Investigation on the Inhibition of Aluminum Dust Explosion by Sodium Bicarbonate and Its Solid Product Sodium Carbonate.

To characterize the inhibiting effects of sodium bicarbonate (NaHCO3) on aluminum dust, the inhibiting capacities of NaHCO3 and its solid product sodium carbonate (Na2CO3) on the explosions of 10 and 20 μm aluminum dusts were studied using a standard 20 L spherical chamber. Explosion parameters were analyzed based on the induction period and explosion stage to evaluate the inhibiting effects. The results show that the induction period of 10 μm aluminum dust explosion is 18.2 ms, which is shorter than that of 20 μm aluminum dust. Two aluminum dust explosions can be completely inhibited during the induction period when inert ratios of NaHCO3 are 350 and 150%, respectively, but that is not observed after adding the corresponding amount of Na2CO3. When the inert ratio ranges from 0 to 150%, the physical effect of NaHCO3 on 10 μm aluminum is poor and the chemical effect is the essential process. But as the inert ratio increased from 200% to 350%, the physical effect of NaHCO3 is higher than the chemical effect, suggesting that the physical effect is the key factor. With the increase of NaHCO3, the physical effect increases gradually. However, the chemical effect changes little. The physical effects of NaHCO3 including heat absorption and isolation play an essential role in the inhibiting process, which has a significant impact on the pyrolysis process and explosion parameters. The results of the present work provide guidance for the prevention and control of aluminum dust explosions.

Introduction

In the aerospace field, metal fuels are superior to fossil fuels, because they provide a high energy density while being relatively nontoxic.[1,2] Aluminum dust is an attractive candidate of metal fuels, as the enthalpy of combustion of aluminum dust (84 kJ/cm3) in oxygen is better than that of energetic materials and gaseous fuel. Aluminum dust was also widely used in other industries.[3] Many industrial processes generate aluminum dust, which is problematic because this material is highly combustible such that aluminum dust clouds can be readily ignited.[4−7] In fact, aluminum explosions have occurred in many countries to date, including the United States, Italy, and China.[8−10] As an example, in 2014, an especially large aluminum dust explosion took place at the Zhongrong Metal Products facility in Kunshan, China, causing 146 deaths and 163 injuries.[10,11] This hinders social progress and economic development and poses a great threat to individuals.[12,13] For these reasons, it is vital to develop better strategies for the mitigation and prevention of such explosions so as to allow the safe handling of aluminum dust.[14] Inhibition systems are an effective approach to mitigating accidental dust explosions.[15−18] In such systems, the inhibitor is sprayed into a restricted space immediately following the initiation of an explosion to prevent the expansion and propagation of the explosion. Numerous inhibitors have been developed and applied, such as phosphates,[19] alkali metal salts,[20−22] and ultrafine water mists,[23,24] each of which involves a different inhibiting function. Thus, it is important to investigate the interaction between a specific inhibitor and a combustible dust.

Sodium bicarbonate (NaHCO3) has been used as an inhibitor because it exhibits superior performance based on physical and chemical effects.[25−35] Wang et al.[29] and Jiang et al.[30] studied the effects of NaHCO3 powder on polyethylene dust and pulverized biomass dust explosions and demonstrated that the explosions could be completely suppressed by NaHCO3. Dounia et al.[31] reported that NaHCO3 particles decomposed in the flame front of premixed hydrocarbon/air, after which the resulting chemical reactions efficiently inhibited combustion. There also have been some works concerning the inhibition of aluminum dust explosion by NaHCO3. Chen et al.[32] and Jiang et al.[33,34] monitored the explosion flame temperature and flame propagation behaviors of aluminum dusts inhibited by NaHCO3, which have different particle size distributions. They found that the flame structures were changed by addition of NaHCO3 and the size of the flame preheat zone was reduced accordingly. In summary, previous studies showed that the inhibiting effects of NaHCO3 included physical and chemical processes and the major research contents were focused on explosion parameters, flame propagation, and inhibition mechanism. They reported that the physical effect of NaHCO3 was achieved by heat absorption and dilution, and the chemical effect was produced by sodium carbonate (Na2CO3), which is generated by NaHCO3. However, a few studies focused on the inhibiting effects of NaHCO3 and its solid product Na2CO3 on aluminum dust explosion to date. On the other hand, explosion parameters including the induction period and explosion pressure rising rate evolutions were also less studied. Therefore, further studies are needed to reveal the inhibiting effects of NaHCO3 and its solid product Na2CO3 on aluminum dust explosion.

