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Experimental investigation on explosion flame propagation of wood dust in a semi-closed tube
Journal Pre-proof Experimental investigation on explosion flame propagation of wood dust in a semiclosed tube Zhihong Pang, Nanfeng Zhu, Yunqi Cui, Wanzhao Li, Changyan Xu PII:
S0950-4230(19)30396-1
DOI:
https://doi.org/10.1016/j.jlp.2019.104028
Reference:
JLPP 104028
To appear in:
Journal of Loss Prevention in the Process Industries
Received Date: 13 May 2019 Revised Date:
4 December 2019
Accepted Date: 5 December 2019
Please cite this article as: Pang, Z., Zhu, N., Cui, Y., Li, W., Xu, C., Experimental investigation on explosion flame propagation of wood dust in a semi-closed tube, Journal of Loss Prevention in the Process Industries (2020), doi: https://doi.org/10.1016/j.jlp.2019.104028. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Experimental investigation on explosion flame propagation of wood dust in a semi-closed tube Zhihong Pang, Nanfeng Zhu*, Yunqi Cui, Wanzhao Li, Changyan Xu College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China
*
Corresponding author. College of Materials Science and Engineering, Nanjing Forestry University,
Nanjing 210037, China. E-mail address: [email protected] (N. Zhu).
Abstract In order to explore flame propagation characteristics during wood dust explosions in a semi-closed tube, a high-speed camera, a thermal infrared imaging device and a pressure sensor were used in the study. Poplar dusts with different particle size distributions (0-50, 50-96 and 96-180 µm) were respectively placed in a Hartmann tube to mimic dust cloud explosions, and flame propagation behaviors such as flame propagation velocity, flame temperature and explosion pressure were detected and analyzed. According to the changes of flame shapes, flame propagations in wood dust explosions were divided into three stages including ignition, vertical propagation and free diffusion. Flame propagations for the two smaller particles were dominated by homogeneous combustion, while flame propagation for the largest particles was controlled by heterogeneous combustion, which had been confirmed by individual Damköhler number. All flame propagation velocities for different groups of wood particles in dust explosions were increased at first and then decreased with the augmentation of mass concentration. Flame temperatures and explosion pressures were almost similarly changed. Dust explosions in 50-96 µm wood particles were more intense than in the other two particles, of which the most severe explosion appeared at a mass concentration of 750 g/m3. Meanwhile, flame propagation velocity, flame propagation temperature and explosion pressure reached to the maximum values of 10.45 m/s, 1373 °C 1
and 0.41 MPa. In addition, sensitive concentrations corresponding to the three groups of particles from small to large were 500, 750 and 1000 g/m3, separately, indicating that sensitive concentration in dust explosions of wood particles was elevated with the increase of particle size. Taken together, the finding demonstrated that particle size and mass concentration of wood dusts affected the occurrence and severity of dust explosions, which could provide guidance and reference for the identification, assessment and industrial safety management of wood dust explosions. Keyword: wood dust explosion; flame propagation behavior; flame propagation velocity; flame temperature; explosion pressure 1. Introduction Large amounts of wood powder are always produced during wood processing such as sawing, milling and sanding, etc. Although a lot of production workshops in timber enterprises have been equipped with conventional dust removal system to improve working environment and meet the requirement of dust-free workshop, it is still hard to completely avoid dust explosions when fine wood powder is accumulated to a certain mass concentration in a semi-closed state and ignited by suitable ignition energy. Dust explosions would become a severe and widespread threat to humans and property if no constructive measures had been for seen (Eckhoff, 2003; Amyotte et al., 2010). Dust explosions are a complex process and usually initiated by exposing a cloud of combustible particles to an ignition source, followed by obvious flame propagation process in heterogeneous
medium
at
high
speed.
Then
dust
particles
are
subjected
to
vaporization/pyrolysis, mixing with oxidizer, ignition, burning and extinction with the rapid and violent increase of pressure (Yuzuriha et al., 2017). Dust explosions result from not only wood dusts but also a lot of other kinds of dust clouds. The earliest known dust explosion accident in the world happened in a flour mill in Turin in 1785. During dust explosions, flame propagation could be affected by thermal radiation and turbulence, as well as different particle sizes (Proust et al. 2006; Gao et al. 2013). Recently, it was also found that flame 2
propagation behaviors in polymethyl methacrylate explosion were varied according to particle size distributions and discrete spot flames appeared in the initial stage of polyethylene dust cloud explosions (Yuzuriha et al. 2017; Ganet al. 2018). These reports are mostly focused on dust explosions in an open space, and little is known about flame propagation and related properties of dust explosions in a semi-closed pipe till now. Therefore, it is worth paying more attention to wood dust explosions that always occur in dust removal pipes. In this study, a high-speed camera, a thermal infrared imaging device and a pressure sensor were utilized to explore the flame propagation behaviors of wood dust explosions in a semi-closed tube, including the effects of dust particle size and mass concentration on flame propagation, flame temperature and explosion pressure. We found that flame propagation in the Hartmann tube was able to be classified into the three stages of ignition, vertical propagation and free diffusion. Particle size affected flame propagation modes and sensitive concentrations in dust explosions, which played a decisive role in the strength of wood dust explosions together with mass concentration. The finding provides theoretical basis and practical reference for dust explosion prevention. 2. Experiments 2.1 Experimental apparatus The experimental apparatus for studying wood dust explosions consisted of a Hartmann tube, a high-speed camera, a thermal infrared imaging device and a pressure test device as shown in Fig. 1. The Hartmann tube is designed based on the British European Standard (BS EN 13821-2002) including a quartz tube, an ignition system and a powder spraying system. It is in a semi-closed transparent cylindrical shape with an open upper end (V=1.2 L, H=300 mm, Inner Diameter=68 mm) to simulate dust removal pipes, which can be used to observe flame structure. Two graphite ignition electrodes are 6 mm apart and located at about 100 mm away from the bottom of the pipe. A protective cover and a ventilation duct were installed outside of the Hartmann tube to ensure the safety of the experiments. Wood dusts were placed on the mushroom-shaped powder spraying device, and the powder spraying pressure was set 3
to be 120 kPa, which was provided by a small air pressure pump. Ignition delay time was 120 ms, referring to the period of time for the dust from ejected to ignited moment. Under such conditions, wood dust could be uniformly dispersed in the Hartmann tube to reduce the effect of initial turbulence on flame propagation (Zhang et al., 2016). The pressure sensor had three stress strain gauges with a diameter of 10 mm labeled as 1, 2 and 3 in Fig. 1, which were installed at 65 mm, 130 mm and 195 mm above the ignition electrodes to measure the pressure of shock wave on the pipe wall, respectively. The high-speed camera (OLYMPUSi-SPEED 3) was used to capture flame images, while the infrared thermal imaging device (FLIR A20) was used to measure flame temperature around the nozzle, which were simultaneously controlled by the computer with the ignition system. The frame rates of the high-speed camera and the thermal infrared imaging device were 1000 fps and 50 fps, respectively. In order to reduce experimental error, all experiments were performed at least three times independently and the data was shown as average values.
