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Experimental study on the explosion-propagation law of coal dust with different moisture contents induced by methane explosion
PTEC-14971; No of Pages 5 Powder Technology xxx (2019) xxx
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Experimental study on the explosion-propagation law of coal dust with different moisture contents induced by methane explosion Yihui Niu a, Leilin Zhang a,b,⁎, Biming Shi a,b a b
School of Mining and Safety Engineering, Anhui University of Science & Technology, Huainan, Anhui 232001, China Key Laboratory of Safety and High-efficiency Coal Mining, Ministry of Education, Anhui University of Science & Technology, Huainan, Anhui 232001, China
a r t i c l e
i n f o
Article history: Received 20 May 2019 Received in revised form 25 September 2019 Accepted 25 November 2019 Available online xxxx Keywords: Moisture content Coal dust Mixed explosion Explosion resistance
a b s t r a c t This study aimed to explore the explosion-propagation law of coal dust with different moisture contents induced by methane explosion. An experimental system of explosion of mixed gas–coal and dust in a straight pipe was established in our laboratory. The evolution of overpressure and the flame-propagation law of deposited-coaldust explosion with different moisture contents were then studied. Experimental results indicate that when the moisture content is less than 15.12%, the deposited coal dust in the pipeline explodes under the induction of gas explosion. Meanwhile, the total amount of deposited coal dust in the pipeline remains constant; the overpressure peak value and flame-propagation velocity after coal-dust explosion initially increase and then decrease with increased coal-dust moisture content. When the moisture content of the deposited coal dust is 9.57%, the overpressure and flame-propagation velocity of the depositional coal-dust explosion reach the maximum values of 0.766 MPa and 468.553 m/s, respectively. However, when the moisture content exceeds 20.28%, coal dust cannot induce an explosion, and the deposited coal dust can resist a gas explosion. Overall, this research provides theoretical references for methane-induced coal-dust-explosion resistance in coal mines. © 2019 Published by Elsevier B.V.
1. Introduction The amount of gas emissions is increasing sharply with the continual increase in coal-mining depth, and the risk of gas explosions rises with the accumulation of underground gas. A huge amount of depositional coal-dust particles exist in roadways and often cause the re-explosion of sedimentary coal dust in the process of gas explosion, causing serious accidents [1–6]. Many scholars believe that the main reason for gas-induced coaldust re-explosion is the deflagration wave acting on the deposited coal dust, which raises the coal dust to form a coal-dust cloud. This cloud is then ignited or detonated by the flame wave generated by the explosion [7–9]. Researchers have also found that adding combustible gas to coal dust and reducing coal-dust particle size can reduce the lower limit of concentration of coal-dust explosion, thereby increasing the coal-dust explosion risk [10,11]. The presence of methane influences both the hybrid flame propagation velocity and the flame front temperature, both of which were also affected by the coal dust concentration [12]. Accordingly, in view of the propagation mechanism of gas–dust explosion in pipelines, research on reducing the damage caused by ⁎ Corresponding author. E-mail address: [email protected] (L. Zhang).
such explosions has progressed. Ajrash et al. established a large cylindrical detonation pipeline and analyzed the propagation characteristics of deflagration wave in gas and coal dust explosion, which laid a foundation for understanding the propagation law of gas and coal dust explosion [13,14]. Liang et al. [15] considered that the volume fraction of active molecules H, OH and O was the key factor affecting gas explosion in the process of chemical reaction of gas explosion and increasing water content could inhibit the formation of CO, NO and NO2 in the process of gas explosion. Yu and Xu [16,17] found that charged water mist can more effectively reduce the pressure peak and flame-propagation velocity of explosion than ordinary water mist. Wang [18] showed that adding the inert gas N2/CO2 reduces the maximum combustion pressure of methane explosion. Jiang et al. [19] concluded that adding ABC dry powder into a semi-enclosed pipeline can effectively restrain gas explosion. Song et al. [20] studied the effect of small-particle rock powder on flame suppression after methane/sedimentary coal-dust explosion. However, studies on the influence of moisture content of sedimentary coal dust on overpressure and flame propagation after a gasinduced coal-dust explosion are few. In the present study, a test system of mixed gas–coal and dust explosion in a horizontal straight pipe was established. The explosion-propagation law of coal dust with different moisture contents induced by methane-gas explosion was examined.
https://doi.org/10.1016/j.powtec.2019.11.089 0032-5910/© 2019 Published by Elsevier B.V.
