Study on Energy Dissipation Law in the Process of Coal and Gas Outburst

Transcription

1 6th International Conference on lectronics, Mechanics, Culture and Medicine (MCM 05) Stud on nerg Dissipation Law in the Process of Coal and Gas Outburst Shugang Li,a, Ning Wang,b * and Tianjun Zhang,c College of nerg Science and ngineering, Xi an Universit of Science & Technolog, Xi an, Shaani, 70054, China School of Science, Xi an Universit of Science & Technolog, Xi an, Shaani, 70054, China a lisg@.ust.edu.cn, bwangningust@63.com, ctianjun hang@6.com Kewords: Coal and gas outburst; nerg; Dissipation law; Transfer process. Abstract. Research from the angle of energ is one of the important wa to eplore the cause of coal and gas outburst for grasping the law of coal and gas outburst. (Objective): In order to stud energ dissipation law in the process of coal and gas outburst. (Methods): Build the relationship between strain and stress of coal and rock according to the phsical equations of elastic mechanics, obtain the deformation potential energ of the coal and rock without considering the change of gas pressure and temperature, establish the practical dnamics equation of gas in coal seams. (Results): The research results show that gas rich coal and rock stores a great deal of deformation energ, adsorption energ of gas adsorbing on coal, gas epansion energ and gas fluid potential energ, etc. (Conclusion): These energ can make coal broken in coal and rock damage fracture process and provide continuous driving energ for coal and gas outburst. Introduction Coal and gas outburst is a dnamic process in which coal eposed surface suddenl and continuousl fling to roadwa space in a relativel short time[-3], continuous coal and gas outburst needs a lot of energ to provide power, so the research from the angle of energ is one of the important wa to eplore the cause of coal and gas outburst for grasping the law of coal and gas outburst[4-6]. In order to stud the total energ dissipation law in the process of coal and gas outburst, anale the energ change process of one micro coal unit in different stages[7, 8]. In a stable state, energ saving in coal and rock mass includes deformation potential energ influenced b crustal stress, adsorption energ of gas adsorbing on coal and dissociative gas flow energ stored in coal and the pores[9-]. To stud the energ dissipation law in coal and gas outburst is actuall to eplore the consumption and transfer process of the three kinds of energ. Deformation Potential nerg of Coal and Rock According to triaial stress test, coal and rock can be seen as elastomer before damage. In the stress stage of coal and gas outburst process, that is, in the original stress state of coal and rock, the stress of the micro unit can be treated as in equilibrium state. In the stress concentration stage of coal and gas outburst, micro coal units produce stress concentration in a certain direction, resulting in changing stress of micro coal units, so focus direction stress decreases and tangential stress of the other two directions increase. The change of stress effects little on the small pore volume of the original coal bod, because of adsorption gas eisting in the small pores, stress changing also influences little to gas pressure within the pores, the increase of the stress just makes deformation potential energ increase of coal skeleton. Therefore, it is reasonable to consider no change of gas pressure while considering deformation potential energ of coal and rock. Crustal stress makes deformation potential energ of coal and rock increase in the particles stress concentration, If the micro units of coal are seen as a closed thermodnamic sstem, and ignore the temperature change of the gas pressure in the process of stress concentration, the work done b coal strata is equal to the increasing of elastic energ 06. The authors - Published b Atlantis Press 79

2 of coal micro units. In balance state, both the kinetic energ and the mechanical energ of the coal and rock have not changed, so the decrease of the potential energ is completel transformed into deformation potential energ storing in the internal of coal and rock. To sum up, deformation potential energ of coal and rock without considering the change of the gas pressure and temperature can be epressed as q.. ( ε ε ε γ γ γ γ )ddd () In q., μ stands for poisson's ratio of coal, mean the stress of coal vertical coordinates,, directions, N m -, ε ε ε mean the strain of coal vertical coordinates,, directions. Obtaining the relationship of strain and stress according to the phsical equations of elastic mechanics shown as q.. ( ) ; γ G ( ) ; γ () G ( ) ; γ G In q., stand for coal shear stresses, N m -, γ γ γ are shear strains of coal. Assume that gas absorbed to coal is applied to coal and rock in uniform, the volume deformation energ of coal element effected b the nominal stress 0 (N m - ) and adsorption pressure P 0 (N m - ) combining q. and q. is defined as q. 3. ( 0 P0 ) ddd 6 (3) V In the coal burst phase, micro units of coal are forward to the eposed surface as the stress peak moves, tangential stress and radial stress effected on the micro units of coal are smaller and smaller, so deformation potential energ of coal micro units also graduall reduces. Decreasing of the deformation potential energ means that deformation potential energ the micro unit of coal is consumed, consumed deformation potential energ is converted into two forms: one is coal breakage under the effect of shear stress,and deformation potential energ is mainl transformed into coal and gas kinetic energ, the other one is the heat generated from the friction of coal particles on the crack surface, and partial deformation potential energ will be transformed into heat energ of coal micro units. Fig. shows the energ distribution in the tunnel when the damage volumes of original coal and rock are respectivel.0v and.5v. Figure. nerg densit changing with length of roadwa in the roadwa 80

