air explosion by kaolinite-based multi-component inhibitor

air explosion by kaolinite-based multi-component inhibitor

Powder Technology 343 (2019) 279–286 Contents lists available at ScienceDirect Powder Technology journal homepage: S...

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Powder Technology 343 (2019) 279–286

Contents lists available at ScienceDirect

Powder Technology journal homepage:

Suppression of methane/air explosion by kaolinite-based multi-component inhibitor Yaru Sun a, Bihe Yuan a,⁎, Xianfeng Chen a,⁎, Kaiyuan Li a, Liancong Wang b,c, Yalong Yun a, Ao Fan a a b c

School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070, China State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230027, China State Key Laboratory of Coal Mine Safety Technology, CCTEG Shengyang Research Institute, Fushun 113122, China

a r t i c l e

i n f o

Article history: Received 11 August 2018 Received in revised form 15 October 2018 Accepted 4 November 2018 Available online 09 November 2018 Keywords: Kaolinite Multi-component inhibitor Gas explosion Mechanism

a b s t r a c t A novel multi-component powder inhibitor was prepared by mixing conventional suppressants (ammonium polyphosphate and aluminium hydroxide) with porous kaolinite. The inhibition performance of this developed inhibitor was investigated using a 20-L spherical experimental explosion system. The explosion parameters, such as the maximum explosion pressure, maximum pressure rising rate, and time to peak explosion pressure, were obtained to evaluate the suppression effects. The results show that the multi-component inhibitor exhibits higher suppression performance than the single components. The thermal stability, decomposition behavior, and gaseous products were investigated using thermogravimetric analysis, differential scanning calorimetry, and thermogravimetric-mass spectrometry. The inhibition mechanisms are illustrated using physical and chemical explanations. The current work introduces a new gas explosion inhibitor with high performance resulting from the synergistic effect between its ingredients. © 2018 Elsevier B.V. All rights reserved.

1. Introduction Prevention and suppression of gas explosions are crucial to industrial and environmental safety in modern society. Over the past decades, researchers have attempted to use different chemical agents to prevent and suppress regular fires while collecting valuable experimental data. Nevertheless, the agents (usually powders) for preventing and suppressing gas explosions are different from those used for traditional fires, as the two processes are quite different in both reaction mechanisms and flow behavior. A gas explosion usually leads to a shock wave and flame, which are devastating to both buildings and human beings. Secondary effects such as oxygen shortages and toxic gases strongly affect the safety of any occupants. Development of high-performance and widely available suppressants has become the main object in this field. Various suppressants have been developed with their own advantages and disadvantages. For example, rock dust presents a strong suppression capability. However, noticeable disadvantages have been identified for both dry and wet rock dust. Dry dust, consisting of undesired respirable dust particles, can impose a severe health hazard on workers, while wet dust is subject to caking issues, leading to a drastic reduction in the dispersibility of particles [1]. Water mist is a substitute for powder inhibitors. It is an economical and environmentally friendly agent. ⁎ Corresponding authors. E-mail addresses: [email protected] (B. Yuan), [email protected] (X. Chen). 0032-5910/© 2018 Elsevier B.V. All rights reserved.

However, it cannot provide effective inhibition in the presence of high concentrations of explosive gas, such as mixtures with a methane concentration over 9.5%. Moreover, the applicability of water mist is limited under both high and low temperatures [2–5]. According to previous reports [6–9], SiO2, NaHCO3, K2CO3, Mg(OH)2, (NH4)2SO4, NaCl, ammonium diphosphate, bauxite, and aluminium hydroxide (ATH) have certain inhibitory effects on gas explosions. These powders have the advantages of more easy transportation and longer-term storage than water mist. Thus, they can be used as inhibitors of gas explosions. Previous studies have shown that mixtures of several inhibitors is often more effective and can perform much better than a single-component inhibitor [10,11], while conventional inhibitors are usually singlecomponent. A limited number of studies have focused on multicomponent inhibitors to improve suppression performance. Previous research has proved that powder additives exert various effects, such as capturing hydrogen and hydroxyl radicals, absorbing heat released from explosion reactions, and diluting the combustible gases. Radical trapping and endothermic effects are also important for explosion suppression [7,12,13]. Kaolinite (K) is widely used in industrial fields as a paper filler, flame retardant, and polymer additive [14,15]. It has a predominantly platelike structure. Unlike layered double hydroxides and montmorillonite, kaolinite has an asymmetric structure consisting of the superposition of tetrahedral (Al-O4) and octahedral (Si-O8) sheets in 1:1 layers [16]. This layered structure can act as a barrier to gas and heat when used in composites and thus kaolinite-based nanocomposites exhibit


