井下应用综放的突出危险指数外文文献翻译

Application of outburst risk indices in the underground coal mines by sublevel cavingJavier Toran o a,n, Susana Torno a, Eliseo Alvarez b, Pedro Riesgo a School of Mines, Oviedo University, Asturias, SpainMining Engineer of Hullera Vasco Leonesa S.A., Santa Lucia, Leon, Spain articleInfoArticle history:Received 7 January 2011.Received in revised form 30 December 2011Accepted 8 January 2012.Available online 20 January 2012 Keywords: Outburst Methane Heading face Desorption rate Drill cutting Gas pressureAbstract The underground coal mines of Hullera Vasco Leonesa SA (HVL) have exhibited sudden emissions of methane called blows and outbursts.A research study has been conducted in order to analyse those risk indices with a greater impact on the prediction and control of outbursts in a heading driven in a coalbed located 17 m below a roadway in which the shortwall and sublevel caving method is being employed. Extensive measurement programmes have been carried out in an operating coal mine, both in the heading face and in the coal above it. Coal samples have been analysed and tested in thelaboratories of HVL.The aim of this study is to analyse the advantages and drawbacks of the chosen indices, theirrelationship with operating parameters such as distance between the heading face and the shortwall face, height difference between both levels, heading face and roadway optimization of the different borehole degasication patterns and water injection in those boreholes. It is concluded that a simple methodology to predict and control sudden methane outbursts has been developed and put into practice in HVL. This methodology is based on the mentioned indices and it can be partially or totally extrapolated to other underground coal mines.Introduction HVL Company located in the province of Leon in the North of Spain, is an underground coal mine with three shafts connected between them. The annual coal production is 2 million tons and the proved exploitable coal reserves at the end of 2010 are 45 million tons. The average coal properties of the mined coalbed are 12.39% of Ash, 0.75% of Sulfur, 10.41% of volatiles, a caloric value of 7455 Kcal/kg and a density of 1.45 kg/m3Stratigraphically, the coal basin is divided into six well-dened formations. The whole group is about 1500 m thick, formed about 300 million years ago. Pastora Formation shows the best economic prospects and it provides the totality of the present coal production. The Pastora coalbed varies in thickness from 20 to 25 m, its dip is between 401 and 701 and the methane concentration varies between 7 and 10 m3/t of coal.Numerous violent sudden emissions of coal, rock and methane(outbursts) have occurred in underground coal mines worldwide,sometimes with fatal consequences for workers 13. In underground coal mines using sublevel caving combined with shortwall, as it is in HVL, sudden methane emissions can be classied into two groups: those originated in the caved zone as a result only of the gas pressure action, called Gas spallings or Blows and Outbursts generated in roadways driven in coalbeds. where gas pressure energy is added to the energy from coal and rock stress 4,5.Fig. 1 shows the main ventilation network of the mine, which is accessible by means of three vertical shafts connected between them. The area marked with an (A) is the place where Gas spallings or Blows have taken place and that marked with (B) is the place where outbursts have occurred. While in the area (A) the mining method is sublevel caving, in the area (B) the mining method is shortwall combined with sublevel caving. In area (B) the coalbed has a thickness of 9 m, a dip of 701 and its roof and footwall are made up of shales.,Although Gas Spallings or Blows are more frequent than outbursts and they produce greater amount of ejected gas, ve blows in years 2009 and 2010 with values ranging between 3000 and 16,000 m3 of emitted gas have taken place. These emissions were not so instant, and the problems associated with them were solved with an appropriate main and auxiliary ventilation 6.Outbursts are violent phenomena of short duration, with a rst stage in which the disintegration and projection of coal are produced, and a second stage, in which high emissions of methane occur 7,8.The outbursts which happened in HVL can be classied as medium type, with approximately 300 t of coal ejected and about 1000 m3 ofemitted gas.Here outbursts are usually produced shortly (some minutes) after having nished the mining works and the ejected coal comes from the upper part of the heading which is being driven; thatcoal lies on the oor of the heading in an elongated position as a consequence of its pneumatic transport with the formation of canals 9. formation of canals。In Fig. 2a, there is a schematic representation of a panel mined by shortwall and sublevel caving between two levels with a detailed view of the broken coal and shield supports. Between the roadway at level 865 (a) and the roadway at level 740 (b), an incline with a dip of 381 (c) is driven. From these shaft eight crosscuts (d) pass through the coalbed, and from the intersection two roadways of 250 m each are driven to the East (e) and (f) and West (g) and (h) in the coalbed. The coal block between two consecutive sublevels has a difference of 17 m in height (i).The mining method by shortwall and sublevel caving is performed with a particular system of shield supports (section view in Fig. 3). The panel width varies according to the thickness of the coalbed. The large coalbed thickness contributes to generate outbursts 9.Fig. 2b shows, in area B, two places (1) and (2) where the last two outbursts have occurred. In location 1, it happened when the heading face cut through the coalbed