Fig. 1. Schematic diagram for experimental apparatus.
4
2.2 Preparation of particles Poplar (Populus tomentosa), one of the three fast-growing and high-yielding tree species in China, has been widely utilized in the wood processing industry. Poplar shavings were milled into dust powder in the factory by a plant shredder. The powder was sieved into three groups with different particle size distributions (0-50, 50-96 and 96-180 µm) to mimic the dust clouds generated in wood processing (Geng et al., 2012; Ding et al., 2012). The microscopic morphologies of wood dusts with different particle size distributions were shown in Fig. 2. Nearly all wood particles were in irregular shapes. Smaller particles were more preferably agglomerated, which might be affected by the surface area of wood particles. Indeed, it was confirmed in Table 1 that the smaller the particles, the larger the specific surface area of particles. Meanwhile, the significant differences in some other characteristic parameters like volume diameter D [4, 3], sauter diameter D [4, 3], etc. were also observed among the three groups of particles. Pyrolysis features for wood dust particles were collected from thermal gravity (TG) and differential scanning calorimetry (DSC) as shown in Fig. 3. Weight loss for wood pyrolysis was divided into three parts: a 3.41 % reduction from 0 to 82.9°C, a 64.27 % reduction from 82.9 to 370.8°C and a 13.52 % reduction from 370.8 to 800°C, which were mainly caused by water evaporation, sequential degradation of hemicellulose, cellulose and lignin, and residual lignin degradation, respectively, and only about 18.8 % of initial dusts was left. By integrating the DSC curve, the burning energy of dust powder was calculated to be 1157.07 J/g. The data for industrial and elemental analyses was shown in Table 2, and the chemical composition of wood dust particles was abbreviated as C47O45H6N.
5
Fig. 2. Microscopic morphologies of wood dusts with different particle size distributions. 6
Table 1. Characteristic parameters of wood dusts with different particle size distributions. Diameter range
0-50 µm
50-96 µm
96-180 µm
Specific surface area S (m ·g )
0.9280
0.5371
0.2159
Volume diameter D [4,3] (µm)
29.74
67.10
162.20
Sauter diameter D [3,2] (µm)
20.85
36.03
89.62
D (10) (µm)
9.861
14.40
70.45
D (50) (µm)
29.45
60.90
154.5
D (90) (µm)
49.87
127.9
268.8
2
-1
Fig. 3. Pyrolysis characteristics of wood dust particles.
Table 2 Proximate and ultimate analyses of wood dusts.
Sample Wood dusts
Proximate analyses (%)
Ultimate analyses (%)
MC
Aad
Vad
FCad
C
H
O
N
3.36
1.44
82.52
12.68
47.63
5.8
44.75
1.47
MC: moisture content; Aad: ash; Vad: volatile matters; FCad: fixed carbon
3. Results and discussion 7
3.1 Flame propagation mode After shot with the high-speed camera, flame propagation in the Hartmann tube was separated into three stages, which were ignition, vertical propagation and free diffusion as shown in Fig. 4. In the first stage, dust powder was ignited by the ignition electrodes, emitting a faint glow to slowly spread around. Then the flame propagated vertically, and high pressure was rapidly generated in the limited space of the pipe. After the flame was released from the pipe, flame propagation entered the stage of free diffusion. Hot air spread to the surrounding, settled, and restarted to go up when it was heated by the rising hot air below, which caused the formation of mushroom cloud fireballs on the top of the nozzle as shown in Fig. 5 (Miller et al., 2010; Dolan and Philip, 1977; Jin, 2017). Generally, there are two types of flame propagation modes during dust explosions as shown in Fig. 6. One is homogeneous mode similar to that in premixed gas explosions, mainly controlled by dynamics, and the other is heterogeneous mode, primarily manipulated by vaporization, pyrolysis and/or devolatilization (Dobashi and Senda, 2006). The two modes can not only coexist, but also convert to each other, depending on the volatility and particle size of dust clouds. The flame propagation images of wood dusts in the Hartmann tube were shown in Fig. 7. In the processing of flame propagation, there were many luminous yellow points, which were probably produced by soot particles as a result of the incomplete combustion of dust particles. Small luminous flames gradually grew up to combine with each other, forming a luminous zone in an irregular shape. During free diffusion, the two smaller particles produced a continuous flame with a smooth flame front, indicating that these particles were burned more completely. The flame generated by the largest particles displayed discrete structure with a broken and discontinuous front, which meant large particles reacted partially and only the surface was charred. Thus, flame propagations for the two smaller particles were mainly controlled by homogeneous combustion, while flame propagation for the largest particles was dominated by heterogeneous combustion. The modes of flame propagation can also be defined by Damkӧhler number (Gao et al., 2015). When Damköhler number is close to 1, homogeneous combustion dominates the flame 8
propagation process. On the contrary, heterogeneous combustion is in a leading position when Damköhler number is far away from 1. Damköhler number for 0-50 and 50-96 µm wood dust particles were calculated to be 0.98 and 1.6, respectively, and Damköhler number for 96-180 µm particles was 18.5, which confirmed the classification for flame propagation based on flame shapes.
