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Ti dust explosion hazards during the production of metal hydride TiH2 in a closed vessel
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Hybrid H2/Ti dust explosion hazards during the production of metal hydride TiH2 in a closed vessel Yang-Fan Cheng a,b,*, Shi-Xiang Song a, Hong-Hao Ma b,c, Jian Su a, Ti-Fei Han a, Zhao-Wu Shen b, Xiang-Rui Meng a,** a
School of Chemical Engineering, Anhui University of Science and Technology, Huainan 232001, PR China CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science and Technology of China, Hefei 230027, PR China c State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230026, Anhui, PR China b
article info
abstract
Article history:
The explosibility of hybrid H2/Ti dust in the production of metal hydride TiH2 was simu-
Received 7 January 2019
lated and studied using a 20-L spherical vessel. The influential factors for the explosion
Received in revised form
performance of hybrid H2/Ti dust, including particle size distribution and polydispersity,
14 February 2019
humidity, temperature, hydrogen content, inert gas and degree of reaction, on hybrid ex-
Accepted 22 February 2019
plosion were investigated. Results showed that both the mean particle sizes and particle
Available online 19 March 2019
size polydispersity had significant effects on the dust severity of hybrid H2/Ti dust. The
Keywords:
range, and it presented a trend of increasing at the early stage and then decreased both for
Dust explosion
the increasing humidity and hydrogen pressure. Explosion inhibition effects of typical inert
explosion severity of hybrid H2/Ti dust was enhanced at a higher temperature in a certain
Metallic dust
gases for hybrid H2/Ti dust increased in the following order: argon < helium < nitrogen. The
Particle size polydispersity
values of (dP/dt)ex and Vf decreased along with the reaction process, while the value of Pex
Metal hydride
kept stable, which showed that the hydrogen state had no obvious impact on Pex but
Hydrogen
significantly affected the explosion risk of hybrid H2/Ti dust, and special attention should be paid to the initial stages of the production process of TiH2. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen has prominent advantages such as high energy density, light weight, exceptionally abundant sources, and low environmental impact, which is regarded as one of the most promising alternative energy carriers that can potentially facilitate the transition from fossil fuels to clean energy sources [1]. Nowadays, hydrogen is mainly stored as pressurized gas or cryogenic liquid, but how to store hydrogen
safely and cost-effectively with high energy density remains challenges that restrict the usage of hydrogen energy [2]. Metal hydride is a most promising material for solid-state storage of hydrogen, which has advantages of high hydrogen density and important safety over the conventional methods [3,4]. The probably most conventional hydride synthesis especially for intermetallic hydrides is a reaction in a closed system (autoclave) under high hydrogen pressure and mostly high temperatures around 600e800 K (for the metal
* Corresponding author. School of Chemical Engineering, Anhui University of Science and Technology, Huainan 232001, PR China. ** Corresponding author. E-mail addresses: [email protected] (Y.-F. Cheng), [email protected] (X.-R. Meng). https://doi.org/10.1016/j.ijhydene.2019.02.189 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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particles) [5], and the ball-milling creating fresh surfaces during processing is an economic process that is widely applied to metal hydrides to achieve good surface properties [6]. The creation of defects on the surface and in the interior of metal particles may aid the diffusion of hydrogen in materials by providing many sites with low activation energy of diffusion, and the increased surface contact with catalyst during ball-milling leads to fast kinetics of hydrogen transformations [7]. Huot et al. [8] and Chen et al. [9] produced metal hydrides under H2 atmosphere by ball-milling, the results indicated that pulverization and deformation processes occurring during high-energy ball-milling play a major role in the hydriding reaction. Aoyagi et al. [10] proposed that ballmilling of the alloys under inert environment led to reduction in powder size and creation of new surfaces, which were effective for the improvement of the hydrogen absorption rate. However, there are also numerous explosion risks in the ball-milling of metal hydrides, such as hydrogen explosion, metal dust explosion, and hybrid hydrogen/metal dust explosion [11]. Many factors may affect the explosivity of dust cloud. Ajrash et al. [12] analyzed the impacts of humidities and temperatures on the minimum auto-ignition temperatures of the coal dust layer. Song et al. [13] explored the explosion characteristics of hybrid CH4/coal mixtures with different initial pressures. Gao et al. [14] studied the influence of particle size distributions on the flame propagation characteristics in octadecanol dust clouds in an open space, and the experimental results showed that dust clouds with different particle size distributions would differ obviously in flame front structures. Liu et al. [15] investigated the effect of particle size polydispersity (sD) on explosibility of suspended coal dust using a 20-L spherical vessel, and found that the explosion parameters of coal dust with equal mean diameter (D50) but different sD may varied largely. From the above analysis, there are various influential factors, including self- and external ones, which may affect the explosion risk of metal hydrides in the ball-milling processes. TiH2 is a widely used metal hydride material in the fields of hydrogen storage, production of aluminum foams, explosive compositions, and rocket propellant, etc [16e20]. The raw materials for the production of TiH2 are Ti powders and hydrogen [1]. In the study, the effects of particle size distribution and polydispersity, humidity, temperature, hydrogen content, inert gas and degree of reaction on the dust explosibility of hybrid H2/Ti dust were studied using a 20-L spherical vessel. Furthermore, the explosion characteristics variations in the TiH2 production at different stages were also investigated. The study is significant to the prevention and mitigation of dust explosion of metal hydrides in the production processes.
hybrid H2/Ti dust in the production processes of TiH2, the hydrogen and typical inert gases (nitrogen, helium and argon) were also needed, and the purity of gases were all over 99.99 mass%. The D50 of Ti and TiH2 powders were tested by a laser particle size analyzer (Mastersizer 2000, Malvern, Britain).
