Flame-propagation behavior and a dynamic model for the thermal-radiation effects in coal-dust explosions

Flame-propagation behavior and a dynamic model for the thermal-radiation effects in coal-dust explosions

Journal of Loss Prevention in the Process Industries 29 (2014) 65e71 Contents lists available at ScienceDirect Journal of Loss Prevention in the Pro...

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Journal of Loss Prevention in the Process Industries 29 (2014) 65e71

Contents lists available at ScienceDirect

Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp

Flame-propagation behavior and a dynamic model for the thermal-radiation effects in coal-dust explosions Weiguo Cao a, Wei Gao b, Jiyuan Liang a, Sen Xu a, Feng Pan a, c, * a

School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, PR China School of Chemical Machinery, Dalian University of Technology, Dalian 116024, PR China c National Quality Supervision and Inspection Center for Industrial Explosive Materials, Nanjing, Jiangsu 210094, PR China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 September 2013 Received in revised form 8 February 2014 Accepted 8 February 2014

To reveal the flame-propagation behavior and the thermal-radiation effects during coal-dust explosions, two coal-dust clouds were tested in a semi-enclosed vertical combustion tube. A high-speed video camera and a thermal infrared imaging device were used to record the flame-propagation process and the thermal-radiation effects of the fireball at the combustion-tube outlet. The flame propagated more quickly and with a higher temperature in the more volatile coal-dust cloud. The coal-dust concentration also significantly affected the propagation behavior of the combustion zone. When the coal-dust concentration was increased, the flame-propagation velocity and the fireball temperature increased before decreasing overall. Based on the experimental results, a dynamic model of the thermal radiation was employed to describe the changes in the fireballs quantitatively and to estimate the thermal-radiation effects during coal-dust explosions. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: Coal-dust explosion Flame-propagation behavior Fireball thermal radiation Dynamic model

1. Introduction Dust explosions are phenomena in which a flame propagates through a dust cloud in air with increasingly subdivided combustible solids. These events are common risks in the coal, metallurgy, chemicals, wood, food processing, explosives and other industries (Abbasi & Abbasi, 2007; Eckhoff, 2009). As shown in Table 1 (Zhang, Jiang, & Zheng, 2005), dust explosions in China have caused severe casualties and damage. Many theoretical and experimental studies regarding dust-explosion phenomena have been performed to help prevent uncontrolled explosions in industry. Most previous studies were devoted to measuring the minimum ignition energy, ignition temperature and explosible concentration, as well as the maximum explosion pressure, to assess the risk of dust explosions (Amyotte, Mintz, Pegg, & Sun, 1993; Bouillard, Vignes, Dufaud, Perrin, & Thomas, 2010; Cashdollar & Zlochower, 2007; Gao et al., 2013); other studies have focused on designing new explosion-protection equipment or methods (Holbrow, Hawksworth, & Tyldesley, 2000; van Wingerden, Arntzen, & Kosinski, 2001). Recently, several studies investigated the

fundamental properties of flame propagation through suspended combustible particles (Dobashi & Senda, 2006; Proust, 2006). Various flame-propagation mechanisms have been proposed, particularly regarding the effects of the volatility on the flamepropagation behavior in various systems such as lycopodium (Han et al., 2000), coal (Xu, Li, Zhu, Wang, & Zhang, 2013) and alcohol (Gao, Dobashi, Mogi, Sun, & Shen, 2012). Despite extensive research, few reports have been devoted to studying the relationship between the flame-propagation behavior and the thermal-radiation effects in coal-dust explosions. Therefore, two coal-dust clouds with different volatilities (#1 and #2) were employed to examine the flame-propagation behavior and the thermal-radiation effects of the fireballs (fireball temperatures and diameters were considered) at the combustion-tube outlet using a high-speed video camera and a thermal infrared imaging device. On the basis of the Martinsen dynamic model and the experimental results, the thermal dynamic properties of the fireball were calculated. 2. Experimental 2.1. Experimental apparatus

* Corresponding author. School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, PR China. E-mail address: [email protected] (F. Pan). http://dx.doi.org/10.1016/j.jlp.2014.02.002 0950-4230/Ó 2014 Elsevier Ltd. All rights reserved.

The experimental apparatus is shown schematically in Fig. 1. The entire system consists of a vertical combustion tube, a high-pressure

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Table 1 Frequency of dust-explosion accidents and injuries, including fatalities. Type of dust Metal Farm product Organics chemicals Compound Inorganic compound Fiber Coal Total

Frequency 44 40 37 31 27 17 12 209

Proportion (%)

Number of casualties

Death

21.53 19.23 17.79 14.90 12.98 8.17 5.77 100

155 113 71 51 37 70 45 542

35 16 9 5 9 5 7 86

Table 2 Industrial analysis of the coal-dust samples. Injury 120 97 62 46 28 65 38 456

No.

