 to 30 metres ahead of the face.

[0135] In the long block application, or with subparallel holes, part of the hole is always within the abutment zone and part of the hole is always in the relaxation zone.This means that the amount of gas desorbed is much more uniform in both total flow and concentration.

[0136] The challenge with these holes is that the face end of the hole is always open, so they will suck air if the vacuum pressure exceeds the dynamic loss along the hole.

[0137] In the cross-block application, the hole is:

[0138] • Initially in a “virgin” area where flow rates are low and very little gas flow occurs;

[0139] • When the abutment zone approaches the hole, flow increases slightly as the coal is crushed;

[0140] • When the abutment zone passes the hole, the coal relaxes, permeability increases, and significant quantities of gas are released;

[0141] • As the longwall mines through the relaxation zone, the face gets closer to the hole, the cracks in the face progressively open up and will ultimately allow the holes to “suck air”.

[0142] With a cross block hole, the gas flow varies according to its location relative to the mining face. The biggest challenge is to get as much of the gas out of the hole whilst within the relaxation zone, without diluting it with air sucked through the cracks in the face.

[0143] As the mining face advances towards a particular hole, there is a slight increase in gas desorbed when the abutment zone reaches the hole. The increase is only slight, because even though the extra surface area created by crushing the coal allows more gas to desorb, the compression of the coal keeps those cracks closed, which restricts the gas flow within that zone.

[0144] It is likely that many outbursts are caused by the build-up in pressure within the abutment zone ahead of a mining face. This build-up in pressure is then able to “burst” out through the crushed coal in the relaxation zone. Having a systemthat can reduce the pressure within the abutment zone could therefore significantly reduce the chance of outbursts, where this is the causal factor.

[0145] Once the face advances enough for the drainage hole to be within the relaxation zone, the cracks open and large quantities of gas are released as a result of the increased permeability.

[0146] If the hole is not connected to the vacuum in the gas mains, the gas will leak through the cracks and out of the face to be removed by the ventilation system. This gas requires a lot of ventilation capacity to ensure it is diluted to safe levels during mining.

[0147] Methanometers installed on electric machinery trip the electricity at between 1 % and 1.25% depending on the jurisdiction, and the mining stops until the gas is cleared to below the trip level. This can take 1 or 2 hours and, in a gassy mine without pre-mining drainage, typically results in 2 to 4 hours of lost time per day.

[0148] If the hole remains connected to the vacuum in the gas main, when the face is close enough for cracks in the face to connect to the hole, the mine air can be sucked through the cracks to the drainage holes, diluting the gas.

[0149] At some stage, parts of such a hole will be cut by the mining machinery, or progressively opening cracks in the face will allow lots of air to be sucked into the gas drainage mains.

[0150] Whilst the quantity of gas and air may increase, the concentration of the Methane reduces quickly when the hole is breached as it is diluted by the air also being drawn into the system. This is why cross block drainage holes are generally disconnected well before the abutment and relaxation zones can contribute to gas drainage, and why long holes are generally disconnected when production in the longwall first commences.The cross-block holes are typically disconnected 20 to 30 metres ahead of the face, because measuring and adjusting valve settings cannot be done in a time effective manner with the current systems used.

[0151] Low permeability coals that are difficult to drain will benefit most from relaxation zone drainage, but a system that can respond quickly to changes in Methane concentration is required for relaxation zone drainage to be effective as well as financially viable.

[0152] COMPUTERISED VALVES

[0153] With reference to Figure 6, the invention aims to implement a Computerised Variable flow control valve (CVFG Valve) that measures Methane content at regular intervals of typically 20 seconds, but it is envisaged that any interval may be pre-programmed with a range of proprietary algorithms into a controlling Programmable Logic Controller (PLC). The valve employs either an air, hydraulic, or electrically actuated flow control valve, of a design and rating suitable for the application controlled by the PLC.

[0154] Different equipment can use different electrical and or pneumatic signals to achieve the same effect. The l / P Converter is the electric to pneumatic interface to control the device. It is the effect that is achieved and the strategy to achieve it that is part of the patent being sought.

[0155] Whilst the electrical components are “intrinsically safe”, to ensure the highest level of safety for use in a gaseous environment, or Hazardous Zone 1 area, some of the electrical components of the CVFG valve assembly may also be built into a flame proof enclosure to suit all local regulations.