3 Adsorption nerg of Gas Adsorbing on Coal The coal mine gas mainl comes from coal seams and coal strata, a large number of gas is produced in the process of plants forming into coal, and gas is associated product in the process of coalification. Coal and gas outburst is a ver complicated process, in the man factors causing coal and gas outburst, large amounts of gas desorption is an important factor. Gas adsorption occurs on coal and rock surface and holes, molecules densit of gas fluid on the adsorption surface is higher than gas densit of free fluid in the roadwa. When the adsorption of gas molecules and coal molecules reaches to geometric equilibrium, the calculation formula of adsorption abilit can be epressed as q. 4. ads M O O / M (4) In q. 4, ads is adsorption energ when gas molecular adsorption on coal surface reaches equilibrium state, M is the energ of coal surface molecular fragment before gas adsorption, O is the energ of gas molecular before adsorption, O/M is the total energ of the adsorption sstem after gas molecular absorbed to coal surface molecular fragment. Potential nerg of Gas Flow within the Coal and Rock Adsorption state gas in coal and rock mainl eists in micro holes and small holes, free state gas mainl eists in the coal and rock mesopores, large pores and fracture. These free state gas is approimatel regarded as an ideal gas in the practical engineering, and there are big differences between adsorption gas and free gas in micro holes and small holes, which have on characteristics of an ideal gas. Gas desorption in micro holes and small holes and its transmission wa to mesopores, large pores and fracture are ver comple, free gas under the effect of pressure gradient can be seen as linear seepage flow to stud its deliver process, and then establish the practical dnamic equations of the gas in coal seam. Combined with the Langmuir equation of gas adsorption on coal, the differential form of gas flow potential energ in coal and rock per unit volume can be epressed as q. 5. ab d flo Mvdv vdv (5) ( bp) In q. 5, v is gas flow velocit, m/s, p is free gas pressure, Pa, M is gas molar mass, g/mol, ρ is the densit of coal and rock, kg/m 3, ρ is the gas densit with pressure for p and temperature for T, kg/m 3. Because of gas compressibilit, generall the gas flow in coal seam is approimate to linear seepage, that is, gas flow is directl proportional to the gradient of gas pressure p in coal seam, and in accordance with Darc's law. Stress and temperature field influences the gas movement, penetration abilit of pore-fracture media depends on the number of pores and fractures on flow direction, width, connectivit and differential pressure at the ends of the pores and fractures. The permeabilit of porosit and fracture is derived from the laminar flow equation of flow through the slit. Thus, the equation of gas motion influenced b stress field and temperature field is epressed as q. 6. m ( T) -a ~ v e K P (6) In q. 6, η means gas dnamic viscosit, kg/(m s), K is the seepage coefficient of coal seam (a third-order tensor), m and a are parameters constants, T is free gas temperature, K, is the \ Hamiltonian operator, =i*d/d+j*d/d+k*d/d, K P stands for gas pressure gradient in the flow direction of coal and rock. Combining q. 5 with q. 6, gas flow potential energ in coal and rock is shown as q. 7. m -a ab ab ( T ~ vdv e d P bp bp K ) ( ) ) flo (7) ( In practical engineering, as gas discharge is slow, at the same time free phase and adsorption state gas eist heat echange with coal bod, the thermal change process of gas in coal seams can be similar 8