Experimental procedures The dust explosion of TiH2 in a closed vessel during the production process was simulated and studied using a 20-L spherical vessel. The experiments were carried out according to the standards of ASTM E-1226 [21]. Fig. 1 shows the schematic of the 20-L spherical vessel testing system. In the experiments, the 20-L spherical vessel was pre-evacuated by a vacuum pump with a final internal pressure of 0.06 MPa, and the powders in dust container were ejected into the vessel with pressurized air of 2.0 MPa, and ignition occurred 60 ms delay from the beginning of dust injection when the inner pressure of the vessel reached 0.04 MPa absolute. For the existence of hydrogen, all of the dust samples could be ignited by an electric spark of electrodes with 2 mm diameter and 1.5 mm gap width, and the ignition energy was kept about 500 mJ in every test. The initial temperature and humidity in the closed vessel were controlled by an oil bath heater and an air moistener, respectively. Each dust sample was tested at least three times.
Experimental data processing methods In the study, typical dust explosion characterization parameters were tested using the 20-L spherical vessel testing system, including the maximum rate of pressure rise ((dP/dt)ex), the maximum pressure rise (Pex), the burning time (t) and the dust flame propagation speed (Vf). The maximum values of (dP/dt)ex and Pex in different concentrations were regarded as Pmax and (dP/dt)max, respectively. Fig. 2 shows a typical explosion pressure-time curve of hybrid H2/Ti dust with a Ti (D50 ¼ 30.3 mm)dust concentration of ca. 300 g/m3 and 3.85 mass% of hydrogen content. The pressure increased after dust dispersion (starting from t1) and ignition (at t2) until the maximum value (at t3) was reached, and the time span between t2 and t3 represented the burning time (t) of dust explosion [15]. The dust flame propagation speed Vf could be estimated by the following formula: Vf ¼ R20L t , where R20-L was the inner radius of the 20-L spherical vessel, t was the burning time of hybrid H2/Ti dust cloud.
Results and discussion The effects of dust particle size distribution and polydispersity
Methodology Experimental materials In the study, Ti powders with different D50 and TiH2 powders (with hydrogen content 3.85 mass%) with D50 of 31.6 mm were chosen for the experiments. In order to investigate the influence of hydrogen content and inert gas on the explosibility of
The influence of D50 of Ti particles on (dP/dt)ex and Pex of the hybrid H2/Ti dust samples were studied using a 20-L spherical vessel testing system. The D50 of Ti particles were 15.5, 30.3 and 80.1 mm (as shown in Fig. 3), and the mass ratio of hydrogen content in hybrid H2/Ti dust was fixed at 3.85 mass %. As shown in Fig. 4a and b, the values of (dP/dt)ex and Pex increased in the fuel-lean mixtures and then decreased in
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Fig. 1 e Schematic of the 20-L spherical vessel testing system.
Fig. 2 e Typical explosion pressure-time curve of Ti dust (D50 ¼ 30.3 mm) with a concentration around 300 g/m3.
Fig. 3 e Particle size distributions of Ti and TiH2 powders.
fuel-rich mixtures for Ti dust samples with varying D50, and the error bars represent one standard deviation of the average value obtained from three tests run under identical conditions. The oxygen content of the 20-L spherical vessel was only ca. 4.0 L, and the oxygen was enough for the burning reaction of Ti dust in fuel-lean mixtures, while in fuel-rich mixtures, the oxygen content became insufficient and the unburned Ti dust would absorb the heat of the combustion, and that is why the (dP/dt)ex and Pex both presented a trend of increasing at the early stage and then decreased as the Ti dust concentration kept increasing. Furthermore, the values of (dP/ dt)ex and Pex increased as the D50 of Ti particles decreased. That is because smaller particles had bigger specific surface areas, then the Ti particles with smaller D50 would take fuller advantage of heat and produce more volatile substance, therefore, the burning time t of smaller Ti particles was shorter than the big ones (as shown in Fig. 4c) and the combustion reaction degree would be also intensified with the increasing burning velocity for the reaction time of dust explosion was limited [15]. In addition, compared to the larger Ti particles, the smaller ones would suspend much longer and disperse more uniformly in the 20-L spherical vessel, which also had a positive influence on the values of (dP/dt)ex and Pex. Particle size polydispersity also has a nonnegligible effect on dust explosions [22,23]. To study the influence of sD on (dP/ dt)ex$V1/3 and Pex with a fixed D50, three kinds of Ti blends with similar D50 but different sD were prepared, and the parameter sD characterized by the span of the size distribution was calculated using the following formula [15]: sD ¼ ðD90 D10 Þ=D50 . We systematically combined the original Ti dust samples with the following mean diameters: 5, 14, 27, 67 and 102 mm, and the values of sD of them were 1.2, 1.1, 1.3, 0.6, 0.6. As listed in Table 1, sD of Ti blend 1 to 3 was 1.3, 2.0 and 4.5, respectively, and Fig. 5 shows the SEM images and particle size distributions of resulting Ti blends. The concentration of Ti dust in the experiment was changed from 100 to 700 g m3 with an interval of 100 g/m3, and the hydrogen content in hybrid H2/Ti dust was fixed at 3.85 mass%.
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Fig. 4 e Experimental results of hybrid H2/Ti dust samples with Ti dust of varying D50: (a) (dP/dt)ex, (b) Pex and (C) t versus dust concentration.
