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air explosions in a large-scale tube
Fuel 89 (2010) 329–335
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Coal dust/air explosions in a large-scale tube Qingming Liu *, Chunhua Bai, Xiaodong Li, Li Jiang, Wenxi Dai State Key Lab of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, China
a r t i c l e
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Article history: Received 7 January 2009 Received in revised form 14 July 2009 Accepted 16 July 2009 Available online 6 August 2009 Keywords: Coal dust Dust explosions Overpressure Experimental tube Explosion suppression
a b s t r a c t Coal dust/air mixture explosions under weak ignition conditions have been studied in a horizontal experimental tube of diameter 199 mm and length 29.6 m. The experimental tube is closed at one end and open at the downstream end. An array of 40 equally spaced dust dispersion units was used to disperse coal dust particles into the experimental tube. The coal dust/air mixture was ignited by an electric spark. A constant-temperature hot-wire anemometer was used to measure the gas velocity during the dispersion process. Kistler piezoelectric pressure sensors were used to measure the propagation of the pressure wave during the explosion process. The maximum overpressure of the coal dust explosion under the weak ignition conditions in the tube was 70 kPa and the propagation velocity of the pressure wave along the tube was approximately 370 m/s. The minimum concentration for obtaining a coal dust explosion that propagated along the tube was 120 g/m3. The suppressing effects on the coal dust explosion of two different kinds of suppressing agents have been studied. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The hazards that coal dust presents in underground coal mining were not fully accepted until the turn of the last century, when mine-scale experiments in England, France, and the USA established that coal dust explosion was possible in the absence of methane [1]. Despite the efforts of state bodies and private companies to find means of preventing these accidents, coal dust explosion continues to represent a constant threat in the coal mining industry. A lack of methods for predicting the structures of real dust clouds and flame propagation has been a major obstacle to predicting the courses and consequences of dust explosions in practice. Full-scale coal dust explosion tests have been performed in USA [2] and mine-scale grain dust explosions have been conducted in Poland [3] to investigate the characteristics of dust explosions in the mining and other process industries. Full-scale mine tests are expensive and time-consuming. As a result, researchers have attempted to develop laboratory-scale tests that can reliably reproduce the results from full-scale tests, thereby saving labor and capital. Knowledge of the propagation characteristics of flames in turbulent dust/air mixtures is essential when applying comprehensive numerical models for dust explosion propagation. Understanding flame acceleration due to flame distortion and turbulence produced by the explosion itself is central for understanding both dust * Corresponding author. Address: No. 5 South Zhongguancun Street, Haidian District, Beijing 100081, China. Tel.: +86 10 68914261; fax: +86 10 68914287. E-mail address: [email protected] (Q. Liu). 0016-2361/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2009.07.010
and gas explosions in practice. Extensive experimental research programs have been conducted to resolve the basic flame acceleration mechanism in gas explosions in obstructed geometries [4–6]. In order to evaluate the explosion violence in ducts of different dust/air mixtures, Bartknecht [7] proposed use of the coefficient Kst as a parameter representing the reactivity of the mixture. Nevertheless, experiments performed with two different dust samples having the same Kst coefficient, under exactly the same experimental conditions, indicated that very different levels of explosion violence could be obtained [8]. From the viewpoint of flame propagation, an industrial installation may be represented by ‘‘vessels” (L/D < 3.5) and ‘‘pipes” (L/D > 5). Many experimental studies on dust and gas explosions have been performed in relation to uncontrolled explosion in mining by using a 1.2 L Hartman tube, a 20 L Siwek chamber [9,10], a 1.25 m3 explosion chamber [11], ducts [12–15], ducts connected to a vessel in which the explosion was initiated [16–18], etc. Coal dust/air explosion experiments by Bartknecht [12,13] were performed in two experimental tubes with different diameters and lengths. The dust cloud was generated along the whole experimental tube by injecting dust from a number of equally spaced external pressurized reservoirs. The coal dust/air mixture was ignited by a pocket of exploding methane/air mixture. The maximum flame speed of the explosion in the 2.5 m diameter and 130 m long tube at the dust concentrations of 250 g/m3 and 500 g/m3 were 500 and 700 m/s, respectively. Wolanski, Kauffman and coworkers [14,15] performed dust explosion research by using a vertical experimental tube. The dust cloud was formed by charging dust samples at the top of the vertical experimental tube at a mass rate giving
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the desired dust concentration during the gravity setting down the tube. The dust explosion was initiated by hydrogen/oxygen explosion. Grain dust/air explosions and coal dust/air explosions by Pineau and Ronchail [16,17] were conducted in a duct connected to a vessel in which the explosion was initiated. In their experiments, dust was layered initially at the bottom of the duct and it was dispersed and ignited by the powerful explosion. In the experiments by Gardner et al. [18], the dust/air mixtures were formed by blowing air and coal dust through the experimental tube connected to an ignition chamber from just the upstream of the ignition chamber. The dust explosion was initiated by a flame jet or chemical ignitor. In the present work, experiments on coal dust explosion and its suppression have been conducted in a large-scale horizontal tube with an inner diameter of 199 mm and a length of 29.6 m. To form dust clouds in the experimental tube, samples of coal dust and suppressing agent particles were dispersed into the tube by means of in-house-constructed dispersion systems. Combustion of the coal dust/air mixtures was initiated by an electric spark. The objectives of the research were to investigate the propagation characteristics of coal dust explosion under weak ignition condition and the means to suppress the coal dust explosion using the 199-mmdiameter and 29.6-m-long tube. The study aimed at to investigate the propagation and suppression mechanism of coal dust explosion. The influences of coal dust and suppressing agent dust cloud parameters (particle diameter, dust concentration) on the propagation characteristics of the explosion waves have been investigated. The effective means to prevent coal dust explosion are appreciated in the study.
experimental section is a 199 mm diameter tube with a length of 28 m. It is closed at the ignition end and open at the downstream end. And it is divided equally into four parts so that each part is 7 m long. The ignition system consists of an electric ignition rod and a capacitor discharge apparatus. The electric ignition rod which is mounted at the closed end of the tube is made of two stainless steel electrodes, one of which is a cylinder while the other is a tube. The two electrodes are coaxially mounted with a clearance of 2 mm and a ceramic tube between them is used as an isolator. The electric energy produced by the spark generator is 40 J. Each dust dispersion unit consists of a pressure chamber, a solenoid valve, a directional valve, a sample can, and a spherical nozzle. The pressure chamber is linked to an air pump and the pressure used to disperse the coal or suppressing agent particles is 800 kPa. The dispersion of the dust in the sample cans is controlled by solenoid valves, which are commanded by the control unit. To produce a uniformly dispersed dust cloud in the tube, 164 holes of diameter from 1.2 to 1.8 mm are drilled through each spherical nozzle. The dispersion of the coal/suppressing agent dust, the ignition of coal dust/air mixture, and the triggering of the data acquisition system are all controlled by the control unit. To study the propagation of the coal dust/air mixture explosion, 16 pressure gauges are arranged on the tube wall along the axial direction. The connecting section is 1.6 m long and has the same inner diameter as the experimental section. It consists of an optical visualization part and a wave structure test part. Pressure data are recorded by a data acquisition system with a sampling frequency of 1 MHz.