(a) Ignition
(b) Vertical propagation
(c) Free diffusion
Fig. 4. Flame propagation images of wood dusts with different size distributions at a mass concentration of 500 g/m3.
Fig. 5. Formation principle of mushroom cloud fireballs
9
Fig. 6. Flame propagation mechanisms in dust explosions. (a) Homogeneous propagation; (b) Heterogeneous propagation.
(a) 0–50 µm
(b) 50–96 µm
10
(c) 96–180 µm Fig. 7. Flame propagation images of wood dusts with different particle size distributions at a mass concentration of 500 g/m3.
3.2 Flame propagation velocity The distances from the flame front to the ignition point and flame velocities for three groups of wood particles were measured and the results were shown in Fig. 8. The flame front of 50-96 µm particles was up to 887 mm, while the flame heights of 0-50 and 96-180 µm wood particles were less than 800 mm. Flame propagation velocity was calculated by the ratio of propagation distance and propagation time of flame front, obtained from digital images. Flame propagations for all groups of the wood particles were accelerated obviously when the flames reached up to 50 mm or so. As the flames came out of the pipe, flame propagation velocities reached to the maximum, which then dropped sharply and tended to be stable till the flame disappeared. Actually, flame propagation velocity was fluctuated as reported in previous researches (Proust & Veyssiere, 1988; Goroshin, Bidabadi, & Lee, 1996), which might be caused by unstable combustion of wood dust and convective heat transfer during combustion. In addition, burning time for the dust was extended with the increase of particle size, for large particles subjected to surface burning needed to undergo further combustion. When mass concentration was further increased, flame propagation velocities were also detected for the different groups of wood dusts, and the results were shown in Fig. 9. Flame propagation velocities were all increased at first and then decreased with the augmentation of mass concentration. As soon as flame propagation velocity reached up to the maximum level, the most violent explosive reaction took place. The mass concentration of this moment was 11
called sensitive concentration, which was the most sensitive point of mass concentration in dust explosions. If mass concentration was continued to increase, the excessive dust powder might be agglomerated, some of which was even probably adhered to the inner wall of the pipe and the ignition electrodes due to electrostatic attraction. Like this, wood particles were not dispersed evenly, which caused incomplete combustion and the decrease of mass concentration. Then the volume of combustible gas generated by pyrolysis was decreased, and flame propagation speed was reduced accordingly. The maximum flame propagation velocities for 0-50, 50-96 and 96-180 µm of dust particles were 7.31 m/s, 10.45 m/s and 6.67 m/s, and the corresponding sensitive concentrations were 500 g/m3, 750 g/m3 and 1000 g/m3, respectively. Small particles tended to be stable combustion and large particles were mostly burned on surface, which could attenuate the severity of explosions. Thus, the most intense dust explosion occurred when the flame of the middle size particles reached to the highest flame propagation velocity. In addition, sensitive concentration was increased with the increase of particle size, indicating that mass concentration was also associated with dust explosion intensity.
12
Fig. 8. Flame front locations and flame propagation velocities of wood dusts with different particle size distributions at a mass concentration of 500 g/m3.
Fig. 9. The relation of flame propagation velocities and mass concentrations of wood dusts with different particle size distributions. 13
3.3 Flame temperature After the flames were ejected from the nozzle, flame temperatures for wood dusts with different size distributions were measured as shown in Fig. 10. The dusts insufficiently burned in the semi-closed tube could be thoroughly mixed with oxygen to burn fully when the flame spread out of the nozzle. Consequently, all flame temperatures rose first, and then dropped till burnout. When mass concentration was changed, flame temperatures for different groups of wood dusts were also measured as shown in Fig. 11. The flames immediately reached to the maximum temperatures after they were ejected from the nozzle. As mass concentration was kept going up, the maximum flame temperatures of two smaller particles started to decrease slightly, which might be caused by the conversion from homogeneous to heterogeneous combustion. In contrast, the maximum level of the largest particles was hardly changed with further increase of mass concentration since the excessive amounts of wood dusts could be spread around to maintain sensitive concentration. Notably, flame temperature for 50-96 µm wood particles reached to the highest value of 1373 ℃ at a mass concentration of 750 g/m3, which indicated the occurrence of the worst explosion.
(a) 0-50 µm
14
(b) 50-96 µm
(c) 96-180 µm Fig. 10. Flame temperatures of wood dusts with different particle size distributions after the flames were ejected from the nozzle.
Fig. 11. The relation of flame temperatures and mass concentrations of wood dusts with different particle size distributions.