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improved thermal stability and flame retardant properties [16–18]. Dahoe et al. [19], Kang et al. [20], and Wang et al. [11] investigated the inhibitory effects of porous materials on gas explosion flames and shock waves. Similarly, the highly porous structure of kaolinite increases its contact area with free radicals and reduces the number of active free radicals involved in the chain reactions, resulting in the suppression of gas explosions. Thus, kaolinite, with its large amount of micropores and large surface area, has great potential as a gas explosion suppression material. In this work, multi-component inhibitors for gas explosions were developed based on different inhibition mechanisms. Porous kaolinite and conventional suppressants were used as the components of composite inhibitors. The explosion inhibition effects were investigated in a 20-L stainless steel spherical vessel and the inhibition mechanisms are discussed. 2. Experimental 2.1. Materials and preparation procedures Kaolinite (chemically pure, purity ≥99.5 wt%) and ATH (analytical reagent, purity ≥99.7 wt%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Ammonium polyphosphate (APP, purity ≥99.0 wt%) was provided by Jinan Taixing Fine Chemicals Co., Ltd. The reagents were used without further purification. Particle size distributions of kaolinite-based multi-component inhibitors were measured by a Malvern Mastersizer 2000 Laser Diffraction Device (Malvern Instruments Ltd., Malvern, UK) and the results are presented in Fig. S1 (see the Supplementary material). The specific surface area (SBET) and particle size (dBET) of kaolinite were calculated based on Brunauer–Emmett–Teller (BET) nitrogen adsorption–desorption isotherms, using a Micromeritics Tristar II 3020 M automatic surface area and pore analyser (test results are shown in Fig. S2 and Table S1). Prior to the adsorption tests, the sample was degassed at 300 °C for 3 h. The procedure used to prepare the kaolinite-based multicomponent inhibitor included three steps. Step 1: The raw materials—ATH, kaolinite, and APP—were separately ground into powder. Step 2: The powders of ATH, kaolinite, and APP were mixed using a high-speed blender according to the proportions in Table 1. Step 3: The mixture was dried at 50°C for 24 h, and then a composite inhibitor was obtained. 2.2. Experimental apparatus and testing method Gas explosion experiments were carried out in a standard 20-L stainless steel spherical vessel, as shown in Fig. 1. The system consisted of an explosion reactor, a gas mixing system, an ignition electrode system, an injection system, a measurement system and a data acquisition system. Before explosive tests, the centrally mounted chemical igniter was connected to ignition leads and the explosion chamber was closed [21,22]. The tips of two ignition electrodes, which were positioned at the centre of the explosion reactor, were wound around a chemical igniter pellet (see Fig. S3). In addition, two 9 V dry batteries in series were used to

Table 1 Formulations of inhibitors. Sample

ATH (wt%)

APP (wt%)

K (wt%)