Table 1 e Mass fractions of original Ti dust samples used to generate three blends at similar D50 but varying sD. Blend
1 2 3
Mass fraction ¼ (Original sample mass)/(Total blend mass) 5 mm
14 mm
27 mm
67 mm
102 mm
e 0.1 0.6
e 0.2 0.1
1 0.6 0.1
e 0.1 e
e e 0.2
D10 (mm)
D50 (mm)
D90 (mm)
sD
11.7 5.4 2.8
27.6 26.2 26.3
48.5 58.1 120.4
1.3 2.0 4.5
Fig. 5 e Particle size distributions and SEM images of blended Ti dust samples.
As illustrated in Fig. 6a and b, the optimum concentrations where the maximum values of (dP/dt)ex and Pex obtained for the three hybrid H2/Ti dust samples were equal (600 g m3), but for the three mixtures with the same D50 (27 mm) of Ti particles, the values of (dP/dt)ex and Pex varied largely with different sD, and larger sD ensured bigger (dP/dt)ex and Pex values. Fig. 6c presents that the burning time of Ti blends decreased with the increasing sD, which also shows that larger
sD produced higher burn rate of Ti dust. As the mass ratio of fine dust particles increased (larger sD), the explosion severity of dust blend would be enhanced for the increasing total surface area and volatilization rate [22]. Compared with the large dust particles, the finer ones had a lower ignition temperature [24], a shorter heat diffusion time [25], and a faster burning rate [26]. The fractions of smaller Ti particles were easier to be ignited at lower temperatures and then
Fig. 6 e Experimental results of hybrid H2/Ti dust having D50 of 27 mm with varying sD of Ti particles: (a) (dP/dt)ex, (b) Pex and (C) t versus dust concentration.
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transferred the heat to the larger ones. Therefore, finer Ti particles in the blended samples presented the dominant impact on the combustion process of dust explosion, and that is why the explosion severity of Ti dust samples with equal D50 usually varied largely. Thus, risk assessment evaluation based on hazard associated to Ti dust sample with low sD may result in a serious underestimation.
The effects of initial ambient temperature and humidity In this section, the influences of initial ambient temperature and humidity on dust explosion parameters were studied. The concentration of Ti dust and the hydrogen content in hybrid H2/Ti dust were 300 g/m3 and 3.85 mass%, respectively, and the D50 of Ti particles is 30.3 mm. For the raw materials of intermetallic hydrides synthesis are normally heated to a relative high temperature [5], therefore, the effects of varying initial ambient temperature on the explosion characteristics of Ti dust were studied at a fixed ambient humidity of 30% RH. In the experiments, the maximum internal ambient temperature of the 20-L spherical vessel was around 398 K and did not increase even if continued heating, therefore, the experiments conducted using initial temperature of 298, 323, 348 and 398 K. Fig. 7 shows that the values of (dP/dt)ex, Pex and Vf of Ti dust were all increased at a higher ambient temperature. High temperature was helpful for producing more volatile substance [27] and accelerating the burning rate of metal dust [28], and that is why the explosion severity of hybrid. H2/Ti dust was enhanced with the increasing initial ambient temperature in a certain range. The impacts of ambient humidity on the explosibility of hybrid H2/Ti dust were studied at room temperature. As
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shown in Fig. 8, when the ambient humidity changed from 30 to 50% RH, the values of (dP/dt)ex, Pex and Vf increased, whereas when the ambient humidity raised to 90% RH, the dust explosion severity dropped significantly. In the dust explosion process, Ti particles may react with water vapor and produce a little H2 at a high temperature, and the thermal transfers and heat sink effects inside the 20-L spherical vessel would be also improved due to the presence of water [29], which may account for the dust explosion characteristics of hybrid H2/Ti dust changed along with the increasing ambient humidity at the early stage. However, like the processes of aluminium oxidation described by Trunov et al. [30], several assumptions could be made: when the water was adsorbed onto the Ti particles surface, the phase transformations of titania layer or its growth could be seriously modified by the presence of water, and no more H2 would be produced; furthermore, it could also have an influence on the diffusion flux of the metal from the particle core or the diffusion flux of oxygen from the atmosphere. Therefore, the processes of thermal transfer inhibition and inerting prevailed as the ambient humidity continued to increase, and then the dust explosibility reduced.
The effects of hydrogen and inert gases In the experiments, the dust concentration and hydrogen content in the initial hybrid H2/Ti dust were 300 g/m3 and 3.85 mass%, respectively, and the D50 of Ti particles was 30.3 mm. In order to investigate the effects of gas pressure on the explosion characteristics, the initial ambient pressure in the 20-L spherical vessel was increased by filling hydrogen or typical inert gases (helium, argon and nitrogen). The initial pressure
Fig. 7 e Experimental results of hybrid H2/Ti dust: (a) (dP/dt)ex, (b) Pex and (C) Vf versus initial ambient temperature.
Fig. 8 e Experimental results of hybrid H2/Ti dust: (a) (dP/dt)ex, (b) Pex and (C) Vf versus initial ambient humidity.