Please cite this article as: Y. Niu, L. Zhang and B. Shi, Experimental study on the explosion-propagation law of coal dust with different moisture contents in..., Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.089
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Y. Niu et al. / Powder Technology xxx (2019) xxx Table 1 Distance of each pressure sensor from the explosion chamber end (m). Point Distance
Fig. 1. Field pipe experiment diagram.
The results can provide basis for preventing underground coal-dust reexplosion accidents. 2. Experimental The straight-pipe explosion test system used in this experiment is shown in Fig. 1. The system comprised a gas-distribution subsystem, a pipeline subsystem, an ignition subsystem, and a data-acquisition subsystem. The gas-distribution subsystem consisted of a gas cylinder, a vacuum pump, an air compressor, a circulation pump, and a digital vacuum-pressure gauge. The volume of the explosion chamber in the pipeline subsystem was 0.2 m3, the diameter of the straight pipe was 180 mm, and the length of the pipe was 11 m. The ignition end was closed and the other end was open. A plastic film was placed between the two flanges at the end of the detonation chamber for sealing. The ignition subsystem was composed of power supply, wire, electrode, and fuse. The ignition energy was 10 J. The data-acquisition subsystem included work machine, data-acquisition system, flame sensor, pressure sensor, and transmitter. The experimental sensor layout is shown in Fig. 2. A set of sensors (T1–T8) was arranged along the wall of the pipe at each measurement point, with a total of eight sets of sensors each comprising a pressure sensor (P1–P8) and a flame sensor (F1–F8). The piezoresistive pressure sensor (CYG1401) ranged from 0 MPa to 3 MPa with an accuracy grade of 0.5%. The photoelectric flame sensor (CKG100) had a response spectrum of 320–960 nm and a response time of approximately 3.5 ms, all of which were calibrated before the experiment. The location of each sensor from the end of the explosion chamber is shown in Tables 1 and 2. Coal dust with a length of 2 m and a total amount of 12 g was laid in the pipeline at the end of the explosion chamber (behind the sealing film). The water contents of the coal dust were 1.57%, 4.86%, 9.57%,
P1 0.1
P2 0.6
P3 1.6
P4 2.6
P5 3.6
P6 4.6
P7 7.1
P8 9.6
14.32%, 20.15%, and 28.32%, respectively. The coal-dust samples were obtained from the Baode Coal Mine, Shanxi, Province, China. The proximate analysis of the coal dust sample is given in Table 3. Raw coal was crushed with a pulverizer and ground into fine coal before drying in an electrothermal constant-temperature drying box at 60 °C for 12 h. After screening through a sieve shaker, the fine pulverized coal samples were sieved through a 150 μm screen and then retained on a 64 μm screen to produce samples with particle diameters ranging within 64–150 μm. After installing the piping and test system according to the experimental plan, the airtightness of each joint was checked. Gas (9.5% concentration) was mixed by partial-pressure method, and a vacuum pump was used to pump the explosion chamber into vacuum state. When the vacuum degree reached the requirements of the experiment, the vacuum pump was shut down and delivered the high-purity gas and air required. A circulating pump was then used to circulate the gas for not less than 20 min to ensure a well-balanced mixture of methane and air. After closing the valves and carrying out safety check, the highspeed data collection at the relevant parameters was started. An igniter was used for ignition, and flame and pressure sensors were used to collect and transmit data in real time. A data-acquisition device obtained information, processed them, and displayed graphics. After the experiment, the explosion-pipe system was ventilated with an air compressor for not less than 30 min, and the residual exhaust gas in the pipe was discharged. 3. Results and discussion 3.1. Maximum explosion-overpressure propagation laws of deflagration waves Fig. 3 shows the peak overpressure at each measurement point when coal dust with different moisture contents is induced by gas explosion. When the moisture content of the deposited coal dust is less than 15.12%, the deflagration wave generated by the explosion chamber makes contact and the coal dust is raised. Under the influence of the heat dissipation and air resistance on the pipeline wall, the peak overpressure shows a descending process at the front of the pipeline. The explosion overpressure then reaches its maximum quickly because of the coal-dust-cloud secondary explosion. The formation of new pressure waves results in a sudden increase in overpressure. When water content ranges within 1.57%–4.86%, the longer time required for coal dust explosion is due to insufficient oxygen in the pipeline and the slow decomposition of coal dust to produce enough combustible gases. Therefore, the peak overpressure reaches the peak at measurement point P5. When the moisture content is 9.57%–15.12%, the depositional coal dust explodes in the vicinity of measurement point P4, and the overpressure increases rapidly. The explosion wave is then affected by the air resistance and
Fig. 2. Transducers layout.
