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英文原文2007 China (Huainan) International Symposium on Coal Gas Control TechnologyGas Drainage in High Efficiency Workingsin German Coal MinesDr. Joachim Brandt, DMT GmbH, GermanyAbstractIn the course of increasing production in the workings of German hard coal mining, the part of ventilation techniques as a factor of production has also increasing importance. In view of still increasing production the cooling of the air is critical for the attainable production on the one hand. On the other hand, the increasing gas emissions have to be controlled as well.This is achieved by fans of high capacity as well as by cross-sections in the underground workings, which are as big as possible, especially in the gateways. Furthermore, increasing cooling power is installed.The air volumes cannot be enlarged unlimited, yet, and rapidly reach their limit due to national statutory regulations concerning the maximal allowed air velocities.Besides that, a methane concentration of 1Vol.-% in the maximum must be maintained in general, which is allowed to be exceeded only with the agreement of the mining authority in defined parts of workings up to a limit of 1,5Vol.%.Due to a dense sequence of coal seams in the German hard coal deposits, the firedamp is released during the exploitation not only from the worked seam, but essentially from the seams in the roof and in the floor behind the passage of the longwall. A gas drainage on the base of an efficient technique is necessary, firstly in order to fulfil the safety regulations and secondly to achieve a maximal production in the working.An investigation, recently finished and executed concerning the improvement of the gas drainage indicated that, by means of rock mechanical calculations and interpretations, an increase in the efficiency of gas drainage boreholes is still possible.IntroductionThe methane emissions in mining operation depend principally on geologic conditions. In rock sequences with a small portion of coal in the roof and the floor, only the released gas quantity from the exploited seam also defined as basic gas emission has to be diluted with the air flow. This methane flow can reduce the exploitation relevantly by an increasing desorbable gas content in the coal. In particular, the coal mass flow quickly exploited with high-capacity production can release very high methane quantities from the accompanying seams. This high methane flow generates an exceeding of the threshold values and leads to switch off of the electrical equipment and to the interruption of production.In a rock sequence with a high portion of coal in the roof and the floor additional gas from the accompanying seams in the area of gas emissions is released into the air flow behind the passage of the longwall. In the Ruhr Basin this gas flow, also defined as additional gas emission, is normally many times higher than the basic gas emission.Figure1 illustrates the loosening of layers caused by longwall mining operation in flat layers. These loosenings can be according to their degree - flow ways for the released gas from the accompanying seams. The coloured areas mark the degree of loosening with red for intensive, dark blue for light and grey for no loosening.Figure 1: Example for the loosening of the rocks in the roof and the floor of a longwall operation (view of the left side only; the right side is symmetric)There can be a high rate of additional gas emissions according to the seam thickness in the area of gas emissions and according to their gas content. Figure 2 shows that the production can decrease dramatically already at low gas contents if there is no gas drainage for the suction of the additional gas emission.This extreme reduction of saleable output and advance of production requires extreme increasing of the air flow and furthermore gas drainage is necessary. Thereby the legal regulations for the German hard coal mining have to be observed, which are the following:maximum air velocity6m/smaximum methane concentration1Vol.