3.4 Explosion pressure Explosion pressures at different sites inside of the Hartmann tube were determined as shown in Fig. 12. When the positions rose, explosion pressure for each group of particles was increased and it was elevated the most dramatically in the case of 50-96 µm particles, up to 0.35 MPa at position 3. Subsequently, explosion pressures at position 3 for dust particles with 15
different mass concentrations were measured as shown in Fig. 13. Like flame propagation velocities, explosion pressures for three particles were all increased at first and then decreased with the augmenting of mass concentration. It was understandable that explosion pressure for 50-96 µm particles at 750 g/m3 mass concentration reached to the highest level of 0.41 MPa, which was still lower than that in the closed container (PilãO et al., 2006; APA Zhang et al., 2014; Richard et al., 2015). Flame propagation velocity and flame temperature reflected the severity of wood dust explosions, so did explosion pressure. Corresponding to 0-50, 50-96 and 96-180 µm of dust particles, the maximum explosion pressures were 0.23, 041 and 0.22 MPa, and sensitive concentrations were 500 g/m3, 750 g/m3 and 1000 g/m3, separately, which demonstrated that there was a relation between particle size and mass concentration to affect the strength of dust explosions.
Fig. 12. Explosion pressures at different sites for dust clouds with 500 g/m3 mass density
16
Fig. 13. The relation of explosion pressures and mass concentrations of dust clouds with different particle size distributions .
4. Conclusion In this study, flame propagations in wood dust explosions were divided into three stages including ignition, vertical propagation, and free diffusion. Flame propagations for 0-50 and 50-96 µm particles were found to be dominated by homogeneous combustion, while flame propagation for 96-180 µm particles was mainly controlled by heterogeneous combustion. When mass concentration for 50-96 µm wood particles reached to 750 g/m3, the most severe dust explosion occurred, in which the maximum flame propagation velocity, flame temperature and explosion pressure were 10.45 m/s, 1373 °C and 0.41 MPa, separately. Sensitive concentrations in dust explosions were also influenced by particle size, which were 500 g/m3, 750 g/m3 and 1000 g/m3 corresponding to 0-50, 50-96 and 96-180 µm of dust particles, respectively. Thus, particle size is associated with mass concentration to determine the severity of wood cloud explosions. 5. Acknowledgments
17
The authors gratefully acknowledge the financial support from the National Key R&D Program of China (No. 2016YFD0600703).
18
References Amyotte, P.R., Eckhoff, R.K., 2010. Dust explosion causation, prevention and mitigation: an overview. J. Chem. Health Saf. 17(1), 15-28. British Standards Institution, 2002.Potentially Explosive Atmosphere. Explosion Prevention and Protection. Determination of Minimum Ignition Energy of Dust/air Mixtures (British Standard), BS EN 13821. Eckhoff, R.K., 2003. Dust explosions in the process industries. J. Hazard Mater. 54(3), 266-267. Ding, T., Gu, L., Zhu, N., Guo, X., He W., 2012. Milling process performance of heat-treated wood. China Wood Industry 22-24. (Chinese) Dobashi, R., Senda, K., 2006. Detailed analysis of flame propagation during dust explosions by UV band observations. J. Loss Prev. Process Ind. 19(2), 149-153. Gan, B., Gao, W., Jiang, H., Li, Y., Zhang, Q., Bi, M., 2018. Flame propagation behaviors and temperature characteristics in polyethylene dust explosions. Powder Technol. 328, 345-357. Gao, W., Mogi, T., Sun, J.H., Yu, J.L., Dobashi, R., 2013. Effects of particle size distributions on flame propagation mechanism during octadecanol dust explosions. Powder Technol. 249, 168-174. Gao, W., Mogi, T., Yu, J., Yan, X., Sun, J., Dobashi, R., 2015. Flame propagation mechanisms in dust explosions. J. Loss Prev. Process Ind. 36, 186-194. Geng, J., Ding, T., Zhou, H., 2012. Study on particle size distribution of medium density fiberboard sanding dust. Wood Processing Machinery 23(6), 27-29 (Chinese). Glasstone, S., Dolan, P.J., 1977. The effects of nuclear weapons, US Dept. of Defense and Energy Research and Development Administration, Washington, 408.
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Goroshin, S., Bidabadi, M., Lee, J.H., 1996. Quenching distance of laminar flame in aluminum dust clouds. Combust. Flame 105, 147-160. Kim J., Kim, S., 2017.Simulation of Blast Wave Propagation and Mushroom Cloud formation by a Bomb Explosion. 55th AIAA Aerospace Sciences Meeting - Grapevine, Texas. Miller, F.P., Vandome, A.F., Mcbrewster, J., 2010. External Ballistics. Alphascript Publishing, 105-185. PilãO, R., Ramalho, E., Pinho, C., 2006. Overall characterization of cork dust explosion. J. Hazard Mater. 133(1-3), 183-195. Proust, C., 2006. A few fundamental aspects about ignition and flame propagation in dust clouds. J. Loss Prev. Process Ind. 19(2), 104-120. Proust, C., Veyssiere, B., 1988. Fundamental properties of flame propagating in starch dust-air mixtures. Combust. Sci. and Technol. 62, 149-172. Richard, K., Szabová, Z., Čekan P., 2015. Determination of the maximum explosion pressure and the maximum rate of pressure rise during explosion of wood dust clouds. Res. Pap. Fac. Mat. Sci. Technol. Slovak Univ. Technol. 23(36), 49-56. Yuzuriha, Y., Gao, W., Mogi, T., Dobashi, R., 2017. Effects of particle size distributions on flame propagation behavior through dust clouds of PMMA. J. Loss Prev. Process Ind. 49, 852-858. Zhang, Q., Ma, Q., Zhang, B., 2014. Approach determining maximum rate of pressure rise for dust explosion. J. Loss Prev. Process Ind. 29, 8-12. Zhang, X.Y., Yu, J.L., Sun, J.H., Gao, W., 2016. Effects of turbulent intensity on nano-PMMA flame propagation behaviors. J. Loss. Prev. Process Ind. 44, 119-124.
