- Email: [email protected]
Study on Supporting Wood Fire Induced by Mine Methane Explosions
Available online at www.sciencedirect.com
Procedia Engineering
Procedia Engineering 00 (2011)26000–000 Procedia Engineering (2011) 195 – 203 www.elsevier.com/locate/procedia
First International Symposium on Mine Safety Science and Engineering
Study on Supporting Wood Fire Induced by Mine Methane Explosions Duan yu-longa,b,, Liu xiang-lanc, Zhou xin-quand, Zhao xu-shenga,b, Zhang qinghuaa,b a
Chongqing Research Institute of China Coal Technology & Engineering Group Corp, Chongqing 400037, China National Key Laboratory of Methane Disaster Detecting，Preventing and Emergency Controlling, Chongqing 400037, China c College of Natural Resources and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266510, China d Natural Key Laboratory of Coal Resources and Safe Mining, China University of Mining & Technology(Beijing), Beijing 100083, China b
Abstract The risk of supporting wood fire induced by methane explosions in roadway was studied by doing some research on the mechanism of it. The thermal environment of methane explosions which include the characteristics of fire flame and the temporal and spatial thermal environment was studied. Based on the research of thermal environment, the fast pyrolysis of supporting wood was carried out by using fast pyrolysis apparatus. The experiment temperature is from 592~1 313 K, and the heat duration were 1 s, 2 s, 5 s, 10 s. Connected with combustion theory, the possibility of supporting wood fire induced by methane explosions was discussed from the following several aspects: fuel, combustion limit, the minimum combustion oxygen limit, and fire period. If the initial methane concentration is between 5.0~9.25 % with someone volume, then the sub-fire may occur at a certain distance from explosion source.
© 2011 Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and/or peer-review under responsibility of China Academy of Safety Science and Technology, China University of Mining and Technology(Beijing), McGill University and University of Wollongong. Keywords: methane; explosions; supporting wood; sub-fire
Coal mine methane explosion is a major disasters and accidents, the sub- fire caused by the methane explosion may induce secondary methane explosion again, even for many times of methane explosion, and will lead to much more loss. So, study on the mechanism of sub- fire caused by the methane explosion has great significance for the prevention of such disasters. Supporting wood as one of the main material of coal mine, there are lots of supporting wood fire accidents occurred in the history. Based on
Corresponding author. Tel.: +86-023-65239029; fax: +86-023-65239029. E-mail address: [email protected]
1877-7058 © 2011 Published by Elsevier Ltd. Open access under CC BY-NC-ND license. doi:10.1016/j.proeng.2011.11.2158
196 2
Duan et al. / Procedia Engineering (2011) 195 – 203 duan yu yu-long long/ Procedia Engineering 00 (2011)26000–000
the analysis of thermal environment after methane explosions, and the application of fast pyrolysis apparatus which could satisfy the dynamic thermal environment of methane explosion characteristics, some fast pyrolysis experiments within a temperature and time range were carried out. Combined with the basic theory of combustion to analyze the sub-fire mechanism and possibility, and then get some conclusions. Research on the prevention and control measures related to this kind of disasters could provide some references and proposals to prevent such kind of disasters, it has some practical value[1-3]. Nomenclature
ρ
air concentration, kg/m3
gi
volume force of direction i
δ ij
unit tensor
µ
dynamic viscosity, Pa.s
P
pressure, Pa
h
enthalpy, K
cp
specific heat at constant pressure, J/(kg· K)
µt
viscosity coefficient
Pr
Prandtl number
λ
heat exchange coefficient
Sh
equation source
T
temperature, K
∇
differential coefficient
1 The thermal environment after mine methane explosion Do some research on the thermal environment after methane explosion is main to know about the law of products out of supporting wood under methane explosions. Divide the thermal environment into two aspects: (1) first, the heat effect of high temperature and high pressure fire flame to supporting wood; (2) the heat effect of the temporal and spatial changeable thermal environment to supporting wood. Then the two parts will be discussed separately. 1.1 The characteristics of fire flame under methane explosions 1.1.1 Temperature of fire flame Some experts have done some 9.5 % methane explosions to test the temperature of fire flame after methane explosions, and the experiment results shows that the maximum temperature could reach 1 400
3197
Duan yu-long et /al. / Procedia Engineering 26 (2011) 195 – 203 duan yu long Procedia Engineering 00 (2011) 000–000
K, and some experts got it to 2 300 K through experiments and simulations[4][5]. As for the convenience of calculation, take the temperature of fire flame changes from 923~1 313 K for deflagration, and 2 000 K for explosions. And presume that temperature of fire flame hold in line when propagating in the roadway. 1.1.2 The heat duration of fire flame to supporting wood The heat duration is related to the velocity and the width of fire flame. Heat duration of fire flame to one point in roadway is the time of the fire width divides the fire velocity. Some experts have done some 9.5 % methane explosions to test the duration and relating velocity. Because 9.5 % methane explosion is the most serious status, then choose it to study some data [6-8]. The experiment results show that , the fire flame velocity increase from 10 m/s to 125 m/s and the holding time decrease from 400 ms to 35 ms very fast in L/D=80 along with the propagation of fire flame, then holds flat trend. Because the pipe experiment conditions are better than that of roadway, the tested fire velocity in pipe is lager than in roadway. Generally, most of the coal mine methane explosions are weak deflagration. In order to connect the following works tightly, assume that the duration is in 1 s [9]. 1.2 The dynamic thermal environment after methane explosions There are always abnormal, mass and thermal transfer physical phenomena after methane explosions. And the spatial and temporal temperature of air is related to concentration, velocity, and pressure. etc. Then, the following two dimension controlling equations were established [10-13]: Mass conservation equation:
Momentum conservation equation:
∂ρ ∂ + 0 ( ρu j ) = ∂t ∂x j
∂ (ρui ) + ∂ ∂t ∂x j
∂u j ∂u j + ρu j ui − µ ∂x j ∂xi
(
Energy conservation equation:
Heat radiation equation:
)
(1)
=
∂ ∂P 2 ∂ ∂u i µ ρu i' u 'j − g i − δ ij − − fi ∂xi 3 ∂x j ∂x j ∂x j ∂ (ρh ) ∂ (ρu j h) = ∂ λ ∂h − ρu 'j h ' + S h + ∂t ∂x j ∂x j ∂x j
σT 4 ∇q r = 4π α + E p − (α + α p )G π
(2) (3)
(4)
Based on the research results of the instant temperature distribution after methane explosions by some experts, it was set as an initial temperature state. Then connect it with equations (1)~(4), use Fluent software to calculate the dynamic temperature distribution after methane explosions. The 7.5 %, 100 and 200 m3 conditions are shown as Fig 1, 2. Tab 1 is the dynamic temperature change law which temperature is higher than 588 K under 200 m3, 7.5 % methane explosions. Choose 588 K is just to fulfill the lowest temperature of fast pyrolysis experiments 592 K. The temperature area which is higher than 588 K is moving far away from explosion source along with the increase of propagation distance. The highest temperature, high temperature area, and the holding time all decrease for period of time. Based on the same theory, the temperature distribution of air after different initial volume and concentration could be got.