1 2 3 4 5 6

– 100.0 – 20.0 22.2 25.0

– – 100.0 50.0 55.6 62.5

– – – 30.0 22.2 12.5

ignite the chemical igniter that produced 1 J of energy to detonate the gas mixture. The explosion chamber was first evacuated with a vacuum pump. Then, air and methane were mixed in the gas mixing system and injected into the explosion chamber. The methane concentration was controlled at 9.5% using a computer controlled system. Inhibitors were sprayed into the vessel by a certain air pressure when the valve between the sample container and the vessel was opened, and the chemical igniter was energized after a time delay of 5 s. All of the programs were controlled by a synchronous controller. When the test was finished, the explosion chamber and powder cylinder were thoroughly cleaned for the next test. During the experiments, the explosion pressure was measured using a pressure sensor on the vessel wall and recorded by a data acquisition system [23]. The maximum explosion pressure (Pmax), time to peak explosion pressure (tmax), and the maximum explosion pressure rising rate ((dP/dt)max) were determined. Simultaneous thermal analyses including thermogravimetric analysis (TGA), differential thermogravimetric (DTG) and differential scanning calorimetry (DSC) were conducted with an STA6000 simultaneous thermal analyser (PerkinElmer Inc.). The samples were heated from room temperature to 800.0°C at a heating rate of 20.0°C/min in a N2 atmosphere. Thermogravimetric-mass spectrometry (TG-MS) was performed using a thermogravimetric analyser (STA 449F3, Netzsch Instruments), which was interfaced to a quadrupole mass spectrometer (QMS 403, Netzsch Instruments). The sample was heated in a high-purity N2 atmosphere with a heating rate of 20.0°C/min. 3. Results and discussion 3.1. Explosion suppression effect 3.1.1. Inhibition effect of multi-component inhibitor with different ratios of kaolinite The suppression effect of kaolinite-based multi-component inhibitors with different component ratios of kaolinite (the ratio of ATH:APP was maintained as 5:2) were investigated at a total concentration of 0.100 g/L. The measured explosion pressures are shown in Fig. 2 and Table 2. The gas explosion can be divided into two main stages. The first one is the pressure increase stage, in which the pressure increases rapidly with time. The heat released from combustion exceeds heat loss to the surroundings, which increases the explosion pressure. The second stage is the pressure decay process [21,22]. During the experiments, Pmax and (dP/dt)max were reduced by the kaolinite-based multi-component inhibitors. Furthermore, the time to maximum explosion pressure was increased. In an explosion accident, overpressure is an important and dangerous parameter, and peak overpressure is a significant factor affecting the destructiveness of the explosion [24,25]. When the mass fraction of kaolinite powder was 22.2 wt%, the inhibition effect was the most marked. For example, the Pmax and (dP/dt)max values were greatly decreased by 40.6% and 39.7%, respectively, compared to those of the explosion without any inhibitor. Thus, in further experiments, the composite inhibitor with 22.2 wt% kaolinite powder was used. 3.1.2. Inhibition effect with different concentrations of kaolinite-based multi-component inhibitor The effect of inhibitor concentration on the gas explosion is presented in Fig. 3 and Table 3. The inhibition performance of single components is compared with a composite inhibitor in Fig. 4. When the methane concentration is 9.5%, increasing the mass of kaolinite-based multi-component inhibitor leads to improved suppression while Pmax is decreased. According to the results of Pmax, (dP/dt)max, and time to maximum explosion pressure, the inhibition effect becomes more apparent with increasing mass of inhibitor.

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Fig. 1. Explosion experimental system. (1-explosion reactor; 2-ignition electrode; 3-controller; 4-powder injector; 5-charge amplifier; 6-synchronous controller; 7-data collection instrument; 8-solenoid valve; 9-powder cylinder; 10-high pressure air tank; 11-compressed air cylinder; 12-vacuum pump; 13-gas mixer; 14-computer controlling system; 15methane cylinder; 16-air compressor; 17-pressure sensor).

3.1.3. Inhibition effect of different inhibitors Fig. 4 and Table 4 present the gas explosion pressure with pure ATH, APP, and kaolinite-based multi-component inhibitor. It is obvious that the performance of the multi-component inhibitor is better than those

of pure ATH and APP, indicating the synergistic effects of the three suppressants. Since gas explosions happen frequently and often cause property damage, preventing and mitigating such disasters has become a very important challenge. Inert premixed gas, water mist, rock dust,

Fig. 2. Inhibition effect of multi-component inhibitors with different ratios of kaolinite powder: (a) explosion pressure curves; (b) Pmax; (c) (dP/dt)max; (d) time of maximum explosion pressure achievement.


