A novel reactive P-containing composite with an ordered porous structure for suppressing nano-Al dust explosions

https://doi.org/10.1016/j.cej.2021.129156Get rights and content

Highlights

  • A novel high-efficiency composite named PA@NH2-MCM41 was synthesized via self-assembly.

  • Nano-Al dust explosion was completely suppressed by only 33.3 wt% PA@NH2-MCM41.

  • A complex film based on Alx(HiPhy) was formed during the explosion suppression process.

  • The excellent capacity of PA@NH2-MCM41 was due to the synergy between gas and condensed phase effects.

Abstract

A novel chemical suppressant (PA@NH2-MCM41) was synthesized via self-assembly. NH2-MCM-41 was prepared using diatomaceous earth (DE) as the silica source. FTIR spectra showed that phytic acid (PA) was successfully grafted onto synthesized NH2-MCM-41 via hydrogen bonding. The highly ordered mesoporous structure of PA@NH2-MCM41 was observed from TEM and small angle X-ray diffraction (SAXRD). The effectiveness of PA@NH2-MCM41 in the suppression of nano-Al dust explosions was investigated. Experimental results suggested that the addition of PA@NH2-MCM41 could effectively reduce the average flame propagation velocity (Vavg) and maximum explosion pressure (Pmax). After the addition of 16.7 wt% PA@NH2-MCM41, the Vavg of the gas-phase flame was reduced by 75%. With addition of 28.6 wt% PA@NH2-MCM41, the Pmax of nano-Al dust decreased by 65.7%. As the mass fraction of PA@NH2-MCM41was increased to 33.3%, nano-Al dust explosion was completely suppressed. The SEM, FTIR and EDS results showed that a complex sealing film was formed on the surface of the explosion products, which occupied the open sites on the Al surface and thereby interrupted the Al evaporation and surface reaction. Subsequently, the suppression mechanism by which phosphorus containing suppressants consume free radicals and the interaction of PA molecules with the aluminum particle surface were revealed in depth.

Introduction

Due to the high specific surface area (high reactivity) and excess energy of surface atoms, Al nanoparticles (NPs) are excellent additives for improving the energy performance of energetic materials [1]. With their unique and favorable properties, Al NPs can also be used in the biochemistry, medicine and chemical industries [2], [3]. However, their application is limited due to safety issues because dust explosions are a major hazard frequently encountered. Al NPs have a low ignition temperature and are prone to explode [4], [5]. Al NP manufacturing and operation plants have high potential fire and explosion hazards [6]. A proper understanding of nano-Al dust explosion phenomena and selection of effective explosion protection techniques are necessary to prevent and mitigate the consequences of such accidents.

Adding suppressants (i.e., inert gas/powder, water mist, or dry chemical powder) to the flame zone is an effective means to reduce explosion hazards [7], [8]. Suppressants can control the explosion by absorbing energy produced by the oxidation reaction (“physical” mechanism) and/or by “chemically” participating in the gas or condensed phase reaction [9], [10], [11]. Examples of physical suppressant agents include china clay, talc, inert rock and Al2O3. [8], [9], [12]. They prevent explosion propagation through absorption of the thermal and radiant energy. This absorption competes with the unburned Al NPs. Due to the high energy density of Al particles, a large amount of physical suppressant agent is required to absorb enough of the available energy to prevent Al dust explosion. This situation is the same for N2 and CO2: 95.2% N2 or 93% CO2 is required to completely suppress micron-sized Al explosions [13]. It should be noted that Al particles can react with pure CO2, CO2/N2 or H2O/Ar mixtures under certain circumstances [14], [15], [16]. Hence, N2, CO2 and water mist, which are often used as suppressants for hydrocarbon fuels, do not seem to be suitable as suppressants for Al dust explosion. Another typical case is hydrofluorocarbons (HFCs). HFCs (i.e., CHF3, C2HF5 and C3HF7) show excellent suppression effects for hydrocarbon and hydrogen explosions [17], [18]. Unfortunately, Al metal reacts vigorously with all halogens. F-containing species can increase the ignition sensitivity and exothermicity of Al NPs via a preignition reaction [19]. Thus, HFCs can seriously promote Al dust explosion, and cannot serve as suppressants for Al [13]. Linteris et al. [20] found that metallic compounds exhibit excellent inhibition efficiency compared with HFCs (i.e., CHF3 and CF3Br). Metal suppressants containing Fe (i.e., Fe(CO)5), Cr (i.e., CrO2Cl2), K, and Na (i.e., KHCO3 and NaHCO3) are relatively effective. Since Al metal is highly reactive, the thermite reaction will occur between Al and Fe oxide or Cr oxide. Fe/Cr containing compounds have not been recommended as suppressants for Al dust explosion. Additionally, our experiments revealed that S condensed aerosols (Sr-containing compounds) can also seriously promote Al dust explosion.

Among the many metal compounds, potassium and sodium compounds are well-known effective suppressants because they play a key role in chain-terminating action through gas-phase reactions [21], [22]. Taveau et al. [23] found that KHCO3 and NaHCO3 particles of similar size deliver equal effectiveness. These suppressants with a mass close to or greater than 4 times the mass of Al powder are required to prevent explosions [24]. Phosphorus-containing compounds have also long been known to be capable of explosion suppression. Researchers have built a gas-phase inhibition kinetic model of phosphorus-containing compounds (PCCs) [25], [26]. Gao et al. [27] experimentally and computationally studied the suppression properties of ABC powder (NH4H2PO4 greater than 90%) and melamine polyphosphate (MPP, C3H9N6O4P) for Al dust explosion, which exhibit higher efficiency than NaHCO3. Small P-containing species, such as H3PO4, HOPO, and PO2, can consume many key radicals through catalytic cycles and radical trapping [28], [29]. However, the suppressant-enhanced explosion parameter (SEEP) phenomenon will occur, when the additive concentration is insufficient [30], [31]. Therefore, the suppression performance of traditional suppressants for St3 dust explosions such as Al dust is extremely limited.