-%exceptional methane concentration1,5Vol.-%minimum negative pressure in the gas drainage100hPaFigure 2: Example for the decline of production in case of (increasing) additional gas emissionImprovement of the gas drainage for maximising the output After increasing the air flow following the legal regulations, and with the permission by the authorities for maximum methane concentrations of 1,5 Vol.-%, and after commissioning a gas drainage system, the output can be increased to a quantity, which is again profitable (see figure 3).Normally, the efficiency of the gas drainage system is up to 50% of the total methane flow occurring during exploitation. An additional optimising of the gas drainage above that also increases the face output.A longwall in seam H of the mine Prosper Haniel serves as an example for optimising gas drainage results (figure4).The length of the panel of approx. 960 m was mined with a daily advance of production of approx. 7 m/d in 140 work days although the desorbable gas contents were at approx. 8 m/t and the gas make from the additional gas emission was 30 m on the average per exploited ton. The drained methane quantities were up to approx. 650.000 m per week (approx. 65 m/ min). The gas drainage efficiency reached up to 72%.The gas drainage boreholes were drilled from both gate ways into the roof and the floor. The distance from one another was 10 m. Figure3: Example of increased output by doubling of air flow and installation of a gas drainage system with 50% efficiency.A gas pipe of 500 mm diameter was available in the loader gate. In the tail gate, a gas pipe of 300 mm width was installed behind the face and extended according to the advance of the exploitation.The total air flow for the panel was up to 85 m/s. In the working area methane concentrations of up to 1,5 Vol.-% were locally allowed. Outside the working area towards the air return shaft the limit of 1%-methane concentration had to be observed.Principles for gas drainageA basic draft (figure5) shows the function of a gas drainage: Gas boreholes are drilled along the goaf shortly behind the longwall. In general, the roof emits the most gas. However, from the floor a considerably additional gas flow can be expected in the case of high gas contents. According to the rock properties, the gas boreholes are tubed at their beginning at a length of 7.5m to 20 m and the annular space between tube and fractured rocks near to the roadway is sealed with plastic material (adhesive or foam plastic). This technology reduces unwanted leakages. The diameters of these boreholes are 75mm to 115 mm. Their length depends on the distance of gassy layers (accompanying seams), which have to be drained from the exploited seam. The length varies normally between 30m and 60 m. In particular cases the length can be 100m and more if there are special rock mechanical conditions.The incline of the borehole to the roadway axis is between 75 gon and 90 gon. According to the occurring gas volume, the distance between the boreholes can be 10 m to 50 m. The boreholes are connected to the gas pipes by plastic tubes with adequate adapters.For planning and dimensioning of gas drainage systems, it is necessary to calculate the occurring quantities of gas mixtures in the planned mining operations at an early stage (prediction of gas emissions)Most important factors concerning the technology of gas drainageDimensioning of pipesPipes are the most important part of a functional and high-capacity gas drainage system. If they are not dimensioned according to the flow characteristic of the gas quantities to be expected, the