Duan et al. / Procedia Engineering (2011) 195 – 203 duan yuyu-long long/ Procedia Engineering 00 (2011)26000–000 873 823 773 723 673 623 573 523 473 423 373 323 273
10s 150s 1800s
5s 100s 1200s 3600s
60s 600s 3000s
30s 300s 2400s
723 温度/K
温度/K
198 4
5s 100s 1200s 3600s
673 623
10s 150s 1800s 4080s
30s 300s 2400s
60s 600s 3000s
573 523 473 423 373 323 273
0
50
100
150
200
250
300
350
400 距离/m
Fig.1 The temperature distribution of air in the explosive roadway when 7.5 %, 200 m3 methane
0
50
100
150
200
250
300
350
400 距离/m
Fig.2 The temperature distribution of air in the explosive roadway when 7.5 %, 100 m3 methane
Tab.1 The characteristics of spatial and temporal temperature transformation process in the roadway when 200 m3, 7.5 % methane explosions Distance/m
Starting time/s
Holding time/s
Temperature range/K
Average temperature/K
0~4 4~10 10~17 17~23 24~30 30~36 36~43 43~55
4 6 7 6 6 6 7 12
5 12 8 7 5 5 5 5
588~821 588~750 588~704 588~667 588~641 588~622 588~607 588~595
705 669 646 628 615 605 598 592
2 The products of supporting wood under the thermal environment when methane explosions gas pipe
a
b d
c
a.curie point tinsel b.put sample c. pucker and press d. put the tinsel into the tinsel quartz pipe Fig.3 The schematic of wood particle sample preparation
curie point equipment
GC
Fig.4 The fast pyrolysis experiment system
Based on the study of thermal environment under methane explosions, high temperature and high pressure fire flame has some heat effect to supporting wood; and supporting wood may suffer some heat effect under the dynamic thermal environment. So, the two aspects will be discussed separately.
5199
Duan yu-long et /al. / Procedia Engineering 26 (2011) 195 – 203 duan yu long Procedia Engineering 00 (2011) 000–000
Because the fast pyrolysis experiment equipment could fulfill the fire and dynamic thermal environments, some supporting wood fast pyrolysis experiments were carried out. Experiment temperature: 592 K, 673 K, 773 K, 923 K, 1 023 K, 1 173 K, 1 313 K; heating rates: 5 000 K/s; initial supporting wood temperature: 293 K; sample mass: 10 mg(±0.5 mg); sample diameter: 0.1~1 mm; heat time: 1 s, 2 s, 5 s, 10 s; analysis objects: CH4, C2H4, C2H6, C2 H2, C3H6, H2, CO, CO2, tar, char. The sample was deal with industry and chemical analysis before experiments. The sample preparation course and experiment system are shown as Fig 3,4. The sample preparation process is as follows: choose a suitable alloy tinsel, put the powdery sample onto the tinsel, pucker and press the tinsel, put the tinsel into quartz pipe. The experiment process is as follows: connect the curie point fast pyrolysis equipment with GC, open N2, open the analysis software system, set the curie point system to a selected condition, start experiments, collect data and do some analysis. 2.1 The products of supporting wood under fire flame Take experiment temperature 923~1 313K as the fire flame temperature, and holding time 1 s, the fast pyrolysis experiment results are shown as Fig 5,6. From Fig.5, under the fast heat effect of fire flame, the pyrolysis products include CH4, C2H4, C2H6, C3H6, H2, CO, CO2 and so on. CO is much more than other gas, it could get to 23 % when 1 023 K, then CO2, CH4, C2H4. Other gas are very few. From Fig.6, char decreases along with the increase of temperature, about from 26 % when 923 K to 18% when 1 313 K. The total gas is increases along with temperature, about 30 % when 1 023 K. Tar decreases from 52 % when 923 K to 45 % when 1 313 K. The total loss increases from 73 % when 923 K to 82 % when 1 313 K. 95 85
20 75
15 CH4 C3H6 CO2
10
C2H4 H2
C2H2 CO
5 0
Mass fraction wt%
Mass fraction of gas wt%
25
65
gas tar
55
char total loss
45 35 25 15
923
1023
1123
1223
1323
Temperature/K Fig.5 Supporting wood pyrolysis mass fraction of gas when 1 s
923
1023
1123
1223
1323
Temperature/K Fig.6 Supporting wood pyrolysis mass fraction of gas, tar, char and total loss when 1 s
2.2 The products of supporting wood under dynamic thermal environment Take experiment temperature 588~773 K as the fire flame temperature, and holding time 2 s, 5 s, 10 s, the fast pyrolysis experiment results are shown as Fig.7,8. Fig.7 shows the relationship between output and temperature under dynamic thermal environment. CO and CO2 increase linearly with temperature increase. CO2 is more than CO, CO could reach 4 %
6200
Duan yu-long et al. / Procedia Engineering (2011) 195 – 203 duan yu long/ Procedia Engineering 00 (2011)26 000–000
8 7 6 5
1 CO2
Mass fraction
Mass fraction
of CO,CO2 wt%
when 773 K; Fig.8 shows the output of CH4, C2 H4, C2 H2, C2 H6, H2 under dynamic thermal environment. CH4 and C2H4 change with temperature obviously, and the output turning point is near 673 K. Fig.9 shows the output of gas, tar, char, total loss under dynamic thermal environment. Except char all decreese with temperature. Char total loss is 50 %, and gas 10% when 773 K.