In this study, NaHCO3 and its solid product Na2CO3 were adopted as suppressants to reveal physical and chemical effects of NaHCO3 quantitatively. In addition, the control variable method was also used to evaluate the inhibiting effects of NaHCO3 on aluminum dust explosion. The thermal decompositions and inhibiting effects of NaHCO3 and Na2CO3 were conducted by a simultaneous thermal analyzer (STA 449F3) and a 20 L spherical chamber, respectively. Furthermore, the characterizations for the physical and chemical effects of NaHCO3 were studied during the induction period and explosion stage based on the explosion propagation characteristics and the micromorphology of explosive residues. The results of this work provide guidance for the prevention and control of aluminum dust explosions.

Experiment

Experimental Materials

Aluminum dust samples (10 and 20 μm) were obtained from Henan Yuanyang Powder Technology Co., Ltd. (Xinxiang, Henan Province, China). NaHCO3 and Na2CO3 (analytically pure grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The surface morphologies and chemical compositions of the aluminum samples, NaHCO3, and Na2CO3 were analyzed using scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS, FEC-450), and the results are shown in Figures and 2. In addition, the particle size distributions of aluminum dusts, NaHCO3, and Na2CO3 were determined by a Malvern Mastersizer 2000 laser particle size analyzer, adopting water and absolute alcohol as dispersants, as presented in Figure . The medium mean diameters, D50, of aluminum dusts and NaHCO3 and Na2CO3 powders are found to be 9.67, 19.13, 96.89, and 122.72 μm, respectively, and the specific surface areas of these four materials are 0.62, 0.31, 0.93, and 1.55 m2/g, respectively. The elemental compositions of aluminum dusts are summarized in Table , and the results show that the aluminum contents of the two samples are 99.846 and 99.762%, respectively. Aluminum dusts, NaHCO3, and Na2CO3 were dried at 50 °C in a vacuum oven for more than 6 h before the explosion tests.

Figure 1

SEM of 10 and 20 μm aluminum dusts (a, b), NaHCO3 (c), and Na2CO3 (d).

Figure 2

EDS of 10 and 20 μm aluminum dusts (a, b), NaHCO3 (c), and Na2CO3 (d).

Figure 3

Particle size distribution of aluminum dusts (a) and NaHCO3 and Na2CO3 (b).

Table 1

Elemental Composition of Aluminum Dusts

element	Al	Cu	Fe	Si
10 μm Al (%)	99.846	0.001	0.101	0.042
20 μm Al (%)	99.762	0.001	0.099	0.138

Experimental Apparatus

A diagram of the experimental device is shown in Figure . The device included a spherical test vessel, an ignition device, a control system, and a data acquisition system. The test vessel comprised a double-walled stainless steel sphere with a volume of 20 L and was attached to a 0.6 L dust container. The ignition device consisted of two spark rods and an ignition control panel that generated a discharge to ignite a 0.48 g charge made of zirconium powder, barium nitrate, and barium peroxide (4:3:3 by mass) that released 2 kJ of energy. The control system was used to input the desired ignition time and data acquisition time span, while the data acquisition system recorded pressure values throughout the trial.