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of 0.1 MPa corresponded to no addition of gas in the initial hybrid H2/Ti dust. As shown in Fig. 9, when the hydrogen pressure kept increasing, the values of (dP/dt)ex, Pex and Vf all presented a trend of increasing at the early stage and then decreased. The hydrogen content in the 20-L spherical vessel increased with the increasing hydrogen pressure, and the two fuels of hybrid H2/Ti dust competed for the oxygen. When the hydrogen pressure is low, the oxygen was enough for the mixture, so the values of (dP/dt)ex, Pex and Vf all enhanced with the higher hydrogen pressure, while the oxygen became insufficient as the hydrogen pressure continued to increase, and for the burning rate of combustible gas was much faster than that of solid dust [31,32], and then more Ti particles would be remained in the combustion products. For both the hybrid explosion pressures and pressure rise rates are higher than that of either H2 or metal dusts [33], and the burning velocity has a positive correlation with (dP/dt)ex [13], and that is why the values of (dP/dt)ex, Pex, and Vf of hybrid H2/Ti decreased when the hydrogen pressure continued to increase. Inert gases are normally used in the production of metal hydrides as protective gas and they are effective for the improvement of the hydrogen absorption rate [10]. Fig. 9 also demonstrates the influences of typical inert gases (helium, argon and nitrogen) at different gas pressures on the explosion severity of hybrid H2/Ti dust. As elaborated in Fig. 9, the values of (dP/dt)ex and Vf both decreased along with the increasing inert gas pressure from 0.11 to 0.14 MPa, while the value of Pex progressively increased, and the experimental results showed that the explosion inhibition effects of these inert gases increased in the following order: argon < helium < nitrogen. Explosion inhibition mechanism of hybrid H2/Ti
dust with inert gas was rather complicated, which may related to the specific heat and thermal conductivity of inert gas, and the heat and mass diffusivity of the mixture [28] as well as reactions between inert gas and Ti particle (such as 2Ti þ N2/2TiN) in dust explosion, etc. Therefore, detailed experimental analysis should be performed to explain these phenomena and would be the subject of subsequent publication.
Explosion characteristics at different production stages Explosion characteristics at different production stages of TiH2 were studied by adjusting the mass ratios of hydrogen, Ti and TiH2 particles in the mixture. The values of degree of reaction of 0 and 1 corresponded to the mixture included hybrid H2/Ti mixture and only TiH2 powders, respectively, and the mixture consisted of H2, Ti and TiH2 powders when the value of degree of reaction between 0 and 1. The concentration of Ti dust (with D50 of 30.3 mm) and hydrogen content in the initial hybrid H2/Ti mixture were 300 g/m3 and 3.85 mass%, respectively. The initial gas pressure was adjusted to 0.14 MPa by nitrogen and the inner temperature of the 20-L spherical vessel was fixed at 333 K. Fig. 10 shows that the values of (dP/dt)ex and Vf of mixture decreased along with the reaction process (represented by degrees of reaction), and surprisingly, the value of Pex kept stable. The mass ratios of hydrogen and Ti elements were tantamount but the compositions of the mixtures were different in the whole production process, which showed that the hydrogen state had no obvious impact on Pex of dust mixtures. While the hydrogen gas content would be reduced
Fig. 9 e Experimental results of hybrid H2/Ti dust: (a) (dP/dt)ex, (b) Pex and (C) Vf versus initial gas pressure for hydrogen and typical inert gases.
Fig. 10 e Experimental results of hybrid H2/Ti dust mixture with different degrees of reaction: (a) (dP/dt)ex, (b) Pex and (C) Vf.
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as the production process continued, and for the burning rate of hydrogen was much higher than that of Ti or TiH2 particles [1], which may account for the deduction of (dP/dt)ex and Vf values, and the explosion risk was decreased with a higher degree of reaction in the metal hydrides production. From the above analysis, it could be concluded that the existence of hydrogen would significantly affect the explosion risk of dust mixture. Therefore, much attention should be paid to the production process of TiH2 which includes hybrid H2/Ti dust normally in a closed vessel with high pressure and high temperature, and especially in the initial stages. Furthermore, the storage of TiH2 should be also carefully checked as it may have an explosion risk of hybrid H2/Ti/TiH2 dust due to the thermal decomposition of TiH2 [16].
Conclusions In the study, dust explosion hazards during the production of metal hydride TiH2 in a closed vessel were simulated and studied using a 20-L spherical vessel. The main conclusions of this experimental work were as follows: The values of (dP/dt)ex$V1/3 and Pex of hybrid H2/Ti dust samples increased as the D50 of Ti particles decreased, and their values varied largely with equal D50 but different sD of Ti particles, and larger sD ensured bigger (dP/dt)ex and Pex values. Thus, explosion risks of hybrid H2/Ti dust should be evaluated in terms of D50 and sD, and risk assessment evaluation based on hazard associated to Ti dust sample with low sD may result in a serious underestimation. The explosion severity of hybrid H2/Ti dust was enhanced with the increasing initial ambient temperature in a certain range, while its dust explosibility increased at first and then dropped significantly as the ambient humidity increased. Therefore, a suitable ambient temperature and humidity are very important for the production quality and safety of metal hydride TiH2. As the hydrogen pressure kept increasing, the values of (dP/dt)ex, Pex and Vf of hybrid H2/Ti dust all presented a trend of increasing at the early stage and then decreased. The values of (dP/dt)ex and Vf both decreased along with the increasing inert gas pressure, while the value of Pex progressively increased, and the explosion inhibition effects of these inert gases increased in the following order: argon < helium < nitrogen. The values of (dP/dt)ex and Vf decreased along with the reaction process, while the value of Pex kept stable, which showed that the hydrogen state had no obvious impact on Pex but significantly affected the explosion risk of dust mixture. Therefore, much attention should be paid to the production process of TiH2 (especially in the initial stages), which includes hybrid H2/Ti dust normally in a closed vessel with high pressure and high temperature.
Acknowledgements This work was supported by National Natural Science Foundation of China (Nos. 11602001; 51874267 and 51674229) and
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Open Project Foundation of CAS Key Laboratory of Mechanical Behavior and Design of Materials (No. lmbd201701).