Mad (%)

Aad (%)

Vad (%)

FCad (%)

Coal dust #1 Coal dust #2

3.54 3.93

14.46 19.72

41.75 35.40

40.25 40.95

temperatures and diameters of the fireball located at the combustion-tube outlet were recorded with a thermal infrared imaging device. 2.2. Experimental materials

dispersion system, an ignition system, a high-speed video camera, a thermal infrared imaging device and a control system. The vertical cylindrical combustion tube is 600 mm high with a 68 mm inner diameter when the top is open. The coal particles were placed evenly in the tube base and were dispersed using a high-pressure powderspray machine, forming uniform coal-dust clouds in the combustion tube. The experimental conditions are as follows: 1) The dispersion pressure is 0.7 MPa. 2) The ignition system is positioned 100 mm above the bottom of the tube. 3) The electrode gap is 6 mm. 4) The igniter voltage is 8000 V. 5) The ignition energy is 5 J. 6) The frame rate of the high-speed video camera and the thermal infrared imaging device are 1000 frames/s and 100 frames/s, respectively.

The industrial analysis of the two types of coal dust, which were supplied by the National Quality Supervision and Inspection Center for Industrial Explosive Materials, is summarized in Table 2. To ensure that the particle sizes were in the same range and to reduce the effect of the particle-size distributions on the flamepropagation process, the coal-dust samples were crushed and sifted in a 200-mesh vibrating sieve to obtain median diameters of 32 mm and 34 mm for the two types of coal dust. The nitrogen adsorptionedesorption isotherms and the pore distributions in the two types of coal dust are presented in Fig. 2. The profiles have representative “type-IV” hysteresis loops at P/ P0 ¼ 0.45e1.0, indicating that the samples are mesoporous. The specific surface areas of coal dusts #1 and #2 are 4.6 m2/g and 2.6 m2/g, respectively. The pore-size distributions displayed in the figure were calculated using the BarreteJoynereHalenda (BJH) model with the desorption branch. A sharp peak for the two coaldust samples was centered at 3.3 nm. 3. Experimental results and discussion

The weighed coal dust was placed evenly on the bottom of the vertical combustion tube and dispersed into the tube under high pressure to form a uniform coal-dust cloud. The suspended particles were ignited by an electric spark after reaching a height of 300 mm to guarantee a consistent concentration of coal-dust clouds and reduce the influence of residual turbulence on the flame propagation. After ignition, the flame-propagation process was recorded using a high-speed video camera, and the

3.1. Flame-propagation behavior The flame propagation of the two coal-dust clouds in and above the combustion tube is depicted in Fig. 3, which shows data that was recorded by the high-speed video camera. The concentration of the coal-dust clouds was 500 g/m3. A time series of the flame-front position was analyzed using the recorded images. It is presumed

Fig. 1. Experimental apparatus: 1) electric-spark generator; 2) programmable logic controller; 3) pneumatic piston; 4) combustion tube; 5) ignition electrodes; 6) nozzle; 7) powder-injection valve; 8) gas tank; 9) air-inlet valve; 10) pressurized air; 11) piston-actuated valve; 12) powder tank; 13) high-speed camera; and 14) infrared imager.

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Fig. 4. The flame structure for coal dust with increasing time.

Fig. 2. The nitrogen adsorptionedesorption isotherms and the pore-size distributions of the coal-dust samples.

that the coal particles first vaporize to yield a gaseous fuel, which is oxidized in the gas phase. To simulate the combustion process, the flame structure is composed of three zones, including a preheat

zone where the rate of chemical reaction and vaporization is small, an extensive-particle vaporization and reaction zone where the vaporization is followed by reaction, and finally, a post-flame zone. As revealed in Fig. 4, the flame propagated slowly during the initial stage after ignition. Upon electric-spark discharge, the flamepropagation process began to enter the brief preheat zone, during which the spark of energy transferred to the surface of the dust particles. Next, the flame propagation reached the vaporization and reaction zone, where the volatile material burned immediately as it evaporated. As a result, vaporization and reaction exist simultaneously in this zone, and this stage determines the behavior of the flame propagation. As shown in Figs. 3 and 5, the flame-

Fig. 3. High-speed photographs of the coal-dust cloud flame at various times. The ignition energy is 5 J and the concentration is 500 g/m3.

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Fig. 5. Relationships between the flame-propagation velocity and the flame-front position at various concentrations in the coal-dust clouds.

propagation velocity of coal dust #1 is higher than that of coal dust #2, which may be due to the smaller Damköhler number of coal dust #1 than that of coal dust #2. The Damköhler number is defined as the ratio of the vaporization time to the characteristic reaction

time or the ratio of the chemical reaction rate to the vaporization rate. The Damköhler number is inversely proportional to the rate of pulverized-coal flame propagation. Similar phenomena were observed by Bidabadi, Haghiri, and Rahbari (2010). This implies that

Fig. 6. Thermal infrared images of the fireballs. The ignition energy is 5 J, and the concentration is 500 g/m3.

