Comparing potentials for gas outburst in a Chinese anthracite and an Australian bituminous coal mine

International Journal of Mining Science and Technology 24(2014)391-396Contents lists available at Science DirectInternational Journal of Mining Science and TechnologyELSEVIERurnalhomepagewww.elsevier.com/locate/ijmstComparing potentials for gas outburst in a Chinese anthraciteand an australian bituminous coal mineLi Guoqing D, * Saghafi AbounaFaculty of earthces, China University of Geosciences, Wuhan 430074, ChinaCSIRO Energy Technology, North Ryde 2113, australiaARTICLE INFOA BSTRACTArticle historGas outbursts in underground mining occur under conditions of high gas desorption rate and gas content,Received 10 October 2013combined with high stress regime, low coal strength and high Young s modulus. This combination of gaReceived in revised5 November 2013Accepted 8 December 2013and stress factors occurs more often in deep mining. Hence, as the depth of mining increases, the potenAvailable online 30 April 2014tial for outburst increases. This study proposes a conceptual model to evaluate outburst potential interms of an outburst indicator The model was used to evaluate the potential for gas outburst in twomines, by comparing numerical simulations of gas flow behavior under typical stress regimes in aIAustralian gassy mine extracting a medium-volatile bituminous coal, and a Chinese gassy coal mine inQinshui Basin( Shanxi province) extracting anthracite coal. We coupled the stress simulation programGas content(FLAC3D)with the gas simulation program (SIMED Il)to compute the stress and gas pressure and gaermescontent distribution following development of a roadway into the targeted coal seams. The data fromgas content and stress distribution were then used to quantify the intensity of energy release in the eventof anc 2014 Published by Elsevier B V on behalf of China University of Mining Technology1 IntroductionGas outburst occurs when all or some of the followingns are present: high gas content, high rate of gas desorption,In China, underground coal mining is often associated with the high stress level, low strength but high Youngs modulus of coalrisk of gas outburst. This risk grows as the depth of mininggeological structure and fast advance rate of mining [1-5]. othercreases. Currently, most Chinese coal mines extract coal at depths factors, such as coal permeability, mining depth and coal thicknessof more than 500 m; these coals usually contain large volume of may also influence the occurrence of outburst [6-9 Gas drainage(up to 40 m /t in anthracite). The combination of highand stress relief are the most effective primary approaches totent and stress magnitude at these depths leads to frequent gas reducing outbursts in collieries [10-13outbursts. Chinas National Energy Administration recorded 350In this work, we developed a conceptual model for occurrencefatalities from 72 outburst events in Chinese mines in 2012. The of outburst and used it to compare the gas outburst potentials ofreduction in the number of events and fatalities is mainly due to two typical Chinese and Australian outburst-prone coal mines. Thislosure of small mines using unsafe mining methods and no gas model is expressed in terms of an index, which we call the outburstdrainage planindicator The indicator is based on the ratio of availableFig. 1 shows the evolution of outburst events and fatalities in from compressed gas and strained rocks to the required enerChinese coal mines since 2005. The occurrence of outbursts in to crush and eject coal into the open voids. To compute stressAustralian mines of similar depth is, however, very rare: no fatali- redistribution due to excavation, the FLac3d code was used. Simties from gas outburst events have been recorded over the past two ilarly, SIMED ll was used to compute the redistribution of gas condecades. The aim of this study was to compare the geo-mechanical tent and pressure due to the excavation. We then used the modeland gas reservoir factors that influence the occurrence of gas out- to compare the potential for outbursts from coal seams with abursts in typical Chinese and Australian coal mines prone to roadway constructed into the seam.outburst2. Development of a conceptual model of outburstCorrespondingr.Tel:+862767884179th中国煤化工 nd gas