Comparison of behavior and microscopic characteristics of first and secondary explosions of coal dust

https://doi.org/10.1016/j.jlp.2017.08.005Get rights and content

Highlights

  • Experimental simulation of secondary explosions at the site of the initial explosion.

  • The distribution and contents of the main chemical elements in the explosion products.

  • Differences in pore structures between first and secondary explosion residues.

Abstract

Differences in behaviors of primary and secondary explosions of a bituminous coal dust were investigated using a standard 20 L spherical test vessel. The aim was to compare the explosibility and microstructures of residues of first and secondary explosions to obtain reference data for investigation of accidental explosions. The residues were analyzed by energy-dispersive spectrometry, scanning electron microscopy, and automatic surface area and pore analyzer. The first explosion showed a higher rate of heat release and higher total heat release, which indicated greater severity of explosion. The spatial distributions of C, Cl, and Si in the residues were uniform, whereas those of Ca, O, and Al occurred as discrete points. The carbon content decreased and Ca, Al, Si, Cl, and Fe contents increased as the reaction proceeded. Unexpectedly, the oxygen content was reduced by 4.41% during the primary explosion, but increased by 9.69% during the secondary explosion. The scanning electron microscopy picture of the secondary explosion residue exhibited a higher degree of fragmentation and a more developed pore structure. All isotherms of dust samples were of Type II and the shapes of the hysteresis loops were of Type H3 or Mixed Type H3–H4. Change of fractal dimension D2 illustrated increasingly complicated pore structures with advance of the explosion process.

Introduction

In China, the annual yield of combustible dust products and related products has reached tens of thousands of tons. Coal dust ranks first in volume, followed by food and metal dusts. When combustible dust suspended in air reaches a certain concentration, the presence of an ignition source or high temperature can lead to an explosion. Based on an incomplete survey, 46 dust explosion incidents occurred in China between 2010 and 2016, in which 210 people were killed and 327 injured (SAWS, 2016). Two of the most serious explosions took place in Jiangsu Province (146 fatalities, 114 injuries) and Taiwan (12 fatalities, 500 injuries), which attracted the attention of the State Administration of Work Safety (SAWS). To prevent and mitigate dust explosions, SAWS drafted China's first guidelines for the prevention of flammable dust explosions in the workplace (SAWS, 2015). Surveys of these accidents showed that their extent often comprised the entire workshop, roadway, and even the whole facility. This can cause huge casualties, economic losses, and even endanger the safety of rescue workers. Therefore, further studies and new technology of dust control are still needed as strategies for explosion prevention (Han et al., 2016, Ji et al., 2016).

The shock wave produced by local suspended dust or other explosive material raises deposited dust. When the dust concentration reaches the lower limit of explosion, the first explosion can develop into a wide range of further systematic explosions, i.e., secondary explosions. Secondary explosions often occur in or within a certain distance from the area of the initial explosion. The first explosion creates a negative pressure zone in the area, into which surrounding air soon flows. Under conditions of high temperature or the presence of an ignition source, secondary explosions may occur in the area of the initial explosion. In addition, the shock wave and flame array created by the first explosion are distributed throughout the area. Secondary explosions may also be produced when the flame front meets suspended coal dust. Compared with the primary explosion, secondary explosions are often more dangerous: the first explosion is typically restrained by protective devices in the building, so damage is relatively less. Uncertainty of the time and place of secondary explosions greatly increases the difficulty of rescue. Improved understanding of the nature of such secondary explosions is therefore urgently required.

Research on secondary explosions has a long history in the West, but started relatively late in China. Cybulski, who was the first to undertake experimental studies in this field, proposed a mechanism for the generation of secondary explosions in coal mine roadways and laboratories (Cybulski, 1975). The detonation of corn starch was studied by Fan and Gröning using a horizontal channel apparatus (1990); however, due to limitations of the experimental scale, the entire secondary dust explosion process could not be simulated. The first of simulation of the entire secondary dust explosion process in China was conducted by Bai et al. (1995), who found that the accelerating flame and shock wave were not the direct cause of the secondary explosion. Italian scholars (Virgopia and Ferraioli, 1982) studied the evolution of weak perturbations and the condition of non-impact between a weak perturbation and primary shock, and proved that secondary shocks can occur if appropriate conditions of the amplitude of the initial perturbations are satisfied. Other studies (Fan and Jiang, 2005, Yan et al., 2012, Yan et al., 2013, Popov, 2016, Song et al., 2009, Shuangqi et al., 2010, Jiang et al., 2005) explored relationships between coal dust density, initial explosion intensity, length of relief duct, and characteristic parameters of secondary explosions during dust explosion venting. In-depth experimental and numerical simulation studies on external flow-field structures for explosion venting and the generation mechanism of secondary explosions have also been conducted.

A review of the literature concerning coal dust explosions found that most studies focused on investigating secondary explosions during venting; experimental simulation of secondary explosions at the site of the initial explosion has been very limited. In the present work, a 20 L spherical vessel was used to systematically assess the evolution of explosion characteristics and the distribution and contents of the main chemical elements in the explosion residues. Differences in pore structures between first and secondary explosion residues were compared and discussed. The results may be useful for accident prevention.

Section snippets

Equipment

Full-scale explosion tests were conducted in a standard 20 L stainless-steel spherical vessel (HY16426C, Jilin Hongyuan Scientific Instrument Co., Ltd., China). The set-up consisted of three main parts: the main spherical body, the control system, and the data acquisition system. The body of the apparatus was the key part of the testing system and consisted of a double-walled stainless-steel sphere with a water-cooled jacket and a gas distribution system. The control system was used to control

Explosion characteristics and analysis of mechanism

To study the difference in dust explosion characteristics between the first and secondary explosions, two samples, with particle sizes of 250–425 μm and <75 μm (labelled as A and B, respectively), were tested at a coal dust concentration of 400 g/m3 and using an ignition energy of 10 kJ. After the first explosion, the residues were collected and used to conduct a second explosion under the same conditions. A typical coal dust explosion overpressure evolution (Pt) curve, based on the 20 L

Conclusions

Differences in explosion behaviors and microstructures of the residues of first and secondary explosions of coal dust were investigated using standard methods, with the aim of improving industrial safety. Two samples, with particle sizes of 250–425 μm and <75 μm, were tested at a coal dust concentration of 400 g/m3 and using an ignition energy of 10 kJ. According to the overpressure evolution (Pt) curve, the explosion was divided into two stages: pressure increase, followed by pressure decay.

Acknowledgements

Financial supports from the Fundamental Research Funds for the Central Universities (2017CXNL02), the program for Innovative Research Team in University of Ministry of Education of China (IRT13098) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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