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Flame propagation characteristics were studied during the explosion of fast-growing poplar wood particle cloud in a semi-closed tube. Particle size of wood dust affected sensitive concentration of dust explosion. Particle size was associated with mass concentration to decide the severity of dust explosion. Dust explosion in 50-96 µm of wood particles was the most severe when mass concentration was 750 g/m3, and flame propagation velocity, flame propagation temperature and explosion pressure reached maximum values of 10.45 m/s, 1373 °C and 0.41 MPa, respectively.
Author Statement N.Z. conceived and designed the study. Z.P., Y.C. and W.L. performed the experiments. N.Z. and Z.P. analyzed the data, Z.P., N.Z. and C.X. wrote and reviewed the manuscript. All authors read and approved the manuscript and agree to be accountable for all aspects of the research in ensuring that the accuracy or integrity of any part of the work are appropriately investigated and resolved.
S0950-4230(19)30396-1
DOI:
https://doi.org/10.1016/j.jlp.2019.104028
Reference:
JLPP 104028
To appear in:
Journal of Loss Prevention in the Process Industries
Received Date: 13 May 2019 Revised Date:
4 December 2019
Accepted Date: 5 December 2019
Please cite this article as: Pang, Z., Zhu, N., Cui, Y., Li, W., Xu, C., Experimental investigation on explosion flame propagation of wood dust in a semi-closed tube, Journal of Loss Prevention in the Process Industries (2020), doi: https://doi.org/10.1016/j.jlp.2019.104028. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Experimental investigation on explosion flame propagation of wood dust in a semi-closed tube Zhihong Pang, Nanfeng Zhu*, Yunqi Cui, Wanzhao Li, Changyan Xu College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China
*
Corresponding author. College of Materials Science and Engineering, Nanjing Forestry University,
Nanjing 210037, China. E-mail address: [email protected] (N. Zhu).
Abstract In order to explore flame propagation characteristics during wood dust explosions in a semi-closed tube, a high-speed camera, a thermal infrared imaging device and a pressure sensor were used in the study. Poplar dusts with different particle size distributions (0-50, 50-96 and 96-180 µm) were respectively placed in a Hartmann tube to mimic dust cloud explosions, and flame propagation behaviors such as flame propagation velocity, flame temperature and explosion pressure were detected and analyzed. According to the changes of flame shapes, flame propagations in wood dust explosions were divided into three stages including ignition, vertical propagation and free diffusion. Flame propagations for the two smaller particles were dominated by homogeneous combustion, while flame propagation for the largest particles was controlled by heterogeneous combustion, which had been confirmed by individual Damköhler number. All flame propagation velocities for different groups of wood particles in dust explosions were increased at first and then decreased with the augmentation of mass concentration. Flame temperatures and explosion pressures were almost similarly changed. Dust explosions in 50-96 µm wood particles were more intense than in the other two particles, of which the most severe explosion appeared at a mass concentration of 750 g/m3. Meanwhile, flame propagation velocity, flame propagation temperature and explosion pressure reached to the maximum values of 10.45 m/s, 1373 °C 1
and 0.41 MPa. In addition, sensitive concentrations corresponding to the three groups of particles from small to large were 500, 750 and 1000 g/m3, separately, indicating that sensitive concentration in dust explosions of wood particles was elevated with the increase of particle size. Taken together, the finding demonstrated that particle size and mass concentration of wood dusts affected the occurrence and severity of dust explosions, which could provide guidance and reference for the identification, assessment and industrial safety management of wood dust explosions. Keyword: wood dust explosion; flame propagation behavior; flame propagation velocity; flame temperature; explosion pressure 1. Introduction Large amounts of wood powder are always produced during wood processing such as sawing, milling and sanding, etc. Although a lot of production workshops in timber enterprises have been equipped with conventional dust removal system to improve working environment and meet the requirement of dust-free workshop, it is still hard to completely avoid dust explosions when fine wood powder is accumulated to a certain mass concentration in a semi-closed state and ignited by suitable ignition energy. Dust explosions would become a severe and widespread threat to humans and property if no constructive measures had been for seen (Eckhoff, 2003; Amyotte et al., 2010). Dust explosions are a complex process and usually initiated by exposing a cloud of combustible particles to an ignition source, followed by obvious flame propagation process in heterogeneous
medium
at
high
speed.