success of the gas drainage is put into question. Even high-capacity pumping stations cannot compensate the pressure loss due to pipe cross sections, which are too small. In the range of negative pressure there is only a very small margin of 300 hPa to 400 hPa available for the compensation of pressure consumption (pipe friction, water accumulation, installations, bends of the pipeline).The normal operating range of the pumps is in general at a negative pressure of 400 hPa to 450 hPa. At the end of the gas collection pipe in the mining area there should be a negative pressure of at least 100 hPa according to the German rules. That means that only approx. 300 hPa to 350 hPa are available for all pressure losses in the gas piping net.Figure4: Arrangement of gas boreholes and air supply of a longwall in seamH, mine Prosper-Haniel.The context between quantities of gas mixtures, which have to be drained, and pressure losses due to small-dimensioned pipes is illustrated in the following diagram (figure6):Figure5:Basic draft for gas drainage boreholes (schematic diagrams)A pipe diameter of less than 400 mm is unfavourable. A tolerable pressure loss in the system normally occurs, when there are flow velocities of between 10 m/s and 15 m/s. However, the length of the pipeline has to be taken into consideration (in fig.6 for 1000m).Even at diameters of 300 mm the pressure loss is four times higher compared to a pipe width of 400 mm. The pressure consumption graph rises steeply, when there is a diameter below 300 mm.The false gas pipe cross section has quickly negative effects on the accessible output of a mining operation due to reduced efficiency of the gas drainage.High capacity pumpsThe pumps to be used should produce negative pressure of up to 500 hPa. Water ring pumps or rotary pumps are suitable here, which are available on the market for any capacity. When planning a pumping station a certain tolerance for capacity modification has to be kept in mind in case the gas quantities increase during the life of the mine. Figure6: Pressure consumption depending on pipe diameterThe single capacities of the pumps should be dimensioned according to the minimum and maximum of the gas quantities to be expected (pumps with graduated capacities). A reserve pump is prescribed by law. Water ring pumps have certain advantages compared to rotary pumps if higher negative pressures have to be reached. In case of a pipe system sufficiently dimensioned, the construction types are equivalent.The drive capacities installed in the gas drainage stations of the German hard coal mines for high gas quantities are 1.5 MW and more, if required. This can cope with volume flows of up to 175 m/min pure methane, which corresponds to gas mixture quantities of 21000 m/h at a methane concentration of 50%.Rock mechanical aspects for the optimisation of the gas drainageConsidering rock mechanical aspects helps to optimise the gas drainage. A preferably precise reproduction of the rock sequence in a rock mechanical computer model offers conclusions for the ideal arrangement and length of gas boreholes. A visual implementation of rock mechanical calculations of the processes of loosening allows insights into the rocks and therefore an idea about the area of gas emissions in the area of loosening of mining operations.The following figure7 of a working at the mine Walsum, shows as an example of a single case plastifications and consequently the occurring flow ways - reaching far into the roof (visible by the light red areas). The dip angle of the rock sequence amounts 20gon.Figure7: Plastifications in the roof and the floor of a working at Walsum mineBy use of this model of rock mechanical reactions, the existing drilling schema was modified (figure8):Figure8: Increase of gas emissions in the case of incline and length modification of the gas boreholes (Walsum mine)After modifying the bore angle from approx. 