Combustion behaviors and residues characteristics in hydrogen/aluminum dust hybrid explosions
Introduction
Aluminum dust is an important raw material in industrial production (Sun et al., 2006; Myers, 2008). It has a wide range of uses, and it may explode easily. The use of aluminum dust in Chinese industry started in the 1950s and has a history of nearly 70 years. With the expansion of the scale of production of the aluminum powder industry, accidents caused by aluminum dust explosions are becoming more and more serious. In many aluminum dust explosion accidents, hydrogen was released due to the presence of damp aluminum dust, the explosive reactions of hydrogen and aluminum dust mixtures were violent, resulting in disasters that should not be overlooked (Li et al. 2016).
On August 2, 2014, an explosion occurred at China Kunshan Metal Products Co., Ltd., causing 146 deaths and an economic loss of 351 million yuan. The cause for the explosion was that the aluminum powder in the dust collector was damp and an oxidation reaction occurred. The reaction gave off hydrogen and a large amount of heat, and then the explosion occurred (Li, G. et al., 2016). On July 7, 2018, a similar accident occurred at the Okayama Aluminum Plant in Japan, which was caused by the explosion of hydrogen generated by moisture reacting with aluminum powder. In the petrochemical industry, triethyl aluminum is a catalyst for the preparation of tert-alcohols and secondary alcohols, and it can also be used for aluminum plating and the preparation of other metal–organic compounds. The pure product can also be used in the metal–organic chemical vapor deposition industry. In addition, triethyl aluminum is a key material for hypergolic rocket fuel and solar silicone production. Micron-sized aluminum powder is widely used in chemical synthesis and industrial polishing production lines, especially in hydrogenation reactions. On June 28, 2015, an explosion occurred at the Alkyl Aluminum Plant in Liaoyang, China, and residual aluminum powder and hydrogen were detected on the reaction vessel. In the production process of triethyl aluminum, aluminum powder and hydrogen are successively added into the hydrogenation kettle to produce diethyl aluminum. After hydrogenation, residual hydrogen needs to be removed, and the hydrogen/aluminum dust mixtures are at risk of combustion or explosion. Hydrogen and aluminum powder are also important raw materials for industrial production of sodium aluminum hydride (Vitalie et al., 2012). Denkevits and Hoess (2015) mentioned that in the design of the International Thermonuclear Experimental Reactor, the LOCA/LOVA scheme involves a case in which hydrogen is accidentally released into the surrounding environment containing aluminum powder, resulting in an explosion of hydrogen/aluminum dust mixtures.
Scholars have been studying the flame or explosion characteristics of gaseous fuel and dust mixtures for many years (Amyotte et al., 1993). Due to the demand of the coal industry, many studies were based on coal dust and methane mixtures. Liu et al. (2007) studied the flame propagation process of coal dust and methane mixtures. The results showed that the co-existence of coal dust and methane significantly increases the flame propagation velocity and maximum flame temperature. Ajrash et al. (2017) studied the deflagration of methane/coal dust mixtures in a large straight tube. It was found that the flames of the methane/coal dust mixtures were hemispherical with a slight oscillation. Bai et al. (2011) studied the explosion of methane/coal dust/air mixtures in a 10 m3 container. The flame structure can be divided into the red zone, the yellow illuminating zone, and the bright white illuminating zone. Gan et al. (2018a, b) systematically studied the flame propagation characteristics of ethylene/polyethylene hybrid explosions, and found that the addition of ethylene can increase the flame propagation velocity of the mixture. All these studies made an important contribution to revealing the explosion mechanism of a hybrid gaseous fuel/organic dust mixture.
The pyrolysis and volatilization of organic dust is a main factor affecting the hybrid explosion mechanism. The combustion mechanism of metal dust is different from that of organic dust, although they have many similar phenomena. Bryant and Sippel (1971) studied the combustion of amorphous boron suspensions in propane–oxygen mixtures. Soo et al. (2013) conducted combustion experiments of micron-sized aluminum powder in premixed methane/air mixtures. The experiments showed that the addition of metal to the mixture always resulted in a burning velocity lower than that of the methane flame. On this basis, Vickery et al. (2017) continued to study the role of geometry in flame propagation through aluminum-methane-oxidizer mixtures in latex balloons. Denkevits (2010); Denkevits and Hoess (2015) studied explosion characteristics of hydrogen/metal dust mixtures and found that the explosion overpressure and rate of pressure rise increased first and then decreased slightly with increasing dust concentration.
In this paper, in order to further study the interaction between hydrogen and aluminum dust during hybrid explosions, the flame propagation characteristics of aluminum dust clouds in different hydrogen concentrations were studied by a high-speed camera. The surface morphology and chemical composition of the combustion residues were also analyzed. The objective of the present work is to investigate how hydrogen affects the flame propagation behavior and combustion residues of hybrid mixtures, and to explore the explosion mechanism of hydrogen/aluminum dust mixtures.
Section snippets
Experimental apparatus
The open-space dust explosion experimental apparatus is shown schematically in Fig. 1 (Zhang et al., 2016). The apparatus consists of a combustion tube of 1 L in size, an ignition system, a dispersion system, a hydrogen/air premixed gas cylinder, a high-speed camera system, a time controller system and a data acquisition system.
The open space which was created by the moveable tube is 115 mm high with a diameter of 95 mm. The air–fuel dispersion system used 0.5 MPa compressed hydrogen/air mixtures
Flame morphology and microstructures
The flame propagation and structures of 2000 g/m3 aluminum dust explosions with 0 %, 5 % and 10 % hydrogen, are shown in Fig. 6. The aluminum dust clouds formed an approximate hemispherical shape during the propagation process. This is helpful to accurately measure the flame propagation velocities. During the combustion of the hydrogen/aluminum dust hybrid mixtures, a phenomenon similar to the combustion of gaseous fuel/organic dust mixtures appeared. When hydrogen was added into the combustion
Conclusions
Flame propagation behaviors and combustion reaction mechanisms in aluminum dust explosions and hydrogen/aluminum dust hybrid explosions were experimentally studied in an open-space apparatus.
In the flame propagation of aluminum dust explosions, when aluminum dust exploded in hydrogen concentration environment, the brightness of the flame increased obviously, the flame front become continuous. In the microstructure of the flame, the aluminum dust burned in hydrogen concentration environment
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.
Acknowledgements
The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (51574056), (51604057), (51904094) and (51904170), and Shandong Natural Science Foundation (ZR2018BEE006).
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