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Effect of monoammonium phosphate particle size on flame propagation of aluminum dust cloud
Journal of Loss Prevention in the Process Industries 60 (2019) 311–316
Contents lists available at ScienceDirect
Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Effect of monoammonium phosphate particle size on flame propagation of aluminum dust cloud
T
Haipeng Jiang, Mingshu Bi, Wei Gao∗ School of Chemical Engineering, Dalian University of Technology, Dalian, 116024, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Aluminum dust explosions Flame propagation behaviors Particle size distribution Surface kinetics
The effect of monoammonium phosphate (NH4H2PO4) particles on 5 μm aluminum dust flames is investigated experimentally and computationally. NH4H2PO4 in three particle size is employed to determine the inhibition efficiency on aluminum flame propagation. Flame inhibition mechanism considering both gas and surface chemistry of aluminum particles is developed. Results show that the inhibition effectiveness monotonously increases as NH4H2PO4 particle size is reduced to 25 μm. Flame morphology and flame microstructure change with the addition of different particle size NH4H2PO4. Small NH4H2PO4 particles within the range studied have a greater reduction in average flame propagation compared to the coarser one. Meanwhile, the fine NH4H2PO4 particles almost decompose completely during the penetration of aluminum flame and then undergo a sufficient chemical interaction with the flame. The simulations indicate that the decomposition products of NH4H2PO4 particles obstruct the oxidation of aluminum particles through flame radical consumption. Additionally, the addition of NH4H2PO4 can reduce the vaporization rate and surface reaction rate of aluminum particles.
1. Introduction In recent years, aluminum dust explosion accidents occur frequently during the grinding/buffing, finishing and polishing process of aluminum products (Myers, 2008; Taveau et al., 2018). A catastrophic explosion of aluminum dust occurred during the wheel hub polishing process at zhongrong metal production company in the city of Kunshan, China, causing lots of casualties. Many injured in the explosion died of terrible burns soon after. Therefore, it is essential to effectively mitigate the serious damage and prevent the explosion accidents of aluminum dust. Flame inhibition of combustible dust cloud using solid inhibitors is a common practice in industrial explosion inhibition. For example, solid inhibitor is discharged into the hazard volume upon detection of an incipient explosion in process equipment (Chatrathi and Going, 2000). The effectiveness of inhibitor in mitigating explosion damage depends on the selection of inhibitor (Amyotte, 2006), particle size (Chen et al., 2017; Zhang et al., 2017), and concentration (Sun et al., 2019), etc. Knuth et al. (2007) experimentally investigated the flame inhibition effectiveness of dry chemical powders for a diffusion flame inhibition. Comparison of the thermal and chemical effects of these inhibitors indicated that the inhibition effectiveness of NH4H2PO4 was more effective than NaHCO3, KHCO3 and Al2O3. They also found that the
∗
inhibition effectiveness was a function of the specific surface area (particle size). Investigations on particle size effect of solid inhibitors were extensive. Chelliah et al. (2003) studied the effect of NaHCO3 particle size on suppression of methane-air flames. Flame suppression results suggested that the inhibition efficiency increased with NaHCO3 particle size decreased. The effect of NaHCO3 particle size on the inhibition of aluminum dust explosions was investigated by Jiang et al. (2018). This latter study also indicated that small NaHCO3 particles were more effective than larger particles. Sridhar Iya et al. (1975) and Birchall (1970) demonstrated that particles below 10 μm would be completely decomposed in the preheat zone, and large particles (> 200 μm) would penetrate the flame with litter reaction. Although NH4H2PO4 particles have showed excellent flame inhibition performance in previous studies. So far, very little information is available for the effect of NH4H2PO4 particle size on the flame, especially aluminum flame. At the same time, there are few studies to understand flame inhibition mechanism of phosphates on aluminum particles. In this study, the inhibition effectiveness of NH4H2PO4 with various particle sizes on flame propagation of aluminum dust cloud is determined systematically. The effects of NH4H2PO4 particle size on flame morphology and flame propagation velocity of aluminum dust cloud are studied. In order to deeply investigate the flame inhibition mechanism, a tentative kinetic model considering both gas and surface
Corresponding author. E-mail address: [email protected] (W. Gao).
https://doi.org/10.1016/j.jlp.2019.05.009 Received 1 February 2019; Received in revised form 16 May 2019; Accepted 17 May 2019 Available online 20 May 2019 0950-4230/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Loss Prevention in the Process Industries 60 (2019) 311–316
H. Jiang, et al.
Fig. 2. Illustration of the process of aluminum vaporization.
performed at constant volume using 0‒D Homogeneous reactor in the Chemkin Pro. The attention of modeling study for surface reaction suppression was paid to the surface and gas/surface interactions at the vicinity of an aluminum particle. The detailed description of the gas‒phase mechanism of Al/air flames doped with P‒containing species and NH3 was given by Jiang et al. (2019). The gas‒phase mechanism was drawn from previous work on reactions of aluminum (Catoire et al., 2003), NH3 (Konnov, 2009; Li et al., 2013), H3PO4 (Twarowski, 1996; Jayaweera et al., 2005; Korobeinichev et al., 2007), involving 87 species and 429 elementary reactions. Surface chemistry of aluminum particles combustion was adopted from the previous work of Glorian et al. (2016), involving 19 surface species and 49 surface reactions. Surface chemistry considered species adsorbed on the solid surface. In this study, species were adsorbed on the open sites Al(L) of the aluminum surface. Surface site species and bulk-phase species were noted using suffixes (S) and (B), respectively. In the surface mechanism, the vaporization process of aluminum particles was described by reaction Al(B) + Al(L) = Al + Al(L). The illustration of the process of aluminum vaporization was shown in Fig. 2. In this reaction, the bulk aluminum Al(B) reacted with the open sites Al(L) to form the gaseous Al and the open sites Al(L).