and in location 2 drivage of the heading face using drilling and blasting method.The shape and dimensions of the outbursts were those shown in section view (2.1) of Fig. 2b.HVL Mining Company in collaboration with Oviedo University has conducted a research work in order to understand the mechanism of the outbursts, and then predict and control similar potential outbursts may take place in the future. With regard to the desorption characteristics of the methane in this coalbed, the following indices, considered individually or combined, of outburst prediction have been selected and studied: methane concentration in coalbed 10, desorption velocity of methane V1 11,methane concentration in ventilation, V30 and the German Regulation 12, analyses of drill cuttings, Jahns test 13 and measurements of gas pressure in the roadway and coalbed 2.The relationships between those indices and the inuence of them on the extraction method by shortwall and sublevel caving have been analysed in detail. Additionally, the degasication by means of boreholes in the heading face and the water injection in those boreholes has been also studied2. Methane concentration in the coalbedAs shown in Fig. 3, above the heading (h) there is a roadway(g) in which the retreating shortwall sublevel caving methods is being used. The height difference between roadways (h) and(g) driven in the coalbed is 17 m. This gure also shows the boreholes performed in face (j) and the roof of the heading (k) in order to study outburst risk assessment indices.The heading (h) is driven in two daily shifts by drilling and blasting with an average advance rate per shift of 1.2 m. The shortwall sublevel caving advances 1 m/day in the opposite direction to the advance of the heading (h). Methane contained in coal and which can migrate to the heading and shortwall as outbursts, is named desorbable methane content, a parameter which characterises the gassiness of coal.It is considered that there is a minimum gas content a coalbed needs to have to cause an outburst. Although this value differs in each country, it is generally estimated that a gas content higher 9 m in length and 105 mm in diameter were opened at the heading face, Fig. 3j. In twenty of them the heading face was degassed and twenty were performed in not degassed heading faces.Samples of coal were taken every 1.5 m in each borehole,analysing a total of 180 samples as a result of the fact that 10 boreholes got blocked at a length of about 4 m. This is due to the excess of cuttings caused by both the methane ow and the overpressures on the bores in the not degassed heading faces.Likewise, 20 boreholes of 9 m in length and 105 mm in diameter were performed in the roof of the heading, Fig. 3k, analysing 120 coal samples in all.Different positions, lengths and diameters of the boreholes have been tested in the degasication of the heading face in order to avoid outbursts. The drilling pattern of degasication with six boreholes of 4 m in length and 65 mm in diameter, at the crown of the heading face, and three boreholes of 10 m in length and 105 mm in diameter in the centre of the heading face, was nally selected (Fig. 4).Every 3.75 m of advance, new degasication boreholes at the crown were performed and they overlap the previous ones 0.25 m. Every 8 m of advance, the three central boreholes, which overlap the previous ones by 2 m, were repeated.Fig. 5 illustrates the evolution of the values of Q (m3/aft) according to the borehole lengths. The black dots and lines correspond to the heading face and the red dot and lines correspond to the values of Q in the roof of the heading. The reference value Q=9m3/aft is also depicted.No remarkable effects on the value of Q in the heading face because of the degasication was noticed. It can be seen how the average values of Q from points further than 1.5 m in the above mentioned face are higher than 9 m3/aft with frequent maximum values over 12 m3/aft.However, the average values of Qm3/aft at the crown of the heading face are clearly lower than 9 m3/aft with average values of 5m3/aft in points further than 1.5 m as a consequence of the coalbed degasication produced by the upper driven roadway and the extraction method employed, that is, shortwall subleve caving.If X is the distance between the heading face and the shortwall sublevel caving located in the upper roadway, Fig. 6, the evolution of the average values of Q m3/aft, obtained in each borehole according to that distance, is shown in Fig. 6. In this gure it can be seen that there is no inuence of the shortwall sublevel caving on the value of Q in the heading face, whereas at the crown that value decreases and remains about 5 m3/aft.Therefore, if the heading is driven inside the area of the degasication inuence of the shortwall sublevel caving located in the upper roadway, in this studied case 12 m, there would befavourable conditions to avoid outbursts as far as methane content in the coalbed is concerned.,3. Desorption rate V1Research conducted by Noack and Heinderlfd 14 has highlighted the importance of the methane desorption kinetics in order to establish the risk of having an outburst. One of the most frequently used indices is V1 (cm3/10 g/35 s) which establishes the quantity of methane in cm3 which becomes desorbed in a 10 g coal sample with a grain size between 0.5 and 0.8 mm andbetween 35 and 70 s after having been taken from the coalbed11.Although there are countries and mining areas with slightly different values, in Spain the outburst probability is considered high when the values of V1 are higher than 2.In order to study the desorption rate in the coalbed, 30 boreholes of 9 m in length and 105 mm in diameter were carried out in the heading face not degassed. Fig. 