CO
4 3 2 1 0
0.8
CH4
0.6 0.4
C2H4
0.2
H2 C2H2、C2H6
0 573
623
673
573
723 773 823 Temperature/K
Fig7 Mass fraction of CO, CO2 under spatial
Mass fraction wt%
673
723
773 823 Temperature/K
Fig.8 Mass fraction of CH4, C2 H4, C2 H6, C3H6 under spatial
and temporal environment
100 90 80 70 60 50 40 30 20 10 0 573
623
and temporal environment
char
total loss
tar gas
623
673
723
773 823 Temperature/K
Fig.9 Mass fraction of gas, tar, char and total loss under spatial and temporal environment
3 Combustion analysis of gas separated out from supporting wood Based on the analysis of output of supporting wood under fire flame and dynamic thermal environment, and combustion theory, the possibility of supporting wood whether could be fired was discussed. Main from the following several aspects: (1) the fired temperature of gas; (2) the lowest oxygen concentration; (3) the combustion concentration; (4) the fire induced time. (1) The fired temperature of gas
Duan yu-long et /al. / Procedia Engineering 26 (2011) 195 – 203 duan yu long Procedia Engineering 00 (2011) 000–000
Based on Fig (5) and (6), the gas out from supporting wood includes CH4, C2H4, C2 H6, C3H6, H2, CO and so on. As to this kind of flammable mixed gas, the fired temperature is main that of the stable. From the experiment data, CO, H2, CH4 are relatively large, and its fired temperature is about 783 K[14]. So, if the air temperature in the roadway after methane explosions is higher than 783 K, the mixed flammable gas separated out from supporting wood may be fired. (2) The lowest oxygen concentration In many cases, flammable gas is mixed gas, no single kind. The lowest oxygen concentration could not substitute that of mixed gas. There are several flammable gases, such as CH4, C2 H4, C 2H6, C3 H6, H2, CO. Based on some conference, the lowest oxygen concentration of supporting wood under 1 s heat effect could be expressed as equation (5) [15-17]: 5 × CH 4 ( % ) +6.25 × CO ( % ) +8.25 × C2 H 4 ( % ) +8.75 × C2 H 6 ( % ) + 6.98 × C3 H 6 ( % ) (5) CO2 min = PT Tab.2 The minimum combustion oxygen limit of gas out of supporting wood at different temperature under 1 s The minimum combustion oxygen limit of gas CO2 min % 923 K/1s
1023 K/1s
1173 K/1s
1313 K/1s
5.09 %
5.61 %
4.98 %
4.44 %
Tab 2 shows the minimum combustion oxygen limit of gas out of supporting wood at different temperature under 1 s based on those experiment data and equation (5). Because the higher the temperature, the lower the oxygen concentration, choose 923 K as the lowest temperature to calculate the minimum combustion oxygen limit is enough. The average minimum combustion oxygen limit is nearly around 4.5 %. Based on some experiment which done by other experts, only the initial methane concentration is betw een 5.0~9.25 %, the minimum combustion oxygen limit of gas could fulfill the need of Tab 2[15]. (3) The combustion concentration Two cases: ① the order of gas separated out from supporting wood is based on molecular weight, small molecular weight gas separated out first, then big molecular weight gas such as CO2, stratification is obvious; ② all gas mixed equably. Based on the study of thermal environment after explosions, the combustion concentration of supporting wood under 1 s, 2 s, 5 s, 10 s could be got as Tab 3. If only take the flammable gas into account, the combustion concentration increase first, then decreease, then increase again. As to all gas, the combustion concentration increase along with temperature and then become clam gradually. Contrast to each other, the max and min combustion concentration range of all gas is narrower than that of flammable gas. This could explain the reason of inert gases or incombustible gas could increase the combustion difficult of flammable gas. The min combustion concentration of supporting wood under fire flame is only 5.5 %, it will be very possible to be fired. Tab.3 The concentration limit for combustion of mixture gas out of supporting wood when heating 1 s, 2 s, 5 s, 10 s
7201
202 8
Duan et al. / Procedia Engineering (2011) 195 – 203 duan yu yu-long long/ Procedia Engineering 00 (2011)26000–000
592 K/2 s、 5 s、10 s
673 K/2 s、 5 s、10 s
min 10.5
max 67.0
min 9.3
max 61.1
min 15.1
max 75.3
min 12.4
max 68.3
Heat conditions 773 K/2 s、 923 K/1s 1023 K/1s 5 s、10 s The combustion concentration limit of flammable gas min max min max min max 6.6 51.9 5.5 46.5 5.8 46.2 The combustion concentration limit of all gas min max min max min max 8.2 57.6 6.1 49.3 6.3 48.4
1173 K/1s
1313 K/1s
min 5.5
max 47.7
min 5.6
max 56.4
min 5.8
max 49.3
min 6.0
max 58.0
(4) The fire induced time The fire induced time refers to the time of separated gas to be fired. There are many effect factors such as component, pressure, temperature. Etc. As to carbon-hydrogen fuel, its fire induced time is about 1 min when 573 K, about 1 s when 623~673 K. it will be much shorter if temperature is much higher. Because the fired temperature of gas 783 K is much higher than 623 K, the gas separated out from supporting wood must be shorter than 1 s, or even 0.1 s [14]. Based on the four aspects analysis above, there are several conclusions as follows: if the initial methane concentration is between 5.0~9.25 %, there are three conditions could be satisfied: ① there are mixed flammable gas;② the lowest oxygen concentration of mixed gas could be fulfilled; ③ there is a dynamic thermal environment which has a temperature area higher than 783 K. If all those three conditions satisfied at the same time and the same place, the sub-fire of supporting wood could be induced. So, it’s necessary to do some prevention and preparation to prevent such kind of disasters. 