2. Experimental
The coal samples used for the study were Qitaihe soft mine, Shenhua soft mine, and Zhungeer soft mine, with volatile contents of 36%, 32%, and 26%, respectively. In the laboratory, the coal samples were ground and sieved to produce coal dusts with different size distributions. Coal dusts with two different size distributions were used in the experimental studies. One was a fine coal dust sample with particle diameters in the range 45–75 lm. The other was sieved through a 105 lm screen but then retained on a 75 lm screen in order to obtain a coarse sample. To eliminate moisture and prevent the loss of volatiles and surface oxidation, the coal dust was kept in airtight driers. 0.5–1% very fine (16 nm diameter) SiO2 (Acrosil) fluidizing agent was added to the samples to increase the dispersibility of the coal dust by decreasing its surface binding energy. Thus, uniformly dispersed coal dust clouds could be formed in the experimental tube, which could be ignited by an electric spark with an ignition energy of 40 J.
2.1. Experimental set-up The experimental set-up is shown schematically in Fig. 1. It consists of an experimental tube, an electric ignition system, a control unit, a data acquisition system, a venting system, a vacuum pump, an air pump, and a 10 m3 dumping tank, which is shared by three different experimental tubes. The experimental tube is composed of an experimental section, 40 sets of dust/liquid dispersion systems, and a connecting section. The dispersion systems are mounted horizontally on both sides of the experimental section, regularly spaced at intervals of 0.7 m in the axial direction of the tube. The electric ignition system consists of an electric ignition rod and an electric spark generator. The pressure measurement system comprises 16 Kistler pressure gauges mounted on the wall of the experimental tube. A plastic film is placed between the experimental tube and the dumping tank to facilitate the establishment of vacuum conditions in the experimental tube and to prevent the cloud of dispersed coal dust from escaping the experimental tube before passage of the explosion wave. The
2.2. Coal dust samples
2.3. Experimental procedure and conditions Before each test, coal dust or suppressing agent samples were weighed by means of an electric balance according to the required
Fig. 1. Schematic diagram of the experimental set-up. (1) experimental tube, (2) dispersion system, (3) ignition system, (4) data acquisition system, (5) pressure sensor, (6) control unit, (7) vacuum pump, (8) venting system, (9) air pump, (10) connecting section, (11) plastic film, and (12) dumping tank.
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3. Results and discussion The propagation characteristics of the coal dust/air mixture explosions were studied by using coal dust samples with different particle size distributions and different volatile contents. The influences of concentration, particle size distribution, and volatile content of the coal dust on the coal dust/air mixture explosions were investigated. To suppress the explosive combustion of the coal dust/air mixtures, two different kinds of suppressing agents were used, namely ABC fire extinguisher powder and SiO2 powder. The overpressures of the coal dust/air mixture explosions were detected by pressure sensors and recorded by a data acquisition system. 3.1. Pressure wave of coal dust explosion propagating along the tube From the pressure histories at different points along the tube, the trajectory and velocity of each compression wave could be obtained. At the same time, the maximum overpressure of the explosion process could easily be obtained. The overpressure inside the explosion tube resulting from the dispersion process of the coal dust was taken as a reference overpressure. This overpressure was the pressure growth from the pneumatic dispersion of the coal dust samples. In the experiments described herein, the reference overpressure was 44–46 kPa, as shown in Fig. 2. Pressure increase above the reference overpressure resulted from the explosion process inside the tube. The pressure histories at different points during an explosion process and the trajectory of the pressure wave of the coal dust explosion are shown in Fig. 2. The dash-dotted line represents the incident wave and the dotted line represents the reflection wave from the tube end. The volatile content of the coal sample was 32% and the concentration of the dust cloud was 243 g/ m3. The particle diameters of the sample were distributed in the range 45–70 lm. The variations of the velocity and overpressure of the coal dust/air mixture explosion wave with propagation distance are shown in Fig. 3. It can be seen that coal dust explosion and propagation were successfully accomplished in the 199 mm inner diameter tube. The propagation velocity of the coal dust
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average concentrations in the tube (the dust concentration was evaluated from the volume of the tube and the mass of the dust sample dispersed into it), and then each sample can was filled with the same weight. The experimental tube was then evacuated to an absolute pressure of 50 kPa and the pressure chamber was filled with pressurized air. Once the start command was given by the control unit, the sample was dispersed into the experimental tube by the pressurized air and a dust cloud was formed in the tube. After a given delay time, the dust cloud was ignited by means of the electric ignition rod and the flame propagated along the tube. Our experimental tests showed that the coal dust/air mixture could be ignited 270–370 ms after the beginning of the injection. In the experiments described herein, an ignition delay time of 350 ms was chosen. The air pressure in the chamber was 800 kPa and its release process required 260 ms. The equilibrium pressure in the tube after the injection of dust was 100 kPa. Therefore, the ambient pressure of the coal dust explosion experiments was 100 kPa. After ignition, a combustion wave of coal dust/air mixture accelerated and propagated from the ignition end to the other end of the tube. The pressure histories of the coal dust/air mixture explosions were recorded by pressure gauges linked to the data acquisition system. After each explosion experiment, the explosion products were drawn-up and released at a height of 40 m to decrease the influence of the dust release on air pollution, and then the experimental tube was filled with fresh air in preparation for the next experiment.