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ScienceDirect journal homepage: www.elsevier.com/locate/he
Hybrid H2/Ti dust explosion hazards during the production of metal hydride TiH2 in a closed vessel Yang-Fan Cheng a,b,*, Shi-Xiang Song a, Hong-Hao Ma b,c, Jian Su a, Ti-Fei Han a, Zhao-Wu Shen b, Xiang-Rui Meng a,** a
School of Chemical Engineering, Anhui University of Science and Technology, Huainan 232001, PR China CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science and Technology of China, Hefei 230027, PR China c State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230026, Anhui, PR China b
article info
abstract
Article history:
The explosibility of hybrid H2/Ti dust in the production of metal hydride TiH2 was simu-
Received 7 January 2019
lated and studied using a 20-L spherical vessel. The influential factors for the explosion
Received in revised form
performance of hybrid H2/Ti dust, including particle size distribution and polydispersity,
14 February 2019
humidity, temperature, hydrogen content, inert gas and degree of reaction, on hybrid ex-
Accepted 22 February 2019
plosion were investigated. Results showed that both the mean particle sizes and particle
Available online 19 March 2019
size polydispersity had significant effects on the dust severity of hybrid H2/Ti dust. The
Keywords:
range, and it presented a trend of increasing at the early stage and then decreased both for
Dust explosion
the increasing humidity and hydrogen pressure. Explosion inhibition effects of typical inert
explosion severity of hybrid H2/Ti dust was enhanced at a higher temperature in a certain
Metallic dust
gases for hybrid H2/Ti dust increased in the following order: argon < helium < nitrogen. The
Particle size polydispersity
values of (dP/dt)ex and Vf decreased along with the reaction process, while the value of Pex
Metal hydride
kept stable, which showed that the hydrogen state had no obvious impact on Pex but
Hydrogen
significantly affected the explosion risk of hybrid H2/Ti dust, and special attention should be paid to the initial stages of the production process of TiH2. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen has prominent advantages such as high energy density, light weight, exceptionally abundant sources, and low environmental impact, which is regarded as one of the most promising alternative energy carriers that can potentially facilitate the transition from fossil fuels to clean energy sources [1]. Nowadays, hydrogen is mainly stored as pressurized gas or cryogenic liquid, but how to store hydrogen
safely and cost-effectively with high energy density remains challenges that restrict the usage of hydrogen energy [2]. Metal hydride is a most promising material for solid-state storage of hydrogen, which has advantages of high hydrogen density and important safety over the conventional methods [3,4]. The probably most conventional hydride synthesis especially for intermetallic hydrides is a reaction in a closed system (autoclave) under high hydrogen pressure and mostly high temperatures around 600e800 K (for the metal
* Corresponding author. School of Chemical Engineering, Anhui University of Science and Technology, Huainan 232001, PR China. ** Corresponding author. E-mail addresses: [email protected] (Y.-F. Cheng), [email protected] (X.-R. Meng). https://doi.org/10.1016/j.ijhydene.2019.02.189 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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particles) [5], and the ball-milling creating fresh surfaces during processing is an economic process that is widely applied to metal hydrides to achieve good surface properties [6]. The creation of defects on the surface and in the interior of metal particles may aid the diffusion of hydrogen in materials by providing many sites with low activation energy of diffusion, and the increased surface contact with catalyst during ball-milling leads to fast kinetics of hydrogen transformations [7]. Huot et al. [8] and Chen et al. [9] produced metal hydrides under H2 atmosphere by ball-milling, the results indicated that pulverization and deformation processes occurring during high-energy ball-milling play a major role in the hydriding reaction. Aoyagi et al. [10] proposed that ballmilling of the alloys under inert environment led to reduction in powder size and creation of new surfaces, which were effective for the improvement of the hydrogen absorption rate. However, there are also numerous explosion risks in the ball-milling of metal hydrides, such as hydrogen explosion, metal dust explosion, and hybrid hydrogen/metal dust explosion [11]. Many factors may affect the explosivity of dust cloud. Ajrash et al. [12] analyzed the impacts of humidities and temperatures on the minimum auto-ignition temperatures of the coal dust layer. Song et al. [13] explored the explosion characteristics of hybrid CH4/coal mixtures with different initial pressures. Gao et al. [14] studied the influence of particle size distributions on the flame propagation characteristics in octadecanol dust clouds in an open space, and the experimental results showed that dust clouds with different particle size distributions would differ obviously in flame front structures. Liu et al. [15] investigated the effect of particle size polydispersity (sD) on explosibility of suspended coal dust using a 20-L spherical vessel, and found that the explosion parameters of coal dust with equal mean diameter (D50) but different sD may varied largely. From the above analysis, there are various influential factors, including self- and external ones, which may affect the explosion risk of metal hydrides in the ball-milling processes. TiH2 is a widely used metal hydride material in the fields of hydrogen storage, production of aluminum foams, explosive compositions, and rocket propellant, etc [16e20]. The raw materials for the production of TiH2 are Ti powders and hydrogen [1]. In the study, the effects of particle size distribution and polydispersity, humidity, temperature, hydrogen content, inert gas and degree of reaction on the dust explosibility of hybrid H2/Ti dust were studied using a 20-L spherical vessel. Furthermore, the explosion characteristics variations in the TiH2 production at different stages were also investigated. The study is significant to the prevention and mitigation of dust explosion of metal hydrides in the production processes.
hybrid H2/Ti dust in the production processes of TiH2, the hydrogen and typical inert gases (nitrogen, helium and argon) were also needed, and the purity of gases were all over 99.99 mass%. The D50 of Ti and TiH2 powders were tested by a laser particle size analyzer (Mastersizer 2000, Malvern, Britain).