Fig. 1. Experiment apparatus.
chemistry is developed. The effects of NH4H2PO4 on gas and surface reaction of aluminum particles burning are further discussed. 2. Experimental The experimental apparatus and methods were similar to our previous work (Jiang et al., 2019). As shown in Fig. 1, the apparatus consists of cylindrical combustion tubes, an air-fuel dispersion system, a high-speed camera, an ignition system, a data acquisition system unit, and a time controller. The mixture powder was placed in a hemispherical cup of the dispersion unit. The dispersion pressure was 0.46 MPa, and the dispersion duration was 0.5 s. The spark lasted 0.25 s. Flame propagation processes and the flame microstructures were visualized with a Photron SA4 high‒speed video camera, operated at 10000 frames per second. 5 μm aluminum dust is employed as the experimental sample. The real mass density of aluminum dust cloud is 300 g/m3. Monoammonium phosphate powder (NH4H2PO4 > 99%, Macklin Inc.) is selected as inhibitor, since it is more effective and widely used in explosion prevention system. The NH4H2PO4 particles are sifted into size ranges of 25–53 μm, 53–100 μm, and 100–212 μm. All samples are dried before experiments. The inerting ratio (α) that referrers to the ratio of mass of inhibitor to mass of explosible dust is used in this study. Al/NH4H2PO4 dust mixture is fully premixed with an inerting ratio ranging from 0 to 2.0.
4. Experimental results 4.1. Flame morphology and microstructures Fig. 3 shows the variation of aluminum flame morphology doped with different particle size NH4H2PO4. 100–212 μm NH4H2PO4 particles have a slight impact on aluminum flame. It is indicated that larger NH4H2PO4 particles are partially decomposed and have little interaction with the flame. For addition of 53–100 μm NH4H2PO4 particles, the luminosity of the aluminum flame is obviously weakened at α = 2.0. The results for 25–53 μm NH4H2PO4 particles show a clear change in 5 μm aluminum flame morphology. Aluminum flame exhibits a dark yellow flame, just like ammonia (NH3) flame. Previous studies (AbdelKader et al., 1991) have revealed that NH4H2PO4 particles decompose to generate NH3 at approximately 210 °C. Hence, it can be inferred that 25–53 μm NH4H2PO4 particles can reach the flame front, decompose completely, and have a fully chemical interaction with aluminum flame. These results demonstrate that 25–53 μm particles are the most effective in flame inhibition of aluminum dust, followed by 53–100 μm, and then 100–212 μm. The effects of NH4H2PO4 particle size are further explored by observing the variation in the microstructures of aluminum flame. Flame microstructures in Al/NH4H2PO4 mixture are shown in Fig. 4. These obtained images indicate different flame structures of aluminum flame doped with various particle size NH4H2PO4. As shown in Fig. 4a, aluminum flame front is similar to the gas flame, perhaps indicating a premixed ammonia flame. In contrast, aluminum particles surrounded by gas phase flame are observed for the two size groups 53–100 μm and 100–212 μm. These above results confirm that 25–53 μm NH4H2PO4 particles can penetrate the flame front, fully decompose within aluminum flame, and exert the greatest inhibition effectiveness.
3. Numerical methods 3.1. Initial conditions This paper deals with nascent 5 μm aluminum particles by ignoring the presence of the alumina shell. Both gas and surface reactions of aluminum combustion were considered in the numerical model. Based on the work of Glorian et al. (2016), the temperature of aluminum particles was assumed to be 2700 K, since it should be higher than the melting temperature of the alumina shell (≈2300K). Initial pressure was 1 atm. The mole ratio of N2 and O2 for air was 3.76. In the reaction zone, NH4H2PO4 particles had been decomposed to form H3PO4 and NH3, NH4H2PO4 → H3PO4 + NH3. The condensed phase products of NH4H2PO4 decomposition were not considered. To further reveal the gas-phase and surface inhibition mechanism of NH4H2PO4 on aluminum dust explosion, an investigation of stoichiometric concentration (ϕ Al = 1) under different inerting ratios was performed. 3.2. Kinetic mechanism The modeling studies for the undoped and doped Al/air flames were 312
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velocity decreases considerably with the addition of NH4H2PO4 particles. Fig. 6 plots the average flame propagation velocity as a function of NH4H2PO4 concentration and particle size. Results show that the average flame propagation velocity decreases with the NH4H2PO4 concentration increases. Meanwhile, with the reduction of NH4H2PO4 particle size, the results show a clear decrease in flame propagation velocity of aluminum flame. It is indicated that small NH4H2PO4 particles in the particle size range studied exert a greater inhibition effectiveness in flame speed reduction compared to the coarser one.