4 Conclusions Based on analysis of the characteristics of fire flame, and the dynamic thermal environment, by used fast pyrolysis equipment to do pyrolysis experiment under different heat effect and different temperature, connected with combustion theory, the fired mechanism of the gas separated out from supporting wood was studied. The main conclusions are as follows: (1) Most of the mine methane explosions are weak deflagration, and the heat holding time is between tens of MSEL to hundreds of MSEL. (2) The dynamic thermal environment was studied, there is high temperature and high temperature holding time area. (3) Choose supporting wood to do some fast pyrolysis experiment to study the law of gas separated out from itself under methane explosions. The gas out from supporting wood under fire flame is much more than that of under dynamic thermal environment. (4) The possibility of gas to be fire was discussed from four aspects:① the fired temperature of gas; ② the lowest oxygen concentration; ③ the combustion concentration; ④ the fire induced time. (5) If the initial methane concentration is between 5.0~9.25 %, there are three conditions could be satisfied: ① there are mixed flammable gas; ② the lowest oxygen concentration of mixed gas could be fulfilled; ③ there is a dynamic thermal environment which has a temperature area higher than 783 K. If all those three conditions satisfied at the same time and the same place, the sub-fire of supporting wood could be induced. Acknowledgements First, I appreciate my country who gives me a comfortable learning atmosphere. Second, I would like to show my deepest gratitude to my supervisor, Prof. Zhou xin-quan (China university of mining &
Duan yu-long et /al. / Procedia Engineering 26 (2011) 195 – 203 duan yu long Procedia Engineering 00 (2011) 000–000
technology(Beijing)), who has walked me through all the stages of the writing of this thesis. Without his illuminating instruction and patience, this thesis could not have reached its present form. I am also greatly indebted to all my colleagues who have helped me to develop the fundamental and essential academic competence. Especially appreciate for the support by National Natural Science Foundation of China (50874062). References [1].Wang Hai-yan, Cao Tao, Zhou Xin-quan. Research and application of attenuation law about gas explosion shock wave in coal mine. Journal Of China Coal Society. 2009.34(6):778-782. [2]. Lin Bo-quan, Jian Cong-guang, Zhang Hui. The Influence of Heat Dissipated in a Conduit Wall on the Transmission Characteristics of Gas Explosions. Journal of China University of Mining & Techenology. 2009,38(1):1-4. [3]. I.Glassman. Combustion[M]. Beijing: Science Press.1983. [4]. Yang Yi, He Xue-qiu, Liu Jian-zhang. Fractal characteristics of flame inner flow field in methane/air explosion. EXPLOSION AND SHOCK WAVES. 2004.24(1):30-36. [5]. Qu zhi-ming. Study on Attenuation Law of Shock wave and Damage Mechanism in Roadway of Coal Mine during Gas Explosion. China unive rsity of mining &technology(bei jing). 2007. [6]. Zhou Xin-quan,Wu Bing, Xu Jing-de. Basic characters of gas explosion in underground coal mine. China Coal. 2002.28(9):8-11. [7]. Lin Bo-quan, Jian Cong-guang. Energy changing characteristics of explosion wave and heating effect of wall. Journal Of China Coal Society. 2004.29(4):429-433. [8]. Lin Bo-quan, Jian Cong-guang, Zhang Hui. The Influence of Heat Dissipated in a Conduit Wall on the Transmission Characteristics of Gas Explosions. Journal of China University of Mining & Techenology. 2009.38(1):1-4. [9]. Wang Cong-yin, He Xue-qiu. Experimental research on flame thickness and sustaining time of gas explosion. Coal Science and Technology. 2001.29(8):27-30. [10]. Li Wei-xin. One-dimensional unsteady flow and shock waves. Beijing:National Defence Industry press. 2003: 202-255. [11]. F. Cammarota, A. Di Benedetto, V. Di Sarli, E. Salzano, G. Russo. Combined effects of initial pressure and turbulence on explosions of hydrogen-enriched methane/air mixtures. Journal of Loss Prevention in the Process Industries. 2009,22(5):607-613. [12]. Yoshitomo Inaba, Tetsuo Nishihara, Mark A Groethe, Yoshikazu Nitta. Study on explosion characteristics of natural gas and methane in semi-open space for the HTTR hydrogen production system. Nuclear Engineering and Design. 2004,232(1):111-119. [13]. Duan Yu-long, Zhou Xin-quan, Gong Wu., etal. The Temperature Distribution of Laneway Air after Mine Methane Explosions. Journal of China University of Mining & Techenology. 2010.39(3): 318-323. [14]. Tongji University, Chongqing Architecture University, Harbin Architectural University. Gas burner and Applications (third edition) . Beijing: China Building Engineering Industry Press. 2000 [15]. Zhang Zeng-liang, Li Ge-mei. Combustible gas (liquid vapor) Estimation of the minimum oxygen concentration and its influencing factors. Hunan University (Natural Science). 2006,21(3):13-15. [16]. Zhou Xin-quan. Mine Fire Disaster Theory and Practice. Beijing: Coal Industry Publishing House .1996. [17]. Liang Yun-tao. Enclosure Kinetics in Gas Explosion Analysis. Journal of China Unive rsity of Mining & Techenology. 2010,39(2):196-200.