[0156] When first installed, the valve will be closed. When the power is turned on to the valve, it will open to 5% and measure the gas flowing from the hole. If there is gas present it will stay open when the Methane concentration is higher than a pre-set trigger level of say 20%. If there is no gas, it will re-close and thenopen and test at preprogrammed intervals until gas at a concentration above the preset trigger level is detected.

[0157] The valve has a PLC controller to manage its functions. The PLC can vary the sampling intervals or compare the most recent measurement to a running average of past measurements and so on, to ensure that stable operations can be achieved in the prevailing environment.

[0158] Measurements of the gas concentration and flow rate can be stored with a time signature, downloaded and matched to a distance from the face for analysis to help with Gas Management planning.

[0159] The PLC also monitors mass flow, temperature and pressure and transmits all acquired information to the surface if a communication system to allow this is available in the mine. This information will assist in planning subsequent drainage systems to reduce gas management costs to the lowest possible level without compromising safety or environmental performance.

[0160] Cross Block Holes

[0161] A cross block hole layout with the holes as close as parallel as possible to the face will maximise the time during which the valve can operate most effectively. The spacing of the holes matches the geometry of the abutment and relaxation zones, so that there is always at least one hole in the relaxation zone. This probably necessitates a spacing of 5 to 10 metres to maximise the results. The spacing will be determined by gas parameters specific to the particular coal seam as well as other circumstances encountered.

[0162] Again, referring to Figure 6, the mass flow meter which measures flow, pressure, temperature and gas (Methane) concentration within the CVFG valve assembly, feeds this data into the PLC, and the PLC sends an electrical signal (4 to 20 Ma) to an I.P converter which converts it to a pneumatic signal (20 to 100 kpa)sent along a narrow tube or an electrical signal along wires, to open or close the valve in 5% increments.

[0163] The valve deals with this variable flow situation by measuring the gas concentration every 20 seconds and compares that to the previous measurement. The valve progressively opens in 5% increments when the measurement is higher than the previous one.

[0164] The valve stays fully open whenever the gas concentration exceeds a pre-set level of say 70%, determined for each particular seam. The variations in gas levels to trigger different actions can be varied with a range of pre-programmed proprietary software.

[0165] When the face is close enough that cracks in the face will allow air to be sucked through the face to the drainage hole, the diluting air will reduce the gas concentration.

[0166] When the valve measures a reduction in gas concentration below the previous level, it will react by closing by 5%. The valve will continue to close in 5% increments whenever the concentration of the gas reduces in comparison to the previous measurement. This reduces flow rates so that less air is sucked through the face, while the gas is still recovered at a high concentration. The valve will close when the Methane concentration falls below a preset limit.

[0167] The aim is for the valve to control the suction pressure so that the gas in the hole is recovered, but the flow rate is not sufficient to draw air through the cracks in the face until the longwall mines through the hole. Restrictor plates may be used in the valve if necessary to reduce total flow rates if needed and this is considered within the ambit of the present invention.

[0168] As the gas is diluted and the valve progressively closes, the flow rate will reach a point where the gas may be desorbed at a faster rate than the air is sucked in through the face. In this circumstance, the valve will open again and thenhunt around the equilibrium point in order to collect as much gas as possible at the highest possible concentration. The extent of hunting can be governed by varying sample intervals and or comparing to running averages as opposed to just the previous measurement.

[0169] The amount of gas recovered, and the concentration are therefore both maximised by collecting the gas desorbed immediately in front of the face, that would otherwise be collected by the mine’s ventilation.

[0170] The valve will close completely if the gas concentration is less than a pre-set level of say 20%, which can also be varied to suit the circumstances at the mine.

[0171] After closing, the valve opens and tests the gas at predetermined intervals and when the gas falls below another pre-set level, an alarm can sound, indicating the valve should be disconnected. This will indicate that the hole is sucking too much air. The valve can then be leapfrogged to the next hole in the sequence.

[0172] The valve will allow the crushed and relaxed coal immediately ahead of the face to continue to be drained to the last possible moment, then be removed, and reinstalled by leapfrogging past other holes just prior to the borehole being mined out.

[0173] Long Block and Subparallel Holes

[0174] There is also a flowmeter included in the valve, so that the actual gas content in m3 / s can be determined in addition to the concentration of the gas in percentage terms.

[0175] Long block or subparallel holes are open ended. With these holes, the valve will initially measure the concentration and the flow and calculate how many m3 / s of Methane are being desorbed into the hole. The valve will then close to the point where just that flow rate is achieved. At this point the hole should be sucking gas but very little air. This will sometimes be made easier by blockagesin the hole in the relaxation zone when the broken coal in the face may reduce the amount of air that can get into the hole.