Figure 4

Dust explosion testing apparatus. 1, 20 L spherical test vessel; 2, electrode cap; 3, spark rod; 4, chemical igniter; 5, rebound nozzle; 6, outlet valve; 7, dust container; 8, pressure gauge; 9, pressure sensor; 10 and 11, control instrumentation; 12, computer; 13, vacuum pump; 14, compressed air tanks.

Prior to each trial, the preweighed portions of either aluminum powder, aluminum/NaHCO3, or aluminum/Na2CO3 mixtures were transferred into the dust container of the test apparatus. After loading each sample into the spherical test vessel, the vessel was evacuated to a pressure of 0.06 MPa using a vacuum pump. Following this, aluminum dust was sprayed into the vessel by a flow of high-pressure air and ignited 60 ms later. The pressure and pressure rising rate in the vessel were recorded over time using a piezoelectric pressure sensor. The surface characteristics and chemical compositions of the solid residues were also examined using SEM-EDS. The ambient temperature and humidity were about 13–15 °C and 71–82%, respectively.

Results and Discussion

Thermal Behaviors of NaHCO3 and Na2CO3 Powders

The thermal behaviors of NaHCO3 and Na2CO3 powders are essential in the inhibition process. The thermal behaviors of the samples were tested with the heating rate of 20 °C/min at the temperature range of 40–800 °C under the air atmosphere. Figure shows the thermogravimetric-derivative thermogravimetric-differential scanning calorimetry (TG-DTG-DSC) curves of NaHCO3 and Na2CO3 powders. It can be seen from Figure that the decomposition of NaHCO3 began at 140.0 °C and ended at 182.3 °C. During this stage, carbon dioxide (CO2) and water vapor (H2O) were generated by NaHCO3, resulting in mass loss. Meanwhile, the mass loss of Na2CO3 began at 80.1 °C and ended at 101.3 °C due to the fact that water evaporated from it. The residual qualities of NaHCO3 and Na2CO3 powders were 64.24 and 98.49%, respectively. Figure shows that the DSC curves of NaHCO3 and Na2CO3 had obvious endothermic peaks at 161.5 and 98.6 °C, respectively, and the areas of the endothermic peak of NaHCO3 and Na2CO3 were 594.7 and 59.7 J/g, respectively. The findings indicate that heat absorption by NaHCO3 is much higher than that by Na2CO3. Therefore, this is the obvious inhibiting difference between NaHCO3 and its solid product Na2CO3 during the inhibition process.

Figure 5

TG-DTG-DSC curves of NaHCO3 (a) and Na2CO3 (b).

The inhibiting effects of NaHCO3 on aluminum dust explosion are not only heat absorption but also isolation and chemical inhibition,[29−35] and those of Na2CO3 contain isolation, chemical process, and little heat absorption. The heat absorbed by NaHCO3 is much higher than Na2CO3 based on the DSC curves. In addition, the NaHCO3 and Na2CO3 particles act as a physical barrier to reduce the collision probability of aluminum dust with an oxidant and also dilute the concentration of aluminum dust. The chemical effects of NaHCO3 and Na2CO3 resulting from it are able to mitigate the explosion severity by generating sodium oxide and sodium hydroxide (NaOH) at high temperatures. NaOH is especially able to combine with H and OH free radicals during the explosion to interrupt the oxidation of aluminum[30−35] so that the chemical effects of NaHCO3 and its solid product Na2CO3 are similar under a certain ratio. During the analysis, it is reasonable to assume that NaHCO3 is completely decomposed according to the thermal decomposition characteristics. Therefore, the different inhibiting effects between NaHCO3 and its solid product Na2CO3 are heat absorption and dilution by CO2 and H2O under one certain ratio.