9203
Procedia Engineering
Procedia Engineering 00 (2011)26000–000 Procedia Engineering (2011) 195 – 203 www.elsevier.com/locate/procedia
First International Symposium on Mine Safety Science and Engineering
Study on Supporting Wood Fire Induced by Mine Methane Explosions Duan yu-longa,b,, Liu xiang-lanc, Zhou xin-quand, Zhao xu-shenga,b, Zhang qinghuaa,b a
Chongqing Research Institute of China Coal Technology & Engineering Group Corp, Chongqing 400037, China National Key Laboratory of Methane Disaster Detecting，Preventing and Emergency Controlling, Chongqing 400037, China c College of Natural Resources and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266510, China d Natural Key Laboratory of Coal Resources and Safe Mining, China University of Mining & Technology(Beijing), Beijing 100083, China b
Abstract The risk of supporting wood fire induced by methane explosions in roadway was studied by doing some research on the mechanism of it. The thermal environment of methane explosions which include the characteristics of fire flame and the temporal and spatial thermal environment was studied. Based on the research of thermal environment, the fast pyrolysis of supporting wood was carried out by using fast pyrolysis apparatus. The experiment temperature is from 592~1 313 K, and the heat duration were 1 s, 2 s, 5 s, 10 s. Connected with combustion theory, the possibility of supporting wood fire induced by methane explosions was discussed from the following several aspects: fuel, combustion limit, the minimum combustion oxygen limit, and fire period. If the initial methane concentration is between 5.0~9.25 % with someone volume, then the sub-fire may occur at a certain distance from explosion source.
© 2011 Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and/or peer-review under responsibility of China Academy of Safety Science and Technology, China University of Mining and Technology(Beijing), McGill University and University of Wollongong. Keywords: methane; explosions; supporting wood; sub-fire
Coal mine methane explosion is a major disasters and accidents, the sub- fire caused by the methane explosion may induce secondary methane explosion again, even for many times of methane explosion, and will lead to much more loss. So, study on the mechanism of sub- fire caused by the methane explosion has great significance for the prevention of such disasters. Supporting wood as one of the main material of coal mine, there are lots of supporting wood fire accidents occurred in the history. Based on
Corresponding author. Tel.: +86-023-65239029; fax: +86-023-65239029. E-mail address: [email protected]
1877-7058 © 2011 Published by Elsevier Ltd. Open access under CC BY-NC-ND license. doi:10.1016/j.proeng.2011.11.2158
196 2
Duan et al. / Procedia Engineering (2011) 195 – 203 duan yu yu-long long/ Procedia Engineering 00 (2011)26000–000
the analysis of thermal environment after methane explosions, and the application of fast pyrolysis apparatus which could satisfy the dynamic thermal environment of methane explosion characteristics, some fast pyrolysis experiments within a temperature and time range were carried out. Combined with the basic theory of combustion to analyze the sub-fire mechanism and possibility, and then get some conclusions. Research on the prevention and control measures related to this kind of disasters could provide some references and proposals to prevent such kind of disasters, it has some practical value[1-3]. Nomenclature
ρ
air concentration, kg/m3
gi
volume force of direction i
δ ij
unit tensor
µ
dynamic viscosity, Pa.s
P
pressure, Pa
h
enthalpy, K
cp
specific heat at constant pressure, J/(kg· K)
µt
viscosity coefficient
Pr
Prandtl number
λ
heat exchange coefficient
Sh
equation source
T
temperature, K
∇
differential coefficient
1 The thermal environment after mine methane explosion Do some research on the thermal environment after methane explosion is main to know about the law of products out of supporting wood under methane explosions. Divide the thermal environment into two aspects: (1) first, the heat effect of high temperature and high pressure fire flame to supporting wood; (2) the heat effect of the temporal and spatial changeable thermal environment to supporting wood. Then the two parts will be discussed separately. 1.1 The characteristics of fire flame under methane explosions 1.1.1 Temperature of fire flame Some experts have done some 9.5 % methane explosions to test the temperature of fire flame after methane explosions, and the experiment results shows that the maximum temperature could reach 1 400
3197
Duan yu-long et /al. / Procedia Engineering 26 (2011) 195 – 203 duan yu long Procedia Engineering 00 (2011) 000–000
K, and some experts got it to 2 300 K through experiments and simulations[4][5]. As for the convenience of calculation, take the temperature of fire flame changes from 923~1 313 K for deflagration, and 2 000 K for explosions. And presume that temperature of fire flame hold in line when propagating in the roadway. 1.1.2 The heat duration of fire flame to supporting wood The heat duration is related to the velocity and the width of fire flame. Heat duration of fire flame to one point in roadway is the time of the fire width divides the fire velocity. Some experts have done some 9.5 % methane explosions to test the duration and relating velocity. Because 9.5 % methane explosion is the most serious status, then choose it to study some data [6-8]. The experiment results show that , the fire flame velocity increase from 10 m/s to 125 m/s and the holding time decrease from 400 ms to 35 ms very fast in L/D=80 along with the propagation of fire flame, then holds flat trend. Because the pipe experiment conditions are better than that of roadway, the tested fire velocity in pipe is lager than in roadway. Generally, most of the coal mine methane explosions are weak deflagration. In order to connect the following works tightly, assume that the duration is in 1 s [9]. 1.2 The dynamic thermal environment after methane explosions There are always abnormal, mass and thermal transfer physical phenomena after methane explosions. And the spatial and temporal temperature of air is related to concentration, velocity, and pressure. etc. Then, the following two dimension controlling equations were established [10-13]: Mass conservation equation:
Momentum conservation equation:
∂ρ ∂ + 0 ( ρu j ) = ∂t ∂x j
∂ (ρui ) + ∂ ∂t ∂x j
∂u j ∂u j + ρu j ui − µ ∂x j ∂xi
(
Energy conservation equation:
Heat radiation equation:
)
(1)
=
∂ ∂P 2 ∂ ∂u i µ ρu i' u 'j − g i − δ ij − − fi ∂xi 3 ∂x j ∂x j ∂x j ∂ (ρh ) ∂ (ρu j h) = ∂ λ ∂h − ρu 'j h ' + S h + ∂t ∂x j ∂x j ∂x j
σT 4 ∇q r = 4π α + E p − (α + α p )G π
(2) (3)
(4)
Based on the research results of the instant temperature distribution after methane explosions by some experts, it was set as an initial temperature state. Then connect it with equations (1)~(4), use Fluent software to calculate the dynamic temperature distribution after methane explosions. The 7.5 %, 100 and 200 m3 conditions are shown as Fig 1, 2. Tab 1 is the dynamic temperature change law which temperature is higher than 588 K under 200 m3, 7.5 % methane explosions. Choose 588 K is just to fulfill the lowest temperature of fast pyrolysis experiments 592 K. The temperature area which is higher than 588 K is moving far away from explosion source along with the increase of propagation distance. The highest temperature, high temperature area, and the holding time all decrease for period of time. Based on the same theory, the temperature distribution of air after different initial volume and concentration could be got.