80 gon to 90 gon and the bore length from 55 m to 110 m, there was a significantly higher efficiency of the single boreholes (see top graph). As a result, at this working and also in the following working the efficiency of gas drainage could be increased up to more than 70%.Due to a study of the years 2004 and 2005 led by support of the DMTGmbH, rock mechanical calculations were made for a high number of workings with various rock sequences of the Carboniferous in the Ruhr Basin. Thereby, processes of rock loosening, which can influence the gas emission, were analysed.At the same time, occurring rock tensions behind passage of the longwall in the rocks were analysed, which might influence the height and the time of the gas release. This study offers an extensive collection of experiences in this regard, which allow to evaluate future methane flows better and to plan gas drainage systems more reliable.The following illustration (figure9) serves as a last example of the complexity of the relations between rock mechanics and gas release.Here, the processes of loosening (on the left) are compared to the occurring rock tensions (on the right). A sandstone layer of 25 m to 30 m thickness near above the worked seam is remarkable in this example. Compared to the softer kinds of rocks this layer shows only minor plastifications. This fact corresponds to the present mining experiences.Concerning the rock mechanical comparison to rocks with less solidness, an essentially higher and longer lasting pressure relief of the layers lying above this sandstone occurs.This means with respect to the gas emissions that the seams lying above the sandstone also underly to a higher and longer lasting pressure relief than it is normally the case for less solid accompanying rocks. For this reason, they emit in total much more methane than one has to expect according to a conventional gas emission.Figure 9: Plastification and relief of mechanical pressure in the area of looseningof a longwall operation with a thick, hard sandstone layer in the roofIn a single case up to three times higher methane inflows occurred in a seam in a working at the mine Ost than calculated before.In the meantime the gas drainage system was extended to a capacity of 15.000m gas mixture per hour. Furthermore, a special piping method was developed for the safe suction of the additional gas emission above the sandstone layer. This method guarantees a lifetime of the gas boreholes of more than 12 months.Additionally, the underground piping net, which has to bridge a length of approx. 12 km from the drainage station at the surface to the exploitation, was extended in a way that two thirds of the pipelines consist of parallel strings with 500 to 600mm diameter.Completing CommentsOn the whole, increasing gas contents and consequently higher additional gas emissions limit the increase of production.According to the national laws there must be a high expense for ventilation as well as gas sucking technology to achieve a maximum output. Safety, which is an important factor of production, as well as economical aspects will be maintained.Additionally, the utilisation of the methane emissions can compensate financial expenses at least partially. However, the gas drainage systems have to be dimensioned optimal. Here, it is important to coordinate the underground gas pipe nets and the above ground gas drainage stations for the gas flows to be expected in the mining operations.Essen, 04.05.2007References1Gao Y F,Shi L Q,Lou H J,et al.Water-Inrush Regularity and Water-Inrush Preferred Plane of Coal Floor.Xuzhou:China University of Mining&Technology Publishing House,1999.(In Chinese)2Qian M G,Miao X X,XU J L.The Key Strata Theory of Controlling the Rock Seam.Xuzhou:China University of Mining &Technology Publishing House,2000.