Then
dust
particles
are
subjected
to
vaporization/pyrolysis, mixing with oxidizer, ignition, burning and extinction with the rapid and violent increase of pressure (Yuzuriha et al., 2017). Dust explosions result from not only wood dusts but also a lot of other kinds of dust clouds. The earliest known dust explosion accident in the world happened in a flour mill in Turin in 1785. During dust explosions, flame propagation could be affected by thermal radiation and turbulence, as well as different particle sizes (Proust et al. 2006; Gao et al. 2013). Recently, it was also found that flame 2
propagation behaviors in polymethyl methacrylate explosion were varied according to particle size distributions and discrete spot flames appeared in the initial stage of polyethylene dust cloud explosions (Yuzuriha et al. 2017; Ganet al. 2018). These reports are mostly focused on dust explosions in an open space, and little is known about flame propagation and related properties of dust explosions in a semi-closed pipe till now. Therefore, it is worth paying more attention to wood dust explosions that always occur in dust removal pipes. In this study, a high-speed camera, a thermal infrared imaging device and a pressure sensor were utilized to explore the flame propagation behaviors of wood dust explosions in a semi-closed tube, including the effects of dust particle size and mass concentration on flame propagation, flame temperature and explosion pressure. We found that flame propagation in the Hartmann tube was able to be classified into the three stages of ignition, vertical propagation and free diffusion. Particle size affected flame propagation modes and sensitive concentrations in dust explosions, which played a decisive role in the strength of wood dust explosions together with mass concentration. The finding provides theoretical basis and practical reference for dust explosion prevention. 2. Experiments 2.1 Experimental apparatus The experimental apparatus for studying wood dust explosions consisted of a Hartmann tube, a high-speed camera, a thermal infrared imaging device and a pressure test device as shown in Fig. 1. The Hartmann tube is designed based on the British European Standard (BS EN 13821-2002) including a quartz tube, an ignition system and a powder spraying system. It is in a semi-closed transparent cylindrical shape with an open upper end (V=1.2 L, H=300 mm, Inner Diameter=68 mm) to simulate dust removal pipes, which can be used to observe flame structure. Two graphite ignition electrodes are 6 mm apart and located at about 100 mm away from the bottom of the pipe. A protective cover and a ventilation duct were installed outside of the Hartmann tube to ensure the safety of the experiments. Wood dusts were placed on the mushroom-shaped powder spraying device, and the powder spraying pressure was set 3
to be 120 kPa, which was provided by a small air pressure pump. Ignition delay time was 120 ms, referring to the period of time for the dust from ejected to ignited moment. Under such conditions, wood dust could be uniformly dispersed in the Hartmann tube to reduce the effect of initial turbulence on flame propagation (Zhang et al., 2016). The pressure sensor had three stress strain gauges with a diameter of 10 mm labeled as 1, 2 and 3 in Fig. 1, which were installed at 65 mm, 130 mm and 195 mm above the ignition electrodes to measure the pressure of shock wave on the pipe wall, respectively. The high-speed camera (OLYMPUSi-SPEED 3) was used to capture flame images, while the infrared thermal imaging device (FLIR A20) was used to measure flame temperature around the nozzle, which were simultaneously controlled by the computer with the ignition system. The frame rates of the high-speed camera and the thermal infrared imaging device were 1000 fps and 50 fps, respectively. In order to reduce experimental error, all experiments were performed at least three times independently and the data was shown as average values.
Fig. 1. Schematic diagram for experimental apparatus.
4
2.2 Preparation of particles Poplar (Populus tomentosa), one of the three fast-growing and high-yielding tree species in China, has been widely utilized in the wood processing industry. Poplar shavings were milled into dust powder in the factory by a plant shredder. The powder was sieved into three groups with different particle size distributions (0-50, 50-96 and 96-180 µm) to mimic the dust clouds generated in wood processing (Geng et al., 2012; Ding et al., 2012). The microscopic morphologies of wood dusts with different particle size distributions were shown in Fig. 2. Nearly all wood particles were in irregular shapes. Smaller particles were more preferably agglomerated, which might be affected by the surface area of wood particles. Indeed, it was confirmed in Table 1 that the smaller the particles, the larger the specific surface area of particles. Meanwhile, the significant differences in some other characteristic parameters like volume diameter D [4, 3], sauter diameter D [4, 3], etc. were also observed among the three groups of particles. Pyrolysis features for wood dust particles were collected from thermal gravity (TG) and differential scanning calorimetry (DSC) as shown in Fig. 3. Weight loss for wood pyrolysis was divided into three parts: a 3.41 % reduction from 0 to 82.9°C, a 64.27 % reduction from 82.9 to 370.8°C and a 13.52 % reduction from 370.8 to 800°C, which were mainly caused by water evaporation, sequential degradation of hemicellulose, cellulose and lignin, and residual lignin degradation, respectively, and only about 18.8 % of initial dusts was left. By integrating the DSC curve, the burning energy of dust powder was calculated to be 1157.07 J/g. The data for industrial and elemental analyses was shown in Table 2, and the chemical composition of wood dust particles was abbreviated as C47O45H6N.
5
Fig. 2. Microscopic morphologies of wood dusts with different particle size distributions. 6
Table 1. Characteristic parameters of wood dusts with different particle size distributions. Diameter range
0-50 µm
50-96 µm
96-180 µm
Specific surface area S (m ·g )
0.9280
0.5371
0.2159
Volume diameter D [4,3] (µm)
29.74
67.10
162.20
Sauter diameter D [3,2] (µm)
20.85
36.03
89.62
D (10) (µm)
9.861
14.40
70.45
D (50) (µm)
29.45
60.90
154.5
D (90) (µm)
49.87
127.9
268.8
2
-1
Fig. 3. Pyrolysis characteristics of wood dust particles.
Table 2 Proximate and ultimate analyses of wood dusts.
