Acoustic emission generated during the gas sorption-desorption process in coal

International Journal of Mining Science and Technology 22(2012)391-397SciVerse Science DirectInternational Journal of Mining Science and TechnologyELSEVIERjournalhomepagewww.elsevier.com/locate/iimstAcoustic emission generated during the gas sorption-desorption process in coalMa Yankun, Wang Enyuan, Xiao Dong, Li Zhonghui", Liu Jie, Gan Lijiaend Mining Resource Engineering, Southern Minois Universiry Carbondale, Carbondale 62901, UAARTICLE INFOA BSTRACTyrevised form 22 october 2011ssures.nline 17 Mayntially stable Cycles of sorption and desorptioneffect, which depends upon the maximurexposed to in earliimpulse frequency to be lower than previously. On the contrary.Cycleboth thehatpressure seen by the sample during earlier cyclesMemory effecte 2012 Published by Elsevier B.V. on behalf of China University of Mining Technology.Recently, scholars have conducted a great deal of research ongas sorption during the replacement of CHa by CO2. Goodmancoupling principles, of et al., studied the variaby Co, adsorpcoal seam gas are basic to research on gas disaster control Gasced gas consisting of CHa and Coz and a coal matrixmal forces and theby a factor ofholars have conducted a great deal of work on gas sorptionpal and therebyvelling deformation of a coal rock mass andResearch results concening coal swelling during gas sorption.nical theory and analyzed the erosion sorption-desorption cycles under different gas pressures are rareffect gasuilibrium gas can generaemission. The variation of acoustic emission signals ca中国煤化coal can cause a variation in the surface potential ofHYH曾NMHGimede础b你Corresponding author. Tel: +86 15862186412.na tne acoustic emission generated during cyclic gasE-mail address: mykunbesre126com(E Wanglsorption and desorption have been studied and are repor095-2686s-see front matter c 2012 Published by Elsevier B.V. on behalf of China University of Mining Technologyhttp://dxdoiorg/10.1016j.jmst.2011.11.00Y. Ma er s /mermetiomal Joumal of Mining Sience and Technolog 22(2012)391-397The acoustic emission generated during cyclic gas sorptionProximate analysis of the coal sample(A)search results are of theoretical significance to the dynamicsolat le mattergas sorption and flow. They can be applied in the forecasting of73z. Experimentaldifferent cyclic conditions: (1)the gas2. 1. Acoustic signal collection and materiaisincreases; and(3)the gas adsorption pressby tal resuits of the present authors, suggest thattthe Physical Actime, Thus only signals gener-signals. Abody, a high pressure gas cylinder, a pipe- ated during the first 30-60 min after gas filling were monitoret mine anin tand the sampling frequency was set at 500 KHz. The iments. The pressure range was 0. 1 MPa during the ycliesxper-sealed cylinder body is a rigid structure the middle part of which ments of gas sorption -desorpwas cosamples were raw coal collected from the Yanjiao Coal3. Results and discussioncylindrical coal samples with 50 mm in diameter by 50mmng, The petrographic characteristics of the coal areFig 2 shows the acoustic emission signals from the experimensystem before gas filling. Note the weak backgrounthe experiment2.2. procedureexample of signals generated during the gased te be gas tight and that he s linder body was set led Tre: axis star ting as 3400 is g. 3a The caoustic neg tssibno signascylinder was then evacuated with the air pump for shown in Fig. 3b are an enlarged part ofthat shows onlyhe CTA-l acoustic emission signal collection system the signal obtained during desorption. An indhe stander body were observe ont ignis gwas atew bin whi t mhe causes tc e mission signas rsoubaly defore gas was back filled into the chamber. Then the fl gas, kept the acoustic emission signals during desorption remain stablering adsorpnd the acobheedingandom, as shown in Figs 3-6. This indicatescoal may occur through layers andthe first sorption processtures that form in it As the amount of gasgain evacuated with the air pump gradually incrntroduced as before, except at an- of the coal matrixmicrodeformations or micro-fractureswere mon. Thus, the acoustic emission signals are generated.nd, agm the cylinder and the acoustic enfrom a cyclon were monitored for about 10 min. This process wasted at as many different pressure levels as were req0.6, 0. 4. and then 0.7 MPa. The cyclic experiment shownurther investigated by conducting the experiment under three formed using the same adsorption pressure, 0.6 MPacrease. Fig. 4if the gas pressfrst adsorption cycle. Fig. 5 shows that when the gasemission IH中国煤化工 increases e agna energyCNMH Gerstage thePg 1. Schematic of the experimental sysremergy and impulse rates in the second cycle compared to the first.Y Ma et al/Intemational Jourmal o Mining Science and Technology 22(2012)391-397acteristics of the coal sampleReflectance of vitrinite(zss(kgmm") Metamophic stageVitrinite Inertinite Total amour2. Acoustic emission signals from a cylinder without gas.