[0176] Advantageously, it may be possible to achieve a vacuum pressure that matches the dynamic losses occurring in the hole at that flow rate. This may mean that the hole may need to be throttled more than can be achieved with the computerised valve on its own, but this may be achieved by adding however many restrictor or diffuser plates are required to achieve the desired outcome.

[0177] In such instances, the valve would act to not only keep air from entering the hole, but also to keep sufficient vacuum pressure in the hole to stop it leaking Methane into the face.

[0178] The reason for this is to help avoid the situation that occurred at Russell Vale Colliery at the end of 2023, where the gas exuding from a pre-mining gas drainage hole was ignited.

[0179] In such instances, the time reaction for the flow rates along the hole would need to be taken into account to stop significant hunting, but establishing long term averages should facilitate drainage of these holes as well. There would be an initial period of hunting around a long-term average with the PLC programmed to work on running averages instead of just instantaneous measurements, although instantaneous measurements could have an override function.

[0180] Goaf Drainage Holes

[0181] Another source of gas in the longwall or even pillar extraction areas is from seam splits or other seams in the roof and floor. The fractures created by the mining process create the extra surface area and permeability required for this coal to desorb gas, sometimes in very large quantities.In some circumstances, this is dealt with by creating a negative ventilation pressure behind the face to pull the gas away from the face. This is sometimes called “h” ventilation.

[0182] Other mines drill goaf drainage holes from the surface, return headings or even roadways above or below the seam level. Some mines even bury gas drainage pipes under the goaf.

[0183] Where boreholes, or suction pipes are used to drain goaf gas, the CVFG Valve can be used to suck just the gas liberated by multiplying the gas concentration by the flow rate and then controlling the total flow to this level. The CVFG Valve can then “hunt” around this level until the desired suction pressure is being achieved. This will draw the gas away from the face, again reducing the amount of gas that needs to be managed by the ventilation system.

[0184] Sealed Sections and Mines

[0185] Sealed sections within a mine are not just affected by the concentration of the gas behind the seals, but also by changes in atmospheric pressure. When the atmospheric pressure drops, the gas pressure behind the seal is higher and gas leaks out. and the concentration of the gas immediately behind the seals increases.

[0186] When the atmospheric pressure increases, the gas pressure behind the seal is lower and air is sucked in, so the concentration of gas immediately behind the seals reduces.

[0187] Some mines use “balanced” airways to prevent the seals from sucking air or exuding gas.

[0188] The CVFG Valve can measure the gas concentration behind the seals and extract gas when the concentration is high but shut down when theconcentration is low. This again maximises gas extracted at the highest possible concentrations.

[0189] Capped Municipal Waste Dumps

[0190] Capped municipal waste dumps act in the same way as a sealed area in a mine. When the atmospheric pressure drops, the gas pressure under the cap is higher and gas leaks out. When the atmospheric pressure increases, the gas pressure under the cap is lower and air is sucked in.

[0191] The CVFG Valve may be used to maximise gas extracted and purity in this environment as well.

[0192] Outcomes

[0193] The gas that is desorbed due to the crushing in the abutment zone, that would otherwise be released into the mine’s ventilation system, is instead captured by the CVFG Valve and taken to the surface through the gas mains. The gas drained from the relaxation zone will be maximised, without compromising the purity any more than necessary.

[0194] Gas exuded in the goaf can also be captured in greater quantities in a purer form.

[0195] It should be noted that the gas desorbed due to the mining machinery crushing the coal and breaking it into smaller pieces will still be generated and will still need to be removed by the mine’s ventilation system. By draining the gas from the relaxation zone and the goaf however, the quantity of gas that needs to be removed by the ventilation system will remain significant but will be less than what would otherwise be experienced.Risk Management

[0196] It is envisaged that a comprehensive risk assessment will be conducted at each mine to determine the trigger levels set into the PLC, utilising what is known at that mine regarding the gas characteristics of the coal and the ventilation system.

[0197] It is envisaged that the CVFG Valve technology will be designed to service mines with low permeability coal and high production levels but will increase gas drainage ahead of mining and increase average gas concentrations in the gas recovered at any mine with gas.

