Flame suppression mechanism of aluminum dust cloud by melamine cyanurate and melamine polyphosphate
Graphical abstract
Introduction
As a kind of recyclable and zero-carbon energy, metal fuels are an attractive candidate of fossil fuels [1,2]. Of all metals, aluminum is most promising, because it has a high energy density and it is, therefore, a main components within many batteries and propellants [3]. The enthalpy of combustion of aluminum particles is 84 kJ/cm3 in oxygen at stoichiometric conditions, and is greater than those of monomolecular energetic materials, biomass fuel and gaseous fuel [4]. In addition, micro aluminum particles are often dispersed in industries processes, such as polishing plants or dust collection systems [5]. Once the ignition source is encountered, the aluminum dust particles will burn intensely, and even an aluminum dust explosion will occur. In the past few decades, many studies have been intensively investigated to prevent the loss of dust explosions. The maximum explosion pressure, pressure rise rate, minimum ignition temperature, and minimum ignition energy are measured in most of these studies [[6], [7], [8], [9], [10]]. However, it remains challenging to prevent or mitigate dust explosion.
As a high reactive metal, aluminum flame has extremely high temperature and fast flame velocity, which could result in the seriously thermal damages and pressure built-up. Flame propagation velocity of aluminum dust cloud various with concentration [11], particle size [12], oxidizer composition [13], and the scale of dust cloud [14]. The measured average temperature of aluminum–air flames is approximately 3370 K, which could cause serious thermal burn injuries [15]. For example, the 2014 Kunshan explosion accident caused burns to the bodies of numerous patients [16]. It can be seen that fatal consequences will happen when aluminum flame is formed. Therefore, study on the suppression mechanisms of flame propagation is also a key part to prevent the dust explosion. However, there are relatively few studies devoted to determination of performance and suppression mechanism of suppressants for aluminum flame.
Flame suppression is an effective means to mitigate the consequence of combustible dust flame [17]. Extensive experimental studies have been made on explosion suppression, and results show that phosphates, carbonates, alkali metal salts and flame retardants all have a certain flame suppression performance [78,1819]. However, suppressants vary widely in their ability to suppress the flames [20,21]. The suppression of aluminum dust explosion could be achieved using a high-performance suppressant. As a kind of low-cost, environment-friendly material and an effective replacement of halogenated hydrocarbons, nitrogen containing flame retardants (such as Melamine cyanurate, MCA) and phosphorus containing flame retardants (such as melamine polyphosphate, MPP) have received considerable attention [22,23]. Fires extinguishing properties of MCA and MPP have been widely investigated [[24], [25], [26], [27]]. These studies indicated that suppression capacity of MCA and MPP is achieved through physical and chemical effects. Thermogravimetric study demonstrates that the main product of the decomposition of MCA is gaseous, and there is nearly no residue left [28]. This indicates that the mechanism behind flame suppression by MCA is chemical and homogeneous. Melamine polyphosphate (MPP) is a reaction product of melamine (MEL) and polyphosphoric acid (PPA). Hence, MPP decomposition produces phosphorus containing species and nonflammable gases [29]. According to previous literatures, phosphorus containing species are most active in the high-temperature region of the flame [30]. Small P-containing species, as the gaseous decomposition products, could alter flame chemistry by catalytic recombination of key radicals and lead to a lower flame burning velocity [31].
Even though MCA and MPP, as fire suppressants, have demonstrated its effectiveness, few researchers have systematically concerned on the suppression effectiveness of MCA and MPP for aluminum dust flame. In this study, the suppression capacity of MCA and MPP on the flame propagation of aluminum dust cloud is evaluated systematically. The effect of MCA and MPP on flame morphology, flame propagation velocity, and flame temperature of 5 μm and 30 μm aluminum dust are firstly investigated. The surface morphology and chemical composition of explosion products, respectively, are analyzed by SEM and XRD. The suppression mechanisms of MCA and MPP are further discussed in detail, combining the pyrolysis characteristic tests and gas phase chemical kinetics analysis.
Section snippets
Experimental apparatus
The experimental apparatus shown schematically in Fig. 1 was similar to our previous study [32]. The apparatus consisted of cylindrical combustion tubes, an air-fuel dispersion system, a high-speed camera, an ignition system, a data acquisition system unit, a time controller, and a thermocouple.
The experimental material was sprayed upward into the tubes at a pressure of 0.46 MPa, the duration time for spray was 0.5 s. The moveable tube subsequently dropped down at the end of spray. Then, the
Effect of MCA and MPP powder on flame morphology and microstructures
In order to avoid the emergence of unstable flame propagation which could hinder the analysis of the effects of suppressant on aluminum flame, the results with stoichiometric Al/air flames are chosen to evaluate the inhibition capacity of MCA and MPP particles. As shown in Fig. 7a, the flame luminous intensity of 5 μm aluminum dust cloud is strong. Aluminum flame emits intense white light. Flame morphology of 5 μm aluminum dust cloud are relatively regular and symmetric. Fig. 7b and c show that
Conclusions
Flame suppression of 5 μm and 30 μm aluminum dust cloud by MCA and MPP has been investigated experimentally. Flame propagation behaviors and flame temperatures of Al dust doped with MCA and MPP are studied in an open-space apparatus. The conclusions can be summarized as follows:
- (1)
The amount of MPP that required to prevent the aluminum dust explosion is lower in comparison with MCA. By increasing the inerting ratio, the acceleration and the maximum flame speed significantly decrease. For MPP
Acknowledgments
The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 51406023 and 51674059), Key Laboratory of Building Fire Protection Engineering and Technology of MPS (KFKT2016ZD01), and the Fundamental Research Funds for the Central Universities (DUT16RC(4)04).
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