expansionug. edu. cn(G. Li)energy contributeCNMHGnd ejection of coalhttp://dx.doiorg/10.1016/j.ijmst.2014.03.0182095-2686 2014 Published by Elsevier B V on behalf of China University of Mining Technology92G. Li, A Saghafi/ International Journal of Mining Science and Technology 24(2014)391-3962500Assuming that the crushed coals have spherical shapes, then theGas events numbersurface area and its increments can be expressed in terms of partiSwhere V is the volume; and d the diameter of spherical particlesThe surface increment due to crushing of a volume Ve of coal is1052006200720082009201020112012Fig. 1. Gas outbursts and fatalities in China since 2005where D and d are the diameters of spherical coal particles beforeand after crushing, respectively.during a gas outburst event. In this analysis, we assume that theIn practice, the diameter d of coal before crushing is muchexpansion of desorbed gthermal. In this model, an outburst larger than the particle diameter after it is crushed. Therefore,occurs if the sum of gas and strain energy is higher than the energy Eq ( 7)can be simplified torequired for coal pulverization and ejection into the roadway, i. eWe+ws>ec+ Ekwhere We and Ws are the gas expansion energy and strain energy:The energy of crushing coal isand Ec and Ek the energy required to crush coal and to eject coalWe define an outburst indicator, u, to evaluate the proneness of ee=ivceoutburst in a location in the mineThe energy required to eject the crushed coal mass isEc + ExEx=2mu =plwvThe gas expansion energy is estimated as followswhere m and v are the mass and velocity of the ejected coalWe= pcv Pa= pclwhrespectively.where Pa and ve are the atmospheric pressure and volume of coal inLastly the outburst indicator u can be evaluated as followsplace prior to the outburst; c the gas content of coal; p the coal den-sity: and L, w and h are the length, width and height of the roadway, u3 cplhwPa+] Ihwo2(E)(11)respectiveldelhi+2 plhwuStrain energy is estimated using the following equationW=Vc0=v2/1p+02(e2)e+3Pu2where a is the average strain in coal; o the average effective stressIf u>1, an outburst may occur. A ladicates a strongerE the Young s modulus; and D the Poissons ratio [3]probability of an outburstRittinger's law states that the work required to crush a solid isIn next sections, we will use this model and outburst indicateto compare an Australian and a Chinese outburst-prone coal minespecific crushing energy, or the amount of energy required for In applying this model, it is assumed thatincreasing the coal surface by a unit area [14 Hence, the energyrequired to crush a coal volume of a surface area S is(1) The length of the outburst into the coal face zone ()is equalto the distance between the coal face and the location of theEc=e△s(5)intact coal in front of the facewhere e is the specific crushing energy; and As the surface incre(2)The outburst is only due to the combined effect of coal stressment after the coal is crushed. Cai and Xiong measured the specificand coal mechanical and gas reservoircrushing energy of several Chinese coals [14. It varied from 10.7 to(3)It is assumed that only 50% of volume of gas initially trapped28.8 J/m", and was positively correlated with the hardness coeffiin coal would be released during the outburst event.cient of these coals(Protodyakonov's coefficient)(Table 1).It should be noted that the geological factors, such as faults andstructures. are not taken into account in this modelTable 1Crushing energy of some Chinese coals [1413. Materials and methodsCoalsHardnescoefficient3. 1. Geological conditions of the studied minesnergy(/m)To use the approach developed in Section 2, publically availableoft coal from Taiji Colliery Beipiao city0.3008.8Coal from Sanbao Colliery, Beipiao city0.240224data from two typical gassy mines in Australia and China wereNo 6 coal from Weitang Colliery, Tonghua city 0.225used to estimate potNo 6 coal from Weitang Colliery, Tonghua city 0. 