[0198] The problem at mines with low permeability coal is that only perhaps 25% to 30% of the gas can be drained prior to mining and then only with long lead times. The high production levels then cause gas problems because a lot of tonnes of coal, times the remaining amount of gas per tonne, can still equal a lot of gas.

[0199] An advantage of the proposed CVFG Valve is that all of the gas measurements (once every 20 seconds or so) can be recorded so that gas capture can be graphed with respect to a distance ahead of the face. This means that after the valves have been used for a short period of time, enough data will be collected to enable the hole spacing to be more precisely designed to drain the amount of gas needed to meet the required objective.

[0200] Application of the CVFG Valve

[0201] The valve, including the programmable logic, and all data obtained from the CVFG valve is particularly useful for the controlled drainage within the relaxation zone as described above.

[0202] The application of the valve is not, however, limited to draining gas from longwalls. It is envisaged that this can be applied to any situation where gasflow rate needs to be adjusted to avoid dilution from external sources. This applies equally to:

[0203] • flanking holes drilled next to development tunnels.

[0204] • goaf drainage holes drilled above the longwall from either the surface or underground.

[0205] • seam drainage holes in adjacent seams.

[0206] • sealed mines where the contained gas is planned for capture for electricity generation; and

[0207] • landfill applications.

[0208] In all cases, the flow rate and hence vacuum pressure on these holes can be adjusted so that the source is constantly draining as much gas as possible, at the required concentration, whilst minimizing suction of air, and varying the suction pressure as circumstances such as atmospheric pressure change.

[0209] Flanking holes drilled parallel and offset say 1 to 2 metres from development tunnels will have the same issues as longwall relaxation zone drainage holes. There will be an equilibrium point where gas captured at a minimum concentration can be optimised by the valve. The gas flow and dilution from flanking holes drilled to extract gas ahead of development sections can be measured by the CVFG Valve. The CVFG Valve will detect where a lot of air is being sucked into the system. Alarms can be sounded at predetermined limits and shotcreting of the ribs to seal cracks can be undertaken if the hole needs to be kept open.

[0210] Goaf drainage holes are a little more challenging, but the same principles generally apply. These holes are likely to be breached by goaf falls, but the vacuum pressure can be controlled so that positive flow can continue from the hole at a rate that matches the Methane rising in the goaf as opposed to the hole sucking air behind the wall. The same longer-term averaging over several measurements or using running averages, as envisaged for long block boreholes, or those subparallel to the longwall face would apply.Holes in overlying and underlying seams are generally drilled from the main headings longitudinally along the wall. Goaf fractures can compromise these holes in the same way as goaf drainage holes, creating an opportunity for the CVFG Valve to again maximise gas recovery without unnecessarily compromising purity.

[0211] Landfill applications capture gas generated beneath an “impervious” cap. Under high pressures, the cap can leak, particularly in mature landfills where plant roots can create leakage pathways. The effectiveness of the cap can also vary due to rainfall, atmospheric pressure and other factors. The valve would find an equilibrium point at the desired gas concentration that would maximise gas recovered without unnecessarily compromising purity.

[0212] The valve does not have to have all of the added features described above to achieve the base task.

[0213] ADVANTAGES

[0214] There are two significant advantages arising from the use of the valve in any gas drainage systems:

[0215] • gas recovered is maximised;

[0216] • gas recovered will have a higher average purity.

[0217] In addition, with respect to underground coal mining, because more of the gas in the seam is captured and not dealt with by the mine’s ventilation system, the production can be increased with the same ventilation quantities or the ventilation flow rates can be reduced for a particular production level, significantly reducing mine ventilation electricity consumed.

[0218] The source of gas that needs to be managed by the ventilation system is generally from:

[0219] • The relaxation zone;

[0220] • The goaf;The coal cut from the face by the mining machinery.

[0221] The CVFG Valve may maximise the gas recovered from the first two sources listed above. It will not have an impact on the amount of gas desorbed from the process of mining the coal.

[0222] The amount of gas recovered by the CVFG Valve is however expected to be sufficient to allow, in mines where the ventilation capacity has been matched to the amount of gas liberated by the mining machinery, for the mining process to continue, uninterrupted by stoppages to clear gas. This is a significant advantage of the proposed methodology.

[0223] If after the utilisation of the CVFG Valve, stoppages to clear gas still occur, the ventilation system may need to be upgraded to prevent such stoppages, but the upgrades required will be less than would be necessary if the CVFG Valves were not being used.