5. Inhibition mechanism 5.1. Physical inhibition mechanism With regard to particle-flame interaction, the inhibition mechanism of solid chemical inhibitors occurs in four determinant steps (Mitani and Niioka, 1984). First, NH4H2PO4 particles undergo heating due to initiated aluminum combustion. Second, NH4H2PO4 particles begin to decompose. Third, gas and condensed phase decomposition products are produced. Fourth, chemical interaction with the flame occurs. Chemical inhibition cannot be accomplished unless the total duration of these four steps (ts) is shorter than the burning time (tb) of 5 μm Al particles. If tb < ts, NH4H2PO4 particles have less capacity to exert the chemical effect in addition to the physical effect of dilution and thermal absorption. The effectiveness of the physical effect depends on the ability of solid inhibitors to absorb heat. The endothermic decomposition process of solid particles is limited by the rate of heat transfer (Hp) and the thermal relaxation timescale (τT) (Cloney et al., 2018):
Hp = πDp λs Nu (T − Tp)
(1)
Nu = 2 + 0.6 Re1/2 Pr1/3
(2)
Fig. 3. Flame propagations of undoped and doped 5 μm Al/air mixture.
τT = 4.2. Flame propagation velocities
mp CPp πDp λs Nu
(3)
where Dp denotes particle diameter, λs is the thermal conductivity, T is the fluid temperature, Tp is the particle temperature, CPp is specific heat at constant pressure, Nu is the non-dimensional Nusselt number and Pr is the non-dimensional Prandlt number. Based on equations (1)–(3), it can be concluded that fine solid particles have a shorter characteristic time of decomposition/evaporation, resulting in a greater degree of decomposition than coarse particles. Therefore, small NH4H2PO4 particles will decompose most or completely during passage through aluminum flame and are most effective. Even so, as the particle size of solid inhibitor continues to reduce, the inhibition effectiveness of solid inhibitor may decrease, since ultra-fine particles have a short residence time in the flame and may tend to agglomerate. However, our findings indicate that the inhibition effectiveness monotonously increases with the decrease in NH4H2PO4 particle size in the range studied.
The flame edges are recognized through a MATLAB program based on the Roberts operator. The flame propagation velocity can be determined by examining the movement of the flame front. Flame contour on each frame was matched with an ellipse, since the flame is not always spherically symmetrical. The flame radius of undoped and doped Al/air mixture was calculated using the method in the literature (Julien et al., 2015). Flame propagation velocity is derived from the flame radius vs. time. The distances from the flame front to the ignition point is evaluated by the most advanced point along the horizontal direction in the flame front. The distances from the flame front to the ignition point and flame propagation velocity of undoped and doped aluminum flame are shown in Fig. 5. It can be seen that the flame front position is forward with the reduction of particle size. As shown in Fig. 5b, the flame propagation
Fig. 4. Flame microstructures in 5 μm Al/air mixture doped with different particle size NH4H2PO4. (a. 25–53 μm, b. 53–100 μm, c. 100–212 μm). 313
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Fig. 5. Flame front position and flame propagation velocity of 5 μm aluminum dust cloud doped with different particle size NH4H2PO4.
surface chemistry is developed. Fig. 7 plots the concentration of AlO and O across Al flame doped with NH4H2PO4. The calculations show that the concentration of AlO and O atom decreases considerably with the inerting ratio increases. Reactions of P‒containing species (such as O + HPO3 < = > O2+HOPO, HOPO + O + M < = > HPO3+M, and HPO3+O < = > OH + PO3) and ammonia combustion (such as NH3 + O2 < = > NH2+HO2 and NH3 + O < = > NH2 + OH.) can cause a significant decrease in O and O2. Therefore, the gas-phase aluminum flame can be effectively suppressed by NH4H2PO4, since AlO and O play a key role in gaseous-Al/air reaction system. The rate of surface reaction and Al particles vaporization are limited by the number of Al(L), since surface reaction considers species adsorbed on Al(L), and the adsorption capacity of the surface of aluminum particles and the number of Al(L) are limited. The illustration of a surface reaction of O + Al(L) → AlO(s) is shown in Fig. 8. As shown in Fig. 9, Al(L) site fraction decreases with the inerting ratio increases. Therefore, an increase in the concentration of NH4H2PO4 can reduce the vaporization rate of aluminum particles, thereby reducing the surface reaction rate. Nevertheless, the increase in NH4H2PO4 concentration has a slightly effect on O(S), AlO(S) and AlO2(S). These results indicate that the diffusion rate of oxidizers close to the droplet surface slightly depends on the concentration of NH4H2PO4. Fig. 10 shows the results of sensitivity analysis of Al(L) site fraction at α = 0.4, 1.2 and 2.0. Negative sensitivity coefficients represent a reduction of Al(L) fraction due to P‒containing compounds. It can be seen that HPO3+M = PO2+OH + M has the biggest negative sensitivity coefficient and plays an important role of the inhibition effect on
Fig. 6. Average flame propagation velocity of 5 μm aluminum dust cloud doped with different particle size NH4H2PO4.