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explosion ranged from 380 to 420 m/s and the maximum overpressure was between 58 and 63 kPa. After ignition, the coal dust flame accelerated due to turbulent combustion and expansion of the products; this produced a pressure wave that propagated along the tube, as shown in Fig. 2. At points 5 and 7.35 m distant from the closed end, obvious negative pressure formed and the velocity of the pressure wave reached 345–375 m/s. The acceleration of the pressure wave was the main cause of negative pressure. The pressure wave then propagated at a speed ranging from 380 to 420 m/s. The overpressure of the coal dust explosion reached 58–63 kPa. From the shape of the pressure wave and the velocity data, we concluded that the reaction regime of the coal dust explosion was deflagration. The coal dust was burned under a turbulent combustion regime. Flow parameters such as gas or dust particle velocity and turbulence have a significant effect on the ignition and the turbulent flame velocity and thus influence the explosion propagation process. To investigate the flow velocity and turbulence during the dispersion process, a constant-temperature hot-wire anemometer was used to measure the velocity history at different points in the tube. The velocity, mean velocity, and root-mean-square (RMS) velocity at the ignition point during the dispersion process are shown in Fig. 4. From the results, it can be seen that the turbulent RMS velocity at the ignition point was 0.2 m/s when the coal dust was ignited. From the ignition study, we had established that the coal dust could be ignited by an electric spark of 40 J with an ignition delay ranging from 270 to 370 ms, and that the corresponding respective turbulent RMS velocity ranged from 5.0 to 0.1 m/s. Greenwald and Wheeler [19] studied coal dust explosions in a tube of diameter 2.3 m and length 230 m with 800 g of black powder as the ignition source. The particle size of the coal used in their studies was less than 74 lm and its volatile content was 33%. The measured flame speed of the coal dust explosion was up to 800 m/s. Bartknecht [12,13] studied coal dust explosions in a tube of diameter 0.4 m with ignition at the closed end by a pocket of methane/air mixture. The maximum flame speed of and overpressure of the explosion in the 0.4 m diameter and 40 m long tube with the mean particle diameter of 22 lm were 250 m/s and 100 kPa, respectively. The mine-scale coal dust explosion tests by
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the ignition point with coal/air mixtures burning in a 1 m3 vessel connected to an 40-m-long tube, while in the duct alone the flame did not propagate the entire length. The coal dust explosion tests by Gardner et al. [18] conducted in a 0.6 m diameter and 42 m long tube connected to a 20 m3 ignition chamber showed that detonation wave could propagate in coal (particle size: 87% < 71 lm, MV = 41.7%) dust/air mixtures with a velocity and overpressure of 2200 m/s and 8.1 MPa, respectively. The dust explosion experiments conducted in a vertical tube [14,15] showed that grain dust/air mixture may sustain detonation in small tube diameter (60–130 mm) with the initiation of hydrogen/oxygen explosion. The velocity and overpressure of the detonation wave propagated in oat/air mixture were 1540 m/s and 2.4 MPa, respectively. 3.2. Influence of dust cloud parameters on dust explosion
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Sapko et al. [2] were conducted in the experimental mine in PRL, USA. The dust was initially placed on the floor and roof-shelves and the dust explosion was initiated by methane/air explosion. The ignition was sufficiently powerful to start dust entrainment and flame propagation through the mine. The effective length of the dust cloud ranged from 76 to 152 m with a averaged cross-sectional area of 2.1 m 6 m. Their experiments showed that the maximum speed of flame propagation with coal dust concentration of 100 and 200 g/m3 were 440 and 740 m/s, respectively, and the corresponding overpressure were 390 and 600 kPa, respectively. The coal dust explosion experiments by Pineau and Ronchail [17] showed that coal dust detonation can be obtained at 20 m from
The influences of dust cloud parameters, such as particle size, dust concentration, and volatile content, on the propagation characteristics have been studied, as described in the following. 3.2.1. Influence of particle size on coal dust explosion The influence of particle size on the coal dust explosions was studied by using coal dust samples with particle diameters distributed in the ranges 45–70 and 90–105 lm, respectively. A coal sample from Qitaihe mine with 36% volatile content was ground and sieved for coal dust explosion tests. The dust concentration used in the experiments to study the influence of particle size was 480 g/m3. In these experiments, coal dust was only dispersed into two sections of the tube, so that the length of the coal dust cloud was 14 m. The variation of overpressure with propagation distance is shown in Fig. 5. It can be seen that particle size had an obvious influence on the overpressure in the coal dust explosions. The maximum overpressure of the coal dust explosion was 74 kPa with the particle size distribution of 45–70 lm, but this decreased to 66 kPa when the particle size was increased to 90–105 lm. Even though the mass concentration, volatile content, and other parameters were the same for the two different coal dust samples, the combustion and explosion processes occurred at different rates. This indicates that the explosion process was dominated by the rate of the devolatilization process and the surface combustion of the coal dust. Fine particles have a greater specific area and devolatilization rate, and the surface reaction rate is higher than in the case of a coarse coal dust cloud. So, the combustion and explosion process
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120 g/m3. The optimum concentration was 370 g/m3 for the fine coal dust and 480 g/m3 for the coarse coal dust. The concentration of coal dust particles clearly has a significant effect on the coal dust explosion. A coal dust explosion cannot propagate in the tube if the concentration is less than 120 g/m3. With increasing coal dust concentration, the explosion overpressure is seen to increase. At the optimum concentration, the overpressure reaches its peak value. On further increasing the dust concentration, the overpressure from the dust explosion gradually decreases. As the concentration was increased from 380 to 960 g/m3, the overpressure decreased from 78 to 64 kPa; no obvious upper concentration limit for explosion was apparent. In this respect, the characteristics of the coal dust explosion clearly differ from those of a gas explosion. The variations in the explosion overpressure with propagation distance of coal dust/air mixtures at different dust concentrations are shown in Fig. 7. It can be seen that the overpressure from the coal dust explosion at a concentration of 487 g/m3 was greater than that at a concentration of 243 g/m3 but less than that at a concentration of 367 g/m3.
Fig. 5. Variation of overpressure with propagating distance for coal dust samples with different particle sizes.
of a fine coal dust cloud develops more rapidly than that of a coarse coal dust cloud. The extent of this influence of dust particle size on the coal dust explosion is affected by the coal dust concentration. With increasing dust concentration, the influence of particle size on the dust explosion process diminishes. When the concentration is greater than 960 g/m3, the explosion overpressure with the fine coal dust cloud is very close to that with the coarse coal dust cloud, as shown in Fig. 6. 3.2.2. Influence of dust concentration on coal dust explosion To study the influence of coal dust concentration on coal dust explosions, experiments were conducted using Qitaihe mine coal with a volatile content of 36%. The coal sample was ground and sieved. The particle sizes of the coal dust used for this study were 45–70 and 90–105 lm, respectively. Coal dust was dispersed into two sections of the tube so that the length of the dust cloud was 14 m. The variations in the maximum overpressures with coal dust concentration for coal dust explosions of samples with different particle size distribution are shown in Fig. 6. It can be seen that the minimum concentration giving a coal dust explosion was
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3.2.3. Influence of volatile content of the coal on the explosion process To investigate the effect of the volatile content of the coal on the coal dust explosion process, soft coal samples from Qitaihe mine, Shenhua mine, and Zhungeer mine with volatile contents of 36%, 32%, and 26%, respectively, were used. The particle size distribution of the coal samples used for this study was 45–75 lm and the concentrations used for the explosion tests ranged from 120 to 960 g/m3. The variations in overpressure and propagation velocity of the respective coal dust explosions with propagation distance for the samples with volatile contents of 36%, 32%, and 26% at the same concentration of 243 g/m3 are shown in Fig. 8. It can be seen from the figure that with the increase of coal volatile content from 26% to 36%, the overpressures of the coal dust explosions increased from 50 to 72 kPa and the velocity increased from 360 to 410 m/s. The variations in the maximum overpressure during the explosion process with dust concentration for the different kinds of coal with different volatile contents are shown in Fig. 9. From the figure, it can be seen that the coal dust explosion with a higher volatile content was more violent than that with a lower volatile content. Moreover, the influence of the volatile content of the coal on the dust explosion is affected by the concentration of the coal dust. With increasing volatile content, the increase in explosion overpressure of oxidant-rich coal dust/air mixtures is greater than that of fuel-rich coal dust/air mixtures.