Experimental procedures The dust explosion of TiH2 in a closed vessel during the production process was simulated and studied using a 20-L spherical vessel. The experiments were carried out according to the standards of ASTM E-1226 [21]. Fig. 1 shows the schematic of the 20-L spherical vessel testing system. In the experiments, the 20-L spherical vessel was pre-evacuated by a vacuum pump with a final internal pressure of 0.06 MPa, and the powders in dust container were ejected into the vessel with pressurized air of 2.0 MPa, and ignition occurred 60 ms delay from the beginning of dust injection when the inner pressure of the vessel reached 0.04 MPa absolute. For the existence of hydrogen, all of the dust samples could be ignited by an electric spark of electrodes with 2 mm diameter and 1.5 mm gap width, and the ignition energy was kept about 500 mJ in every test. The initial temperature and humidity in the closed vessel were controlled by an oil bath heater and an air moistener, respectively. Each dust sample was tested at least three times.
Experimental data processing methods In the study, typical dust explosion characterization parameters were tested using the 20-L spherical vessel testing system, including the maximum rate of pressure rise ((dP/dt)ex), the maximum pressure rise (Pex), the burning time (t) and the dust flame propagation speed (Vf). The maximum values of (dP/dt)ex and Pex in different concentrations were regarded as Pmax and (dP/dt)max, respectively. Fig. 2 shows a typical explosion pressure-time curve of hybrid H2/Ti dust with a Ti (D50 ¼ 30.3 mm)dust concentration of ca. 300 g/m3 and 3.85 mass% of hydrogen content. The pressure increased after dust dispersion (starting from t1) and ignition (at t2) until the maximum value (at t3) was reached, and the time span between t2 and t3 represented the burning time (t) of dust explosion [15]. The dust flame propagation speed Vf could be estimated by the following formula: Vf ¼ R20L t , where R20-L was the inner radius of the 20-L spherical vessel, t was the burning time of hybrid H2/Ti dust cloud.
Results and discussion The effects of dust particle size distribution and polydispersity
Methodology Experimental materials In the study, Ti powders with different D50 and TiH2 powders (with hydrogen content 3.85 mass%) with D50 of 31.6 mm were chosen for the experiments. In order to investigate the influence of hydrogen content and inert gas on the explosibility of
The influence of D50 of Ti particles on (dP/dt)ex and Pex of the hybrid H2/Ti dust samples were studied using a 20-L spherical vessel testing system. The D50 of Ti particles were 15.5, 30.3 and 80.1 mm (as shown in Fig. 3), and the mass ratio of hydrogen content in hybrid H2/Ti dust was fixed at 3.85 mass %. As shown in Fig. 4a and b, the values of (dP/dt)ex and Pex increased in the fuel-lean mixtures and then decreased in
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Fig. 1 e Schematic of the 20-L spherical vessel testing system.
Fig. 2 e Typical explosion pressure-time curve of Ti dust (D50 ¼ 30.3 mm) with a concentration around 300 g/m3.
Fig. 3 e Particle size distributions of Ti and TiH2 powders.
fuel-rich mixtures for Ti dust samples with varying D50, and the error bars represent one standard deviation of the average value obtained from three tests run under identical conditions. The oxygen content of the 20-L spherical vessel was only ca. 4.0 L, and the oxygen was enough for the burning reaction of Ti dust in fuel-lean mixtures, while in fuel-rich mixtures, the oxygen content became insufficient and the unburned Ti dust would absorb the heat of the combustion, and that is why the (dP/dt)ex and Pex both presented a trend of increasing at the early stage and then decreased as the Ti dust concentration kept increasing. Furthermore, the values of (dP/ dt)ex and Pex increased as the D50 of Ti particles decreased. That is because smaller particles had bigger specific surface areas, then the Ti particles with smaller D50 would take fuller advantage of heat and produce more volatile substance, therefore, the burning time t of smaller Ti particles was shorter than the big ones (as shown in Fig. 4c) and the combustion reaction degree would be also intensified with the increasing burning velocity for the reaction time of dust explosion was limited [15]. In addition, compared to the larger Ti particles, the smaller ones would suspend much longer and disperse more uniformly in the 20-L spherical vessel, which also had a positive influence on the values of (dP/dt)ex and Pex. Particle size polydispersity also has a nonnegligible effect on dust explosions [22,23]. To study the influence of sD on (dP/ dt)ex$V1/3 and Pex with a fixed D50, three kinds of Ti blends with similar D50 but different sD were prepared, and the parameter sD characterized by the span of the size distribution was calculated using the following formula [15]: sD ¼ ðD90 D10 Þ=D50 . We systematically combined the original Ti dust samples with the following mean diameters: 5, 14, 27, 67 and 102 mm, and the values of sD of them were 1.2, 1.1, 1.3, 0.6, 0.6. As listed in Table 1, sD of Ti blend 1 to 3 was 1.3, 2.0 and 4.5, respectively, and Fig. 5 shows the SEM images and particle size distributions of resulting Ti blends. The concentration of Ti dust in the experiment was changed from 100 to 700 g m3 with an interval of 100 g/m3, and the hydrogen content in hybrid H2/Ti dust was fixed at 3.85 mass%.
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Fig. 4 e Experimental results of hybrid H2/Ti dust samples with Ti dust of varying D50: (a) (dP/dt)ex, (b) Pex and (C) t versus dust concentration.