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Table 2 Distance of each flame sensor from the explosion chamber end (m). Point Distance
F1 0.15
F2 0.65
F3 1.65
F4 2.65
F5 3.65
F6 4.65
F7 7.15
F8 9.65
friction force of the pipe wall, and the overpressure decreases gradually. When the moisture content is more than 20.28%, the water content is larger, coal dust cannot be raised, and secondary explosion does not occur. Consequently, the peak value of the explosive-wave overpressure decreases. 3.2. Fitting analysis of deflagration-wave overpressure The propagation characteristics of explosion-pressure waves generated by gas in coal dust deposited with different moisture contents are shown in Fig. 4. With increased water content, the peak overpressure of the pressure wave initially increases and then decreases. When the moisture content is 9.57% coal dust, the overpressure peak produced by the secondary explosion of sedimentary coal dust is the largest, i.e., 0.766 MPa. Using the least-square method to fit the data, the quadratic-function relation between the peak overpressure and the function water rate is obtained: y = −6.803x2 + 1.515x + 0.612. According to the quadratic-function formula, under the action of gas explosion, depositional coal dust must be raised to form the coal-dust cloud. This cloud reaches the explosion condition and then produces the secondary explosion, thereby increasing the explosion power. When the water content is low, all the coal dust deposited in the pipeline is raised after the gas explosion, forming a coal-dust cloud. However, due to the oxygen content in the pipeline is not enough, coal dust cannot participate in the explosion completely. Coal-dust clouds also hinder the explosion wave and thus reduce the peak overpressure. When the water content is higher, part of the coal dust rises, the oxygen in the pipeline supplies the coal-dust cloud only to the second explosion; oxygen and coal dust react more completely, and the deflagration wave generated by the explosion increases. When the moisture content is more than 9.57%, as the water content increases, a layer of fog is formed on the surface of the particles，which increase the effective particle size of the coal dust and reduce the dispersion of the coal dust, so that some of the deposited coal dust cannot be lifted, causing the power of explosion is reduced. When the moisture content reaches 20.28%, the agglomeration of coal dust will be caused by the increase of moisture content in coal dust, the deposited coal dust cannot be raised and secondary explosion cannot occur. At the same time, the coal dust becomes an obstacle in the pipeline, which affects the propagation of the deflagration wave produced by the gas explosion, thereby reducing the peak overpressure. Under the coal mine, we can use this principle to sprinkle a large amount of water. When the quantity of water is appropriate, the coal dust cannot be raised, which hinders the propagation of gas explosion.
Fig. 3. Maximum explosion overpressure of coal dusts with different moisture contents at measuring points.
the pressure and light signals obviously increase, but the peak duration relatively shortens and the deflagration wave decays faster. The pressure and light signals between 0.17 and 0.18 s rise and then a second peak point appears, which proves that the coal dust is ignited by the explosion wave to produce a second explosion. The coal-dust cloud is formed from the static state to the rise of coal dust, and reaching the explosion concentration takes a long time, which causes the pressure to show a second peak value and receive the optical signal twice. 3.4. Maximum value of optical signal In the process of the experiment, the optical signal required by the experiment is measured by the flame sensor. This sensor uses a fastresponse photodiode as a sensitive element to exert a photoelectric effect immediately after exposure to visible light and convert the optical signal into an electrical signal output. Through data acquisition, the presence or absence of flame and the time of arrival of the flame can be accurately measured. Fig. 6 shows the maximum-value curve of the optical-signal intensity of coal dust with different moisture contents under the induction of methane-gas explosion. When the moisture content of coal dust is
3.3. Propagation law of deflagration wave at measurement point T4 Fig. 5 shows the variation in pressure and optical-signal intensity with time at measurement point T4 when the moisture content of coal dust is 9.57%. The first peak value of pressure and optical-signal intensity are clearly found between 0.14 and 0.15 s, i.e., 0.766 MPa and 1.799 V, respectively. Due to the explosion of depositional coal dust, Table 3 Proximate analysis of the coal dust (on the dry basis). Sample
Coal dust (Baode)
Proximate analysis (%) Mad
Aad
Vad
FCad
25.15
23.56
1.57
50.09
Mad: moisture content; Aad: ash; Vad: volatile matters; FCad: fixed carbon.
Fig. 4. Maximum overpressure at different moisture contents.