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diameter of 16 nm, were used. The suppressing mechanisms of these two suppressing agents are different. At the present time, limestone dust instead of ABC powder or SiO2 powder was used as a coal dust inhibitor in coal mine. From the study by Amyotte [20] we know that ABC powder can inert coal dust explosion more efficiently than limestone and it is explained by the inability of limestone to decompose in the rapidly advanced flame front [21]. So, in this paper, ABC fire extinguisher powder was chosen as a chemical suppressing agent, while SiO2 was chosen as a physical suppressing agent. The decomposition reactions of ABC powder are endothermic processes and thus decrease the temperature of the dust cloud. The decomposition products can interrupt the chain reactions of the explosion process. In contrast to the coal dust explosions inerting study by Dastidar and Amyotte [22,23], the suppressing agent and coal dust were injected and dispersed into the experimental tube separately from different nozzles and mixed in the tube. The time delay between the dispersions of dust and the suppressing agent was adjustable. For this coal dust explosion suppression study, fine coal dust with a volatile content of 36% and a particle diameter of 45–70 lm was dispersed into all four sections of the tube from one side so that the length of the dust cloud was 28 m. The suppressing agent was injected and dispersed into the third section of the tube from the other side. In this way, the suppressing agent and the coal dust were kept separate prior to injection into the tube and were then mixed within the tube. The length of the region in which coal dust was mixed with suppressing agent was 7 m, in the section of the tube 14–21 m away from the ignition point. The concentration of the coal dust was 243 g/m3, and its volatile content was 36%. The concentration of the suppressing agent was 360 g/m3. The variations in explosion overpressure and velocity with propagation distance under different suppressing conditions are shown in Fig. 10. From the curves, it can be seen that ABC fire extinguisher powder was more effective in suppressing the explosion than SiO2. In the two situations, the starting point of the explosion suppression was 14 m away from the ignition point and the pressure wave propagated at a speed of approximately 400 m/s. Even though the combustion was depressed, the pressure wave propagated along the tube by inertia. To minimize the hazard from the overpressure, an explosion vent was required to release the pressure load.
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4. Conclusions
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Coal dust samples from different coal mines have been found to be explosible in a tube with an inner diameter of 199 mm. The tube is closed at the ignition end and open at the downstream end. The dust/air mixture was ignited by an electric spark of 40 J. Pressure gauges have been used to measure the pressure histories at different points along the tube during the explosion process. The velocity of the pressure wave that propagated along the tube ranged between 370 and 420 m/s and the maximum overpressure of the coal dust explosion was between 50 and 72 kPa. Explosions of coal dust samples with different particle sizes, concentrations, and volatile contents have been studied. The combustion and explosion of fine particle coal dust (diameter 45– 70 lm) was faster than that of coarse particle coal dust (diameter 70–105 lm). The rate of combustion of the dust cloud was mainly controlled the devolatilization process of the volatile matter of the coal. The minimum concentration of coal dust for obtaining an explosion that propagated along the tube was 120 g/m3, and the optimum explosion concentration ranged from 370 to 480 g/m3 for the different particle sizes. The influences of coal particle size and coal dust concentration on the coal dust explosion process are closely related. If the coal dust concentration is increased beyond 960 g/m3, the violence of the coal dust explosion is only slightly influenced by the particle size.
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3.3. Coal dust explosion suppression To suppress coal dust combustion and explosion in the experimental tube, two suppressing agents, ABC fire extinguisher powder with particle size less than 70 lm and fine SiO2 powder with a
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distance / m Fig. 10. Variation of overpressure and velocity with propagating distance with different suppressing conditions. (a) overpressure and (b) velocity.
ABC fire extinguisher powder dispersed into the coal dust clouds can suppress the combustion to some extent. However, if the suppressing section is far from the ignition point, venting is required to mitigate the damage caused by the pressure wave. Early detection and dispersion of the suppressing agent may efficiently suppress the coal dust explosion before the pressure wave becomes too strong and causes damage to the environment. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 10772032) and Foundation of State Key Lab of Explosion Science and Technology (Grant No. ZDKT08-02-6).
[1] Verakis HC, Nagy J. A brief history of dust explosions. In: Cashdollar KL, Hertzberg M, editors. Industrial dust explosion, STP 958. Philadelphia, PA: American Society For Test and Materials; 1987. p. 342–51. [2] Sapko MJ, Weiss ES, Cashdollar KL, Zlochower IA. Experimental mine and laboratory dust explosion research at NIOSH. J Loss Prevent Process Ind 2000;13:229–42. [3] Lebecki K, Cybulski K, Sliz J, Dyduch Z, Wolanski P. Large scale grain dust explosions-research in Poland. Shock Waves 1995;5:109–14. [4] Moen IO, Lee JHS, Hjertager BH, Fuhre K, Eckhoff RK. Pressure development due to turbulent flame propagation in large-scale methane/air explosions. Combust Flame 1982;47:31–52. [5] Hjertager BH, Fuhre K, Bjørkhaug M. Gas explosion experiments in 1:33 and 1:5 scale offshore separator and compressor modules using stoichiometric homogeneous fuel/air clouds. J Loss Prevent Process Ind 1988;1:197–205. [6] Bakke JR, van Wingerden K. Guidance for designing offshore modules evolving from gas explosion research. In: SPE annual technical conference and exhibition, SPE 24617. Richardson, Texas: Society of Petroleum Engineers Inc; 1992. p. 763–70. [7] Bartknecht W. Dust explosion: course, prevention, protection. Berlin: Springer Verlag; 1987. [8] Pineau JP, Konchail G. Propagation of coal dust explosion in pipes. In: Paper presented in the symposium on industrial explosion, Pittsburg, USA; 1986. [9] Amyotte PR, Khan FI, Basu A, Dastidar AG, Dumeah RK, Erving WL. Explosibility parameters for mixtures of pulverized fuel and ash. J Loss Prevent Process Ind 2006;19:142–8. [10] Amyotte PR, Basu A, Khan FI. Dust explosion hazard of pulverized fuel carryover. J Hazard Mater 2005;122:23–30. [11] Klemens R, Gieras M, Kaluzny M. Dynamics of dust explosions suppression by means of extinguishing powder in various industrial conditions. J Loss Prevent Process Ind 2007;20:664–74. [12] Bartknecht W. Brenngas-und Staubexplosionen, Forschungsbericht F45. Koblenz, W. Germany: Bundesinstitut fur Arbeitsschutz (Bifa); 1971. [13] Bartknecht W. Staubexplosionen. Heidelberg: Springer Verlag; 1987. [14] Wolanski P, Sacha W, Zelesinski M. Effect of dust concentration on detonation parameter parameters in grain dust–air mixtures. In: Proceedings of the 4th international colloquium on dust explosion, Porabka – Kozubnik, Poland; 1990. p. 355–70. [15] Kauffman CW, Wolanski P, Ural E, Nicholls JA. Detonation waves in confined dust clouds. In: Proceedings of the 19th international symposium on combustion, Israel; 1982. [16] Pineau JP, Ronchail G. Propagation of dust explosions in Ducts. In: Paper presented in the international symposium on control and prevention of dust explosions, Basle, Switzerland; 1982. [17] Pineau JP, Ronchail G. Propagation of coal dust explosions in pipes. In: Paper presented in the symposium on industrial explosions, Pittsburg, USA; 1986. [18] Gardner BR, Winter RJ, Moore MJ. Explosion development and deflagration to detonation in coal dust/air suspensions. In: Proceedings of the 21st symposium (international) on combustion. Pittsburg: The combustion institute; 1986. p. 335–43. [19] Greenwald HP, Wheeler RV. Coal dust explosions: the effect of release of pressure on their development. Safety in mines research board paper no. 14. London; 1925. p. 3–12. [20] Amyotte PR. Solid inertants and their use in dust explosion prevention and mitigation. J Loss Prevent Process Ind 2006;19:161–73. [21] Chatrathi K, Going J. Dust deflagration extinction. Process Saf Prog 2000;19:146–53. [22] Dastidar AG, Amyotte PR, Going J, Chatrathi K. Inerting of coal dust explosions in laboratory and intermediate scale chambers. Fuel 2001;80:1593–602. [23] Dastidar AG, Amyotte PR, Pegg MJ. Factors influencing the suppression of coal dust explosions. Fuel 1997;76:663–70.