5.2. Chemical inhibition mechanism To gain more insight for the chemical suppression effect of NH4H2PO4 on aluminum dust explosion, the general behavior of the Al/ air and Al/NH4H2PO4/air is examined in an adiabatic, constant-pressure environment using CHEMKIN software package. Since the burning of 5 μm aluminum particles is a combination of the gas-phase and surface reaction processes, a kinetics model considering both gas and
Fig. 7. Temporal evolution of (a) AlO and (b) O atom in Al/NH4H2PO4/air flame. 314
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conclusions are given as follows. Flame morphology and color change with the addition of different particle size NH4H2PO4. For 25–53 μm NH4H2PO4 particles, aluminum flame front is as smooth as a gas flame. Aluminum particles surrounded by discrete gas phase flame are observed for the other two size distributions of NH4H2PO4. Small NH4H2PO4 particles within the range studied have a greater reduction in average flame propagation compared to the larger NH4H2PO4 particles. Hence, the inhibition effectiveness of aluminum flame depends on the particle size of NH4H2PO4. Aluminum flame inhibition is achieved by particle-flame interaction, including thermal effects and chemical inhibition. Small NH4H2PO4 particles have a better thermal effect, since fine solid particles have a shorter characteristic time of decomposition/evaporation and a larger degree of decomposition than the coarse particles. NH4H2PO4 can suppress the gas phase reaction of aluminum combustion by flame radical consumption. The surface reaction rate and the vaporization rate of aluminum particles are reduced by NH4H2PO4 particles addition. Small NH4H2PO4 particles can be almost completely decomposed to form more gas phase compounds, which produces better chemical inhibition for aluminum flame.
Fig. 8. Illustration of a surface reaction, O + Al(L) → AlO(s).
Acknowledgments The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 51874066 and No. 51674059), Key Laboratory of Building Fire Protection Engineering and Technology of MPS (KFKT2016ZD01), and the Fundamental Research Funds for the Central Universities (DUT16RC(4)04). Appendix A. Supplementary data
Fig. 9. Occupied site fractions at aluminum surface as a function of inerting ratio.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jlp.2019.05.009. References Abdel-Kader, A., Ammar, A.A., Saleh, S.I., 1991. Thermal behaviour of ammonium dihydrogen phosphate crystals in the temperature range 25-600° C. Thermochim. Acta 176, 293–304. Amyotte, P.R., 2006. Solid inertants and their use in dust explosion prevention and mitigation. J. Loss Prev. Process. Ind. 19, 161–173. Birchall, J.D., 1970. On the mechanism of flame inhibition by alkali metal salts. Combust. Flame 14, 85–95. Catoire, L., Legendre, J.F., Giraud, M., 2003. Kinetic model for aluminum‒sensitized ram accelerator combustion. J. Propuls. Power 19, 196–202. Chatrathi, K., Going, J., 2000. Dust deflagration extinction. Process Saf. Prog. 19, 146–153. Chelliah, H.K., Wanigarathne, P.C., Lentati, A.M., Krauss, R.H., Fallon, G.S., 2003. Effect of sodium bicarbonate particle size on the extinction condition of non-premixed counterflow flames. Combust. Flame 134, 261–272. Chen, X., Zhang, H., Chen, X., Liu, X., Niu, Y., Zhang, Y., Yuan, B., 2017. Effect of dust explosion suppression by sodium bicarbonate with different granulometric distribution. J. Loss Prev. Process. Ind. 49, 905–911. Cloney, C.T., Ripley, R.C., Pegg, M.J., Amyotte, P.R., 2018. Laminar burning velocity and structure of coal dust flames using a unity Lewis number CFD model. Combust. Flame 190, 87–102. Glorian, J., Gallier, S., Catoire, L., 2016. On the role of heterogeneous reactions in aluminum combustion. Combust. Flame 168, 378–392. Jayaweera, T.M., Melius, C.F., Pitz, W.J., Westbrook, C.K., Korobeinichev, O.P., Shvartsberg, V.M., Shmakov, A.G., Rybitskaya, I.V., Curran, H.J., 2005. Flame inhibition by phosphorus-containing compounds over a range of equivalence ratios. Combust. Flame 140, 103–115. Jiang, H., Bi, M., Gao, W., Gan, B., Zhang, D., Zhang, Q., 2018. Inhibition of aluminum dust explosion by NaHCO3 with different particle size distributions. J. Hazard Mater. 344, 902–912. Jiang, H., Bi, M., Li, B., Ma, D., Gao, W., 2019. Flame inhibition of aluminum dust explosion by NaHCO3 and NH4H2PO4. Combust. Flame 200, 97–114. Julien, P., Vickery, J., Whiteley, S., Wright, A., Goroshin, S., Bergthorson, J.M., Frost, D.L., 2015. Effect of scale on freely propagating flames in aluminum dust clouds. J. Loss Prev. Process. Ind. 36, 232–238. Knuth, E.L., Ni, W.F., Seeger, C., 2007. Molecular-beam sampling study of extinguishment of methane-air flames by dry chemicals. Combust. Sci. Technol. 28, 247–262. Konnov, A.A., 2009. Implementation of the NCN pathway of prompt‒NO formation in the detailed reaction mechanism. Combust. Flame 156, 2093–2105. Korobeinichev, O.P., Shvartsberg, V.M., Shmakov, A.G., Knyazkov, D.A., Rybitskaya, I.V.,
Fig. 10. Sensitivity of Al(L) due to NH4H2PO4 addition.