Table 1 e Mass fractions of original Ti dust samples used to generate three blends at similar D50 but varying sD. Blend
1 2 3
Mass fraction ¼ (Original sample mass)/(Total blend mass) 5 mm
14 mm
27 mm
67 mm
102 mm
e 0.1 0.6
e 0.2 0.1
1 0.6 0.1
e 0.1 e
e e 0.2
D10 (mm)
D50 (mm)
D90 (mm)
sD
11.7 5.4 2.8
27.6 26.2 26.3
48.5 58.1 120.4
1.3 2.0 4.5
Fig. 5 e Particle size distributions and SEM images of blended Ti dust samples.
As illustrated in Fig. 6a and b, the optimum concentrations where the maximum values of (dP/dt)ex and Pex obtained for the three hybrid H2/Ti dust samples were equal (600 g m3), but for the three mixtures with the same D50 (27 mm) of Ti particles, the values of (dP/dt)ex and Pex varied largely with different sD, and larger sD ensured bigger (dP/dt)ex and Pex values. Fig. 6c presents that the burning time of Ti blends decreased with the increasing sD, which also shows that larger
sD produced higher burn rate of Ti dust. As the mass ratio of fine dust particles increased (larger sD), the explosion severity of dust blend would be enhanced for the increasing total surface area and volatilization rate [22]. Compared with the large dust particles, the finer ones had a lower ignition temperature [24], a shorter heat diffusion time [25], and a faster burning rate [26]. The fractions of smaller Ti particles were easier to be ignited at lower temperatures and then
Fig. 6 e Experimental results of hybrid H2/Ti dust having D50 of 27 mm with varying sD of Ti particles: (a) (dP/dt)ex, (b) Pex and (C) t versus dust concentration.
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transferred the heat to the larger ones. Therefore, finer Ti particles in the blended samples presented the dominant impact on the combustion process of dust explosion, and that is why the explosion severity of Ti dust samples with equal D50 usually varied largely. Thus, risk assessment evaluation based on hazard associated to Ti dust sample with low sD may result in a serious underestimation.
The effects of initial ambient temperature and humidity In this section, the influences of initial ambient temperature and humidity on dust explosion parameters were studied. The concentration of Ti dust and the hydrogen content in hybrid H2/Ti dust were 300 g/m3 and 3.85 mass%, respectively, and the D50 of Ti particles is 30.3 mm. For the raw materials of intermetallic hydrides synthesis are normally heated to a relative high temperature [5], therefore, the effects of varying initial ambient temperature on the explosion characteristics of Ti dust were studied at a fixed ambient humidity of 30% RH. In the experiments, the maximum internal ambient temperature of the 20-L spherical vessel was around 398 K and did not increase even if continued heating, therefore, the experiments conducted using initial temperature of 298, 323, 348 and 398 K. Fig. 7 shows that the values of (dP/dt)ex, Pex and Vf of Ti dust were all increased at a higher ambient temperature. High temperature was helpful for producing more volatile substance [27] and accelerating the burning rate of metal dust [28], and that is why the explosion severity of hybrid. H2/Ti dust was enhanced with the increasing initial ambient temperature in a certain range. The impacts of ambient humidity on the explosibility of hybrid H2/Ti dust were studied at room temperature. As
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shown in Fig. 8, when the ambient humidity changed from 30 to 50% RH, the values of (dP/dt)ex, Pex and Vf increased, whereas when the ambient humidity raised to 90% RH, the dust explosion severity dropped significantly. In the dust explosion process, Ti particles may react with water vapor and produce a little H2 at a high temperature, and the thermal transfers and heat sink effects inside the 20-L spherical vessel would be also improved due to the presence of water [29], which may account for the dust explosion characteristics of hybrid H2/Ti dust changed along with the increasing ambient humidity at the early stage. However, like the processes of aluminium oxidation described by Trunov et al. [30], several assumptions could be made: when the water was adsorbed onto the Ti particles surface, the phase transformations of titania layer or its growth could be seriously modified by the presence of water, and no more H2 would be produced; furthermore, it could also have an influence on the diffusion flux of the metal from the particle core or the diffusion flux of oxygen from the atmosphere. Therefore, the processes of thermal transfer inhibition and inerting prevailed as the ambient humidity continued to increase, and then the dust explosibility reduced.
The effects of hydrogen and inert gases In the experiments, the dust concentration and hydrogen content in the initial hybrid H2/Ti dust were 300 g/m3 and 3.85 mass%, respectively, and the D50 of Ti particles was 30.3 mm. In order to investigate the effects of gas pressure on the explosion characteristics, the initial ambient pressure in the 20-L spherical vessel was increased by filling hydrogen or typical inert gases (helium, argon and nitrogen). The initial pressure
Fig. 7 e Experimental results of hybrid H2/Ti dust: (a) (dP/dt)ex, (b) Pex and (C) Vf versus initial ambient temperature.
Fig. 8 e Experimental results of hybrid H2/Ti dust: (a) (dP/dt)ex, (b) Pex and (C) Vf versus initial ambient humidity.