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Y. Niu et al. / Powder Technology xxx (2019) xxx
Fig. 5. Pressure, optical signal of the T4 point.
less than 15.12%, the maximum value of optical-signal intensity slightly increases at measurement point F4. With the coal dust participating in the secondary explosion, the flame energy moves all the way to the end of the pipeline. The maximum intensity of the optical signal at the F8 measurement point is about 0.24 V. When the water content is more than 20.28%, the optical signal cannot be detected at the F8 point, indicating that the flame can only propagate a short distance when the coal dust does not participate in the secondary explosion. 3.5. Flame Propagation speed In this experiment, the flame-propagation velocity is calculated by Eq. (1): v¼
xn t nþ1 −t n
ð1Þ
where v is the flame-propagation velocity, xn is the distance from the n + 1-th flame sensor to the n-th flame sensor, tn+1 is the time for the n + 1-th flame front end to arrive at the flame sensor, and tn is the time for the n-th flame front end to arrive at the flame sensor [21]. After calculation, the distribution of flame propagation speed of coal dust with different moisture contents induced by methane-gas explosion is shown in Fig. 7. When the moisture content of coal dust is less
Fig. 6. Maximum optical signal value of coal dusts with different moisture contents at measuring points.
Fig. 7. Flame propagation speed of coal dusts with different moisture contents at measuring points.
than 15.12%, the deflagration wave rises and ignites the coal dust in the pipeline, resulting in a secondary explosion, which accelerates the speed of flame combustion and reaches its peak at 2.65 m. However, when the water content is low, all the deposited coal dust is raised. These coal dusts absorb a large amount of flame heat, whereas some does not reach the explosion ignition point. At the same time, the amount of oxygen in the pipeline is certain and cannot participate in the secondary explosion, so the flame speed decreases. With increased moisture content of coal dust, some coal-dust particles cannot be raised; at this time all coal-dust clouds explode, resulting in the most powerful, maximum flame velocity [22]. When the moisture content of coal dust is greater than 20.28%, the moisture content is too large, so the coal dust cannot form coal-dust cloud and no explosion ensues. Under the influence of the energy loss of the open pipe and the heat absorption of coal-dust evaporation, the flame speed continues to decrease. A larger water content results in a faster decrease in flame-propagation speed. Thus, coal dust plays an important role in suppressing flame propagation after a gas explosion.
Fig. 8. Flame sustaining time of coal dusts with different moisture contents at measuring points.
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3.6. Flame-sustaining time
References
The flame-sustaining time at a certain point can be measured by the time differences between the vanishing moment and initial rising time of the optical signal at this point [23]. According to this method, the flame duration of coal dust with different moisture contents induced by methane-gas explosion is shown in Fig. 8. Before the coal dust is ignited by the deflagration wave, it stretches the flame surface and causes the flame duration to increase gradually because coal dust hinders the deflagration wave. However, when the coal-dust explosion is induced by gas explosion, flame propagation accelerates under the pressure of overpressure and the flame duration suddenly shortens. The flame duration of coal dust with 9.57% moisture content is the shortest, i.e., 22.7 ms. Subsequently, under the influence of the rarefaction wave at the closed end of the pipeline, the flame in front of the maximum pressure moves forward faster, whereas the flame behind the maximum pressure moves forward slowly. Thus, the flame duration increases gradually [24].
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4. Conclusions (1) When coal dust with 9.57% moisture content is laid, the peak value of overpressure (0.766 MPa) is the largest when the explosion induced by methane-gas explosion. The maximum value of explosion flame-propagation speed is 468.553 m/s. (2) When the moisture content of coal dust is between 1.57% and 9.57%, the peak overpressure of the deflagration wave increases with increased coal-dust moisture content and decreases with increased water content when the coal-dust moisture content exceeds 9.57%. After more than 20.28%, the depositional coal dust does not participate in the secondary explosion and even plays an explosion-suppressing role. (3) When the moisture content of coal dust is between 1.57% and 15.12%, the maximum value of flame-propagation velocity increases with increased moisture content of coal dust. When the moisture content exceeds 20.28%, the moisture evaporation absorbs heat and the flame-propagation speed decreases. A larger moisture content of coal dust means a faster decrease in flamepropagation speed. Thus, coal dust with a large moisture content plays a role in suppressing flame propagation after a gas explosion. Acknowledgments This research was financially supported by National Key R&D Program of China (2018YFC0808101), National Natural Science Foundation of China (Nos.51474010, 51874008, 51504009), Anhui Provincial Natural Science Foundation (No.1608085QE114).