the surface reaction during aluminum particles burning. The reaction containing NH3 can also decrease the surface reaction rate and the vaporization rate of aluminum particles. Moreover, the inhibition effect on the surface reaction increases with NH4H2PO4 concentration increases. The above findings lead us to conclude that small NH4H2PO4 particles can be almost completely decomposed to form more gas phase compounds, which produces better chemical inhibition for aluminum flame. 6. Conclusions The effect of NH4H2PO4 particle size on inhibition effectiveness of aluminum flame is systematically discussed. Experimental and numerical investigation is performed to investigate the detailed inhibition mechanism of NH4H2PO4 particles for aluminum flame. The 315
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H. Jiang, et al. 2007. Inhibition of atmospheric lean and rich CH4/O2/Ar flames by phosphorus‒containing compound. Proc. Combust. Inst. 31, 2741–2748. Li, B., He, Y., Li, Z.S., Konnov, A.A., 2013. Measurements of NO concentration in NH3‒doped CH4 + air flames using saturated laser‒induced fluorescence and probe sampling. Combust. Flame 160, 40–46. Mitani, T., Niioka, T., 1984. Extinction phenomenon of premixed flames with alkali metal compounds. Combust. Flame 55, 13–21. Myers, T.J., 2008. Reducing aluminum dust explosion hazards: case study of dust inerting in an aluminum buffing operation. J. Hazard Mater. 159, 72–80. Sridhar Iya, K., Wollowitz, S., Kaskan, W.E., 1975. The mechanism of flame inhibition by sodium salts. Symp. (Int.) Combust. 15, 329–336.
Sun, Y., Yuan, B., Chen, X., Li, K., Wang, L., Yun, Y., Fan, A., 2019. Suppression of methane/air explosion by kaolinite-based multi-component inhibitor. Powder Technol. 343, 279–286. Taveau, J., Hochgreb, S., Lemkowitz, S., Roekaerts, D., 2018. Explosion hazards of aluminum finishing operations. J. Loss Prev. Process. Ind. 51, 84–93. Twarowski, A., 1996. The temperature dependence of H + OH recombination in phosphorus oxide containing post‒combustion gases. Combust. Flame 105, 407–413. Zhang, H., Chen, X., Zhang, Y., Niu, Y., Yuan, B., Dai, H., He, S., 2017. Effects of particle size on flame structures through corn starch dust explosions. J. Loss Prev. Process. Ind. 50, 7–14.
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Contents lists available at ScienceDirect
Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Effect of monoammonium phosphate particle size on flame propagation of aluminum dust cloud
T
Haipeng Jiang, Mingshu Bi, Wei Gao∗ School of Chemical Engineering, Dalian University of Technology, Dalian, 116024, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Aluminum dust explosions Flame propagation behaviors Particle size distribution Surface kinetics
The effect of monoammonium phosphate (NH4H2PO4) particles on 5 μm aluminum dust flames is investigated experimentally and computationally. NH4H2PO4 in three particle size is employed to determine the inhibition efficiency on aluminum flame propagation. Flame inhibition mechanism considering both gas and surface chemistry of aluminum particles is developed. Results show that the inhibition effectiveness monotonously increases as NH4H2PO4 particle size is reduced to 25 μm. Flame morphology and flame microstructure change with the addition of different particle size NH4H2PO4. Small NH4H2PO4 particles within the range studied have a greater reduction in average flame propagation compared to the coarser one. Meanwhile, the fine NH4H2PO4 particles almost decompose completely during the penetration of aluminum flame and then undergo a sufficient chemical interaction with the flame. The simulations indicate that the decomposition products of NH4H2PO4 particles obstruct the oxidation of aluminum particles through flame radical consumption. Additionally, the addition of NH4H2PO4 can reduce the vaporization rate and surface reaction rate of aluminum particles.
