Mine gas
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Mine gas is a gas mixture that was created in the history of the earth through coalification and is released during the modern mining of hard coal . It is known in mining jargon as beating weather . The main component of mine gas is methane (CH 4 ).
| Composition of the mine gas | |||
|---|---|---|---|
| component | unscratched coal | active mine | disused shafts |
| Methane (CH 4 ) | 90-95% by volume | 25-60% by volume | 30-95% by volume |
| Carbon dioxide (CO 2 ) | 2-4% by volume | 1-6% by volume | 1-15% by volume |
| Carbon monoxide (CO) | 0 vol% | 0.1-0.4% by volume | 0 vol% |
| Oxygen (O 2 ) | 0 vol% | 7-17 vol% | 0 vol% |
| Nitrogen (N 2 ) | 1-8% | 4-40% | 5-32% |
Formation of the mine gas
About 300 million years ago, large parts of northwestern Europe were covered with a sea basin. The shallow sea silted up and moorlands formed. In the humid and warm climate, dead organic plant materials got under the water surface. The peatization followed in the absence of air. Sediments were again deposited on this layer. This process was repeated several times and led to the seam structure . By the pressure of the deposited layer of sediment and increasing with the depth of temperature, the formation of coal, which set coalification a. Carbon and the hydrogen contained in the organic material formed methane. Depending on the gas permeability of the surface layers, the methane remained in the coal as seam gas. Today methane continues to be formed in many (former) coal mining areas. However, this does not happen geochemically through coalification, but biochemically through microbes . That happens deep under the Ruhr area and under the Münsterland.
Release of mine gas from mining activities
The mine gas is the seam gas that is released by scratching the coal in mining . The composition can, if necessary, change due to the inflowing air. The coal mining leads to a loosening and pressure reduction of the seams, with the result that the methane in the bewetterten routes of mines diffused and the weather accumulate with methane.
Mine gas species
Seam gas
CBM ( Coal Bed Methane (CBM)) is carried bores of over days promoted.
Mine gas
The actual mine gas ( Coal Mine Methane (CMM)) is the gas that is released during active hard coal mining . The gas can be removed via the mine ventilation system and / or through active mine gas extraction. The gas extraction serves primarily to avoid bad weather as a measure of occupational safety. An energetic utilization takes place with the strong gas exhaust and the mine gas is used as fuel for the boiler firing or also for gas engines and gas turbines for the thermal power coupling.
Mine gas from disused mines
Coal seam gases from disused coal mines are known as Abandoned Mine Methane . This is very common in the Saar district . As with the CMM, there is no unique name for this type of coal seam gas.
Strong gas extraction
In the event of a high methane concentration in a face prepared for mining, the gas is sucked out of the seams by applying a negative pressure to avoid a dangerous explosive concentration in the weather. In addition, sections that are no longer used and separated by dams are connected to the extraction system in order to reduce diffusion into open sections. This extracted gas is called mine gas. Since the bores, dams and the pipe connections are only partially sealed off from the ventilated route, a considerable amount of air is always sucked in. Therefore, the mine gas from active mines always has a high air content; the ratio of oxygen and nitrogen corresponds to the composition of the atmosphere. The permissible lower limit of the methane content for mine gas extraction is 22 vol%. If the values are lower, the suction is switched off in order to prevent methane from falling below the upper explosion limit (UEL) (15% by volume) and thus conveying an explosive gas mixture. This form of gas evacuation is known as strong gas evacuation, as the methane concentration is above the upper explosion limit. The regulations for explosion protection must be observed.