Duan et al. / Procedia Engineering (2011) 195 – 203 duan yuyu-long long/ Procedia Engineering 00 (2011)26000–000 873 823 773 723 673 623 573 523 473 423 373 323 273
10s 150s 1800s
5s 100s 1200s 3600s
60s 600s 3000s
30s 300s 2400s
723 温度/K
温度/K
198 4
5s 100s 1200s 3600s
673 623
10s 150s 1800s 4080s
30s 300s 2400s
60s 600s 3000s
573 523 473 423 373 323 273
0
50
100
150
200
250
300
350
400 距离/m
Fig.1 The temperature distribution of air in the explosive roadway when 7.5 %, 200 m3 methane
0
50
100
150
200
250
300
350
400 距离/m
Fig.2 The temperature distribution of air in the explosive roadway when 7.5 %, 100 m3 methane
Tab.1 The characteristics of spatial and temporal temperature transformation process in the roadway when 200 m3, 7.5 % methane explosions Distance/m
Starting time/s
Holding time/s
Temperature range/K
Average temperature/K
0~4 4~10 10~17 17~23 24~30 30~36 36~43 43~55
4 6 7 6 6 6 7 12
5 12 8 7 5 5 5 5
588~821 588~750 588~704 588~667 588~641 588~622 588~607 588~595
705 669 646 628 615 605 598 592
2 The products of supporting wood under the thermal environment when methane explosions gas pipe
a
b d
c
a.curie point tinsel b.put sample c. pucker and press d. put the tinsel into the tinsel quartz pipe Fig.3 The schematic of wood particle sample preparation
curie point equipment
GC
Fig.4 The fast pyrolysis experiment system
Based on the study of thermal environment under methane explosions, high temperature and high pressure fire flame has some heat effect to supporting wood; and supporting wood may suffer some heat effect under the dynamic thermal environment. So, the two aspects will be discussed separately.
5199
Duan yu-long et /al. / Procedia Engineering 26 (2011) 195 – 203 duan yu long Procedia Engineering 00 (2011) 000–000
Because the fast pyrolysis experiment equipment could fulfill the fire and dynamic thermal environments, some supporting wood fast pyrolysis experiments were carried out. Experiment temperature: 592 K, 673 K, 773 K, 923 K, 1 023 K, 1 173 K, 1 313 K; heating rates: 5 000 K/s; initial supporting wood temperature: 293 K; sample mass: 10 mg(±0.5 mg); sample diameter: 0.1~1 mm; heat time: 1 s, 2 s, 5 s, 10 s; analysis objects: CH4, C2H4, C2H6, C2 H2, C3H6, H2, CO, CO2, tar, char. The sample was deal with industry and chemical analysis before experiments. The sample preparation course and experiment system are shown as Fig 3,4. The sample preparation process is as follows: choose a suitable alloy tinsel, put the powdery sample onto the tinsel, pucker and press the tinsel, put the tinsel into quartz pipe. The experiment process is as follows: connect the curie point fast pyrolysis equipment with GC, open N2, open the analysis software system, set the curie point system to a selected condition, start experiments, collect data and do some analysis. 2.1 The products of supporting wood under fire flame Take experiment temperature 923~1 313K as the fire flame temperature, and holding time 1 s, the fast pyrolysis experiment results are shown as Fig 5,6. From Fig.5, under the fast heat effect of fire flame, the pyrolysis products include CH4, C2H4, C2H6, C3H6, H2, CO, CO2 and so on. CO is much more than other gas, it could get to 23 % when 1 023 K, then CO2, CH4, C2H4. Other gas are very few. From Fig.6, char decreases along with the increase of temperature, about from 26 % when 923 K to 18% when 1 313 K. The total gas is increases along with temperature, about 30 % when 1 023 K. Tar decreases from 52 % when 923 K to 45 % when 1 313 K. The total loss increases from 73 % when 923 K to 82 % when 1 313 K. 95 85
20 75
15 CH4 C3H6 CO2
10
C2H4 H2
C2H2 CO
5 0
Mass fraction wt%
Mass fraction of gas wt%
25
65
gas tar
55
char total loss
45 35 25 15
923
1023
1123
1223
1323
Temperature/K Fig.5 Supporting wood pyrolysis mass fraction of gas when 1 s
923
1023
1123
1223
1323
Temperature/K Fig.6 Supporting wood pyrolysis mass fraction of gas, tar, char and total loss when 1 s
2.2 The products of supporting wood under dynamic thermal environment Take experiment temperature 588~773 K as the fire flame temperature, and holding time 2 s, 5 s, 10 s, the fast pyrolysis experiment results are shown as Fig.7,8. Fig.7 shows the relationship between output and temperature under dynamic thermal environment. CO and CO2 increase linearly with temperature increase. CO2 is more than CO, CO could reach 4 %
6200
Duan yu-long et al. / Procedia Engineering (2011) 195 – 203 duan yu long/ Procedia Engineering 00 (2011)26 000–000
8 7 6 5
1 CO2
Mass fraction
Mass fraction
of CO,CO2 wt%
when 773 K; Fig.8 shows the output of CH4, C2 H4, C2 H2, C2 H6, H2 under dynamic thermal environment. CH4 and C2H4 change with temperature obviously, and the output turning point is near 673 K. Fig.9 shows the output of gas, tar, char, total loss under dynamic thermal environment. Except char all decreese with temperature. Char total loss is 50 %, and gas 10% when 773 K.