1. Introduction In recent years, aluminum dust explosion accidents occur frequently during the grinding/buffing, finishing and polishing process of aluminum products (Myers, 2008; Taveau et al., 2018). A catastrophic explosion of aluminum dust occurred during the wheel hub polishing process at zhongrong metal production company in the city of Kunshan, China, causing lots of casualties. Many injured in the explosion died of terrible burns soon after. Therefore, it is essential to effectively mitigate the serious damage and prevent the explosion accidents of aluminum dust. Flame inhibition of combustible dust cloud using solid inhibitors is a common practice in industrial explosion inhibition. For example, solid inhibitor is discharged into the hazard volume upon detection of an incipient explosion in process equipment (Chatrathi and Going, 2000). The effectiveness of inhibitor in mitigating explosion damage depends on the selection of inhibitor (Amyotte, 2006), particle size (Chen et al., 2017; Zhang et al., 2017), and concentration (Sun et al., 2019), etc. Knuth et al. (2007) experimentally investigated the flame inhibition effectiveness of dry chemical powders for a diffusion flame inhibition. Comparison of the thermal and chemical effects of these inhibitors indicated that the inhibition effectiveness of NH4H2PO4 was more effective than NaHCO3, KHCO3 and Al2O3. They also found that the
∗
inhibition effectiveness was a function of the specific surface area (particle size). Investigations on particle size effect of solid inhibitors were extensive. Chelliah et al. (2003) studied the effect of NaHCO3 particle size on suppression of methane-air flames. Flame suppression results suggested that the inhibition efficiency increased with NaHCO3 particle size decreased. The effect of NaHCO3 particle size on the inhibition of aluminum dust explosions was investigated by Jiang et al. (2018). This latter study also indicated that small NaHCO3 particles were more effective than larger particles. Sridhar Iya et al. (1975) and Birchall (1970) demonstrated that particles below 10 μm would be completely decomposed in the preheat zone, and large particles (> 200 μm) would penetrate the flame with litter reaction. Although NH4H2PO4 particles have showed excellent flame inhibition performance in previous studies. So far, very little information is available for the effect of NH4H2PO4 particle size on the flame, especially aluminum flame. At the same time, there are few studies to understand flame inhibition mechanism of phosphates on aluminum particles. In this study, the inhibition effectiveness of NH4H2PO4 with various particle sizes on flame propagation of aluminum dust cloud is determined systematically. The effects of NH4H2PO4 particle size on flame morphology and flame propagation velocity of aluminum dust cloud are studied. In order to deeply investigate the flame inhibition mechanism, a tentative kinetic model considering both gas and surface
Corresponding author. E-mail address: [email protected] (W. Gao).
https://doi.org/10.1016/j.jlp.2019.05.009 Received 1 February 2019; Received in revised form 16 May 2019; Accepted 17 May 2019 Available online 20 May 2019 0950-4230/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Loss Prevention in the Process Industries 60 (2019) 311–316
H. Jiang, et al.
Fig. 2. Illustration of the process of aluminum vaporization.
performed at constant volume using 0‒D Homogeneous reactor in the Chemkin Pro. The attention of modeling study for surface reaction suppression was paid to the surface and gas/surface interactions at the vicinity of an aluminum particle. The detailed description of the gas‒phase mechanism of Al/air flames doped with P‒containing species and NH3 was given by Jiang et al. (2019). The gas‒phase mechanism was drawn from previous work on reactions of aluminum (Catoire et al., 2003), NH3 (Konnov, 2009; Li et al., 2013), H3PO4 (Twarowski, 1996; Jayaweera et al., 2005; Korobeinichev et al., 2007), involving 87 species and 429 elementary reactions. Surface chemistry of aluminum particles combustion was adopted from the previous work of Glorian et al. (2016), involving 19 surface species and 49 surface reactions. Surface chemistry considered species adsorbed on the solid surface. In this study, species were adsorbed on the open sites Al(L) of the aluminum surface. Surface site species and bulk-phase species were noted using suffixes (S) and (B), respectively. In the surface mechanism, the vaporization process of aluminum particles was described by reaction Al(B) + Al(L) = Al + Al(L). The illustration of the process of aluminum vaporization was shown in Fig. 2. In this reaction, the bulk aluminum Al(B) reacted with the open sites Al(L) to form the gaseous Al and the open sites Al(L).
Fig. 1. Experiment apparatus.
chemistry is developed. The effects of NH4H2PO4 on gas and surface reaction of aluminum particles burning are further discussed. 2. Experimental The experimental apparatus and methods were similar to our previous work (Jiang et al., 2019). As shown in Fig. 1, the apparatus consists of cylindrical combustion tubes, an air-fuel dispersion system, a high-speed camera, an ignition system, a data acquisition system unit, and a time controller. The mixture powder was placed in a hemispherical cup of the dispersion unit. The dispersion pressure was 0.46 MPa, and the dispersion duration was 0.5 s. The spark lasted 0.25 s. Flame propagation processes and the flame microstructures were visualized with a Photron SA4 high‒speed video camera, operated at 10000 frames per second. 5 μm aluminum dust is employed as the experimental sample. The real mass density of aluminum dust cloud is 300 g/m3. Monoammonium phosphate powder (NH4H2PO4 > 99%, Macklin Inc.) is selected as inhibitor, since it is more effective and widely used in explosion prevention system. The NH4H2PO4 particles are sifted into size ranges of 25–53 μm, 53–100 μm, and 100–212 μm. All samples are dried before experiments. The inerting ratio (α) that referrers to the ratio of mass of inhibitor to mass of explosible dust is used in this study. Al/NH4H2PO4 dust mixture is fully premixed with an inerting ratio ranging from 0 to 2.0.