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Coal dust/air explosions in a large-scale tube Qingming Liu *, Chunhua Bai, Xiaodong Li, Li Jiang, Wenxi Dai State Key Lab of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, China
a r t i c l e
i n f o
Article history: Received 7 January 2009 Received in revised form 14 July 2009 Accepted 16 July 2009 Available online 6 August 2009 Keywords: Coal dust Dust explosions Overpressure Experimental tube Explosion suppression
a b s t r a c t Coal dust/air mixture explosions under weak ignition conditions have been studied in a horizontal experimental tube of diameter 199 mm and length 29.6 m. The experimental tube is closed at one end and open at the downstream end. An array of 40 equally spaced dust dispersion units was used to disperse coal dust particles into the experimental tube. The coal dust/air mixture was ignited by an electric spark. A constant-temperature hot-wire anemometer was used to measure the gas velocity during the dispersion process. Kistler piezoelectric pressure sensors were used to measure the propagation of the pressure wave during the explosion process. The maximum overpressure of the coal dust explosion under the weak ignition conditions in the tube was 70 kPa and the propagation velocity of the pressure wave along the tube was approximately 370 m/s. The minimum concentration for obtaining a coal dust explosion that propagated along the tube was 120 g/m3. The suppressing effects on the coal dust explosion of two different kinds of suppressing agents have been studied. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction The hazards that coal dust presents in underground coal mining were not fully accepted until the turn of the last century, when mine-scale experiments in England, France, and the USA established that coal dust explosion was possible in the absence of methane [1]. Despite the efforts of state bodies and private companies to find means of preventing these accidents, coal dust explosion continues to represent a constant threat in the coal mining industry. A lack of methods for predicting the structures of real dust clouds and flame propagation has been a major obstacle to predicting the courses and consequences of dust explosions in practice. Full-scale coal dust explosion tests have been performed in USA [2] and mine-scale grain dust explosions have been conducted in Poland [3] to investigate the characteristics of dust explosions in the mining and other process industries. Full-scale mine tests are expensive and time-consuming. As a result, researchers have attempted to develop laboratory-scale tests that can reliably reproduce the results from full-scale tests, thereby saving labor and capital. Knowledge of the propagation characteristics of flames in turbulent dust/air mixtures is essential when applying comprehensive numerical models for dust explosion propagation. Understanding flame acceleration due to flame distortion and turbulence produced by the explosion itself is central for understanding both dust * Corresponding author. Address: No. 5 South Zhongguancun Street, Haidian District, Beijing 100081, China. Tel.: +86 10 68914261; fax: +86 10 68914287. E-mail address: [email protected] (Q. Liu). 0016-2361/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2009.07.010
and gas explosions in practice. Extensive experimental research programs have been conducted to resolve the basic flame acceleration mechanism in gas explosions in obstructed geometries [4–6]. In order to evaluate the explosion violence in ducts of different dust/air mixtures, Bartknecht [7] proposed use of the coefficient Kst as a parameter representing the reactivity of the mixture. Nevertheless, experiments performed with two different dust samples having the same Kst coefficient, under exactly the same experimental conditions, indicated that very different levels of explosion violence could be obtained [8]. From the viewpoint of flame propagation, an industrial installation may be represented by ‘‘vessels” (L/D < 3.5) and ‘‘pipes” (L/D > 5). Many experimental studies on dust and gas explosions have been performed in relation to uncontrolled explosion in mining by using a 1.2 L Hartman tube, a 20 L Siwek chamber [9,10], a 1.25 m3 explosion chamber [11], ducts [12–15], ducts connected to a vessel in which the explosion was initiated [16–18], etc. Coal dust/air explosion experiments by Bartknecht [12,13] were performed in two experimental tubes with different diameters and lengths. The dust cloud was generated along the whole experimental tube by injecting dust from a number of equally spaced external pressurized reservoirs. The coal dust/air mixture was ignited by a pocket of exploding methane/air mixture. The maximum flame speed of the explosion in the 2.5 m diameter and 130 m long tube at the dust concentrations of 250 g/m3 and 500 g/m3 were 500 and 700 m/s, respectively. Wolanski, Kauffman and coworkers [14,15] performed dust explosion research by using a vertical experimental tube. The dust cloud was formed by charging dust samples at the top of the vertical experimental tube at a mass rate giving
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the desired dust concentration during the gravity setting down the tube. The dust explosion was initiated by hydrogen/oxygen explosion. Grain dust/air explosions and coal dust/air explosions by Pineau and Ronchail [16,17] were conducted in a duct connected to a vessel in which the explosion was initiated. In their experiments, dust was layered initially at the bottom of the duct and it was dispersed and ignited by the powerful explosion. In the experiments by Gardner et al. [18], the dust/air mixtures were formed by blowing air and coal dust through the experimental tube connected to an ignition chamber from just the upstream of the ignition chamber. The dust explosion was initiated by a flame jet or chemical ignitor. In the present work, experiments on coal dust explosion and its suppression have been conducted in a large-scale horizontal tube with an inner diameter of 199 mm and a length of 29.6 m. To form dust clouds in the experimental tube, samples of coal dust and suppressing agent particles were dispersed into the tube by means of in-house-constructed dispersion systems. Combustion of the coal dust/air mixtures was initiated by an electric spark. The objectives of the research were to investigate the propagation characteristics of coal dust explosion under weak ignition condition and the means to suppress the coal dust explosion using the 199-mmdiameter and 29.6-m-long tube. The study aimed at to investigate the propagation and suppression mechanism of coal dust explosion. The influences of coal dust and suppressing agent dust cloud parameters (particle diameter, dust concentration) on the propagation characteristics of the explosion waves have been investigated. The effective means to prevent coal dust explosion are appreciated in the study.
