△P index with different gas compositions for instantaneous outburst prediction in coal mines

Availableonlineatwww.sciencedirect.comMININGScience DirectSCIENCE ANDTECHNOLOGYELSEVIERMining Science and Technology 20(2010)0723-0726www.elsevier.com/locate/jcumtAP index with different gas compositions forinstantaneous outburst prediction in coal minesWU Dongmei, ZHAO Yuemin, CHENG Yuanping, AN FernghuaNational Engineering Research Center of Coal Gas Control, China University of Mining Technology, Xuzhou 221008, ChinaKey Laboratory of Gas and Fire Control for Coal Mines, China University of Mining Technology, Xuzhou 221008, ChinaSchool of Chemical Engineering and Technology, China University of Mining Technology, Xuzhou 221116, ChinaAbstract: In this study we measured the AP (initial speed of gas emission)index with different gas concentrations of carbon dioxide(pure CO2, 90%CO2+10%CH4. 67%COz+33% CH4, 50% CO2+50% CH4, 30%COz+10% CHa and pure CH4) of coal samplesfrom the No. 2 coal seam in the Yaojie Coal Mine, Gansu province, China. The effect of carbon dioxide concentration, gas composition, coal strength and particle size of coal samples on the AP index was investigated. The experimental results show that withof various compositions, the AP value of three samples were clearly different. The AP index of coal samples A, B and C0.2-0.25 mm)were 4, 6 and 7 with pure CH4 and 22, 30 and 21 when pure CHa was used. Carbon dioxide concentration affects theAP index markedly. The AP index increases with anin carbon dioxide concentration, especially for coal B. Hence, theAP index and K(another outburst index)values testedpure CH4 for prediction of the danger of outburst is not accurateIt is important to determine the initial speed of gas emission given the gas composition of the coal seam to be tested for exact outburst prediction.Keywords: prediction; AP index; outbursts; coal mining: gas composition; carbon dioxide1 Introductionelement in gas, coal and rock outbursts is the amountof gas in a coal seam. It is important to estimateInstantaneous outbursts in underground coal mines, the gas pressure that is likely to occurinvolving both methane(CHa)and carbon dioxide and geological structures play a very dominant role in(CO2)or mixtures of both, have occurred in at least outbursts/5-1. They are also affected by sorp.20 countries". Outbursts are dynamic, energetic tion/desorption properties of coal 2-16. Because inevents which may result in the projection of frag- situ measurements of stress are difficult, the mecha-mented coal and rapid release of gases from the nism of gas outbursts has not been well understoodworking face. The most intensive outbursts, more up to the present, due to the complexity of storage forthan one-third of the total, have occurred in China. methane(CH)and carbon dioxide (CO2)in coalThese disastrous mine outbursts have resulted in theseams. Therefore, it is important to collect sufficientloss of lives of many miners, great losses of equip- data for predicting the probability of instantaneousment, production time and even entire mines all over outbursts in underground coal mines. Most currentlythe world 2. The gas released is predominantly com- used assessment indices in outburst predictions areposed of methane, carbon dioxide or their mixtures. based on the amount of gas and coal strengthHowever. carbon dioxide outbursts tend to be morThe AP index based on the initial rate of gas de-nt and occur often. Studies of this problem have sorption from coal has been widely used for predictng instantaneous outbursts in underground coalThe most important factoors affecting the occur- mines in China and other countries. AP is used torence of outbursts are: 1)the amount of gas in a condicate the tendency to outbursts, based on theseam, 2)geologic structures, 3)properties of coal and method developed by Ettinger 7. A high AP is usu-rock,4)vertical and lateral stresses occurring in the allydered to he an important indicator for a higlseam and rock or part of the coal seam". An essential probal中国煤化工 as been discussedReceived 16 December 2009: accepted 20 April 2010author.Te:8651683990201theCNMHGement techniquesE-mailnewdm9@163.comhave been employed in past work!".However,thedoi:|00l6s16745264(09)60270AP indices were previously measured with pure CHMining Science and TechnologyVol 20 No 5In our study, we first measured the AP index with seam No. 2 in the Yaojie Coal Mine, in Gansu provdifferent concentrations of carbon dioxide, such as ince, China. The original carbon dioxide concentra-pure CO2, 90% CO2+10% CH4, 67%CO2+33%CH4, tion in this mine ranges from 30%to 98%, which has50% CO2+50% CH4, 30%CO2+70% CH4 and pure experienced violent outbursts of carbon dioxide inCHA. We combined the Protodyakonov coal strength 1978. Based on the test data, we intented to analyzecoefficient and the AP index for calculating another the effect of carbon dioxide concentration, theoutburst index, i.e., the K value. Coal samples used amount of gas, coal strength and particle size on thefor these measurements were collected from coal AP index.Table 1 Worldwide occurrence of outburstsSize: coal+rock(gas)(mlI Australia( Bowen)CH/COz5045001400CHacO21,19274 Canada(Sydney)1977,1984375 Czech Republic(Ostra1894,19819851330(130007 France(Grad)b608 Poland(Lower Silesia>63005000(7500009 Russia(KuznetskI1 Germany2500(12002500(600001977,197815 China(Shanxi D250000(1894t2 Experimof numerous fissures and cracks, of which wereempty, while others were filled with pyrite. In addi2.1 Coal samplestion, several macerals of coal B were porous, so thatThree crushed samples with moderate levels of the semi-fusinite had an easily recognized cellularmetamorphism, referred to as A, B and C, were structure, of which some were filled with clay, occa-screened and divided into two groups, one of particle sionally with resinite. Unlike coal B, the vitrinite ofsize of 0. 