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zone (Qiu, Cao, Huang, Zhang, & Pan, 2012). Consequently, the flame-propagation velocity decreased with further increases in the concentration. 3.3. Descriptive parameters of the fireballs

Fig. 7. Relationship between the fireball temperature and time.

the coal-dust clouds contain large particles and that the combustion process is controlled by the vaporization rate, if the vaporization rate (pyrolysis of volatile particles) becomes lower than the reaction rate. In this zone, the vaporization rate dominates the flame-propagation behavior (Haghiri & Bidabadi, 2010). Finally, the flame propagation entered into the post-flame zone. This region mainly involves the incomplete combustion of fixed carbon. In addition, the burning rate of the fixed carbon is lower than that of the volatilized organics; therefore, the propagation rate of the flame declines in this zone. 3.2. Flame-propagation velocities The relationships between the flame-propagation velocity and the flame-front position of the two types of coal-dust clouds at various concentrations are displayed in Fig. 5. The flames propagated in all directions inside the combustion tube. After reaching the bottom of the tube, the flame accelerated the propagation process, proceeding only upward due to the expansion of the combustion products in the confined tube. Over time, the flame propagated upward within the vertical tube, and the flamepropagation velocity peaked when the concentration of the coaldust cloud was 500 g/m3 (Fig. 5(a) and (b)). As presented in Fig. 5(c) (d), the flame-propagation velocity peaked before decreasing overall. When the concentrations of the coal-dust clouds were 250 g/m3, 500 g/m3 and 750 g/m3, the velocities of coal dust #1 reached their maxima of 7.8 m/s, 11.7 m/s and 10.9 m/s at 105 ms, 85 ms and 90 ms after ignition, respectively; however, the velocities of coal dust #2 reached their maxima of 5.4 m/s, 10.2 m/s and 9.5 m/s at 115 ms, 90 ms and 95 ms after ignition, respectively. Both of the clouds reached their maximum flame-propagation velocities at approximately 100 ms after ignition. However, coal dust #1 propagated faster, taking less time to reach its maximum than coal dust #2 did under the same conditions. In addition, the flame-propagation velocities of the two types of coal dust reached their maximum values at 500 g/m3. At lower concentrations, the number of ignited particles per unit volume and the number of effective collisions between the reactant molecules increased with increasing coal-dust concentrations, thus accelerating the flame propagation. However, at higher concentrations, the quantity of ignited particles per unit volume was limited by the lack of oxygen; the heat released by the combustion reaction was absorbed by the unburned particles in the preheat

The descriptive parameters for the fireballs at the combustiontube outlet were recorded using a thermal infrared imaging device, as shown in Fig. 6. During the experiment, the initial time was the moment when the fireballs reached the combustion-tube outlet. Because the surface temperature of the fireball was nonuniform, the mean temperature was utilized to obtain reasonable results. The mean temperature was obtained using the infrared thermal imager software. Due to their short duration, the fireballs were regarded as uniform spherical radiation. The corresponding temperatures and diameters of the fireballs are listed in Figs. 7 and 8. The maximum average temperatures of coal dusts #1 and #2 were 1298.4  C and 1273.2  C, respectively. As shown in Fig. 8, the maximum diameters of the fireballs were 31 cm and 29 cm, respectively. To estimate the thermal-radiation effects accurately, thermal dynamic calculation methods were developed based on the Martinsen dynamic model and the experimental results. 3.4. Dynamic model of thermal radiation The received powder flux (Q) for the thermal radiation generated in the flame-propagation process of dust explosion can be estimated by integrating the thermal flux (q) over the duration (ta) of the fireball (Martinsen & Marx, 1999). Thermal flux (q) is a function of the surface-powder density (E(t)), the view factor for a fireball F(x,t) and the atmospheric transmissivity (s(x,t)). The received powder flux (Q) is as follows:

Zta Q ¼

Zta qðx; tÞdt ¼

0

EðtÞFðx; tÞsðx; tÞdt:

(1)

0

3.4.1. Fireball surface-powder density The high temperatures generated during the flame propagation can radiate light with various wavelengths that can be observed as a continuum spectrum. Continuum-spectrum radiation may be regarded as gray-body radiation; the principles of temperature of

Fig. 8. Relationship between the fireball diameter and time.

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Fig. 9. Geometric perspective of a fireball.