To improve the capacity of suppressants, researchers began to try to synthesize composite powders to exert a synergistic suppression effect. For instance, composite powders with core–shell structures, layered structures, and porous structures facilitate loading of chemical components. Wang et al. [32] prepared a NaHCO3/red mud (RM) composite powder to suppress methane explosion. RM has numerous pores and thus can capture free radicals and be used as a carrier with excellent thermal stability. Meng et al. [33] selected modified RM as the carrier and Ca(H2PO4)2 as the loaded particles to fabricate a core–shell structure composite powder. The results showed that Ca(H2PO4)2/RM is more effective for Al dust explosion suppression than either Ca(H2PO4)2 or RM powder alone. However, it was still necessary to add 200 wt% composite powder to completely prevent Al dust explosion. Wang et al. [34] prepared carbamide/fly ash cenosphere suppressants with bilayer spherical shell structures for coal dust explosion suppression. However, NH3 will be generated during the decomposition of carbamide powder, which can enhance Al combustion. Recently, a NaHCO3/diatomaceous earth (DE) composite powder suppressant with unique clustered structures was prepared by Meng et al [35]. The results showed that the suppression capacity of the NaHCO3/DE composite powder was double that of Ca(H2PO4)2/RM. Another advantage of DE is that it is affordable and economical because it is abundant and environmentally. However, the pore structure of DE is irregular, and its particle size distribution is large. The concentration of these suppressants required to suppress Al dust explosion is still too high.

Knowing the fuel combustion mechanism is also essential to suppress explosions. This mechanism is something that is often overlook but should be given special attention. Al dust with a large particle size burns predominantly via homogeneous gas-phase reactions [36]. Small P-containing radical species released by the lower oxidation state phosphorus (3+) in suppressants can terminate the chain branching reaction via flame radical capture, thereby interrupting gas-phase Al combustion. Al NPs predominantly burn via heterogeneous reactions [37]. Several mechanisms have been proposed for its reaction, including the diffusion mechanism [38], the polymorphic phase transformation mechanism [39], and the melt dispersion mechanism (MDM) [40]. No matter which kind of the mechanisms Al NPs follow, the core Al cations will diffuse outward and a melted film is formed during heating, especially in a situation where the heating rate is reduced due to the addition of an explosion suppressant. In this case, higher oxidation state phosphorus (5+) is required to suppress surface reactions. Phytic acid (PA) (C6H6(H2PO4)6) is a natural and nontoxic source and is recognized as an effective flame retardant [41], [42]. The active groups of PA can strongly chelate with Al cations to form stable chelation complexes, which can be deposited and coated on the Al particle surface [43]. Therefore, PA is expected to exhibit an excellent suppression performance against nano-Al dust explosions.

Herein, ordered mesoporous MCM-41 functionalized with –NH2 groups was first synthesized using DE as the silica source. Phytic acid was then grafted onto the MCM-41 carrier surface via self-assembly. A novel reactive P-containing composite suppressant (PA@NH2-MCM41) with a porous structure was then prepared. The effect of PA@NH2-MCM41 on the flame propagation behaviors and explosion pressure of nano-Al dust explosions was systematically investigated. The suppression mechanism is then discussed in depth. This paper provides a new prospect for the design of economic and environmental suppressants by considering both gas and surface chemistry, which will achieve metal (even nanometal) dust explosion protection.

Section snippets

Explosion suppression tests

The experimental apparatus is illustrated in Fig. 1 and consisted of a 1.47 L explosion chamber, a dispersion system, an ignition system, a high-speed camera, a data acquisition system and a time controller system. The explosion chamber was a stainless-steel cuboid vessel (300 mm × 70 mm × 70 mm) with optical glass (150 mm × 30 mm × 30 mm) on the front side. Al NPs and suppressants were fully mixed prior to being placed in the hemispherical cup at the bottom of the combustion chamber. A 0.5 L

Characterization of PA@NH2-MCM41

The morphology and microstructures of PA@NH2-MCM41 are shown in Fig. 4. The Sauter mean diameter D32 of PA@NH2-MCM41 was 2.066 μm. Highly ordered channels of PA@NH2-MCM41 were observed based on the TEM results. The number of grafted organic moieties determined by elemental analysis was 3.12 mmol/g. The chemical structures of DA, NH2-MCM-41, and PA@NH2-MCM41 were investigated via FTIR. As shown in Fig. 5a, the typical bands of DA at 3440 cm−1 can be attributed to Si-OH and absorbed water. The

Conclusion

In this paper, a novel high-efficiency P-containing composite suppressant named PA@NH2-MCM41 was prepared via self-assembly. The suppression performance of PA@NH2-MCM41 for the Al NP dust explosion was evaluated. The conclusions can be summarized as follows.

  • (1)

    PA@NH2-MCM41 has a highly ordered mesoporous structure. The selection of diatomaceous earth as a silicon source makes it economical and environmentally friendly. The chemical component PA was successfully grafted onto its surface via

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors appreciate the financial support by National Natural Science Foundation of China (No. 52006026, No. 51922025 and No 51874066.), China Postdoctoral Science Foundation (No. 2020 M670759), Science and Technology Major Project of Liaoning Province (2019JH1/10300002) the Elite Foundation of Revitalizing Liaoning (No. XLYC1907161).

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