Please cite this article as: Y. Niu, L. Zhang and B. Shi, Experimental study on the explosion-propagation law of coal dust with different moisture contents in..., Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.089
Contents lists available at ScienceDirect
Powder Technology journal homepage: www.elsevier.com/locate/powtec
Experimental study on the explosion-propagation law of coal dust with different moisture contents induced by methane explosion Yihui Niu a, Leilin Zhang a,b,⁎, Biming Shi a,b a b
School of Mining and Safety Engineering, Anhui University of Science & Technology, Huainan, Anhui 232001, China Key Laboratory of Safety and High-efficiency Coal Mining, Ministry of Education, Anhui University of Science & Technology, Huainan, Anhui 232001, China
a r t i c l e
i n f o
Article history: Received 20 May 2019 Received in revised form 25 September 2019 Accepted 25 November 2019 Available online xxxx Keywords: Moisture content Coal dust Mixed explosion Explosion resistance
a b s t r a c t This study aimed to explore the explosion-propagation law of coal dust with different moisture contents induced by methane explosion. An experimental system of explosion of mixed gas–coal and dust in a straight pipe was established in our laboratory. The evolution of overpressure and the flame-propagation law of deposited-coaldust explosion with different moisture contents were then studied. Experimental results indicate that when the moisture content is less than 15.12%, the deposited coal dust in the pipeline explodes under the induction of gas explosion. Meanwhile, the total amount of deposited coal dust in the pipeline remains constant; the overpressure peak value and flame-propagation velocity after coal-dust explosion initially increase and then decrease with increased coal-dust moisture content. When the moisture content of the deposited coal dust is 9.57%, the overpressure and flame-propagation velocity of the depositional coal-dust explosion reach the maximum values of 0.766 MPa and 468.553 m/s, respectively. However, when the moisture content exceeds 20.28%, coal dust cannot induce an explosion, and the deposited coal dust can resist a gas explosion. Overall, this research provides theoretical references for methane-induced coal-dust-explosion resistance in coal mines. © 2019 Published by Elsevier B.V.
1. Introduction The amount of gas emissions is increasing sharply with the continual increase in coal-mining depth, and the risk of gas explosions rises with the accumulation of underground gas. A huge amount of depositional coal-dust particles exist in roadways and often cause the re-explosion of sedimentary coal dust in the process of gas explosion, causing serious accidents [1–6]. Many scholars believe that the main reason for gas-induced coaldust re-explosion is the deflagration wave acting on the deposited coal dust, which raises the coal dust to form a coal-dust cloud. This cloud is then ignited or detonated by the flame wave generated by the explosion [7–9]. Researchers have also found that adding combustible gas to coal dust and reducing coal-dust particle size can reduce the lower limit of concentration of coal-dust explosion, thereby increasing the coal-dust explosion risk [10,11]. The presence of methane influences both the hybrid flame propagation velocity and the flame front temperature, both of which were also affected by the coal dust concentration [12]. Accordingly, in view of the propagation mechanism of gas–dust explosion in pipelines, research on reducing the damage caused by ⁎ Corresponding author. E-mail address: [email protected] (L. Zhang).
such explosions has progressed. Ajrash et al. established a large cylindrical detonation pipeline and analyzed the propagation characteristics of deflagration wave in gas and coal dust explosion, which laid a foundation for understanding the propagation law of gas and coal dust explosion [13,14]. Liang et al. [15] considered that the volume fraction of active molecules H, OH and O was the key factor affecting gas explosion in the process of chemical reaction of gas explosion and increasing water content could inhibit the formation of CO, NO and NO2 in the process of gas explosion. Yu and Xu [16,17] found that charged water mist can more effectively reduce the pressure peak and flame-propagation velocity of explosion than ordinary water mist. Wang [18] showed that adding the inert gas N2/CO2 reduces the maximum combustion pressure of methane explosion. Jiang et al. [19] concluded that adding ABC dry powder into a semi-enclosed pipeline can effectively restrain gas explosion. Song et al. [20] studied the effect of small-particle rock powder on flame suppression after methane/sedimentary coal-dust explosion. However, studies on the influence of moisture content of sedimentary coal dust on overpressure and flame propagation after a gasinduced coal-dust explosion are few. In the present study, a test system of mixed gas–coal and dust explosion in a horizontal straight pipe was established. The explosion-propagation law of coal dust with different moisture contents induced by methane-gas explosion was examined.
https://doi.org/10.1016/j.powtec.2019.11.089 0032-5910/© 2019 Published by Elsevier B.V.