CO
4 3 2 1 0
0.8
CH4
0.6 0.4
C2H4
0.2
H2 C2H2、C2H6
0 573
623
673
573
723 773 823 Temperature/K
Fig7 Mass fraction of CO, CO2 under spatial
Mass fraction wt%
673
723
773 823 Temperature/K
Fig.8 Mass fraction of CH4, C2 H4, C2 H6, C3H6 under spatial
and temporal environment
100 90 80 70 60 50 40 30 20 10 0 573
623
and temporal environment
char
total loss
tar gas
623
673
723
773 823 Temperature/K
Fig.9 Mass fraction of gas, tar, char and total loss under spatial and temporal environment
3 Combustion analysis of gas separated out from supporting wood Based on the analysis of output of supporting wood under fire flame and dynamic thermal environment, and combustion theory, the possibility of supporting wood whether could be fired was discussed. Main from the following several aspects: (1) the fired temperature of gas; (2) the lowest oxygen concentration; (3) the combustion concentration; (4) the fire induced time. (1) The fired temperature of gas
Duan yu-long et /al. / Procedia Engineering 26 (2011) 195 – 203 duan yu long Procedia Engineering 00 (2011) 000–000
Based on Fig (5) and (6), the gas out from supporting wood includes CH4, C2H4, C2 H6, C3H6, H2, CO and so on. As to this kind of flammable mixed gas, the fired temperature is main that of the stable. From the experiment data, CO, H2, CH4 are relatively large, and its fired temperature is about 783 K[14]. So, if the air temperature in the roadway after methane explosions is higher than 783 K, the mixed flammable gas separated out from supporting wood may be fired. (2) The lowest oxygen concentration In many cases, flammable gas is mixed gas, no single kind. The lowest oxygen concentration could not substitute that of mixed gas. There are several flammable gases, such as CH4, C2 H4, C 2H6, C3 H6, H2, CO. Based on some conference, the lowest oxygen concentration of supporting wood under 1 s heat effect could be expressed as equation (5) [15-17]: 5 × CH 4 ( % ) +6.25 × CO ( % ) +8.25 × C2 H 4 ( % ) +8.75 × C2 H 6 ( % ) + 6.98 × C3 H 6 ( % ) (5) CO2 min = PT Tab.2 The minimum combustion oxygen limit of gas out of supporting wood at different temperature under 1 s The minimum combustion oxygen limit of gas CO2 min % 923 K/1s
1023 K/1s
1173 K/1s
1313 K/1s
5.09 %
5.61 %
4.98 %
4.44 %
Tab 2 shows the minimum combustion oxygen limit of gas out of supporting wood at different temperature under 1 s based on those experiment data and equation (5). Because the higher the temperature, the lower the oxygen concentration, choose 923 K as the lowest temperature to calculate the minimum combustion oxygen limit is enough. The average minimum combustion oxygen limit is nearly around 4.5 %. Based on some experiment which done by other experts, only the initial methane concentration is betw een 5.0~9.25 %, the minimum combustion oxygen limit of gas could fulfill the need of Tab 2[15]. (3) The combustion concentration Two cases: ① the order of gas separated out from supporting wood is based on molecular weight, small molecular weight gas separated out first, then big molecular weight gas such as CO2, stratification is obvious; ② all gas mixed equably. Based on the study of thermal environment after explosions, the combustion concentration of supporting wood under 1 s, 2 s, 5 s, 10 s could be got as Tab 3. If only take the flammable gas into account, the combustion concentration increase first, then decreease, then increase again. As to all gas, the combustion concentration increase along with temperature and then become clam gradually. Contrast to each other, the max and min combustion concentration range of all gas is narrower than that of flammable gas. This could explain the reason of inert gases or incombustible gas could increase the combustion difficult of flammable gas. The min combustion concentration of supporting wood under fire flame is only 5.5 %, it will be very possible to be fired. Tab.3 The concentration limit for combustion of mixture gas out of supporting wood when heating 1 s, 2 s, 5 s, 10 s
7201
202 8
Duan et al. / Procedia Engineering (2011) 195 – 203 duan yu yu-long long/ Procedia Engineering 00 (2011)26000–000
592 K/2 s、 5 s、10 s
673 K/2 s、 5 s、10 s
min 10.5
max 67.0
min 9.3
max 61.1
min 15.1
max 75.3
min 12.4
max 68.3
Heat conditions 773 K/2 s、 923 K/1s 1023 K/1s 5 s、10 s The combustion concentration limit of flammable gas min max min max min max 6.6 51.9 5.5 46.5 5.8 46.2 The combustion concentration limit of all gas min max min max min max 8.2 57.6 6.1 49.3 6.3 48.4
1173 K/1s
1313 K/1s
min 5.5
max 47.7
min 5.6
max 56.4
min 5.8
max 49.3
min 6.0
max 58.0
(4) The fire induced time The fire induced time refers to the time of separated gas to be fired. There are many effect factors such as component, pressure, temperature. Etc. As to carbon-hydrogen fuel, its fire induced time is about 1 min when 573 K, about 1 s when 623~673 K. it will be much shorter if temperature is much higher. Because the fired temperature of gas 783 K is much higher than 623 K, the gas separated out from supporting wood must be shorter than 1 s, or even 0.1 s [14]. Based on the four aspects analysis above, there are several conclusions as follows: if the initial methane concentration is between 5.0~9.25 %, there are three conditions could be satisfied: ① there are mixed flammable gas;② the lowest oxygen concentration of mixed gas could be fulfilled; ③ there is a dynamic thermal environment which has a temperature area higher than 783 K. If all those three conditions satisfied at the same time and the same place, the sub-fire of supporting wood could be induced. So, it’s necessary to do some prevention and preparation to prevent such kind of disasters. 