(In Chinese)3Zhang J C,Zhang Y Z,Liu T Q.The Seepage Flow in Rock and the Water Inrush in Coal Floor.Beijing:Geological Publishing House,1997.(In Chinese)4Wang L G,Song Y.The Non-Linear Characteristic and the Forecast of Water Inrush from Coal Floor.Beijing:Coal Industry Press,2001.(In Chinese)5Gong S G.The Basic Application and Example Analysis of ANSYS.Beijing:Machine Press,2003.(In Chinese)6Li H Y,Zhou T P,Liu X X.The Tutorial of Engineering Application of ANSYS.Beijing:China Railway Press,2003.(In Chinese)7Wang L G,Song Y.A model to risk assessment for mine water-inrush.Journal of Engineering Geology,2001,09(02):158163.8Miao X X,Lu A H,Mao X B,et al.Numerical simulation for roadways in swelling rock under coupling function of water and ground pressure.Journal of China University of Mining&Technolog,2002,12(2):121125.9Wang L G,Bi S J,Song Y.Numerical simulation research on law of deformation and breakage of coal floor.Group Pressure and Strate Control,2004,(4):3537.(In Chinese)10Wang L G,Song Y,Miao X X.Study on prediction of water-inrush from coal floor based on cusp catastrophic model.Chinese Journal of Rock Mechanics and Engineering,2003,22(4):573577.11Jiang J Q.The Stress and the Movement of the Rock Around the Stope.Beijing:Coal Industry Press,1997.(In Chinese)中文译文2007年中国（淮南）煤层气控制技术国际座谈会瓦斯抽放在德国煤矿的高效运作Joachim Brandt博士DMT GmbH公司，艾森，德国Essen，2007年4月5日摘要：在提高德国硬煤开采生产过程中，通风技术部分作为生产要素的也变得越来越重要。一方面，针对还在增加的生产空气冷却十分重要。另一方面,也必须对不断增加的瓦斯释放加以控制。可以通过使用大功率通风机或在井下，尤其是在巷道内设置尽可能大的联络巷，可以实现这一点。此外，越来越多的制冷设备也得以安装。除此之外，甲烷的浓度必须控制在1Vol.-%常规以内，常规规定甲烷最高浓度可以在采矿专家定义的井下浓度上限1,5Vol.%。由于德国的硬煤沉积中煤层沉积致密，瓦斯不仅从废弃煤层释放，而且尤其从顶板岩层和长壁工作面后方的两巷释放。高效的瓦斯抽采很有必要，首先是为了达到安全规定，其次是达到生产最大化。最经一项已经完成并投入使用的实验表明，通过岩体力学计算和解释，提高瓦斯抽放钻孔的效率仍有可能。说明在采矿工程中甲烷排放量主要取决于地质条件。在那些顶、底板含有少量煤炭的岩层中，只有那些从已开采煤层且被定义为基本瓦斯排放的瓦斯释放量才需要用气流加以稀释。煤炭中瓦斯含量越高，这种瓦斯气流就越容易导致开采工作的减缓。特别是，煤炭开采速度越快、量越大，就越容易导致工作面瓦斯的大量释放。这种高甲烷产生的流量超过阈值并导致关掉电气设备和中断生产。在那些顶底板含有大量煤的岩层中，从位于瓦斯卸压释放区的临近层中释放的额外瓦斯被四方到工作面后方的废弃两巷。在鲁尔盆地，这种被称为额外瓦斯释放的瓦斯气流通常比基本瓦斯释放大好几倍。图1说明了在水平层中因长壁采煤作业而引起的煤层卸压。依据它们的卸压程度，这种卸压可以为临近煤层中的瓦斯释放提供缝隙。彩色区域标志着卸压程度：红表示强烈，深蓝表示轻微，灰表示未卸压。根据瓦斯释放区的煤层厚度和瓦斯含量，会有大量的额外瓦斯释放。图2表示如果没有瓦斯抽放来吸收额外瓦斯释放，采煤作业能够迅速的把瓦斯含量降到一个较低值。这种极端的输出下降和生产进尺需要急剧的增大风流，并且，瓦斯抽采也是必要的。因此，针对德国硬煤开采的合法规定必须被遵守，规定如下：最大风速：6m/s最大甲烷浓度：1Vol.-%特殊甲烷浓度：1,5Vol.-%瓦斯抽采最低负压：100hPa图1：长壁采煤作业的顶底板卸压的示例（只看左边，右边为对称）基于提高产量的瓦斯抽采技术改进通过遵循法定的条例来增加风量，经过官方允许的最大瓦斯含量（1,5 Vol.-%），并且经过调试瓦斯抽放系统，产量又可以重新达到一个可观值。（见图3）图2：额外瓦斯释放量递增时引起的生产下降。通常，瓦斯抽放系统的效率是可以抽出煤炭开采过程中50%的瓦斯释放量。一个经优化后数值大于前面数值的系统也能带来工作面产量的提高。Prosper Haniel煤矿的一个位于H煤层的长壁工作面作为优化瓦斯抽采系统的结果的示例（图4）该区段推进长度约960m,在140个工作日内日进7m，尽管瓦斯解析量约为8 m/t且加上额外瓦斯释放得到的总瓦斯含量为平均每吨30 m。每周的瓦斯抽放量达到650.000 m左右（排放速度65 m/ min左右）。瓦斯抽放时间利用率达到72%。从工作面两顺槽向顶底板打抽放钻孔，钻孔间距10m。一个直径500mm的瓦斯抽放管可以布置在运输巷，在轨道巷，在工作面后面布置一个直径300mm的管子，并依据开采适时延长。区段总风流可达85米/ s。在工作区域的局部可以允许甲烷浓度达到1、5 Vol. - %。工作面以外的浓度不得高于1%。图3：风量加倍且安装一套效率50%的瓦斯抽采系统后的产量增长示例图4：Prosper-Haniel矿一个长壁工作面的瓦斯抽放钻孔的布置及风量供给瓦斯抽放的原则一个基本的草案(图5)表明瓦斯抽放的机理:紧随工作面，沿着采空区打抽放钻孔。一般来说,顶板释放大部分瓦斯。然而,若地板含有大量瓦斯，就有可能出现大量的额外瓦斯气流。根据岩石特性、气体钻井是布置在他们开头长度为7.5米到20米长。另外，抽放管和钻空间的环状空间由塑料材料(胶或泡沫塑料)充填。该技术减少不必要的泄漏。这些钻孔直径为75毫米到115毫米。它们的长度取决于含瓦斯层(临近层)的距离，这些瓦斯必须从已经形成的缝隙加以抽放。不同的长度通常在30米,60米之间。在特定的情况下, 如果有特殊的岩石力学状况，长度可以达到100米或更多。钻孔与顺槽轴间倾角介于75到90几何角。根据已测得的瓦斯体积，钻孔间距可以从10m到50m。用带有足够的适配器的塑料管来将钻孔和瓦斯管道连接起来。图5：瓦斯抽放钻孔基本草稿（原理图）对于规划与瓦斯抽放系统定型,有一步是必要的,那就是在早期(瓦斯释放预测)的计算出区段回采作业工作中既得混合气体的量。有关瓦斯抽放技术的最重要因素管道尺寸管道是可用和大容量的瓦斯抽放系统中最重要的部分。如果它们的尺寸不是根据预期的瓦斯流量特性来定制，瓦斯抽放就很难成功。即便大容量泵站也不能补偿因管道交叉环节太小引起的压力损失。在负压阶段，只有一个非常小范围的300到400hpa的压力可用于补偿的压力消耗(管摩擦、积水、设施、弯曲的管道)。在泵的正常工作范围，负压一般在400到450hpa。根据德国规定，在回采区的瓦斯回收管的末尾应该至少有100hpa的负压。这意味着只有大约300到350hpa的压力可作用于所有的管道网络压力损失。下表（图6）解释了必须被抽放的混合气的量和由小尺寸管路引起的压力损失间的关系。图6:压力消耗取决于管的直径管道直径小于400毫米是不利的。当流速介于10 m/s和15m/s,一些在允许范围内的压力损失会经常发生。然而,管道的长度必须被考虑(图6为1000m)。即使在直径300毫米,压力损失高出管道宽度400毫米时的四倍。当有一个直径300毫米以下，压力损耗图大幅上涨。瓦斯管路交叉环节的无效会很快对因瓦斯抽放效率的降低而引起的回采作业的产出产生负面影响。高容量泵泵的使用将产生达到500hpa的负压。水环