experimental section is a 199 mm diameter tube with a length of 28 m. It is closed at the ignition end and open at the downstream end. And it is divided equally into four parts so that each part is 7 m long. The ignition system consists of an electric ignition rod and a capacitor discharge apparatus. The electric ignition rod which is mounted at the closed end of the tube is made of two stainless steel electrodes, one of which is a cylinder while the other is a tube. The two electrodes are coaxially mounted with a clearance of 2 mm and a ceramic tube between them is used as an isolator. The electric energy produced by the spark generator is 40 J. Each dust dispersion unit consists of a pressure chamber, a solenoid valve, a directional valve, a sample can, and a spherical nozzle. The pressure chamber is linked to an air pump and the pressure used to disperse the coal or suppressing agent particles is 800 kPa. The dispersion of the dust in the sample cans is controlled by solenoid valves, which are commanded by the control unit. To produce a uniformly dispersed dust cloud in the tube, 164 holes of diameter from 1.2 to 1.8 mm are drilled through each spherical nozzle. The dispersion of the coal/suppressing agent dust, the ignition of coal dust/air mixture, and the triggering of the data acquisition system are all controlled by the control unit. To study the propagation of the coal dust/air mixture explosion, 16 pressure gauges are arranged on the tube wall along the axial direction. The connecting section is 1.6 m long and has the same inner diameter as the experimental section. It consists of an optical visualization part and a wave structure test part. Pressure data are recorded by a data acquisition system with a sampling frequency of 1 MHz.
2. Experimental
The coal samples used for the study were Qitaihe soft mine, Shenhua soft mine, and Zhungeer soft mine, with volatile contents of 36%, 32%, and 26%, respectively. In the laboratory, the coal samples were ground and sieved to produce coal dusts with different size distributions. Coal dusts with two different size distributions were used in the experimental studies. One was a fine coal dust sample with particle diameters in the range 45–75 lm. The other was sieved through a 105 lm screen but then retained on a 75 lm screen in order to obtain a coarse sample. To eliminate moisture and prevent the loss of volatiles and surface oxidation, the coal dust was kept in airtight driers. 0.5–1% very fine (16 nm diameter) SiO2 (Acrosil) fluidizing agent was added to the samples to increase the dispersibility of the coal dust by decreasing its surface binding energy. Thus, uniformly dispersed coal dust clouds could be formed in the experimental tube, which could be ignited by an electric spark with an ignition energy of 40 J.
2.1. Experimental set-up The experimental set-up is shown schematically in Fig. 1. It consists of an experimental tube, an electric ignition system, a control unit, a data acquisition system, a venting system, a vacuum pump, an air pump, and a 10 m3 dumping tank, which is shared by three different experimental tubes. The experimental tube is composed of an experimental section, 40 sets of dust/liquid dispersion systems, and a connecting section. The dispersion systems are mounted horizontally on both sides of the experimental section, regularly spaced at intervals of 0.7 m in the axial direction of the tube. The electric ignition system consists of an electric ignition rod and an electric spark generator. The pressure measurement system comprises 16 Kistler pressure gauges mounted on the wall of the experimental tube. A plastic film is placed between the experimental tube and the dumping tank to facilitate the establishment of vacuum conditions in the experimental tube and to prevent the cloud of dispersed coal dust from escaping the experimental tube before passage of the explosion wave. The
2.2. Coal dust samples
2.3. Experimental procedure and conditions Before each test, coal dust or suppressing agent samples were weighed by means of an electric balance according to the required
Fig. 1. Schematic diagram of the experimental set-up. (1) experimental tube, (2) dispersion system, (3) ignition system, (4) data acquisition system, (5) pressure sensor, (6) control unit, (7) vacuum pump, (8) venting system, (9) air pump, (10) connecting section, (11) plastic film, and (12) dumping tank.
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3. Results and discussion The propagation characteristics of the coal dust/air mixture explosions were studied by using coal dust samples with different particle size distributions and different volatile contents. The influences of concentration, particle size distribution, and volatile content of the coal dust on the coal dust/air mixture explosions were investigated. To suppress the explosive combustion of the coal dust/air mixtures, two different kinds of suppressing agents were used, namely ABC fire extinguisher powder and SiO2 powder. The overpressures of the coal dust/air mixture explosions were detected by pressure sensors and recorded by a data acquisition system. 3.1. Pressure wave of coal dust explosion propagating along the tube From the pressure histories at different points along the tube, the trajectory and velocity of each compression wave could be obtained. At the same time, the maximum overpressure of the explosion process could easily be obtained. The overpressure inside the explosion tube resulting from the dispersion process of the coal dust was taken as a reference overpressure. This overpressure was the pressure growth from the pneumatic dispersion of the coal dust samples. In the experiments described herein, the reference overpressure was 44–46 kPa, as shown in Fig. 2. Pressure increase above the reference overpressure resulted from the explosion process inside the tube. The pressure histories at different points during an explosion process and the trajectory of the pressure wave of the coal dust explosion are shown in Fig. 2. The dash-dotted line represents the incident wave and the dotted line represents the reflection wave from the tube end. The volatile content of the coal sample was 32% and the concentration of the dust cloud was 243 g/ m3. The particle diameters of the sample were distributed in the range 45–70 lm. The variations of the velocity and overpressure of the coal dust/air mixture explosion wave with propagation distance are shown in Fig. 3. It can be seen that coal dust explosion and propagation were successfully accomplished in the 199 mm inner diameter tube. The propagation velocity of the coal dust
incident wave track reflection wave track
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average concentrations in the tube (the dust concentration was evaluated from the volume of the tube and the mass of the dust sample dispersed into it), and then each sample can was filled with the same weight. The experimental tube was then evacuated to an absolute pressure of 50 kPa and the pressure chamber was filled with pressurized air. Once the start command was given by the control unit, the sample was dispersed into the experimental tube by the pressurized air and a dust cloud was formed in the tube. After a given delay time, the dust cloud was ignited by means of the electric ignition rod and the flame propagated along the tube. Our experimental tests showed that the coal dust/air mixture could be ignited 270–370 ms after the beginning of the injection. In the experiments described herein, an ignition delay time of 350 ms was chosen. The air pressure in the chamber was 800 kPa and its release process required 260 ms. The equilibrium pressure in the tube after the injection of dust was 100 kPa. Therefore, the ambient pressure of the coal dust explosion experiments was 100 kPa. After ignition, a combustion wave of coal dust/air mixture accelerated and propagated from the ignition end to the other end of the tube. The pressure histories of the coal dust/air mixture explosions were recorded by pressure gauges linked to the data acquisition system. After each explosion experiment, the explosion products were drawn-up and released at a height of 40 m to decrease the influence of the dust release on air pollution, and then the experimental tube was filled with fresh air in preparation for the next experiment.