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Table 2 Explosion suppression results of multi-component inhibitors (0.100 g/L) with different ratios of kaolinite powder. Sample 1 4 5 6

Formulation (wt%) ATH APP – 20.0 22.2 25.0

– 50.0 55.6 62.5


tmax (s)

Pmax (MPa)

(dP/dt)max (MPa/s)

– 30.0 22.2 12.5

0.28 0.36 0.34 0.31

0.69 0.51 0.41 0.51

5.92 3.52 3.57 3.60

porous materials, and aerosols are common gas explosion suppressants. Compared to a 9.5% methane/air explosion, Luo et al. found that the maximum explosion pressure could be reduced by 14.0% and 12.0% by using superfine ABC powder and diatomite, respectively [8]. Compared with an unsuppressed gas explosion of a stoichiometric mixture (with methane concentration of 9.5%), the optimal rock dust loading was found to be 36.0 kg/m3, resulting in the best inhibitory effect such that Pmax declined by 40.0%, the peak flame speed by 50.0%, and the flame length by 42.0% [26]. At 9.5% methane, adding 0.25 g/L Mg(OH)2 powder reduced the maximum explosion pressure by no N22.0% [6]. In this work, the prepared kaolinite-based multi-component inhibitor exhibited a significant inhibitory effect. Compared to a gas explosion without any powder, the values of Pmax and (dP/dt)max decreased by 40.6% and 39.7%, respectively, with the use of a kaolinite-based multi-component inhibitor (22.2 wt% ATH, 55.6 wt% APP, and 22.2 wt% kaolinite).

Table 3 Explosion suppression results with different concentrations of multi-component inhibitor. Sample

Concentration of inhibitor g/L

tmax (s)

Pmax (MPa)

(dP/dt)max (MPa/s)

1 7 8 5

– 0.050 0.075 0.100

0.28 0.27 0.29 0.34

0.69 0.86 0.52 0.41

5.92 5.60 3.77 3.57

spherical chamber (V), Kst is a widely used value and it can be calculated by Eq.(1): 1 K ¼ ðdP=dtÞ  V =3 ð1Þ st


For a 20-L spherical explosion vessel, a typical gas explosion overpressure evolution curve (P-t) is shown in Fig. 5. The time of maximum explosion pressure is defined as the period from ignition to the time when the overpressure reaches its maximum. Fig. 6a shows the deflagration indexes of pure APP, ATH, and ATHAPP-K with different ratios of kaolinite. The lowest deflagration index was achieved by the inhibitor with 30.0 wt% kaolinite and the three multi-component inhibitors' values are extremely close. From Fig. 6b, with increasing concentration of composite inhibitor, the deflagration index decreases, indicating higher inhibition effect. 3.2. Thermal stability and decomposition of inhibitors

3.1.4. Deflagration index The deflagration index was employed to evaluate the gas explosion characteristics [27–29]. Based on (dP/dt)max and the volume of the

TG and DTG are commonly employed to evaluate the thermal stability and thermal decomposition behavior of materials at various

Fig. 3. Inhibition effect with different concentrations of inhibitors (component ratios of ATH, APP, and K are 22.2 wt%, 55.6 wt% and 22.2 wt%, respectively): (a) explosion pressure curves; (b) Pmax; (c) (dP/dt)max; (d) time to maximum explosion pressure.