Typical Plots of 10 and 20 μm Aluminum Dust Explosions

The explosion characteristics of aluminum dusts are the basis of the inhibiting effect analysis. To ensure the analysis reliability, the aluminum dusts with different particle sizes including 10 and 20 μm were adopted in the present test. The typical plots of explosion pressure and pressure rising rate versus time of the two aluminum dust explosions at a concentration of 200 g/m3 are shown in Figure . It can be seen from Figure that each explosion pressure profile consists of an initial stage during which dust was injected into the test vessel, a second stage during which the pressure increased (corresponding to combustion of the aluminum), and a third stage associated with a decay in pressure. Also, the evolutions of pressure rising rate are consistent with explosion pressure profiles. The pressure rising rate first rises rapidly and then declines during the pressure rising stage and it remains constant in the pressure decay stage. The time of ignition, maximum pressure rising rate, and maximum explosion pressure were marked by t0, t1, and t2, respectively, as shown in Figure . Hence, the pressure rising stage was divided into two parts by point t1. The first part ranging from t0 to t1 is defined as the induction period, which belongs to the initial stage of explosion, and aluminum dusts decompose into smaller particles during this period. The induction periods of 10 and 20 μm aluminum dusts are 18.2 and 33.3 ms, respectively. Moreover, the second part from t1 to t2 is the violent explosion stage and the intense combustion occurs at the stage.

Figure 6

Pressure profiles obtained from aluminum dust explosions at 200 g/m.

It must be emphasized that a characteristic inflection point appears after ignition on the pressure rising rate profiles of 20 μm aluminum dust but not observed on 10 μm aluminum dust as shown in Figure . The characteristic inflection point indicates that the pressure rising rate of 20 μm aluminum dust decreased in a few milliseconds after ignition. Aluminum dust is easily oxidized and forms an alumina shell on its surface.[36] After ignition, the oxide shell could burst and form aluminum vapor with a much smaller particle size, which reacts with oxygen, leading to explosion.[37,38] Owing to the fact that the particle size of 20 μm aluminum dust is larger than that of 10 μm aluminum dust, it needs much energy to melt the oxide shell, resulting in the pyrolysis of 20 μm aluminum dust, which is slower than that of 10 μm aluminum dust during the induction period. In addition, the change in aluminum dust particle size during explosion can be verified by the morphology of the explosion product as shown in Figure . It can be found from Figure that the morphology of explosion residue is different from that of the raw dust as presented in Figure and the particle size of the product changed much smaller after explosion. As presented in Figure , the particle size of explosion residue produced by 10 μm aluminum dust is smaller than that of 20 μm aluminum dust. Therefore, different pyrolysis of 10 and 20 μm aluminum dusts during the induction period leads to distinct explosion pressure and pressure rising rate. Also, the characteristic inflection point is important for the inhibition investigation and will be discussed later in this article.

Figure 7

SEM (a, b) and EDS (c, d) of 10 and 20 μm aluminum dust explosion residues.

From Figure , the maximum explosion pressure (Pm) values for 10 and 20 μm aluminum dusts were 0.64 and 0.52 MPa, respectively, while the maximum pressure rise rates ((dP/dt)m) were 53.15 and 15.96 MPa/s. In each trial, the combustion time (tc) was defined as the time from the point of ignition to the point at which Pm was obtained,[39,40] and the combustion times for the 10 and 20 μm specimens were 37 and 67 ms, respectively. The deflagration index (Kst) for each trial was calculated as[41−44]where (dP/dt)m is the maximum rate of pressure rise (MPa/s) and V is the volume of the test vessel (m, V = 0.02 m in the present work). The Kst values of 10 and 20 μm aluminum dusts were found to be 14.40 and 4.33 MPa·m/s in these tests, respectively. Furthermore, the explosion intensity of 10 and 20 μm aluminum dusts was studied under different concentrations, and the explosion pressure evolutions are shown in Figure . It can be seen that the Pm values increase first and then decrease both for 10 and 20 μm aluminum dust explosions. When the concentration of aluminum dust is higher than 200 g/m, the Pm values are almost close. Therefore, to ensure the complete decomposition of NaHCO3 in the explosion process, 200 g/m was adopted as the initial explosion concentration because the addition of NaHCO3 will be increased and the complete decomposition of NaHCO3 is difficult when the initial concentration of aluminum dust is high.