(a)Acoustic emission signals generated during sorption-desorption3. Typical acoustic emission signals generated during sorption and desorption: left energy right impulses per second.For thetic emission sigshow a memory effect related to the maximum gas pressure towhich the coal was exposed during an earlier stage.ults in the on reduces the surface free energy of the coal and19.10] According to Bangham, the swelling deformation of a solid4. An analysis of the mechanism by which acoustic emission is (1)191. Eq (2) is an expression for the stress generated by gasdsorptionThe experimental results show that abundant signals are gener. E= AAyuring gas sorption and desorption. The acoustic emission siggenerated during desorption shows a phenomenon中国煤化工that reportedCN Gdb s thed soid:io na scaling rector:aspects. First, during gas filling and gas bleeding the gas pressure factor; K, the swelling factor: a and b adsorption constants; and pgradient causes damage to the coal This generates acoustic emis. the gas pressurmicro-deformations orion and seepage into the coal gas moleculesmicro-fractures in the coal, which also generates signals.gradually enter the matrix of the coal. As the amount of gaY Ma er al/Intemarional Joumal of Mining Soience and Technology 22(2012)391-39725000Adsorption under 0. 6 MPa(first adsorpion cycle))Adsorption under 0. 4 MPa(second adsorption cycle)6.52x1Adsorption under 0.7 MPa (third adsorption cycle)Acoustic emission signals generated during adsorption: left energy. right impulses per second.adsorbed increases the stress in the matrix graduallydesorption varies mainly by restoration of previously induceddeformations. The coal matrix shrinks and the acoustic emissionof coal break and the attractive pin the coal [22]. This also has a certain effeforms micro deformations or micro-fractures in the coal and this expansion. This accelerates the formation of micro-deformationsirface and theauseson of micro-deformations on the miial on the coal gradually decreases.The sudoemission signals may be explThe acoustic emission signals simultaneousg that gas adsorption and seepage in coal may occur by layer中国煤化工 the coal body increases andoal matrix shrinks, BecauseCN Gnal and releases some of the storeption andtion. Mon process are far weaker than those suring adsorp. of the coal As the amount of adsorbed gas continues to increaseure of the coal during the stress in the coal increases until the coal breaks, Gas adsorptioY, Ma et al/Intermational Joumal of Mining Soience and Technotogy 22(2012)391-39732020000(a)Adsorption under 0.5 MPa(firM adsorption cycle)500Time(s)(b)Adsorption under 0.6 MPa(second adsorption cycle)(s)c)Adsoepxon under 0.7 MPa(third adsorption cycle15Flg 5. Acoustic emission signals generated during sorption: left energy, right impulses per second.and seepage into deeper layers of the coal are the next step of en- gas pressure gradient. Pores or tiny cracks with a stronger interac-on force still remain inHowever, during a subsequentI decreases. This indicates that gas adsorption and seepage the first cycle will create a stress high enough so that additionin the coal and that the beginning of the next damagethe acoustic emission中国煤化工 maximum in previous cycles mea large acoustic emission signal. If the adsorptionCNMHGvarious other pores. In the first adsorption cycle the coal adsorbs signals that depends on the maximum gas adsorption pressureases and swells and is damaged by the adsorption stress and seen by the coal in earlier stagY, Ma ef al/lnternational Joumal of Miming Sience and Technoiogy 22(2012)391-397(a)Adsorption under 0. 6 MPa(first cycle)▲M(c)Adsoption under 0. 6 MPa(third cycle)Fig. 6. Acoustic emission signals generated during adsorption: left energy, right impulses per seconing previous stages both the acoustic emission energy and num -.Monitoring acoustic emission signals generated during gasber of impulses increase.orption and or desorption allows the following conclusionsdrawnThis paper describes research on the characteristics of theuring gas sorption and/or desorptin coal. This is of great importance for research on gas properties.(1 )The intensity of the acoustic emissiocoal seam gas drainage, and utilization and gas disaster control.as desorption. As theuent ofAcknowledgmentssignalsFinancial support for this work, provide by the National Naturaldinto coal may occtafety in CUMT (No. SKLCRSM09X01). the(2)The formation of acoustic emission signals has two aspects:( No 2008DFB70100) the Foundation for the Authorrst, gas adsorption forms micro-deformations or micro-fractures in the coal; second, during gas filling and bleeding the for New Century Excellent Talents in University(Nogas pressure gradient causes damage to the coal anKnowledgedes the acoustic signals.中国煤化工(3)During cyclic sorption-desorption the acousticshows a memory effect that depends on the maximumCNMH Ghosting and flow theory. Bejing: China Coalsure the coal was exposed to in earlier stages. when[2] Liu YB, Cao SG, U Y, Wang J, Guo P, Xu L, et al. Experimental study of swthe number of impulses are lower. On the contrary, if the gas p anliNg Zo n& l and gas us wan g hin seary son ggs okay 4 w xatY Mu et al/Mntemaronaf journed of Minang Sciencr and Technology 22(2012)391-397榴m, G、吧知union of deep12ses chin j Rock Mect ErsW Cuo wnges in fracture porosity of coal witMorption and descrption at in situ stressN%a如WaH“E她与四两mc:c图m2H中国煤化工CNMHG