2-0. 25 mm and the other with 1-3 mm par- coal a was mostly devoid of pores because its emptyticles. Parameters of the three coal samples were de- cells were filled with gelinite. The porosity and fis-termined according to China standards: GB/T212, sures of coal C are greater than that of coal A andDL/T 1030-2006 and GB/T 217-1996. Coal density smaller than that of coal Bwas measuerd using the automated apparatus Micro-Table 3 Petrographic analysis of coals A, B and Cpore Structure Analysis Apparatus-Pore Sizer 9310,made in the USA(Table 2).Group of maceralATable 2 Properties of coal samples A, B and C2.85Mincral substance100.8243Depth(top)(m)Mad(%)17242.3 AP index measurement methodVdaf (%)676Fig. I shows a schematic diagram of measuring thePorosity (AP index of 3.5 g coal samples of 0. 2-0.25 mm particle size, fed into the sample cup after being dried by2.2 Petrographic examinationheating and desorbed under vacuum. After that, thesamples were immersed at I atm for 1.5 h in gas.were cut parallel to the bedding plane. Petrographic Then the solenoid valve of the sample cup was auto-analyses were performed with the use of a Jenapol began to record time. After 10 s, the solenoid valvepolarising microscope, equipped with a 50ximmersion lens and a 10xmicroscope eye-piece. Maceralwas closed, the gas was evacuated from the system atanalysis(see Table 3)indicated that the three coallow pressure providing a pressure of PI. After 35 sthesamples were dominated by vitrinite and inertinitennened again automatically, closedwhere the amount of inertinite in both a and B coals at6中国煤化工 ressure reading P2was more than 47%. The moderately ranked coal Awas brittle and fissured, while coal B was more com- latCNMHGmmHg, was calcu-fter bos apact. The distinctive feature of coal B is the presenceressure after 10s: i.e. AP=P-PWU DonAP index with different gas composition2.4 AP index measurementAll the gases used in the measurements, suppliedby the Nan Jing Special Gas Factory Co Ltd,weredifferent in their carbon dioxide concentrations: 1)pure CO2,2)90%CO2+10%CH4,3)67%CO2+33%1. Vacuum pump; 2. Measure gas: 3. Solenoid valve; 4. Sample cupsCH4,4)50%CO2+50%CH,5)30%CO2+70%CHand pure CH4. The AP indices were measured in thelaboratory of the China National Engineering Re-Fig 1 Measure schematic of APsearch Center for Coal Gas Control (Table 4, Fig. 2).coal ACoal B一06080100CO concentration (%CO concentration (%(b)1-~3mmFig 2 Three coal sample AP3 Results and discussion2)indicate that particle size of the samples consid-erably affects the AP index, this is especially true for3.1 Gas compositioncoals a and b with their high concentration of coIn Fig. 2, the different AP values were obtained The results show that the AP index clearly increaseswhen tested with the gases at different compositions. with a decrease in particle size. The change in theThe AP indices of coal A, B and C(0. 2-0.25 mm) AP index was inconspicuous when particle sizesshow values of 4, 6 and 7 when tested with pure CH4 were 1-3 mm and carbon dioxide concentrations lessand 22, 30 and 21 with pure CO. The AP indices of than 50% for all three coal samples. That the changecoal A, B and C (1-3 mm)were 3, 3 and 4 when tested in the AP index was quite marked when particlewith pure CH4 and 11, 14 and 16 with pure CO2, re- sizes were 0. 2-0.25 mm, is because small sized coalspectively. If, in predicting the risk of gas outburst, has a greater capacity for adsorption than larger coalAP is smaller than 10(the critical AP index is 10, 3.4 Coal strength coefficientaccording to the Detailed Rules for the Prevention andControl of Outbursts of Coal and Gas in China and Coal strength is an important factor in the outbursttested with pure CH4), the coal seam is safe. But this is of a coal seam. The f index is very useful and com-completely different from the result using AP values monly used in outburst predictions. In general, atested with pure CO2. Therefore, gas outburst predic- widely applied assessment for regional coal outburststions based on the AP index obtained with pure CH4 in China is a combination of coal strength and the gasalone, is not accurate for coal seams large amounts of emission rate, i.e, the value K9)carbon dioxide(CO2)or mixtures of CO2 and CHaK=△P∫(1)3.2 Gas contentwheref is Protodyakonov strength coefficient of coalThe major factor in gas, coal and rock outbursts is samples. When K<15, a coal seam is not liable tothe amount of methane in a coal seam however oroutbursts when 15