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Fig. 4. Inhibition effect of different inhibitors: (a) explosion pressure curves; (b) Pmax; (c) (dP/dt)max; (d) time to maximum explosion pressure.

temperatures by measuring the degradation temperature and decomposition rate [30]. TG and DTG curves of the samples are presented in Fig. 7. In the TG curve of ATH, noticeable mass losses of 3.4%, 27.0%, and 35.0% were observed in the temperature intervals of 263.0–306.0°C, 306.0–374.0°C, and 374.0–609.0°C, respectively (Fig. 7a), which correspond to the DTG peaks at 291.0°C, 359.0°C, and 576.0°C, respectively (Fig. 7b). The APP sample exhibited two major mass loss stages, corresponding to the removal of H2O and NH3 in the first stage, and further decomposition of phosphorus-based acid in the second stage [31]. The thermal decomposition results of kaolinite are similar to those reported in the literature [11,32,33], which suggest excellent thermal stability and heat resistance. From the DTG curves, the peak mass loss rates of the kaolinite-based multi-component inhibitor appeared at 219.5°C, 379.0°C, and 591.0°C. According to the TG curve, the kaolinite-based multi-component inhibitor is not a thermally stable compound. It begins to dehydrate with a low rate at about 100°C. Its thermal decomposition is accelerated significantly from 137.0°C to 659.5°C. The decomposition process can be divided into three main stages: the first stage from 137.0 to 267.0°C with a mass loss of 21.15% may be attributed to the loss of intercalated moisture and the elimination of NH3 from the decomposition of APP to

generate polyphosphoric acid. The second stage from 267.0 to 405.5°C with a mass loss of 10.56% may be attributed to the further decomposition of APP and the dehydration of ATH, while the third stage from 405.5 to 659.5°C with a mass loss of only 5.22% can be attributed to the evaporation of phosphorus-containing components and possible chemical reactions between ATH and phosphorus based acid. Fig. 8 presents the TG-DSC curves of the kaolinite-based multicomponent inhibitor. As shown in Fig. 8, four clear endothermic peaks can be identified in the temperature ranges of 50.0–200.0°C, 200.0–300.0°C, 300.0–400.0°C, and 400.0–800.0°C, possibly due to the

Table 4 Explosion suppression results of different inhibitors (0.100 g/L). Sample

1 2 3 5

Formulation (wt%)










– 100.0 – 22.2

– – 100.0 55.6

– – – 22.2

0.28 0.27 0.25 0.34

0.69 0.59 0.77 0.41

5.92 3.59 5.18 3.57

Fig. 5. Pressure evolution of gas explosion in the 20-L sphere.


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Fig. 6. Deflagration index of (a) different inhibitors and (b) different concentrations of kaolinite-based multi-component inhibitor.

endothermic evaporation of water (bound and adsorbed water) and endothermic decomposition of APP and ATH. The endothermic effect is one of the main approaches to explosion suppression, and the thermal decomposition of the kaolinite-based multi-component inhibitor can absorb a significant amount of heat released from the explosion reaction to cool the system down and inhibit the propagation of the explosion wave. The release profiles of gaseous products (such as H2O and NH3) from the samples can be obtained using thermogravimetry-infrared spectroscopy (TG-IR) and TG-MS [6,31,34,35]. TG-MS results from the composite inhibitor are presented in Fig. 9. The release of NH3 from the kaolinite-based multi-component inhibitor started at the early stage when the temperature was approximately 116.5°C, and increased rapidly with increasing temperature. Four peaks were observed at 258.7°C, 280.0°C, 342.0°C, and 548.0°C. In addition, the generation of H2O started at approximately 101.0°C, and increased rapidly with increasing temperature. Its four peaks were observed at 185.0°C, 267.7°C, 339.0°C, and 542.0°C. The release profiles of the major gas products of the composite inhibitor correspond quite well with the TG curves, except for the differences in the gas species and the intensities of ion peaks. The releases of NH3 and H2O correspond to the endothermic decomposition of the inhibitor. These gas products can dilute the combustible gases and reduce the gas explosion velocity.