[0224] Furthermore, after a few months of the initial usage of the CVFG Valves in a particular mine, there will be a lot of data collected that will assist in improving the design of the pre-mining gas drainage system that will help improve the resultant outcomes.

[0225] COMMERCIAL VALUE

[0226] One (1) cubic metre of gas contains 35,310 BTUs. The price of gas in Russia, for instance, where there is a lot of mines with high gas content and low permeability coal, is about $US4.00 per MBTUs. This means that the gas price is of the order of $US0.14 per cubic metre.

[0227] In order to work out the probable return at each mine, we can start with the historical records of gas content before draining and gas drained ahead of mining to calculate the gas content at the time of mining.The gas released during mining can be calculated by taking the ventilation flow rate out of the longwall times the gas concentration in each stream. This will provide total gas in cubic metres. This can then be divided by the coal production rate averaged over the same period to determine gas per tonne of coal released during mining. If this is subtracted from the gas content ahead of mining, the gas content remaining is that which remains in the coal until it is burnt or coked.

[0228] Guesses on the above for a typical mine are 20 m3 / t initial gas content. 30% or 6 m3 / t drained ahead of mining, leaving 14 m3 / t.

[0229] If the longwall has 40 m3 / s ventilation at 0.7% Methane, this equals 0.28 m3 / s of Methane. Over 24 hours, this equals 24,192 m3 / day. If there are 7,000 tonnes of coal mined in the day, there is 3.5 m3 / t of gas being released during mining or about 25% of the content remaining in the coal. The gas remaining in the coal is therefore 11.5 m3 / t. This is measured as “volatiles” in the product coal quality parameters.

[0230] If the computerised gas valve can recover just 20% of the gas that would otherwise be released during mining, the gas recovered per day would be 4,838 m3. Over a 24-hour period, this equates to 201 m3 / hr or 0.06 m3 / s. In an extreme situation, a single valve would need to be able to handle this much gas, plus an extra amount for contingency purposes, or say 0.1 m3 / s. Mines with more gas, or more production would need valves with greater capacity, but 0.1 m3 / s should be the minimum size applied.

[0231] At a value of $US0.10 per m3at the mine surface, this equates to $US484 per day.

[0232] With an expected cost of about $US50,000 to $US70,000 depending on equipment specifications and certifications to meet local regulations, this suggests a payback time of 100 days, or about 3 to 4 months. This paybacktime is a separate consideration to the safety benefits and increased production possible.

[0233] It is noted that the value at the mine surface would be significantly increased if gas powered electricity generating units were obtained to reduce electricity purchased from the grid. At an equivalent electricity cost, the gas used in gas engines would be worth much more than $US0.10.

[0234] If the CVFG Valve is able to eliminate say 2 hours of lost time per day due to gas clearance, and the total production time after maintenance is 14 hours, then the production increase would be 14%. As all the costs other than a little electricity are already accounted for, this would be expected to result in a 10 to 15% reduction in production cost.

[0235] Throughout this specification, unless the context requires otherwise, the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated step or element or integer or group of steps or elements or integers, but not the exclusion of any other step or element or integer or group of steps, elements or integers. Thus, in the context of this specification, the term “comprising” is used in an inclusive sense and thus should be understood as meaning “including principally, but not necessarily solely”.

[0236] Unless the context requires otherwise or specifically stated to the contrary, integers, steps or elements of the invention recited herein as singular integers, steps or elements clearly encompass both singular and plural forms of the recited integers, steps or elements.

[0237] It will be appreciated that the foregoing description has been given by way of illustrative example of the invention and that all such modifications and variations thereto as would be apparent to persons of skill in the art are deemed to fall within the broad scope and ambit of the invention as herein set forth.

Claims

CLAIMS1. A method of gas drainage in a coal mine, said method comprising: drilling a plurality of drainage holes into an area that will ultimately fall within a relaxation zone in a coal seam to be mined;attaching a variable flow control valve to a collar of at least a first of said drainage holes, in a mining direction;monitoring methane concentration within said first drainage hole at predetermined time intervals;progressively opening said variable flow control valve with corresponding increases in said methane concentration;fully opening said variable flow control valve if said methane concentration reaches a predetermined upper value;progressively closing said variable flow control valve with corresponding decreases in said methane concentration;closing said variable flow control valve if said methane concentration reaches a predetermined lower value; andremoving said variable flow control valve from said drainage hole.