Figure 8

Explosion pressure curves of 10 μm (a) and 20 μm (b) aluminum dust explosions with various concentrations.

Explosion Parameters of Aluminum Dusts Mixed with NaHCO3 and Its Solid Product Na2CO3

The amount of aluminum used was equivalent to an aluminum dust concentration in the test vessel of 200 g/m3. The inert ratio φ, equal to the mass of NaHCO3 divided by the mass of aluminum dust, is used herein to indicate mass ratios in the test specimens. The inert ratio φ for each trial was calculated aswhere φ is the inert ratio, and and are the masses of NaHCO3 and aluminum dust (g), respectively. The masses of Na2CO3 generated from various NaHCO3 ratios are provided in Table .

Table 2

Amounts of Na2CO3 Generated from NaHCO3 at Various Ratios

inert ratio	50%	100%	150%	200%	250%	300%	350%
MNaHCO3 (g)	2	4	6	8	10	12	14
MNa2CO3 (g)	1.26	2.52	3.79	5.05	6.30	7.56	8.82

Explosion Parameters of Aluminum/NaHCO3 Mixtures

The inhibition experiments of 10 and 20 μm aluminum dust deflagration were conducted by a 20 L spherical chamber. Explosion pressure profiles of the aluminum/NaHCO3 mixtures are presented in Figure . It can be found that the aluminum dust explosions were changed by the addition of NaHCO3. The pressure peaks gradually decreased as the inert ratio of NaHCO3 increased from 50% to 350%. Especially, when the inert ratio was 350%, the Pm value of the mixture was 0.09 MPa, indicating that the explosion was completely inhibited by NaHCO3. For 20 μm aluminum, the pressure peak disappeared and the explosion was suppressed when the inert ratio of NaHCO3 was 150%. The results were obtained by the experiment, which provide guidance for the prevention and control of aluminum dust explosions by using NaHCO3. It is noteworthy that the pressure rising rate evolutions are various under different inert ratios as presented in Figure . Figure a shows that the pressure rising rate of the 10 μm aluminum/NaHCO3 mixtures increased rapidly during the induction period as the inert ratio ranged from 50 to 100%. However, when the inert ratio increased from 150% to 350%, the inflection points appeared on pressure rising rate profiles after ignition, indicating that the pyrolysis of 10 μm aluminum dust was slowed down by the addition of NaHCO3. Figure b presents that the first inflection points appeared on pressure rising rate profiles of 20 μm aluminum dust when the inert ratio of NaHCO3 was 50%. Therefore, these results show that the characteristic inflection point is a sign of inhibition and the inhibition process of NaHCO3 can be described clearly by the explosion pressure rising rate evolution.

Figure 9

Explosion pressure profiles of 10 μm (a) and 20 μm (b) aluminum dusts mixed with NaHCO3.

Figure 10

Explosion pressure rising rate profiles of 10 μm (a) and 20 μm (b) aluminum dusts mixed with NaHCO3.

Figure shows the explosion parameters including Pm, (dP/dt)m, Kst, and tc of 10 and 20 μm aluminum dusts mixed with various inert ratios of NaHCO3. As the inert ratio was increased from 50% to 350%, the Pm values of the 10 μm aluminum/NaHCO3 mixtures decreased from 0.63 MPa to 0.09 MPa, while the (dP/dt)m values of the mixtures decreased from 52.03 MPa/s to 4.8 MPa/s. Also, the tc value increased from 38 ms to 72.8 ms during the 10 μm aluminum explosion as the ratio was increased over this same range. The Kst of the mixtures mixture was reduced from 13.88 MPa·m/s to 1.24 MPa·m/s. The explosion parameters of the 20 μm aluminum/NaHCO3 mixtures show the same trend. These findings illustrate that the inhibition effect of NaHCO3 on 20 μm aluminum dust is superior to that on 10 μm aluminum dust under a certain inert ratio. NaHCO3 powder can absorb the heat of aluminum dust deflagration reactions, and the thermal decomposition of aluminum dust is insufficient. Due to the finding that 20 μm aluminum dust needs much energy to undergo pyrolysis, little heat loss will result in the explosion failure. The results indirectly suggest that the heat absorbed by NaHCO3 has a significant impact on the evolution of pressure rising rate during the induction period.