4 as isothermal process. When gas has no emission in the original coal seams, adsorption state gas and free phase gas are in the thermodnamic equilibrium state, and their macro velocities are eros, with the same pressure, temperature and chemical potential, so the mass and energ of the sstem of coal and rock can be thought of on diffusion. According to q. 7, the gas flow potential energ is ero in ~. coal and rock, that is, flo 0 Due to the adsorption potential energ of adsorption phase gas is greater than that of free state gas, phsical border conditions of gas emission in coal seam changes, the original stress state and thermodnamic equilibrium state in coal seams are destroed. When the gas pressure in coal seam decreases graduall, free gas flow in mesopores, large pores and fracture under the effect of pressure gradient, temperature decreases with the decreasing pressure. Conclusions With coal micro unit as the research object, anale the energ change process of one micro coal unit in different stages to stud the total energ dissipation law in the process of coal and gas outburst. Deformation potential energ of coal and rock is converted into two forms: one is coal breakage under the effect of shear stress, and deformation potential energ is mainl transformed into coal and gas kinetic energ, the other one is the heat generated from the friction of coal particles on the crack surface, and partial deformation potential energ will be transformed into heat energ of coal micro units. Adsorption potential energ of adsorption phase gas is greater than that of free state gas, phsical border conditions of gas emission in coal seam changes, the original stress state and thermodnamic equilibrium state in coal seams are destroed. When the gas pressure in coal seam decreases graduall, free gas flow in mesopores, large pores and fracture under the effect of pressure gradient, temperature decreases with the decreasing pressure. Gas rich coal and rock stores a great deal of deformation energ, adsorption energ of gas adsorbing on coal, gas epansion energ and gas fluid potential energ, etc. These energ can make coal broken in coal and rock damage fracture process and provide continuous driving energ for coal and gas outburst. Acknowledgements This work was carried out in the contet of the Project NO ,57457, supported b the National Natural Science Foundation of China (NSFC). The financial support is greatl acknowledged. References [] XIONG Yangtao, HUANG Gun, LUO Jiauan, et al. Theoretical Anslsis and perimental Stud of nerg Dissipation of Coal and Gas Outburst[J]. Chinese Journal of Rock Mechanics and ngineering, 34(): (05) [] WANG Gang, WU Mengmeng, WANG Haiang, et al. Sensitivit Analsis of Factors Affecting Coal and Gas Outburst Based on A nerg quilibrium Model[J]. Chinese Journal of Rock Mechanics and ngineering, 34(): (05) [3] KANG Xiangtao, HUANG Gun, SONG Zhenlong, et al. Research on Characteristics of nerg Dissipation and Seepage of Coal Containing Gas under Triaial Compression[J]. Rock and Soil Mechanics and ngineering, 36(3): (05) [4] JIANG Chenglin, YU Qiiang. Rules of nerg Dissipation in Coal and Gas Outburst[J]. Journal of China Coal Societ, (): (996) 8

5 [5] WANG Wen, LI Huamin, GU Helong, et al. Feature Analsis of nerg Dissipation of Water-saturated Coal Samples under Coupled Static-dnamic Loads[J]. Chinese Journal of Rock Mechanics and ngineering, 34(): (05) [6] TU Qing-i, CHNG Yuanping, WANG Liang, et al. Stud on Dnamic Process of Coal and Gas Outburst[J]. Coal Science and Technolog, 43(6): (05) [7] CAI Cheng-gong, WANG You-an. Qualitative and Quantitative Analsis of General Regularit of Coal and Gas Outburst[J]. China Safet Science Journal, 4(6):09-. (004) [8] CHN Yun, JIN Yan, CHN Mian. A Rock Brittleness valuation Method Based on nerg Dissipation[J]. Chinese Journal of Theoretical and Applied Mechanics, 47(6): (05) [9] SONG Zhenlong, HAN Peibo, LI Wenpu, et al. Impact of nerg Dissipation of Coal Samples with Rockburst Tendenc from Gas in Its Failure Process[J]. Journal of China Coal Societ, 40(4): (05) [0] ZHANG Hao, GAO Jianning, CUI Yongjie. Thermodnamic Analsis of Gas pansion nerg Calculation Method[J]. Safet in Coal Mines, 46(3): (05) [] LI Shugang. Movement of the Surrounding Rock and Gas Deliver in Full-mechanied Top Coal Caving[M]. Xuhou: China Universit of Mining and Technolog Press. (000) 83