250
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explosion ranged from 380 to 420 m/s and the maximum overpressure was between 58 and 63 kPa. After ignition, the coal dust flame accelerated due to turbulent combustion and expansion of the products; this produced a pressure wave that propagated along the tube, as shown in Fig. 2. At points 5 and 7.35 m distant from the closed end, obvious negative pressure formed and the velocity of the pressure wave reached 345–375 m/s. The acceleration of the pressure wave was the main cause of negative pressure. The pressure wave then propagated at a speed ranging from 380 to 420 m/s. The overpressure of the coal dust explosion reached 58–63 kPa. From the shape of the pressure wave and the velocity data, we concluded that the reaction regime of the coal dust explosion was deflagration. The coal dust was burned under a turbulent combustion regime. Flow parameters such as gas or dust particle velocity and turbulence have a significant effect on the ignition and the turbulent flame velocity and thus influence the explosion propagation process. To investigate the flow velocity and turbulence during the dispersion process, a constant-temperature hot-wire anemometer was used to measure the velocity history at different points in the tube. The velocity, mean velocity, and root-mean-square (RMS) velocity at the ignition point during the dispersion process are shown in Fig. 4. From the results, it can be seen that the turbulent RMS velocity at the ignition point was 0.2 m/s when the coal dust was ignited. From the ignition study, we had established that the coal dust could be ignited by an electric spark of 40 J with an ignition delay ranging from 270 to 370 ms, and that the corresponding respective turbulent RMS velocity ranged from 5.0 to 0.1 m/s. Greenwald and Wheeler [19] studied coal dust explosions in a tube of diameter 2.3 m and length 230 m with 800 g of black powder as the ignition source. The particle size of the coal used in their studies was less than 74 lm and its volatile content was 33%. The measured flame speed of the coal dust explosion was up to 800 m/s. Bartknecht [12,13] studied coal dust explosions in a tube of diameter 0.4 m with ignition at the closed end by a pocket of methane/air mixture. The maximum flame speed of and overpressure of the explosion in the 0.4 m diameter and 40 m long tube with the mean particle diameter of 22 lm were 250 m/s and 100 kPa, respectively. The mine-scale coal dust explosion tests by
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the ignition point with coal/air mixtures burning in a 1 m3 vessel connected to an 40-m-long tube, while in the duct alone the flame did not propagate the entire length. The coal dust explosion tests by Gardner et al. [18] conducted in a 0.6 m diameter and 42 m long tube connected to a 20 m3 ignition chamber showed that detonation wave could propagate in coal (particle size: 87% < 71 lm, MV = 41.7%) dust/air mixtures with a velocity and overpressure of 2200 m/s and 8.1 MPa, respectively. The dust explosion experiments conducted in a vertical tube [14,15] showed that grain dust/air mixture may sustain detonation in small tube diameter (60–130 mm) with the initiation of hydrogen/oxygen explosion. The velocity and overpressure of the detonation wave propagated in oat/air mixture were 1540 m/s and 2.4 MPa, respectively. 3.2. Influence of dust cloud parameters on dust explosion
0 0.0
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Sapko et al. [2] were conducted in the experimental mine in PRL, USA. The dust was initially placed on the floor and roof-shelves and the dust explosion was initiated by methane/air explosion. The ignition was sufficiently powerful to start dust entrainment and flame propagation through the mine. The effective length of the dust cloud ranged from 76 to 152 m with a averaged cross-sectional area of 2.1 m 6 m. Their experiments showed that the maximum speed of flame propagation with coal dust concentration of 100 and 200 g/m3 were 440 and 740 m/s, respectively, and the corresponding overpressure were 390 and 600 kPa, respectively. The coal dust explosion experiments by Pineau and Ronchail [17] showed that coal dust detonation can be obtained at 20 m from
The influences of dust cloud parameters, such as particle size, dust concentration, and volatile content, on the propagation characteristics have been studied, as described in the following. 3.2.1. Influence of particle size on coal dust explosion The influence of particle size on the coal dust explosions was studied by using coal dust samples with particle diameters distributed in the ranges 45–70 and 90–105 lm, respectively. A coal sample from Qitaihe mine with 36% volatile content was ground and sieved for coal dust explosion tests. The dust concentration used in the experiments to study the influence of particle size was 480 g/m3. In these experiments, coal dust was only dispersed into two sections of the tube, so that the length of the coal dust cloud was 14 m. The variation of overpressure with propagation distance is shown in Fig. 5. It can be seen that particle size had an obvious influence on the overpressure in the coal dust explosions. The maximum overpressure of the coal dust explosion was 74 kPa with the particle size distribution of 45–70 lm, but this decreased to 66 kPa when the particle size was increased to 90–105 lm. Even though the mass concentration, volatile content, and other parameters were the same for the two different coal dust samples, the combustion and explosion processes occurred at different rates. This indicates that the explosion process was dominated by the rate of the devolatilization process and the surface combustion of the coal dust. Fine particles have a greater specific area and devolatilization rate, and the surface reaction rate is higher than in the case of a coarse coal dust cloud. So, the combustion and explosion process
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120 g/m3. The optimum concentration was 370 g/m3 for the fine coal dust and 480 g/m3 for the coarse coal dust. The concentration of coal dust particles clearly has a significant effect on the coal dust explosion. A coal dust explosion cannot propagate in the tube if the concentration is less than 120 g/m3. With increasing coal dust concentration, the explosion overpressure is seen to increase. At the optimum concentration, the overpressure reaches its peak value. On further increasing the dust concentration, the overpressure from the dust explosion gradually decreases. As the concentration was increased from 380 to 960 g/m3, the overpressure decreased from 78 to 64 kPa; no obvious upper concentration limit for explosion was apparent. In this respect, the characteristics of the coal dust explosion clearly differ from those of a gas explosion. The variations in the explosion overpressure with propagation distance of coal dust/air mixtures at different dust concentrations are shown in Fig. 7. It can be seen that the overpressure from the coal dust explosion at a concentration of 487 g/m3 was greater than that at a concentration of 243 g/m3 but less than that at a concentration of 367 g/m3.