Sample Wood dusts
Proximate analyses (%)
Ultimate analyses (%)
MC
Aad
Vad
FCad
C
H
O
N
3.36
1.44
82.52
12.68
47.63
5.8
44.75
1.47
MC: moisture content; Aad: ash; Vad: volatile matters; FCad: fixed carbon
3. Results and discussion 7
3.1 Flame propagation mode After shot with the high-speed camera, flame propagation in the Hartmann tube was separated into three stages, which were ignition, vertical propagation and free diffusion as shown in Fig. 4. In the first stage, dust powder was ignited by the ignition electrodes, emitting a faint glow to slowly spread around. Then the flame propagated vertically, and high pressure was rapidly generated in the limited space of the pipe. After the flame was released from the pipe, flame propagation entered the stage of free diffusion. Hot air spread to the surrounding, settled, and restarted to go up when it was heated by the rising hot air below, which caused the formation of mushroom cloud fireballs on the top of the nozzle as shown in Fig. 5 (Miller et al., 2010; Dolan and Philip, 1977; Jin, 2017). Generally, there are two types of flame propagation modes during dust explosions as shown in Fig. 6. One is homogeneous mode similar to that in premixed gas explosions, mainly controlled by dynamics, and the other is heterogeneous mode, primarily manipulated by vaporization, pyrolysis and/or devolatilization (Dobashi and Senda, 2006). The two modes can not only coexist, but also convert to each other, depending on the volatility and particle size of dust clouds. The flame propagation images of wood dusts in the Hartmann tube were shown in Fig. 7. In the processing of flame propagation, there were many luminous yellow points, which were probably produced by soot particles as a result of the incomplete combustion of dust particles. Small luminous flames gradually grew up to combine with each other, forming a luminous zone in an irregular shape. During free diffusion, the two smaller particles produced a continuous flame with a smooth flame front, indicating that these particles were burned more completely. The flame generated by the largest particles displayed discrete structure with a broken and discontinuous front, which meant large particles reacted partially and only the surface was charred. Thus, flame propagations for the two smaller particles were mainly controlled by homogeneous combustion, while flame propagation for the largest particles was dominated by heterogeneous combustion. The modes of flame propagation can also be defined by Damkӧhler number (Gao et al., 2015). When Damköhler number is close to 1, homogeneous combustion dominates the flame 8
propagation process. On the contrary, heterogeneous combustion is in a leading position when Damköhler number is far away from 1. Damköhler number for 0-50 and 50-96 µm wood dust particles were calculated to be 0.98 and 1.6, respectively, and Damköhler number for 96-180 µm particles was 18.5, which confirmed the classification for flame propagation based on flame shapes.
(a) Ignition
(b) Vertical propagation
(c) Free diffusion
Fig. 4. Flame propagation images of wood dusts with different size distributions at a mass concentration of 500 g/m3.
Fig. 5. Formation principle of mushroom cloud fireballs
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Fig. 6. Flame propagation mechanisms in dust explosions. (a) Homogeneous propagation; (b) Heterogeneous propagation.
(a) 0–50 µm
(b) 50–96 µm
10
(c) 96–180 µm Fig. 7. Flame propagation images of wood dusts with different particle size distributions at a mass concentration of 500 g/m3.
3.2 Flame propagation velocity The distances from the flame front to the ignition point and flame velocities for three groups of wood particles were measured and the results were shown in Fig. 8. The flame front of 50-96 µm particles was up to 887 mm, while the flame heights of 0-50 and 96-180 µm wood particles were less than 800 mm. Flame propagation velocity was calculated by the ratio of propagation distance and propagation time of flame front, obtained from digital images. Flame propagations for all groups of the wood particles were accelerated obviously when the flames reached up to 50 mm or so. As the flames came out of the pipe, flame propagation velocities reached to the maximum, which then dropped sharply and tended to be stable till the flame disappeared. Actually, flame propagation velocity was fluctuated as reported in previous researches (Proust & Veyssiere, 1988; Goroshin, Bidabadi, & Lee, 1996), which might be caused by unstable combustion of wood dust and convective heat transfer during combustion. In addition, burning time for the dust was extended with the increase of particle size, for large particles subjected to surface burning needed to undergo further combustion. When mass concentration was further increased, flame propagation velocities were also detected for the different groups of wood dusts, and the results were shown in Fig. 9. Flame propagation velocities were all increased at first and then decreased with the augmentation of mass concentration. As soon as flame propagation velocity reached up to the maximum level, the most violent explosive reaction took place. The mass concentration of this moment was 11
called sensitive concentration, which was the most sensitive point of mass concentration in dust explosions. If mass concentration was continued to increase, the excessive dust powder might be agglomerated, some of which was even probably adhered to the inner wall of the pipe and the ignition electrodes due to electrostatic attraction. Like this, wood particles were not dispersed evenly, which caused incomplete combustion and the decrease of mass concentration. Then the volume of combustible gas generated by pyrolysis was decreased, and flame propagation speed was reduced accordingly. The maximum flame propagation velocities for 0-50, 50-96 and 96-180 µm of dust particles were 7.31 m/s, 10.45 m/s and 6.67 m/s, and the corresponding sensitive concentrations were 500 g/m3, 750 g/m3 and 1000 g/m3, respectively. Small particles tended to be stable combustion and large particles were mostly burned on surface, which could attenuate the severity of explosions. Thus, the most intense dust explosion occurred when the flame of the middle size particles reached to the highest flame propagation velocity. In addition, sensitive concentration was increased with the increase of particle size, indicating that mass concentration was also associated with dust explosion intensity.
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Fig. 8. Flame front locations and flame propagation velocities of wood dusts with different particle size distributions at a mass concentration of 500 g/m3.
Fig. 9. The relation of flame propagation velocities and mass concentrations of wood dusts with different particle size distributions. 13
3.3 Flame temperature After the flames were ejected from the nozzle, flame temperatures for wood dusts with different size distributions were measured as shown in Fig. 10. The dusts insufficiently burned in the semi-closed tube could be thoroughly mixed with oxygen to burn fully when the flame spread out of the nozzle. Consequently, all flame temperatures rose first, and then dropped till burnout. When mass concentration was changed, flame temperatures for different groups of wood dusts were also measured as shown in Fig. 11. The flames immediately reached to the maximum temperatures after they were ejected from the nozzle. As mass concentration was kept going up, the maximum flame temperatures of two smaller particles started to decrease slightly, which might be caused by the conversion from homogeneous to heterogeneous combustion. In contrast, the maximum level of the largest particles was hardly changed with further increase of mass concentration since the excessive amounts of wood dusts could be spread around to maintain sensitive concentration. Notably, flame temperature for 50-96 µm wood particles reached to the highest value of 1373 ℃ at a mass concentration of 750 g/m3, which indicated the occurrence of the worst explosion.