17915.5The australian中国煤化工 bcoal; and is locatedCNMHGeSydney BasinCoal from Jiulishan Colliery, Jiaozuo city119Coal is extracted frousig lOngwall mining. DuringOutburst-prone coal from Xiaosiping CollieryFushun citymining, mechanized shearers cut and remove the coal at the longwall face of the mine Hydraulic-powered supports hold up the roofG. Li, A Saghafi/ Intemational Joumal of Mining Science and Technology 24(2014)391-396Table 2Basic coal seam parameters in two collieries.Colliery GasGas typeCoal rankCoal seamCoal thickMine a12-20CO2, CH4 Bituminous Bulli Seam: Illawarra Coal Measure, Permian A2.3-2.7, average 2.5Minec 11-21(Main West) CHaAnthracite No. 15 Seam: Taiyuan formation of Upper Carboniferous No 15: 2.. 1, average 3.7 No. 3:7-11(Main East)ge No. 3 Seam: Shanxi formation of Lower Permian Age 5.0-7.5, average 6.0as the extraction process proceeds. Following coal extraction, thelongwall panel is allowed to collapse behind the roof supports,forming a goaf Mine a has recorded 252 outbursts since its open-ng in 1976. Most of the outbursts are related to shear zones andmylonite, low permeability and localized high stress (includingmining-induced stress ). Several outburst events in the north-eastern part of the mine have also been related to high gas content(16 m/t)and high CO2 concentration(>95%)[2, 3, 10, 111The chinese mine. here called mine c extracts anthracite andlocated in the southern Qinshui Coal Basin It is a modern coal minewhich started to mine in December 1996 and put into operation inNovember 2002. Mine C currently produces 10.8 x 10 tons of coal口Fhorper year. The mine is divided into two areas: Main West and MainFig. 2. Meshing of coal and strata block(FLAC3D).East. It uses longwall mining and hydraulic-powered roof supportsThe No. 3 Seam is the main target coal seam. Gas emission datashow that the specific gas emission(gas emitted per ton of coal extracted)is 17m/t in the Main West area and 9m/t in the Main 3.3. FLAC3D inputs and meshingEast area [15, 16]. Table 2 shows the basic coal seam parametersn the two collieriesFor computing the stress distribution around a roadway driveninto the target coal seam, a block of coal and strata is meshedover 50(width), 100(length)and 60 m(height ) This block, in a3.2. Numerical simulation methodsvertical direction, contains part of the roof (38 m), part of thefloor (20 m) and the totality of the coal seam (3 m). The roof isof a roadway heading. SIMED Il is a gas flow simulator devel- Table 6The SIMED Il computer code was used to compute gas con- divided into 20 mesh points, the floor into 10 points and the coaltent and gas pressure variation in coal due to the development seam into 5 points(Fig. 2). The input parameters are shown inoped by Csiro and UNSW. It is a two-phase (gas and water)The initial stress regime and mechanical properties of coal andflow, three-dimensional, multi-component(multiple gas species) rocks will significantly influence excavation-induced stress distri-simulator with single and dual porosity reservoirs. The simulator bution. It is assumed that the initial principal stress components.was initially designed to model gas drainage from coal seamsprior to roadway development in the horizontal plane are 10does not simulate rock failure and solely solve gas flow equa- and 20 MPa. The initial vertical component of stress is assumedtions [17-19]. Therefore, another code is required to compute to be 10 MPa (corresponding to the weight of the ground)stress changes due to roadway development, and subsequently [2,3, 10, 11, 15, 16]. Furthermore, it is assumed that the roadway isestimate permeability variation due to mining-induced stress developed in the direction of maximum principal stressBefore running SIMED IL, the FLAC3D computer code is used tocalculate stress changes, coal failure, and the stress redistribution 3. 4. SIMED ll inputs and meshingaround the roadway. Its outputs are fed to siMed Il to calculatethe permeability, and consequently, gas pressure and gas content Once FLaC3d was run, the stress distribution data were fed todistribution over timeSIMed Il to compute gas pressure and gas content distributionsIn SIMED II, the permeability is calculated using the following around the roadway. because gas pressure can be affected withinequation [20, 211a large distance from the roadway, we computed the gas flow overa relatively large area. The dimensions of the area covered by thek=koe- cEa( -ao(13) grid are 140(width), 100 (length)and 3 m(height)For relative permeability, we used a curve recommended inwhere Cog is the cleat compressibility, MPa o the effective SIMED ll manual(Fig 3)stress, MPa: o the initial effective stress, MPa; and ko theOther parameters input to SiMEd Il for the two mines are pre-itial permeability, md. Those variables and their units are used sented in Table 4. Note that for both mines, we assume that coalin AImed IIseam is fully gas saturatedng's modulusPoissonCohesionFriction angle中国煤化工Tensile strengthE(MParatIo pC(MPa)CNMHG T(MPaMine c coal seamMine a coal seam12Roof and floor sandstone1080094G. Li, A. Saghafi/Inal of Mining Science and Technology 24(2014) 391-3960.permeability20m5 mFig. 3. Gas andrelative permeability data used in this study.5m10m15m20m25mTable 4SIMED II input parametersFig. 