Please cite this article as: Y. Niu, L. Zhang and B. Shi, Experimental study on the explosion-propagation law of coal dust with different moisture contents in..., Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.089
2
Y. Niu et al. / Powder Technology xxx (2019) xxx Table 1 Distance of each pressure sensor from the explosion chamber end (m). Point Distance
Fig. 1. Field pipe experiment diagram.
The results can provide basis for preventing underground coal-dust reexplosion accidents. 2. Experimental The straight-pipe explosion test system used in this experiment is shown in Fig. 1. The system comprised a gas-distribution subsystem, a pipeline subsystem, an ignition subsystem, and a data-acquisition subsystem. The gas-distribution subsystem consisted of a gas cylinder, a vacuum pump, an air compressor, a circulation pump, and a digital vacuum-pressure gauge. The volume of the explosion chamber in the pipeline subsystem was 0.2 m3, the diameter of the straight pipe was 180 mm, and the length of the pipe was 11 m. The ignition end was closed and the other end was open. A plastic film was placed between the two flanges at the end of the detonation chamber for sealing. The ignition subsystem was composed of power supply, wire, electrode, and fuse. The ignition energy was 10 J. The data-acquisition subsystem included work machine, data-acquisition system, flame sensor, pressure sensor, and transmitter. The experimental sensor layout is shown in Fig. 2. A set of sensors (T1–T8) was arranged along the wall of the pipe at each measurement point, with a total of eight sets of sensors each comprising a pressure sensor (P1–P8) and a flame sensor (F1–F8). The piezoresistive pressure sensor (CYG1401) ranged from 0 MPa to 3 MPa with an accuracy grade of 0.5%. The photoelectric flame sensor (CKG100) had a response spectrum of 320–960 nm and a response time of approximately 3.5 ms, all of which were calibrated before the experiment. The location of each sensor from the end of the explosion chamber is shown in Tables 1 and 2. Coal dust with a length of 2 m and a total amount of 12 g was laid in the pipeline at the end of the explosion chamber (behind the sealing film). The water contents of the coal dust were 1.57%, 4.86%, 9.57%,
P1 0.1
P2 0.6
P3 1.6
P4 2.6
P5 3.6
P6 4.6
P7 7.1
P8 9.6
14.32%, 20.15%, and 28.32%, respectively. The coal-dust samples were obtained from the Baode Coal Mine, Shanxi, Province, China. The proximate analysis of the coal dust sample is given in Table 3. Raw coal was crushed with a pulverizer and ground into fine coal before drying in an electrothermal constant-temperature drying box at 60 °C for 12 h. After screening through a sieve shaker, the fine pulverized coal samples were sieved through a 150 μm screen and then retained on a 64 μm screen to produce samples with particle diameters ranging within 64–150 μm. After installing the piping and test system according to the experimental plan, the airtightness of each joint was checked. Gas (9.5% concentration) was mixed by partial-pressure method, and a vacuum pump was used to pump the explosion chamber into vacuum state. When the vacuum degree reached the requirements of the experiment, the vacuum pump was shut down and delivered the high-purity gas and air required. A circulating pump was then used to circulate the gas for not less than 20 min to ensure a well-balanced mixture of methane and air. After closing the valves and carrying out safety check, the highspeed data collection at the relevant parameters was started. An igniter was used for ignition, and flame and pressure sensors were used to collect and transmit data in real time. A data-acquisition device obtained information, processed them, and displayed graphics. After the experiment, the explosion-pipe system was ventilated with an air compressor for not less than 30 min, and the residual exhaust gas in the pipe was discharged. 3. Results and discussion 3.1. Maximum explosion-overpressure propagation laws of deflagration waves Fig. 3 shows the peak overpressure at each measurement point when coal dust with different moisture contents is induced by gas explosion. When the moisture content of the deposited coal dust is less than 15.12%, the deflagration wave generated by the explosion chamber makes contact and the coal dust is raised. Under the influence of the heat dissipation and air resistance on the pipeline wall, the peak overpressure shows a descending process at the front of the pipeline. The explosion overpressure then reaches its maximum quickly because of the coal-dust-cloud secondary explosion. The formation of new pressure waves results in a sudden increase in overpressure. When water content ranges within 1.57%–4.86%, the longer time required for coal dust explosion is due to insufficient oxygen in the pipeline and the slow decomposition of coal dust to produce enough combustible gases. Therefore, the peak overpressure reaches the peak at measurement point P5. When the moisture content is 9.57%–15.12%, the depositional coal dust explodes in the vicinity of measurement point P4, and the overpressure increases rapidly. The explosion wave is then affected by the air resistance and
Fig. 2. Transducers layout.