4 Conclusions Based on analysis of the characteristics of fire flame, and the dynamic thermal environment, by used fast pyrolysis equipment to do pyrolysis experiment under different heat effect and different temperature, connected with combustion theory, the fired mechanism of the gas separated out from supporting wood was studied. The main conclusions are as follows: (1) Most of the mine methane explosions are weak deflagration, and the heat holding time is between tens of MSEL to hundreds of MSEL. (2) The dynamic thermal environment was studied, there is high temperature and high temperature holding time area. (3) Choose supporting wood to do some fast pyrolysis experiment to study the law of gas separated out from itself under methane explosions. The gas out from supporting wood under fire flame is much more than that of under dynamic thermal environment. (4) The possibility of gas to be fire was discussed from four aspects:① the fired temperature of gas; ② the lowest oxygen concentration; ③ the combustion concentration; ④ the fire induced time. (5) If the initial methane concentration is between 5.0~9.25 %, there are three conditions could be satisfied: ① there are mixed flammable gas; ② the lowest oxygen concentration of mixed gas could be fulfilled; ③ there is a dynamic thermal environment which has a temperature area higher than 783 K. If all those three conditions satisfied at the same time and the same place, the sub-fire of supporting wood could be induced. Acknowledgements First, I appreciate my country who gives me a comfortable learning atmosphere. Second, I would like to show my deepest gratitude to my supervisor, Prof. Zhou xin-quan (China university of mining &
Duan yu-long et /al. / Procedia Engineering 26 (2011) 195 – 203 duan yu long Procedia Engineering 00 (2011) 000–000
technology(Beijing)), who has walked me through all the stages of the writing of this thesis. Without his illuminating instruction and patience, this thesis could not have reached its present form. I am also greatly indebted to all my colleagues who have helped me to develop the fundamental and essential academic competence. Especially appreciate for the support by National Natural Science Foundation of China (50874062). References [1].Wang Hai-yan, Cao Tao, Zhou Xin-quan. Research and application of attenuation law about gas explosion shock wave in coal mine. Journal Of China Coal Society. 2009.34(6):778-782. [2]. Lin Bo-quan, Jian Cong-guang, Zhang Hui. The Influence of Heat Dissipated in a Conduit Wall on the Transmission Characteristics of Gas Explosions. Journal of China University of Mining & Techenology. 2009,38(1):1-4. [3]. I.Glassman. Combustion[M]. Beijing: Science Press.1983. [4]. Yang Yi, He Xue-qiu, Liu Jian-zhang. Fractal characteristics of flame inner flow field in methane/air explosion. EXPLOSION AND SHOCK WAVES. 2004.24(1):30-36. [5]. Qu zhi-ming. Study on Attenuation Law of Shock wave and Damage Mechanism in Roadway of Coal Mine during Gas Explosion. China unive rsity of mining &technology(bei jing). 2007. [6]. Zhou Xin-quan,Wu Bing, Xu Jing-de. Basic characters of gas explosion in underground coal mine. China Coal. 2002.28(9):8-11. [7]. Lin Bo-quan, Jian Cong-guang. Energy changing characteristics of explosion wave and heating effect of wall. Journal Of China Coal Society. 2004.29(4):429-433. [8]. Lin Bo-quan, Jian Cong-guang, Zhang Hui. The Influence of Heat Dissipated in a Conduit Wall on the Transmission Characteristics of Gas Explosions. Journal of China University of Mining & Techenology. 2009.38(1):1-4. [9]. Wang Cong-yin, He Xue-qiu. Experimental research on flame thickness and sustaining time of gas explosion. Coal Science and Technology. 2001.29(8):27-30. [10]. Li Wei-xin. One-dimensional unsteady flow and shock waves. Beijing:National Defence Industry press. 2003: 202-255. [11]. F. Cammarota, A. Di Benedetto, V. Di Sarli, E. Salzano, G. Russo. Combined effects of initial pressure and turbulence on explosions of hydrogen-enriched methane/air mixtures. Journal of Loss Prevention in the Process Industries. 2009,22(5):607-613. [12]. Yoshitomo Inaba, Tetsuo Nishihara, Mark A Groethe, Yoshikazu Nitta. Study on explosion characteristics of natural gas and methane in semi-open space for the HTTR hydrogen production system. Nuclear Engineering and Design. 2004,232(1):111-119. [13]. Duan Yu-long, Zhou Xin-quan, Gong Wu., etal. The Temperature Distribution of Laneway Air after Mine Methane Explosions. Journal of China University of Mining & Techenology. 2010.39(3): 318-323. [14]. Tongji University, Chongqing Architecture University, Harbin Architectural University. Gas burner and Applications (third edition) . Beijing: China Building Engineering Industry Press. 2000 [15]. Zhang Zeng-liang, Li Ge-mei. Combustible gas (liquid vapor) Estimation of the minimum oxygen concentration and its influencing factors. Hunan University (Natural Science). 2006,21(3):13-15. [16]. Zhou Xin-quan. Mine Fire Disaster Theory and Practice. Beijing: Coal Industry Publishing House .1996. [17]. Liang Yun-tao. Enclosure Kinetics in Gas Explosion Analysis. Journal of China Unive rsity of Mining & Techenology. 2010,39(2):196-200.
9203