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of 0.1 MPa corresponded to no addition of gas in the initial hybrid H2/Ti dust. As shown in Fig. 9, when the hydrogen pressure kept increasing, the values of (dP/dt)ex, Pex and Vf all presented a trend of increasing at the early stage and then decreased. The hydrogen content in the 20-L spherical vessel increased with the increasing hydrogen pressure, and the two fuels of hybrid H2/Ti dust competed for the oxygen. When the hydrogen pressure is low, the oxygen was enough for the mixture, so the values of (dP/dt)ex, Pex and Vf all enhanced with the higher hydrogen pressure, while the oxygen became insufficient as the hydrogen pressure continued to increase, and for the burning rate of combustible gas was much faster than that of solid dust [31,32], and then more Ti particles would be remained in the combustion products. For both the hybrid explosion pressures and pressure rise rates are higher than that of either H2 or metal dusts [33], and the burning velocity has a positive correlation with (dP/dt)ex [13], and that is why the values of (dP/dt)ex, Pex, and Vf of hybrid H2/Ti decreased when the hydrogen pressure continued to increase. Inert gases are normally used in the production of metal hydrides as protective gas and they are effective for the improvement of the hydrogen absorption rate [10]. Fig. 9 also demonstrates the influences of typical inert gases (helium, argon and nitrogen) at different gas pressures on the explosion severity of hybrid H2/Ti dust. As elaborated in Fig. 9, the values of (dP/dt)ex and Vf both decreased along with the increasing inert gas pressure from 0.11 to 0.14 MPa, while the value of Pex progressively increased, and the experimental results showed that the explosion inhibition effects of these inert gases increased in the following order: argon < helium < nitrogen. Explosion inhibition mechanism of hybrid H2/Ti
dust with inert gas was rather complicated, which may related to the specific heat and thermal conductivity of inert gas, and the heat and mass diffusivity of the mixture [28] as well as reactions between inert gas and Ti particle (such as 2Ti þ N2/2TiN) in dust explosion, etc. Therefore, detailed experimental analysis should be performed to explain these phenomena and would be the subject of subsequent publication.
Explosion characteristics at different production stages Explosion characteristics at different production stages of TiH2 were studied by adjusting the mass ratios of hydrogen, Ti and TiH2 particles in the mixture. The values of degree of reaction of 0 and 1 corresponded to the mixture included hybrid H2/Ti mixture and only TiH2 powders, respectively, and the mixture consisted of H2, Ti and TiH2 powders when the value of degree of reaction between 0 and 1. The concentration of Ti dust (with D50 of 30.3 mm) and hydrogen content in the initial hybrid H2/Ti mixture were 300 g/m3 and 3.85 mass%, respectively. The initial gas pressure was adjusted to 0.14 MPa by nitrogen and the inner temperature of the 20-L spherical vessel was fixed at 333 K. Fig. 10 shows that the values of (dP/dt)ex and Vf of mixture decreased along with the reaction process (represented by degrees of reaction), and surprisingly, the value of Pex kept stable. The mass ratios of hydrogen and Ti elements were tantamount but the compositions of the mixtures were different in the whole production process, which showed that the hydrogen state had no obvious impact on Pex of dust mixtures. While the hydrogen gas content would be reduced
Fig. 9 e Experimental results of hybrid H2/Ti dust: (a) (dP/dt)ex, (b) Pex and (C) Vf versus initial gas pressure for hydrogen and typical inert gases.
Fig. 10 e Experimental results of hybrid H2/Ti dust mixture with different degrees of reaction: (a) (dP/dt)ex, (b) Pex and (C) Vf.
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as the production process continued, and for the burning rate of hydrogen was much higher than that of Ti or TiH2 particles [1], which may account for the deduction of (dP/dt)ex and Vf values, and the explosion risk was decreased with a higher degree of reaction in the metal hydrides production. From the above analysis, it could be concluded that the existence of hydrogen would significantly affect the explosion risk of dust mixture. Therefore, much attention should be paid to the production process of TiH2 which includes hybrid H2/Ti dust normally in a closed vessel with high pressure and high temperature, and especially in the initial stages. Furthermore, the storage of TiH2 should be also carefully checked as it may have an explosion risk of hybrid H2/Ti/TiH2 dust due to the thermal decomposition of TiH2 [16].
Conclusions In the study, dust explosion hazards during the production of metal hydride TiH2 in a closed vessel were simulated and studied using a 20-L spherical vessel. The main conclusions of this experimental work were as follows: The values of (dP/dt)ex$V1/3 and Pex of hybrid H2/Ti dust samples increased as the D50 of Ti particles decreased, and their values varied largely with equal D50 but different sD of Ti particles, and larger sD ensured bigger (dP/dt)ex and Pex values. Thus, explosion risks of hybrid H2/Ti dust should be evaluated in terms of D50 and sD, and risk assessment evaluation based on hazard associated to Ti dust sample with low sD may result in a serious underestimation. The explosion severity of hybrid H2/Ti dust was enhanced with the increasing initial ambient temperature in a certain range, while its dust explosibility increased at first and then dropped significantly as the ambient humidity increased. Therefore, a suitable ambient temperature and humidity are very important for the production quality and safety of metal hydride TiH2. As the hydrogen pressure kept increasing, the values of (dP/dt)ex, Pex and Vf of hybrid H2/Ti dust all presented a trend of increasing at the early stage and then decreased. The values of (dP/dt)ex and Vf both decreased along with the increasing inert gas pressure, while the value of Pex progressively increased, and the explosion inhibition effects of these inert gases increased in the following order: argon < helium < nitrogen. The values of (dP/dt)ex and Vf decreased along with the reaction process, while the value of Pex kept stable, which showed that the hydrogen state had no obvious impact on Pex but significantly affected the explosion risk of dust mixture. Therefore, much attention should be paid to the production process of TiH2 (especially in the initial stages), which includes hybrid H2/Ti dust normally in a closed vessel with high pressure and high temperature.
Acknowledgements This work was supported by National Natural Science Foundation of China (Nos. 11602001; 51874267 and 51674229) and
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Open Project Foundation of CAS Key Laboratory of Mechanical Behavior and Design of Materials (No. lmbd201701).
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