Figure 11

Pm and (dP/dt)m (a) and Kst and tc (b) of aluminum/NaHCO3 explosions.

Explosion Parameters of Aluminum/Na2CO3 Mixtures

NaHCO3 could decompose into Na2CO3, CO2, and H2O at 140.0 to 182.3 °C. To qualitatively study the physical and chemical effects of NaHCO3 on aluminum dust explosion, the effects of Na2CO3, which was generated by NaHCO3, were also assessed. Figures and 13 display the explosion pressure and pressure rising rate profiles of the aluminum/Na2CO3 mixtures. It can be noted that the pressure peak of 10 μm aluminum dust decreased slightly after the addition of Na2CO3. Especially, when the corresponding Na2CO3 at inert ratios of 350 and 150% was added, the explosions of 10 and 20 μm aluminum dusts were not suppressed by Na2CO3 and the pressure rising rate peaks of 10 and 20 μm aluminum dusts still exist. Figure a shows that the inflection point of 10 μm aluminum/Na2CO3 mixtures first appeared at an inert ratio of 200%, and it first appeared when 150% NaHCO3 was added. These results indicate that the inhibiting effect of NaHCO3 is better than that of Na2CO3 under a certain inert ratio both in the induction period and explosion stage.

Figure 12

Explosion pressure profiles of 10 μm (a) and 20 μm (b) aluminum dusts mixed with Na2CO3.

Figure 13

Pressure rising rate profiles of 10 μm (a) and 20 μm (b) aluminum dusts mixed with Na2CO3.

Figure plots the relationships between the inert ratio and Pm, (dP/dt)m, Kst, and tc. These data demonstrate that the addition of Na2CO3 reduced the explosion pressure of 10 μm aluminum dust by a little. Thus, the explosion pressure did not significantly decrease with increases in the concentration of Na2CO3, indicating that this compound had minimal additional effects as more was added to aluminum. Although there was only a slight reduction in the explosion pressure, the (dP/dt)m and Kst values were greatly lowered, while tc was increased. The inhibiting effect of Na2CO3 on aluminum is primarily based on chemical inhibition, and any chemical reaction will be limited by the reactant present in the lowest stoichiometric amount.[30] In the present tests, the aluminum dust and air in the test vessel were the limiting reagents, and so, adding more Na2CO3 did not increase the chemical effect.

Figure 14

Pm and (dP/dt)m (a) and Kst and tc (b) of aluminum/Na2CO3 explosions.

Comparative Analysis Inhibition Effects of NaHCO3 and Its Solid Product Na2CO3

Inhibiting Effects of NaHCO3 and Its Solid Product Na2CO3 on the Induction Period