4. Experimental results 4.1. Flame morphology and microstructures Fig. 3 shows the variation of aluminum flame morphology doped with different particle size NH4H2PO4. 100–212 μm NH4H2PO4 particles have a slight impact on aluminum flame. It is indicated that larger NH4H2PO4 particles are partially decomposed and have little interaction with the flame. For addition of 53–100 μm NH4H2PO4 particles, the luminosity of the aluminum flame is obviously weakened at α = 2.0. The results for 25–53 μm NH4H2PO4 particles show a clear change in 5 μm aluminum flame morphology. Aluminum flame exhibits a dark yellow flame, just like ammonia (NH3) flame. Previous studies (AbdelKader et al., 1991) have revealed that NH4H2PO4 particles decompose to generate NH3 at approximately 210 °C. Hence, it can be inferred that 25–53 μm NH4H2PO4 particles can reach the flame front, decompose completely, and have a fully chemical interaction with aluminum flame. These results demonstrate that 25–53 μm particles are the most effective in flame inhibition of aluminum dust, followed by 53–100 μm, and then 100–212 μm. The effects of NH4H2PO4 particle size are further explored by observing the variation in the microstructures of aluminum flame. Flame microstructures in Al/NH4H2PO4 mixture are shown in Fig. 4. These obtained images indicate different flame structures of aluminum flame doped with various particle size NH4H2PO4. As shown in Fig. 4a, aluminum flame front is similar to the gas flame, perhaps indicating a premixed ammonia flame. In contrast, aluminum particles surrounded by gas phase flame are observed for the two size groups 53–100 μm and 100–212 μm. These above results confirm that 25–53 μm NH4H2PO4 particles can penetrate the flame front, fully decompose within aluminum flame, and exert the greatest inhibition effectiveness.
3. Numerical methods 3.1. Initial conditions This paper deals with nascent 5 μm aluminum particles by ignoring the presence of the alumina shell. Both gas and surface reactions of aluminum combustion were considered in the numerical model. Based on the work of Glorian et al. (2016), the temperature of aluminum particles was assumed to be 2700 K, since it should be higher than the melting temperature of the alumina shell (≈2300K). Initial pressure was 1 atm. The mole ratio of N2 and O2 for air was 3.76. In the reaction zone, NH4H2PO4 particles had been decomposed to form H3PO4 and NH3, NH4H2PO4 → H3PO4 + NH3. The condensed phase products of NH4H2PO4 decomposition were not considered. To further reveal the gas-phase and surface inhibition mechanism of NH4H2PO4 on aluminum dust explosion, an investigation of stoichiometric concentration (ϕ Al = 1) under different inerting ratios was performed. 3.2. Kinetic mechanism The modeling studies for the undoped and doped Al/air flames were 312
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velocity decreases considerably with the addition of NH4H2PO4 particles. Fig. 6 plots the average flame propagation velocity as a function of NH4H2PO4 concentration and particle size. Results show that the average flame propagation velocity decreases with the NH4H2PO4 concentration increases. Meanwhile, with the reduction of NH4H2PO4 particle size, the results show a clear decrease in flame propagation velocity of aluminum flame. It is indicated that small NH4H2PO4 particles in the particle size range studied exert a greater inhibition effectiveness in flame speed reduction compared to the coarser one.
5. Inhibition mechanism 5.1. Physical inhibition mechanism With regard to particle-flame interaction, the inhibition mechanism of solid chemical inhibitors occurs in four determinant steps (Mitani and Niioka, 1984). First, NH4H2PO4 particles undergo heating due to initiated aluminum combustion. Second, NH4H2PO4 particles begin to decompose. Third, gas and condensed phase decomposition products are produced. Fourth, chemical interaction with the flame occurs. Chemical inhibition cannot be accomplished unless the total duration of these four steps (ts) is shorter than the burning time (tb) of 5 μm Al particles. If tb < ts, NH4H2PO4 particles have less capacity to exert the chemical effect in addition to the physical effect of dilution and thermal absorption. The effectiveness of the physical effect depends on the ability of solid inhibitors to absorb heat. The endothermic decomposition process of solid particles is limited by the rate of heat transfer (Hp) and the thermal relaxation timescale (τT) (Cloney et al., 2018):
Hp = πDp λs Nu (T − Tp)
(1)
Nu = 2 + 0.6 Re1/2 Pr1/3
(2)
Fig. 3. Flame propagations of undoped and doped 5 μm Al/air mixture.
τT = 4.2. Flame propagation velocities
mp CPp πDp λs Nu
(3)
where Dp denotes particle diameter, λs is the thermal conductivity, T is the fluid temperature, Tp is the particle temperature, CPp is specific heat at constant pressure, Nu is the non-dimensional Nusselt number and Pr is the non-dimensional Prandlt number. Based on equations (1)–(3), it can be concluded that fine solid particles have a shorter characteristic time of decomposition/evaporation, resulting in a greater degree of decomposition than coarse particles. Therefore, small NH4H2PO4 particles will decompose most or completely during passage through aluminum flame and are most effective. Even so, as the particle size of solid inhibitor continues to reduce, the inhibition effectiveness of solid inhibitor may decrease, since ultra-fine particles have a short residence time in the flame and may tend to agglomerate. However, our findings indicate that the inhibition effectiveness monotonously increases with the decrease in NH4H2PO4 particle size in the range studied.