2. A method as claimed in claim 1 , wherein said variable flow control valve is removed from the drainage hole when mining equipment breaches the hole or when cracks from a mining face to the drainage hole allow ventilation air to be sucked into the drainage hole to the point where gas from the hole is diluted below the predetermined lower value.

3. A method as claimed in claim 1 or 2, wherein said predetermined time intervals are from 10-60 seconds, typically about 20 seconds.

4. A method as claimed in any one of the preceding claims, wherein said variable flow control valve progressively opens in 5% increments on monitoring predetermined increases in methane concentration.

5. A method as claimed in any one of the preceding claims, wherein said variable flow control valve fully opens if said methane concentration reaches a predetermined upper value of from 60-80%, typically 70%.

6. A method as claimed in any one of the preceding claims, wherein said variable flow control valve progressively closes in 5% increments on monitoring predetermined decreases in methane concentration.

7. A method as claimed in any one of the preceding claims, wherein said variable flow control valve fully closes if said methane concentration reaches a predetermined lower value of from 10-30%, typically 20%.

8. A method as claimed in any one of the preceding claims, wherein each step is repeated in successive drainage holes in the relaxation zone along the mining direction, and / or several variable flow control valves are attached to adjacent drainage holes at the same time.

9. A method as claimed in any one of the preceding claims, further comprising calculating the flow rate of methane being desorbed from the drainage hole and adjusting the flow rate through said variable flow control valve correspondingly.

10. A method as claimed in any one of the preceding claims, further comprising recovering methane desorbed from said drainage holes, optionally combining the recovered methane with ventilation air, to provide a fuel source.

11. A variable flow control valve comprising:a gas inlet and gas outlet;pipework extending between said gas inlet and gas outlet;a variable flow control valve disposed between said gas inlet and gas outlet;a Programmable Logic Controller (PLC) that controls said variable flow control valve; anda mass flow meter programmed to monitor gas flow rate, pressure, temperature and methane concentration of gas entering said gas inlet at predetermined time intervals and transmit methane concentration to said PLC, wherein said PLC progressively opens said variable flow control valve with corresponding increases in said methane concentration, fully opens said variable flow control valve if said methane concentration reaches a predetermined upper value, progressively closes said variable flow control valve with corresponding decreases in said methane concentration, and closes said variable flow control valve if said methane concentration reaches a predetermined lower value.

12. A variable flow control valve as claimed in claim 11 , wherein said variable flow control valve comprises an air, hydraulic, or electrically activated flow control valve.

13. A variable flow control valve as claimed in claim 11 or 12, wherein said PLC and mass flow meter and any other associated devices are rated and certified for use in an underground application.

14. A variable flow control valve as claimed in any one of claims 11 to 13, wherein said predetermined time intervals are from 10-60 seconds, typically about 20 seconds.

15. A variable flow control valve as claimed in any one of claims 11 to 14, wherein said PLC is programmed to progressively open said variable flow control valve in 5% increments on monitoring predetermined increases in methane concentration.

16. A variable flow control valve as claimed in any one of claims 11 to 15, wherein said PLC is programmed to fully open said variable flow control valve if said methane concentration reaches a predetermined upper value of from 60-80%, typically 70%.

17. A variable flow control valve as claimed in any one of claims 11 to 16, wherein said PLC is programmed to progressively close said variable flow control valve in 5% increments on monitoring predetermined decreases in methane concentration.

18. A variable flow control valve as claimed in any one of claims 11 to 17, wherein said PLC is programmed to fully close said variable flow control valve if said methane concentration reaches a predetermined lower value of from 10-30%, typically 20%.

19. A method of methane gas drainage, said method comprising:drilling at least one drainage hole into strata containing methane to be drained;attaching a variable flow control valve to a collar of said drainage hole; monitoring methane concentration within said drainage hole at predetermined time intervals;progressively opening said variable flow control valve with corresponding increases in said methane concentration;fully opening said variable flow control valve if said methane concentration reaches a predetermined upper value;progressively closing said variable flow control valve with corresponding decreases in said methane concentration;closing said variable flow control valve if said methane concentration reaches a predetermined lower value; andremoving said variable flow control valve from said drainage hole.

20. A method as claimed in claim 19, wherein said method is applied to drainage of methane from:flanking holes drilled next to development tunnels;goaf drainage holes drilled above a longwall from either the surface or underground;seam drainage holes in adjacent seams;sealed mines where the contained gas is planned for capture for electricity generation; andlandfill applications.

21. A method as claimed in claim 20, further comprising recovering said methane.

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