3.3. Discussion of gas explosion suppression mechanism Gas explosions are mainly a result of a chain reaction and thermal explosion. The HO• and H• free radicals are the key free radicals which sustain the chain reaction and initiate the explosion reaction, and these free radicals have the characteristics of both high energy and high activity. Once these free radicals are generated, they trigger more free radicals, thus the cycle can continue, leading to explosion. Meanwhile, a large amount of combustion heat is released from the chain reaction which increases the system temperature. The increase in temperature speeds up chemical reactions and heat release. The heat release of the system overwhelms heat dissipation, leading to the explosion [32,36,37]. (1) Physical inhibition effect

The inhibitors have a special layered silicate and porous structure. A portion of HO• and H• free radicals formed by combustion reactions can be physically adsorbed by ATH and kaolinite, which reduces the collision probability with intermediate products of the methane reaction. Besides, the porous clay presents a wide range of pore sizes, porosities, and pore connectivity. These characteristics reinforce the heat shielding,

Fig. 7. (a) TG and (b) DTG curves of inhibitors.

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Fig. 8. DSC and TG curves of kaolinite-based multi-component inhibitor.

heat consumption, and gas blockage [38]. With the large surface area and abundant pores of kaolinite, the labyrinth effect of the porous mineral can promote the radical trapping effect. Moreover, the endothermic decomposition of ATH and APP can also absorb heat. These effects take place simultaneously during explosion suppression, resulting in the physical synergistic inhibition effect. (2) Chemical inhibition effect The thermal decomposition of ATH and the corresponding H2O vaporization can absorb a large amount of the heat generated from the explosion reaction to reduce the system temperature and mitigate the explosion wave intensity. Thermal decomposition of ATH and APP are shown in Eqs. (2) and (3), respectively. Free radicals containing N and P can bind with H• and HO• quickly, inhibiting the chain reaction and suppressing the explosion [11,39]. Furthermore, polyphosphoric acid, which is a decomposition product of APP, can react with ATH to generate NH4AlP2O7 and AlPO4 at temperatures above about 600.0°C [40]. NH3 generated by the decomposition of APP plays an important role in decreasing the concentration of reactants. Many relevant studies of the inhibitory effect of NH3 on gas explosions have indicated that NH3 has a significant ability to bind with H• and HO• free radicals. For example, HO• free radicals, which are generated by the reaction of oxygen and NH3, are finally consumed by NH3. NH3 can continually promote the consumption of H• and HO• free radicals and oxygen. Thus, these reactions form a positive cycle, and H2O, which is generated by this positive feedback loop, can inhibit the gas explosion [22,41,42]. The chemical reaction equations are listed as Eqs. (4), (5), and (6). 2AlðOHÞ3 →Al2 O3 þ 3H2 0↑



Fig. 9. Selected MS ion intensity curves and TG curve of kaolinite-based multi-component inhibitor.

4. Conclusions The explosion parameters, such as deflagration index and explosion pressure, show that the kaolinite-based multi-component inhibitor is able to significantly suppress a gas explosion and hence the synergy of ATH, APP, and kaolinite can be confirmed. This work proposes a novel method of developing high-performance gas explosion inhibitors on the basis of different inhibition mechanisms. The unique porous structure increases the contact area available for free radicals and enhances the adsorption of free radicals during the explosion. Furthermore, this characteristic reinforces the heat shielding and heat consumption. The results of this study may thus find applications in the prevention and suppression of gas explosions where inhibitor materials are prepared by mixing porous minerals with conventional suppressants. Acknowledgements This work was supported by National Key Technologies Research and Development Program of China (2017YFC0804705), National Natural Science Foundation of China (51703175 and 51774221), Fundamental Research Funds for the Central Universities (WUT: 2018IVB061) and Open Research Fund of State Key Laboratory of Coal Mine Safety Technology (sklcmst103). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.powtec.2018.11.026.

ð3Þ References NH3 þ H•→NH2 • þ H2 ↑


NH2 • þ O2 →NHO þ HO•


NH3 þ HO•→NH2 þ H2 0


In summary, kaolinite combined with APP and ATH can suppress gas explosions efficiently and achieve an excellent inhibitory effect.

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