Fig. 5. Variation of overpressure with propagating distance for coal dust samples with different particle sizes.
of a fine coal dust cloud develops more rapidly than that of a coarse coal dust cloud. The extent of this influence of dust particle size on the coal dust explosion is affected by the coal dust concentration. With increasing dust concentration, the influence of particle size on the dust explosion process diminishes. When the concentration is greater than 960 g/m3, the explosion overpressure with the fine coal dust cloud is very close to that with the coarse coal dust cloud, as shown in Fig. 6. 3.2.2. Influence of dust concentration on coal dust explosion To study the influence of coal dust concentration on coal dust explosions, experiments were conducted using Qitaihe mine coal with a volatile content of 36%. The coal sample was ground and sieved. The particle sizes of the coal dust used for this study were 45–70 and 90–105 lm, respectively. Coal dust was dispersed into two sections of the tube so that the length of the dust cloud was 14 m. The variations in the maximum overpressures with coal dust concentration for coal dust explosions of samples with different particle size distribution are shown in Fig. 6. It can be seen that the minimum concentration giving a coal dust explosion was
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3.2.3. Influence of volatile content of the coal on the explosion process To investigate the effect of the volatile content of the coal on the coal dust explosion process, soft coal samples from Qitaihe mine, Shenhua mine, and Zhungeer mine with volatile contents of 36%, 32%, and 26%, respectively, were used. The particle size distribution of the coal samples used for this study was 45–75 lm and the concentrations used for the explosion tests ranged from 120 to 960 g/m3. The variations in overpressure and propagation velocity of the respective coal dust explosions with propagation distance for the samples with volatile contents of 36%, 32%, and 26% at the same concentration of 243 g/m3 are shown in Fig. 8. It can be seen from the figure that with the increase of coal volatile content from 26% to 36%, the overpressures of the coal dust explosions increased from 50 to 72 kPa and the velocity increased from 360 to 410 m/s. The variations in the maximum overpressure during the explosion process with dust concentration for the different kinds of coal with different volatile contents are shown in Fig. 9. From the figure, it can be seen that the coal dust explosion with a higher volatile content was more violent than that with a lower volatile content. Moreover, the influence of the volatile content of the coal on the dust explosion is affected by the concentration of the coal dust. With increasing volatile content, the increase in explosion overpressure of oxidant-rich coal dust/air mixtures is greater than that of fuel-rich coal dust/air mixtures.
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diameter of 16 nm, were used. The suppressing mechanisms of these two suppressing agents are different. At the present time, limestone dust instead of ABC powder or SiO2 powder was used as a coal dust inhibitor in coal mine. From the study by Amyotte [20] we know that ABC powder can inert coal dust explosion more efficiently than limestone and it is explained by the inability of limestone to decompose in the rapidly advanced flame front [21]. So, in this paper, ABC fire extinguisher powder was chosen as a chemical suppressing agent, while SiO2 was chosen as a physical suppressing agent. The decomposition reactions of ABC powder are endothermic processes and thus decrease the temperature of the dust cloud. The decomposition products can interrupt the chain reactions of the explosion process. In contrast to the coal dust explosions inerting study by Dastidar and Amyotte [22,23], the suppressing agent and coal dust were injected and dispersed into the experimental tube separately from different nozzles and mixed in the tube. The time delay between the dispersions of dust and the suppressing agent was adjustable. For this coal dust explosion suppression study, fine coal dust with a volatile content of 36% and a particle diameter of 45–70 lm was dispersed into all four sections of the tube from one side so that the length of the dust cloud was 28 m. The suppressing agent was injected and dispersed into the third section of the tube from the other side. In this way, the suppressing agent and the coal dust were kept separate prior to injection into the tube and were then mixed within the tube. The length of the region in which coal dust was mixed with suppressing agent was 7 m, in the section of the tube 14–21 m away from the ignition point. The concentration of the coal dust was 243 g/m3, and its volatile content was 36%. The concentration of the suppressing agent was 360 g/m3. The variations in explosion overpressure and velocity with propagation distance under different suppressing conditions are shown in Fig. 10. From the curves, it can be seen that ABC fire extinguisher powder was more effective in suppressing the explosion than SiO2. In the two situations, the starting point of the explosion suppression was 14 m away from the ignition point and the pressure wave propagated at a speed of approximately 400 m/s. Even though the combustion was depressed, the pressure wave propagated along the tube by inertia. To minimize the hazard from the overpressure, an explosion vent was required to release the pressure load.
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4. Conclusions
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Coal dust samples from different coal mines have been found to be explosible in a tube with an inner diameter of 199 mm. The tube is closed at the ignition end and open at the downstream end. The dust/air mixture was ignited by an electric spark of 40 J. Pressure gauges have been used to measure the pressure histories at different points along the tube during the explosion process. The velocity of the pressure wave that propagated along the tube ranged between 370 and 420 m/s and the maximum overpressure of the coal dust explosion was between 50 and 72 kPa. Explosions of coal dust samples with different particle sizes, concentrations, and volatile contents have been studied. The combustion and explosion of fine particle coal dust (diameter 45– 70 lm) was faster than that of coarse particle coal dust (diameter 70–105 lm). The rate of combustion of the dust cloud was mainly controlled the devolatilization process of the volatile matter of the coal. The minimum concentration of coal dust for obtaining an explosion that propagated along the tube was 120 g/m3, and the optimum explosion concentration ranged from 370 to 480 g/m3 for the different particle sizes. The influences of coal particle size and coal dust concentration on the coal dust explosion process are closely related. If the coal dust concentration is increased beyond 960 g/m3, the violence of the coal dust explosion is only slightly influenced by the particle size.
50 40
Volatile content 36% Volatile content 32% Volatile content 26%
30 20 10 0
100 200 300 400 500 600 700 800 900 1000
concentration / g/m3 Fig. 9. Variation of maximum overpressures with coal dust concentration for samples with different volatile contents.
3.3. Coal dust explosion suppression To suppress coal dust combustion and explosion in the experimental tube, two suppressing agents, ABC fire extinguisher powder with particle size less than 70 lm and fine SiO2 powder with a
Q. Liu et al. / Fuel 89 (2010) 329–335
a
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80
overpressure / kPa
70 60 50 40 No suppressing agent ABC suppressing agent SiO2 suppressing agent
30 20 10 0 0
5
10
15
20
25
30
distance / m
b
450 400 350
velocity / m/s
335
300 250 No suppressing agent ABC suppressing agent SiO2 suppressing agent
200 150 100 50 0 0
5
10
15
20
25
30
distance / m Fig. 10. Variation of overpressure and velocity with propagating distance with different suppressing conditions. (a) overpressure and (b) velocity.
ABC fire extinguisher powder dispersed into the coal dust clouds can suppress the combustion to some extent. However, if the suppressing section is far from the ignition point, venting is required to mitigate the damage caused by the pressure wave. Early detection and dispersion of the suppressing agent may efficiently suppress the coal dust explosion before the pressure wave becomes too strong and causes damage to the environment. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 10772032) and Foundation of State Key Lab of Explosion Science and Technology (Grant No. ZDKT08-02-6).
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