(a) 0-50 µm
14
(b) 50-96 µm
(c) 96-180 µm Fig. 10. Flame temperatures of wood dusts with different particle size distributions after the flames were ejected from the nozzle.
Fig. 11. The relation of flame temperatures and mass concentrations of wood dusts with different particle size distributions.
3.4 Explosion pressure Explosion pressures at different sites inside of the Hartmann tube were determined as shown in Fig. 12. When the positions rose, explosion pressure for each group of particles was increased and it was elevated the most dramatically in the case of 50-96 µm particles, up to 0.35 MPa at position 3. Subsequently, explosion pressures at position 3 for dust particles with 15
different mass concentrations were measured as shown in Fig. 13. Like flame propagation velocities, explosion pressures for three particles were all increased at first and then decreased with the augmenting of mass concentration. It was understandable that explosion pressure for 50-96 µm particles at 750 g/m3 mass concentration reached to the highest level of 0.41 MPa, which was still lower than that in the closed container (PilãO et al., 2006; APA Zhang et al., 2014; Richard et al., 2015). Flame propagation velocity and flame temperature reflected the severity of wood dust explosions, so did explosion pressure. Corresponding to 0-50, 50-96 and 96-180 µm of dust particles, the maximum explosion pressures were 0.23, 041 and 0.22 MPa, and sensitive concentrations were 500 g/m3, 750 g/m3 and 1000 g/m3, separately, which demonstrated that there was a relation between particle size and mass concentration to affect the strength of dust explosions.
Fig. 12. Explosion pressures at different sites for dust clouds with 500 g/m3 mass density
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Fig. 13. The relation of explosion pressures and mass concentrations of dust clouds with different particle size distributions .
4. Conclusion In this study, flame propagations in wood dust explosions were divided into three stages including ignition, vertical propagation, and free diffusion. Flame propagations for 0-50 and 50-96 µm particles were found to be dominated by homogeneous combustion, while flame propagation for 96-180 µm particles was mainly controlled by heterogeneous combustion. When mass concentration for 50-96 µm wood particles reached to 750 g/m3, the most severe dust explosion occurred, in which the maximum flame propagation velocity, flame temperature and explosion pressure were 10.45 m/s, 1373 °C and 0.41 MPa, separately. Sensitive concentrations in dust explosions were also influenced by particle size, which were 500 g/m3, 750 g/m3 and 1000 g/m3 corresponding to 0-50, 50-96 and 96-180 µm of dust particles, respectively. Thus, particle size is associated with mass concentration to determine the severity of wood cloud explosions. 5. Acknowledgments
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The authors gratefully acknowledge the financial support from the National Key R&D Program of China (No. 2016YFD0600703).
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Goroshin, S., Bidabadi, M., Lee, J.H., 1996. Quenching distance of laminar flame in aluminum dust clouds. Combust. Flame 105, 147-160. Kim J., Kim, S., 2017.Simulation of Blast Wave Propagation and Mushroom Cloud formation by a Bomb Explosion. 55th AIAA Aerospace Sciences Meeting - Grapevine, Texas. Miller, F.P., Vandome, A.F., Mcbrewster, J., 2010. External Ballistics. Alphascript Publishing, 105-185. PilãO, R., Ramalho, E., Pinho, C., 2006. Overall characterization of cork dust explosion. J. Hazard Mater. 133(1-3), 183-195. Proust, C., 2006. A few fundamental aspects about ignition and flame propagation in dust clouds. J. Loss Prev. Process Ind. 19(2), 104-120. Proust, C., Veyssiere, B., 1988. Fundamental properties of flame propagating in starch dust-air mixtures. Combust. Sci. and Technol. 62, 149-172. Richard, K., Szabová, Z., Čekan P., 2015. Determination of the maximum explosion pressure and the maximum rate of pressure rise during explosion of wood dust clouds. Res. Pap. Fac. Mat. Sci. Technol. Slovak Univ. Technol. 23(36), 49-56. Yuzuriha, Y., Gao, W., Mogi, T., Dobashi, R., 2017. Effects of particle size distributions on flame propagation behavior through dust clouds of PMMA. J. Loss Prev. Process Ind. 49, 852-858. Zhang, Q., Ma, Q., Zhang, B., 2014. Approach determining maximum rate of pressure rise for dust explosion. J. Loss Prev. Process Ind. 29, 8-12. Zhang, X.Y., Yu, J.L., Sun, J.H., Gao, W., 2016. Effects of turbulent intensity on nano-PMMA flame propagation behaviors. J. Loss. Prev. Process Ind. 44, 119-124.
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Flame propagation characteristics were studied during the explosion of fast-growing poplar wood particle cloud in a semi-closed tube. Particle size of wood dust affected sensitive concentration of dust explosion. Particle size was associated with mass concentration to decide the severity of dust explosion. Dust explosion in 50-96 µm of wood particles was the most severe when mass concentration was 750 g/m3, and flame propagation velocity, flame propagation temperature and explosion pressure reached maximum values of 10.45 m/s, 1373 °C and 0.41 MPa, respectively.
Author Statement N.Z. conceived and designed the study. Z.P., Y.C. and W.L. performed the experiments. N.Z. and Z.P. analyzed the data, Z.P., N.Z. and C.X. wrote and reviewed the manuscript. All authors read and approved the manuscript and agree to be accountable for all aspects of the research in ensuring that the accuracy or integrity of any part of the work are appropriately investigated and resolved.