3j. Samples of coal were taken every 1.5 m in each borehole, analysing 120 samples in the laboratory, as in 10 boreholes the rod got blocked at a length of about 4 m due to the excess of cuttings caused by the methane ow and the overpressures on the bores because of the mentioned methane.Twenty boreholes of 9 m in length and 105 mm in diameter were performed in the coalbed located in the roof of the heading. Samples of coal were taken every 1.5 m in each borehole, analysing 120 samples in the laboratory.In Fig. 7 it can be seen that the values of V1, for coal located on the roof of the heading, are lower than those at the heading face,as a consequence of the coalbed degasication due to the mining works by shortwall sublevel caving carried out in the upper roadway. Final values of V1 are lower than 1.In Fig. 8 the results of measurement programmes, which were conducted in the heading face with and without degasication,are shown. The average values of V1 are 2.54 with frequent values higher than 4 in points located 3 m from the face. At a distance of 1.5 m from the face the average values are 1.85.,Most of the values of V1 are lower than 2. In fact these average values are 1 at 3 m from the face and 0.47 at 1.5 m from the face.If in addition to the degasication of the heading face, a mean of 750 l of water is injected to a pressure of 90 kg/cm2 into a borehole of 4 m in length and 46 mm in diameter and located in the middle of the heading face, all the average values of V1 are lower than 0.6.Fig. 9 shows the evolution of V1 in the heading face with the mentioned degasication by water injection in the borehole, as a function of the distance X.When X=0 there is an important increase in V1, which exceeds the value of 2. In order to diminish such a value, it is necessary to reinforce the degasication system with an additional central borehole of 45 mm in diameter and 5.6 m in length, through which 890 l of water at 90 bar pressure are injected.The evolution of V1 as a function of X is similar to that of methane swept by the ventilation of the heading and that of the evolution of Q.,4. Methane concentration in ventilation and V30 parameterMonitoring for methane in the heading face by a methane detector Trolex TX 6321 and monitoring for airow by an anemometer Status Av/05/30, allow us to know methane emission behaviour in the coalbed. This monitoring has been frequently used in Spain 15 and in Germany (Bureau of Mines of the Federal State of North Rhine,Westphalia) in its circulars 12 to predict outbursts.If methane concentration in the coalbed is higher than 9 m3/aft for headings driven by drilling and blasting, the regulation, mentioned above, mandates to use the parameter V30, or relationship between the methane volume emitted in 30 min after blasting and the quantity of coal mined in that blasting.When coal is extracted by blasting a sudden disintegration of the coalbed with a substantial methane emission is produced. Outbursts are sometimes caused by this means, so V30 is a very signicant parameter. If V30 is 40% higher than Q=9m3/aft, local studies must be done, reinforcing degasication.Fig. 10 shows the evolution of the values V30 and maximum Q obtained from tests together with the value of Q indicated in the regulation according to the distance X. Three zones can be considered. When X ranges from 65 to 40 m, 57.14% of values V30 are higher than 9 m3/aft and 7.14% are higher than maximum Q. The average value of V30 is 9.48. When X is between 40 and0 m, 82.61% of values V30 are higher than 9 m3/aft and 34.78% are higher than maximum Q. The average value of V30 is 12.29. The inuence of the shortwall sublevel caving nearness is signicant.When X0, V30 decreases below 9 m3/aft, with average value of 6.08.5. Drill cutting volumeThe analysis of drill cuttings obtained in boreholes carried out in the heading face and its comparison with a certain critical volume, Jahns Test, together with the difculty in performing boreholes as a result of overpressures in the coalbed 13, are good parameters to predict outbursts.Fig. 11 shows the results, of the drill cutting volume per drilled metre, obtained in the tests carried out in eleven boreholes which were performed in the heading face. It can be seen how in six boreholes A, B, C, D, E and F shown in Fig. 11, an increase in the drill cutting production, at a distance of 4 m, which indicates an overpressure and thus, an outburst-prone area. This fact was already noticed in the boreholes performed for the study of methane content in the coalbed.The preceding results are consistent with the methane pressure measurements, which will be analysed below. So that the gas-pressure-measurement tube set works correctly, its sealing system has to be placed 3 m ahead of the heading face in a not fractured zone.,6. Gas pressure measurementsMeasuring gas pressure in the coalbed is of great importance not only individually, but also combined with other risk parameters to forecast outbursts. In order to take gas pressure measurements, a gas-pressure-measurement tube set was designed and manufactured by S.A. Hullera Vasco Leonesa, as it is illustrated in section view of Fig. 12. The gas-pressure-measurement tube set consists of (1) measurement tube, (2) sealing system, (3) borehole rod, (4) manometer or monitoring system and (5) borehole rod to inject water at a pressure of 1.96 MPa used for the sealing system.When it is necessary to insert the gas-pressure-measurement tube into deeper boreholes, it is extended using extensible and exible pipes. The boreholes used for inserting the gas-pressure-measurement tube sets were carried out at the crown of the heading face. They were 8 m in length, 45 mm in diameter and 101 angles in upward direction. The measurement t