Lean gas extraction
In addition to the high-gas extraction, the low-level gas extraction is also set up in special cases. This method can be used if the methane content of the extracted mine gas is below 2% by volume and therefore the lower explosion limit is not exceeded. The gas suction devices must be placed as close as possible to the suction point. A secondary inlet is installed in front of the gas suction devices, through which the methane content is kept below the limit value by the addition of the surrounding weather. The suction and pressure side of the compressor and the secondary inlet are equipped with flame arresters to limit a possible explosion to the interior of the pipeline. The methane level is monitored; the pre-alarm occurs at 2–2.5% by volume and the compressor switches off at 3% by volume. If the compressor fails, valves on the suction side behind the suction points must be closed automatically. The sucked off lean gas is diluted with the surrounding weather in front of the mouth point in the face to the limit value <1 vol .-%, for which an additional dilution fan is necessary. The methane content downstream of the outlet must also be monitored and the extraction system must be shut down when the limit value of 1% by volume is reached. The mining authorities usually only allow lean gas extraction for a limited period and in special cases as part of the operating plan procedure. Relatively large weather flows less contaminated with CH 4 are necessary in order to sufficiently dilute the mine gas.
Extraction from disused mines
The gas extracted from disused shafts has a significantly different composition than the strong gas extracted from active mines. Since there is no longer any direct air access to the coal seam due to the set ventilation, this mine gas hardly contains any free oxygen. The oxygen that penetrated the existing cavities through open shafts during the mining of coal or after the mine was shut down, reacts with the carbon to form CO 2 . In addition to methane, this gas contains the oxygen bound in the CO 2 and the molecular nitrogen .
Hazards from mine gas
In hard coal mining, an impermissibly high concentration of mine gas in the weather can lead to a firedamp explosion . By ventilating the mine workings, the methane content is kept below 1% so that no explosive mixtures can arise. An explosive mixture is formed with a methane content of 4 to 15% by volume in the air. Mine gas can also enter public and private sewer networks in mining areas through leaks .
All electrical systems in the underground coal mining facilities must be protected against firedamp . Approved firedamp-proof electrical devices must not generate sparks or have surface temperatures that could ignite an explosive methane-air mixture.
The volume of mine gas leakage in the Federal Republic of Germany is estimated at around 1.5 billion m³.
Energetic use
Mine gas extraction at an active mine
Mine gas as a dangerous explosive gas is diluted from the underground areas of the mines by the weather flow. The weather is sucked out by pit ventilators after days and discharged into the atmosphere. In the early days of mining, firedamp explosions often occurred as a result of short-circuits in the weather, stretches that were badly weatherable or suddenly high amounts of methane gas escaping during mining . Often the gas explosion was followed by dust explosions due to the whirled up coal dust , which have an even more devastating effect and lead to injuries and fatalities.
In connection with the increasing mechanization of hard coal mining and the associated larger accumulation of methane in the air stream, the gas began to be extracted in a targeted manner in the 1940s. For this purpose, boreholes were drilled into the seam from the mining routes. These boreholes are connected to a gas collecting line through which the mine gas is conveyed to the surface. Gas suction devices are usually set up above ground and convey the gas to the atmosphere or a gas processing plant via flame arresters. Rotary piston blowers or water ring pumps are used as gas suction devices. The extraction is only released if the upper explosion limit is not undershot with a safety margin (min. 22 vol .-% methane).
Mine gas was initially only used by burning the gas in steam boilers. However, mine gas was produced in very different amounts. In addition, the gas content of the seams fluctuates greatly, and because of the effort involved, a gas extraction system was only installed on a mining section if insufficient dilution of the mine gas in the weather stream was forecast. Since gas utilization was not the business objective of the colliery, the amount of gas produced was in no relation to the possible usability. Either the boiler system had to be heated with purchased natural gas or excess gas was burned off in the atmosphere for days.
The first more extensive use of mine gas with electricity generation was advanced at the Haus Aden 1/2 colliery in Oberaden near Bergkamen in the 1980s. The mine gas was compressed to 12 bar and burned in a gas turbine with powered electric generators and thus used to generate high-quality electricity. At the Ewald 3/4 colliery (Herten), the extracted and compressed gas was fed to a motor-driven thermal power station owned by Stadtwerke Gelsenkirchen and used to generate electricity.