The flame edges are recognized through a MATLAB program based on the Roberts operator. The flame propagation velocity can be determined by examining the movement of the flame front. Flame contour on each frame was matched with an ellipse, since the flame is not always spherically symmetrical. The flame radius of undoped and doped Al/air mixture was calculated using the method in the literature (Julien et al., 2015). Flame propagation velocity is derived from the flame radius vs. time. The distances from the flame front to the ignition point is evaluated by the most advanced point along the horizontal direction in the flame front. The distances from the flame front to the ignition point and flame propagation velocity of undoped and doped aluminum flame are shown in Fig. 5. It can be seen that the flame front position is forward with the reduction of particle size. As shown in Fig. 5b, the flame propagation
Fig. 4. Flame microstructures in 5 μm Al/air mixture doped with different particle size NH4H2PO4. (a. 25–53 μm, b. 53–100 μm, c. 100–212 μm). 313
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Fig. 5. Flame front position and flame propagation velocity of 5 μm aluminum dust cloud doped with different particle size NH4H2PO4.
surface chemistry is developed. Fig. 7 plots the concentration of AlO and O across Al flame doped with NH4H2PO4. The calculations show that the concentration of AlO and O atom decreases considerably with the inerting ratio increases. Reactions of P‒containing species (such as O + HPO3 < = > O2+HOPO, HOPO + O + M < = > HPO3+M, and HPO3+O < = > OH + PO3) and ammonia combustion (such as NH3 + O2 < = > NH2+HO2 and NH3 + O < = > NH2 + OH.) can cause a significant decrease in O and O2. Therefore, the gas-phase aluminum flame can be effectively suppressed by NH4H2PO4, since AlO and O play a key role in gaseous-Al/air reaction system. The rate of surface reaction and Al particles vaporization are limited by the number of Al(L), since surface reaction considers species adsorbed on Al(L), and the adsorption capacity of the surface of aluminum particles and the number of Al(L) are limited. The illustration of a surface reaction of O + Al(L) → AlO(s) is shown in Fig. 8. As shown in Fig. 9, Al(L) site fraction decreases with the inerting ratio increases. Therefore, an increase in the concentration of NH4H2PO4 can reduce the vaporization rate of aluminum particles, thereby reducing the surface reaction rate. Nevertheless, the increase in NH4H2PO4 concentration has a slightly effect on O(S), AlO(S) and AlO2(S). These results indicate that the diffusion rate of oxidizers close to the droplet surface slightly depends on the concentration of NH4H2PO4. Fig. 10 shows the results of sensitivity analysis of Al(L) site fraction at α = 0.4, 1.2 and 2.0. Negative sensitivity coefficients represent a reduction of Al(L) fraction due to P‒containing compounds. It can be seen that HPO3+M = PO2+OH + M has the biggest negative sensitivity coefficient and plays an important role of the inhibition effect on
Fig. 6. Average flame propagation velocity of 5 μm aluminum dust cloud doped with different particle size NH4H2PO4.
5.2. Chemical inhibition mechanism To gain more insight for the chemical suppression effect of NH4H2PO4 on aluminum dust explosion, the general behavior of the Al/ air and Al/NH4H2PO4/air is examined in an adiabatic, constant-pressure environment using CHEMKIN software package. Since the burning of 5 μm aluminum particles is a combination of the gas-phase and surface reaction processes, a kinetics model considering both gas and
Fig. 7. Temporal evolution of (a) AlO and (b) O atom in Al/NH4H2PO4/air flame. 314
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conclusions are given as follows. Flame morphology and color change with the addition of different particle size NH4H2PO4. For 25–53 μm NH4H2PO4 particles, aluminum flame front is as smooth as a gas flame. Aluminum particles surrounded by discrete gas phase flame are observed for the other two size distributions of NH4H2PO4. Small NH4H2PO4 particles within the range studied have a greater reduction in average flame propagation compared to the larger NH4H2PO4 particles. Hence, the inhibition effectiveness of aluminum flame depends on the particle size of NH4H2PO4. Aluminum flame inhibition is achieved by particle-flame interaction, including thermal effects and chemical inhibition. Small NH4H2PO4 particles have a better thermal effect, since fine solid particles have a shorter characteristic time of decomposition/evaporation and a larger degree of decomposition than the coarse particles. NH4H2PO4 can suppress the gas phase reaction of aluminum combustion by flame radical consumption. The surface reaction rate and the vaporization rate of aluminum particles are reduced by NH4H2PO4 particles addition. Small NH4H2PO4 particles can be almost completely decomposed to form more gas phase compounds, which produces better chemical inhibition for aluminum flame.
Fig. 8. Illustration of a surface reaction, O + Al(L) → AlO(s).
Acknowledgments The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 51874066 and No. 51674059), Key Laboratory of Building Fire Protection Engineering and Technology of MPS (KFKT2016ZD01), and the Fundamental Research Funds for the Central Universities (DUT16RC(4)04). Appendix A. Supplementary data
Fig. 9. Occupied site fractions at aluminum surface as a function of inerting ratio.
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