Based on the previous research results, the analysis of the inhibiting effects of NaHCO3 and its solid product Na2CO3 on 10 and 20 μm aluminum dust explosions is discussed from two sides including the induction period and explosion stage. Figure shows the induction period duration of aluminum dust explosion under various inert ratios of NaHCO3 and the corresponding Na2CO3. It can be found that the induction period duration of 10 μm aluminum dust was prolonged as the inert ratio of NaHCO3 increased from 0% to 350%. Especially, when the inert ratio was 350%, the induction period infinitely increased due to the finding that the pyrolysis of aluminum dust particles was a failure after ignition. However, when the corresponding Na2CO3 was added, the induction period duration increased slightly. Figure b shows that the induction period of 20 μm aluminum dust infinitely increased at an inert ratio of 150%. When the corresponding Na2CO3 was added, the induction period of aluminum/NaHCO3 mixture was higher than that of aluminum/Na2CO3 mixture both for 10 and 20 μm aluminum dusts under a certain ratio. During the induction period, aluminum dust decomposed into small aluminum vapors, absorbing the energy released by the igniter. As NaHCO3 was added, a lot of heat was absorbed, which resulted in the finding that the pyrolysis of aluminum dust was a failure. Due to the fact that 20 μm aluminum dust possesses a larger particle size, it needs more energy to achieve pyrolysis at the induction period. Therefore, little addition of NaHCO3 could stop it from undergoing pyrolysis. On the contrary, 10 μm aluminum dust with a smaller particle size needs a larger amount of NaHCO3 to achieve prevention. The flame propagation behavior can reflect the induction period more specifically as shown in Figure . Without addition of NaHCO3, the initial flame of the explosion was very bright. However, with addition of NaHCO3, the flame propagation gradually darkened. Especially, when 350 and 150% NaHCO3 was added into 10 and 20 μm aluminum dusts, respectively, the chemical igniters flash and go out. Thus, these findings in this work suggest that the complete inhibition of aluminum dusts was realized during the induction period and the heat absorbed by NaHCO3 is the essential effect in this process.

Figure 15

Induction period of 10 μm (a) and 20 μm (b) aluminum dusts mixed with NaHCO3 and Na2CO3.

Figure 16

Flame propagations of 10 μm (a) and 20 μm (b) aluminum dusts mixed with NaHCO3.

In addition, to verify the above conclusion, the microscopic features of explosion residues of aluminum/NaHCO3 mixture at the inert ratio of 150% were tested as shown in Figures and 18. Figure a shows that the shells and gaps of 10 μm aluminum particles were no longer present and large numbers of particle fragments were evident. However, it can be seen from Figure b that 20 μm aluminum dusts were still in the original shape and the surface of particles was covered with NaHCO3, indicating that the pyrolysis of aluminum powder particles was a failure and combustion did not occur. In addition, the EDS of the explosion product shows that the main elements on the surface of 10 μm aluminum/NaHCO3 explosion residue are aluminum (Al) and oxygen (O), while elements of 20 μm aluminum/NaHCO3 explosion residue are O and sodium (Na). Furthermore, Figure b shows that the particle size distribution of residues was generally consistent with that of the original 20 μm aluminum particles, which means that the pyrolysis of aluminum dust was a failure. These distributions are also in agreement with the SEM images of residues as shown in Figure a,b. The findings provide evidence that the complete inhibition was achieved in the induction period and the pyrolysis was prevented by NaHCO3.

Figure 17

SEM (a, b) and EDS (c, d) of 10 and 20 μm aluminum/NaHCO3 explosion residues.

Figure 18

Particle size distributions of 10 μm (a) and 20 μm (b) aluminum/NaHCO3 explosion residues.