Mine gas utilization in disused shafts
Until the 1980s, the disused coal mining shafts were filled with loose material. The gas migrating through the manhole filling column flows to the manhole plate and from there is conducted to the atmosphere via a pipeline. The mouth of the degassing line is provided with a flame arrester that is resistant to continuous burning. Depending on the air pressure conditions , mine gas either flows into the atmosphere or air is sucked in. The degassing line was built in order to avoid an uncontrolled leakage of mine gas. There is a risk that the gas can accumulate dangerously in basement rooms. Shafts that were filled later were sealed with a concrete and therefore permanent, cohesive filling compound in order to prevent the often occurring waste of the filling column. Most of the existing manhole pipelines were used through the filling column to discharge the accumulated mine gases. However, the execution was z. B. not optimized for gas use due to missing connections to the insulated floors or the small pipe cross-section. The amount of gas that can be used is often limited due to the negative pressure that can be achieved and the pressure drop in the pipeline, which increases with the gas flow.
The profitability of using mine gas from disused shafts has long been questioned. There were some shafts from which noticeable amounts of mine gas leaked out when the pressure was low. This can be recognized by flow noises and streaking in the atmosphere due to different light refraction.
- The first project for the energetic use of mine gas was initiated in 1997 by the Stadtwerke Herne in cooperation with the Fraunhofer Institute for Environmental, Safety and Energy Technology . A mine gas extraction system was installed on the disused Mont Cenis 3 shaft in Herne-Sodingen, which is equipped with a degassing line. The gas can two Jenbacher - gas engines are fed. The electrical output of each module is 253 kW with 378 kW thermal output. The engine waste heat can be used directly at the location (Academy Mont Cenis). The successful operation (and rising energy prices) led to a boom in the energetic use of mine gas in the Ruhr area . This development was also promoted further because the systems are funded under the Renewable Energy Sources Act (EEG). Methane, the main component of mine gas, has a 21 times stronger greenhouse effect than CO 2 . Therefore, the combustion of the mine gas also helps to reduce the greenhouse effect.
- In September 2002, a mobile system went into operation at the Anna mine . This former colliery in the Aachen district was the only one where mine gas was still possible to generate electricity; in all others the groundwater had already risen too high. In 2002 the groundwater on “Anna” was 420 meters deep .
In most cases, the pit suction systems and gas engines are built into mobile containers. This means that the locations of the systems can be changed if the gas source dries up.
Mine gas production in Germany has increased significantly and developed successfully since the 1990s, as the graphic opposite shows.
Individual evidence
- ↑ W. Kuipers et al .: Autonomous flame ionization detector for explosion protection in sewer networks . In: Technical Security . tape 7 , no. 11/12 , ISSN 2191-0073 , p. 19-24 .
- ↑ T. Hegemann, M. Opahle, S. Klaß: Outgassing protection of the permanently filled Grillo shaft 3. In: 7th Altbergbau-Kolloquium at the TU Bergakademie Freiberg. November 2007, accessed December 11, 2017 .
- ↑ The city of Alsdorf has a mine again
literature
- Landesoberbergamt NW, guidelines of the Landesoberbergamt NW for the establishment, operation and monitoring of systems for extracting mine gas, "Gas extraction guideline", from April 26, 1985
- Walter Bischoff , Heinz Bramann, Westfälische Berggewerkschaftskasse Bochum: The small mining dictionary. 7th edition, Verlag Glückauf GmbH, Essen, 1988, ISBN 3-7739-0501-7
- Heinz Kundel: coal production. 6th edition, Verlag Glückauf GmbH, Essen, 1983, ISBN 3-7739-0389-8
- Ernst-Ulrich Reuther: Introduction to mining. 1st edition, Verlag Glückauf GmbH, Essen, 1982, ISBN 3-7739-0390-1
- Thomas Thielemann: Coal seam gas - the rise of an energy source - mining, 2/2008: pages 63–65; Food (RDB)
- Thomas Thielemann, Bernhard Cramer , Axel Schippers: Coal seam gas in the Ruhr basin: fossil or renewable? - Series of publications by the German Geological Society, Volume 34: 193; Hanover ( ISBN 3-932537-06-8 )
- Thomas Thielemann: Coal seam gas in Germany. - BGR Commodity Top News, Volume 17: 4 pages; Hanover