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Table 2 Distance of each flame sensor from the explosion chamber end (m). Point Distance
F1 0.15
F2 0.65
F3 1.65
F4 2.65
F5 3.65
F6 4.65
F7 7.15
F8 9.65
friction force of the pipe wall, and the overpressure decreases gradually. When the moisture content is more than 20.28%, the water content is larger, coal dust cannot be raised, and secondary explosion does not occur. Consequently, the peak value of the explosive-wave overpressure decreases. 3.2. Fitting analysis of deflagration-wave overpressure The propagation characteristics of explosion-pressure waves generated by gas in coal dust deposited with different moisture contents are shown in Fig. 4. With increased water content, the peak overpressure of the pressure wave initially increases and then decreases. When the moisture content is 9.57% coal dust, the overpressure peak produced by the secondary explosion of sedimentary coal dust is the largest, i.e., 0.766 MPa. Using the least-square method to fit the data, the quadratic-function relation between the peak overpressure and the function water rate is obtained: y = −6.803x2 + 1.515x + 0.612. According to the quadratic-function formula, under the action of gas explosion, depositional coal dust must be raised to form the coal-dust cloud. This cloud reaches the explosion condition and then produces the secondary explosion, thereby increasing the explosion power. When the water content is low, all the coal dust deposited in the pipeline is raised after the gas explosion, forming a coal-dust cloud. However, due to the oxygen content in the pipeline is not enough, coal dust cannot participate in the explosion completely. Coal-dust clouds also hinder the explosion wave and thus reduce the peak overpressure. When the water content is higher, part of the coal dust rises, the oxygen in the pipeline supplies the coal-dust cloud only to the second explosion; oxygen and coal dust react more completely, and the deflagration wave generated by the explosion increases. When the moisture content is more than 9.57%, as the water content increases, a layer of fog is formed on the surface of the particles，which increase the effective particle size of the coal dust and reduce the dispersion of the coal dust, so that some of the deposited coal dust cannot be lifted, causing the power of explosion is reduced. When the moisture content reaches 20.28%, the agglomeration of coal dust will be caused by the increase of moisture content in coal dust, the deposited coal dust cannot be raised and secondary explosion cannot occur. At the same time, the coal dust becomes an obstacle in the pipeline, which affects the propagation of the deflagration wave produced by the gas explosion, thereby reducing the peak overpressure. Under the coal mine, we can use this principle to sprinkle a large amount of water. When the quantity of water is appropriate, the coal dust cannot be raised, which hinders the propagation of gas explosion.
Fig. 3. Maximum explosion overpressure of coal dusts with different moisture contents at measuring points.
the pressure and light signals obviously increase, but the peak duration relatively shortens and the deflagration wave decays faster. The pressure and light signals between 0.17 and 0.18 s rise and then a second peak point appears, which proves that the coal dust is ignited by the explosion wave to produce a second explosion. The coal-dust cloud is formed from the static state to the rise of coal dust, and reaching the explosion concentration takes a long time, which causes the pressure to show a second peak value and receive the optical signal twice. 3.4. Maximum value of optical signal In the process of the experiment, the optical signal required by the experiment is measured by the flame sensor. This sensor uses a fastresponse photodiode as a sensitive element to exert a photoelectric effect immediately after exposure to visible light and convert the optical signal into an electrical signal output. Through data acquisition, the presence or absence of flame and the time of arrival of the flame can be accurately measured. Fig. 6 shows the maximum-value curve of the optical-signal intensity of coal dust with different moisture contents under the induction of methane-gas explosion. When the moisture content of coal dust is
3.3. Propagation law of deflagration wave at measurement point T4 Fig. 5 shows the variation in pressure and optical-signal intensity with time at measurement point T4 when the moisture content of coal dust is 9.57%. The first peak value of pressure and optical-signal intensity are clearly found between 0.14 and 0.15 s, i.e., 0.766 MPa and 1.799 V, respectively. Due to the explosion of depositional coal dust, Table 3 Proximate analysis of the coal dust (on the dry basis). Sample
Coal dust (Baode)
Proximate analysis (%) Mad
Aad
Vad
FCad
25.15
23.56
1.57
50.09
Mad: moisture content; Aad: ash; Vad: volatile matters; FCad: fixed carbon.
Fig. 4. Maximum overpressure at different moisture contents.