Fig. 10. Relationship between the thermal-radiation dose and the distance.

the thermal infrared imager device are based on gray-body radiation. According to the StefaneBoltzmann law (Zhang, Li, & Jin, 1995), the surface-powder density of the fireball can be estimated

EðtÞ ¼ εsT 4 ðtÞ;



(2)

(4)

3.4.4. Calculation results Based on the high-speed video camera and the thermal infrared imaging device data, the instantaneous temperature of the fireball was obtained and fitted using the polynomial fitting method. The fitted curves for the temperatures and the diameters of the fireballs are displayed in Figs. 7 and 8, respectively, and the fitted coefficients are summarized in Table 3. During the experiment, the ambient temperature, the relative humidity and the saturated vapor pressure were 25  C, 51% and 3.17 KPa, respectively. The received powder fluxes from the fireballs at the various locations, as shown in Fig. 10, were calculated using the data in Table 3 and equations (1)e(4). As observed in Fig. 10, coal dust #1 produced a larger thermalradiation dose than coal dust #2 did under the same conditions. The thermal-radiation doses of coal dusts #1 and #2 were 59 kJ/m2 and 45 kJ/m2, respectively, when the receptor was 0.1 m away from the center of the fireball; these values decreased as the distance increased. The thermal-radiation doses for coal dusts #1 and #2 were 5 kJ/m2 when the receptor was 0.8 m away from the center of the fireball.

3.4.2. View factors for the fireballs As shown in Fig. 9, because the radiation of each radiation point cannot be received completely at the location of an object, a geometric perspective, namely, the ratio of the radiation per unit area received by the object and released by the fireball, is accepted. It is not only associated with the size of the fireball and the distance between the fireball and the object, but also it is related to the relative orientation of the fireball and the object (Zhong, Wang, & Huang, 2011). Assuming that the receptor and the center of the fireball are always on the same level, the view factor for fireballs is as follows:

  1 DðtÞ 2 ; 4 S

 DðtÞ 0:09 ; 2

where R is the relative humidity during the experiments and PV is the saturated vapor pressure at the corresponding ambient temperature [Pa].

where ε is the emissivity, (0.1); s is the Boltzmann constant, (5.67  108 W m2 K4); and T is the absolute temperature [K].

Fðx; tÞ ¼



sðx; tÞ ¼ 2:02 RPV S 

(3)

where D(t) is the instantaneous diameter [m]; S is the distance from the center of the fireball to the receptor [m]; and t is the time [s]. 3.4.3. Atmospheric transmissivity The thermal radiation absorbed by the atmospheric water vapor and carbon dioxide can significantly reduce the thermal radiation received by an exposed object. A useful relationship developed by Zhang, Liu, Wang, and Li (2007) was adopted in this study

4. Conclusions Two coal-dust clouds with different volatilities were tested in a semi-enclosed vertical combustion tube to reveal the flame-

Table 3 Fitted coefficients for the temperatures and the diameters of the fireballs. Y(t) ¼ a þ bt þ ct2 þ dt3 þ et4 þ ft5

Y

Fitted coefficient

T ( C) D (cm)

Coal Coal Coal Coal

dust dust dust dust

#1 #2 #1 #2

a

b

c

d

440.64 439.51 13.42 0.56

36.53 29.85 1.71  102 2.75  102

0.53 0.38 2.36  103 3.91  104

3.25 2.06 1.25 1.99

e    

103 103 105 106

9.16 5.33 1.61 3.29

Scope (s)

Precision (%)

(0 (0 (0 (0

98.61 99.31 99.10 99.74

f    

106 106 108 109

9.63  109 5.24  109 0 0

0.3) 0.3) 0.3) 0.3)

W. Cao et al. / Journal of Loss Prevention in the Process Industries 29 (2014) 65e71

propagation behavior and thermal-radiation effects during coaldust explosions. The following results were obtained: (1) The flame-propagation behavior was similar in the two types of coal-dust clouds. The flame-propagation velocity and the temperature reached their maximum values before decreasing overall. (2) Under the same experimental conditions, the maximum flame-propagation velocity and temperature of coal-dust cloud #1 were larger than those of coal-dust cloud #2. When the concentration of a coal-dust cloud was increased, the flame-propagation velocity and temperature reached their maxima at 500 g/m3 before decreasing overall. (3) The fireball temperatures and diameters were quantified. The highest flame temperatures for coal-dust clouds #1 and #2 at the combustion-tube outlet were 1298.4  C and 1273.2  C, respectively, and the largest diameters of their fireballs were 31.1 cm and 28.6 cm. Based on the Martinsen dynamic model and the experimental results, the powder flux for the thermal radiation generated during the flame propagation in the dust explosion was estimated to evaluate the thermal-radiation effects on the fireball, thereby providing a theoretical foundation for the damaging power of the fireballs in dust explosions. Acknowledgments The authors appreciate the financial support from the Research and Innovation Project for College Graduates of Jiangsu Province (Grant No. CXZZ13_0216) and the Natural Science Foundation of China (Grant No. 11102091).

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