Inhibiting Effects of NaHCO3 and Its Solid Product Na2CO3 on the Explosion Stage

During the explosion stage, the reduction of Pm was used to evaluate the effect of NaHCO3. Figure shows the reduction of Pm and inhibiting rate of 10 and 20 μm aluminum dusts after the addition of NaHCO3 and Na2CO3. As presented in Figure , the gray-marked part is the disparity of the inhibiting effect generated by NaHCO3 and Na2CO3. According to previous discussion, the disparity of NaHCO3 and Na2CO3 mainly contains a physical effect generated by NaHCO3 including heat absorption and isolation due to the fact that they possess the same chemical effect. It is found that the gray-marked part increases with increasing inert ratio, indicating that the physical effect of NaHCO3 increases. When the inert ratio ranges from 0 to 150%, the physical effect of NaHCO3 on 10 μm aluminum is poor and the chemical effect is the essential process since it involves the chemical reaction between aluminum and NaHCO3. As the inert ratio increased from 200% to 350%, the physical effect of NaHCO3 is higher than the chemical effect, indicating that the physical effect is the key factor. When the inert ratio is 300%, the proportion of the physical effect of NaHCO3 on 10 μm aluminum dust is 62%. As for 20 μm aluminum dust, the physical effect of NaHCO3 worked efficiently at inert ratio ranges from 50 to 150%. Especially, when the inert ratio is 100%, the proportion of the physical effect of NaHCO3 on 20 μm aluminum dust is 80%. On the other hand, the inhibition effect of Na2CO3 increased slowly due to the finding that the chemical effect was related to free radicals, whose concentration remained unchanged. The findings suggest that with the increase in NaHCO3, the physical effect increased gradually, while the chemical effect changed little. In summary, the physical effect plays an important role in the aluminum dust explosion both for 10 and 20 μm aluminum dusts.

Figure 19

Reduction of Pm and inhibition rate of 10 μm (a) and 20 μm (b) aluminum mixed with NaHCO3 and Na2CO3.

On the basis of the explosion parameters and microscopic features of explosion residues, the inhibition process of NaHCO3 on aluminum dust was summarized as shown in Figure . It can be divided into two steps including the induction period and explosion stage. If the inert ratio of NaHCO3 is higher than 350%, 10 μm aluminum dust can be inhibited completely during the induction period and the explosion stage cannot occur. During this period, the heat generated by the igniter was absorbed by NaHCO3, leading to the failure of aluminum dust decomposition, suggesting that the physical effect plays a leading role. The inflection point on the pressure rising rate profiles is significantly characteristic of the inhibition process and it is also related to aluminum dust pyrolysis. When the inert ratio is lower than 350%, the inhibition process will fall into the explosion stage after the induction period. Under these circumstances, the physical and chemical effects will work together to decrease the Pm and (dP/dt)m. Afterward, it needs to increase the inert ratio again to add in the system so that the inhibition process underwent a cycle once until the aluminum dust explosion was completed inhibited. All in all, the aluminum dust was completely inhibited by NaHCO3 during the induction period and the physical effect plays a significant role in the inhibition process.

Figure 20

Inhibition process of NaHCO3 on aluminum dust.

Conclusions

The inhibiting effects of NaHCO3 and its solid product Na2CO3 on 10 and 20 μm aluminum dust explosions were investigated in a standard 20 L spherical chamber. The changes in induction period were revealed by pressure rising rate evolutions and the role of physical and chemical effects was illustrated by the reduction of Pm. The detailed conclusions of this work are as follows.

The inhibition process of NaHCO3 can be described by the explosion pressure rising rate evolution. Based on the pressure rising rate evolution, the induction period of 10 μm aluminum dust explosion is 18.2 ms, which is shorter than that of 20 μm aluminum dust when the concentration of aluminum dust is 200 g/m3. A large amount of heat was absorbed by NaHCO3, causing the pyrolysis of 10 and 20 μm aluminum dusts to be a failure, and the explosion can be inhibited completely during the induction period when the inert ratios of NaHCO3 are 350 and 150%, respectively. In addition, the characteristic inflection point on the pressure rising rate profiles is a sign of the pyrolysis of aluminum dust and it also can characterize the inhibiting process of the inhibitor.

The physical effects of NaHCO3 including heat absorption and isolation play an essential role in the inhibiting process, which have a significant impact on aluminum dust pyrolysis and explosion parameters. With the increase of NaHCO3, the physical effect increased gradually. However, the chemical effect changed little after the addition of the corresponding Na2CO3. When the inert ratio ranges from 0 to 150%, the physical effect of NaHCO3 on 10 μm aluminum is poor and the chemical effect is the essential process. As the inert ratio increased from 200% to 350%, the physical effect of NaHCO3 is higher than the chemical effect, suggesting that the physical effect is the key factor.