5. Maximum principal stress distribution in Mine A( Pa).ParameterMine cMine aSeam depth(m)Absolute permeability (md)Cleat compressibility(kPa)145×104145×10-4Langmuir isotherm propertiesCH4:V=40mV=22m31100 kPa-- Mine CInitial gas content, average(m/t)20Desorption time(day)10.0Fig. 6. Maximum principal stress in the heading directionMoreover, for both mines, we assume that the roadway isdeveloped in an area with 100% CHA. Based on gas content andadsorption isotherm parameters, desorption pressure should be252933 kPa for Mine a and 625 kPa for Mine c4. ResultsMine a4.1. Stress distributionFLAC3D provided the excavation-induced stress distributionaround the roadway. The maximum principal stress distributionsDistance from the face into the coal seam (m)calculated for the two mines are presented in Figs. 4 and 5Figs 6 and 7 show the maximum principal stress distributionsin front of the coal face and laterally in the rib. For stress distribution in the rib the data are for the location of the start of the roadTable 5way. Other FLAC3D outputs are reported in Table 5FLAC3D outputs when roadway advance is 20 m.The results show that, for Mine C, the coal failure length I is 2 mameterin front of the coal face and 4 m in the rib. for mine a, the coal failure length is 4 m in front of the coal face and 6 m in the rib. perme-Heading Sideding Sideection direction directionability is calculated using Eq. (13); for Mine C, it changes from 2.5Failure length I(m)2.0at dividing 14.36to 424.63 md in front of the roadway(coal face)and from 2. 11 to827. 64 md in the rib For Mine A, permeability changes from 2.52.0E+7to 746.88 md in front of the roadway and from 2.05to135322 md in the ribFig 7 shows that there is a narrower stress relief zone from therib into the coal seam in Mine C than in Mine A. Both Figs 6 and 7show a low stress concentration level中国煤化工4.2. Gas pressure andCNMHGFigs. 8 and 9 show the gas pressures three days after roadwayFig 4. Maximum principal stress distribution in Mine C(Paexcavation. Fig 10 shows the variation in gas content at the sectionG. Li, A Saghafi/Intenational Joumal of Mining Science and Technology 24(2014)391-3963952.0Fig. 11. Variation of outburst indicator, in front of coal face in a heading at Mine cto the coal face is higher in Mine C(167 kPa/m)than in Mine a15m20m(145kPa/m)Fig 8. Gas pressure in Mine C(3 days after excavation)4.3. Evaluation of outburst potentialsIt is assumed that the specific crushing energy is 25 J/m foranthracite coal at Mine C and 20 J/m for bituminous Mine A coalUsing Eq.(12), the outburst indicators for the two mines arecalculated using FLAC3D outputs, SIMEd Il data and other data inTable 6Outburst indicators change over time as stress, gas contentand pressure change. Fig 11 shows outburst indicators over timefor the two collieries. The potential for outburst(in terms ofoutburst indicator) is lower and reduces faster for Mine c thafor mine a5 Conclusions010m20m30m40m50m60mWe developed a conceptual model of gas outburst in coal mines,g. 9. Gas pressure in Mine a(3 days after excavationexpressed in terms of an outburst indicator, u, which is defined asthe ratio of available energy to the required energy for crushingand ejecting the coal into the roadwayUsing this model, the occurrence of an outburst in a roadwayheading was evaluated for an Australian bituminous coal mineand a Chinese anthracite coal mine. For a roadway driven approx-imately 20 m into the Australian bituminous coal mine, the risk ofoutburst was 50% higher than for a similar roadway driven into theChinese anthracite coal mineTime(dary)Fig. 10. Total gas content variation over time at dividing point in two collieries. 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