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Fig. 5. Pressure, optical signal of the T4 point.
less than 15.12%, the maximum value of optical-signal intensity slightly increases at measurement point F4. With the coal dust participating in the secondary explosion, the flame energy moves all the way to the end of the pipeline. The maximum intensity of the optical signal at the F8 measurement point is about 0.24 V. When the water content is more than 20.28%, the optical signal cannot be detected at the F8 point, indicating that the flame can only propagate a short distance when the coal dust does not participate in the secondary explosion. 3.5. Flame Propagation speed In this experiment, the flame-propagation velocity is calculated by Eq. (1): v¼
xn t nþ1 −t n
ð1Þ
where v is the flame-propagation velocity, xn is the distance from the n + 1-th flame sensor to the n-th flame sensor, tn+1 is the time for the n + 1-th flame front end to arrive at the flame sensor, and tn is the time for the n-th flame front end to arrive at the flame sensor [21]. After calculation, the distribution of flame propagation speed of coal dust with different moisture contents induced by methane-gas explosion is shown in Fig. 7. When the moisture content of coal dust is less
Fig. 6. Maximum optical signal value of coal dusts with different moisture contents at measuring points.
Fig. 7. Flame propagation speed of coal dusts with different moisture contents at measuring points.
than 15.12%, the deflagration wave rises and ignites the coal dust in the pipeline, resulting in a secondary explosion, which accelerates the speed of flame combustion and reaches its peak at 2.65 m. However, when the water content is low, all the deposited coal dust is raised. These coal dusts absorb a large amount of flame heat, whereas some does not reach the explosion ignition point. At the same time, the amount of oxygen in the pipeline is certain and cannot participate in the secondary explosion, so the flame speed decreases. With increased moisture content of coal dust, some coal-dust particles cannot be raised; at this time all coal-dust clouds explode, resulting in the most powerful, maximum flame velocity [22]. When the moisture content of coal dust is greater than 20.28%, the moisture content is too large, so the coal dust cannot form coal-dust cloud and no explosion ensues. Under the influence of the energy loss of the open pipe and the heat absorption of coal-dust evaporation, the flame speed continues to decrease. A larger water content results in a faster decrease in flame-propagation speed. Thus, coal dust plays an important role in suppressing flame propagation after a gas explosion.
Fig. 8. Flame sustaining time of coal dusts with different moisture contents at measuring points.
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3.6. Flame-sustaining time
References
The flame-sustaining time at a certain point can be measured by the time differences between the vanishing moment and initial rising time of the optical signal at this point [23]. According to this method, the flame duration of coal dust with different moisture contents induced by methane-gas explosion is shown in Fig. 8. Before the coal dust is ignited by the deflagration wave, it stretches the flame surface and causes the flame duration to increase gradually because coal dust hinders the deflagration wave. However, when the coal-dust explosion is induced by gas explosion, flame propagation accelerates under the pressure of overpressure and the flame duration suddenly shortens. The flame duration of coal dust with 9.57% moisture content is the shortest, i.e., 22.7 ms. Subsequently, under the influence of the rarefaction wave at the closed end of the pipeline, the flame in front of the maximum pressure moves forward faster, whereas the flame behind the maximum pressure moves forward slowly. Thus, the flame duration increases gradually [24].
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4. Conclusions (1) When coal dust with 9.57% moisture content is laid, the peak value of overpressure (0.766 MPa) is the largest when the explosion induced by methane-gas explosion. The maximum value of explosion flame-propagation speed is 468.553 m/s. (2) When the moisture content of coal dust is between 1.57% and 9.57%, the peak overpressure of the deflagration wave increases with increased coal-dust moisture content and decreases with increased water content when the coal-dust moisture content exceeds 9.57%. After more than 20.28%, the depositional coal dust does not participate in the secondary explosion and even plays an explosion-suppressing role. (3) When the moisture content of coal dust is between 1.57% and 15.12%, the maximum value of flame-propagation velocity increases with increased moisture content of coal dust. When the moisture content exceeds 20.28%, the moisture evaporation absorbs heat and the flame-propagation speed decreases. A larger moisture content of coal dust means a faster decrease in flamepropagation speed. Thus, coal dust with a large moisture content plays a role in suppressing flame propagation after a gas explosion. Acknowledgments This research was financially supported by National Key R&D Program of China (2018YFC0808101), National Natural Science Foundation of China (Nos.51474010, 51874008, 51504009), Anhui Provincial Natural Science Foundation (No.1608085QE114).
Please cite this article as: Y. Niu, L. Zhang and B. Shi, Experimental study on the explosion-propagation law of coal dust with different moisture contents in..., Powder Technol., https://doi.org/10.1016/j.powtec.2019.11.089