Scholarly article on topic 'Experimental Study on the Influence of Initial Pressure on Explosion of Methane-coal Dust Mixtures'

Experimental Study on the Influence of Initial Pressure on Explosion of Methane-coal Dust Mixtures Academic research paper on "Chemical engineering"

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{explosion / Methane / "Coal dust" / "Initial pressure"}

Abstract of research paper on Chemical engineering, author of scientific article — Yuan Li, Hongli Xu, Xishi Wang

Abstract Methane-coal dust explosion belongs to chemical explosion which always directly causes fire disasters. When it comes to coal mine, the gas explosion easily leads to extensive burning and forms large area stereo fire finally, due to the big blast power and many combustible materials at the scene of an accident. In order to study the explosion characteristics of methane-coal dust mixture explosion as well as to reveal the effects of initial pressure on the explosion, a rectangular explosion test vessel which is 60cm long and with 10cm × 10cm cross-section was used. Different initial pressures, such as 0.2MPa, 0.25MPa, 0.3MPa and 0.35MPa were considered in this paper. The explosion pressure was measured with PCB pressure transducers, and the maximum rate of explosion pressure rise was determined based on these measured data. The results show that the maximum explosion overpressure and maximum rate of overpressure rise increase with the increasing of the initial pressures and coal dust concentration.

Academic research paper on topic "Experimental Study on the Influence of Initial Pressure on Explosion of Methane-coal Dust Mixtures"

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ScienceDirect Procedia

Engineering

Procedia Engineering 62 (2013) 980 - 984

www.elsevier.com/locate/procedia

The 9th Asia-Oceania Symposium on Fire Science and Technology

Experimental study on the influence of initial pressure on explosion of

methane-coal dust mixtures

Yuan Li, Hongli Xu, Xishi Wang*

State Key Laboratory of Fire Science, University of Science & Technology of China, Hefei 230026, China

Abstract

Methane-coal dust explosion belongs to chemical explosion which always directly causes fire disasters. When it comes to coal mine, the gas explosion easily leads to extensive burning and forms large area stereo fire finally, due to the big blast power and many combustible materials at the scene of an accident. In order to study the explosion characteristics of methane-coal dust mixture explosion as well as to reveal the effects of initial pressure on the explosion, a rectangular explosion test vessel which is 60 cm long and with 10 cm x 10 cm cross-section was used. Different initial pressures, such as 0.2 MPa, 0.25 MPa, 0.3 MPa and 0.35 MPa were considered in this paper. The explosion pressure was measured with PCB pressure transducers, and the maximum rate of explosion pressure rise was determined based on these measured data. The results show that the maximum explosion overpressure and maximum rate of overpressure rise increase with the increasing of the initial pressures and coal dust concentration.

© 2013 InternationalAssociation for FireSafetyScience. Published byElsevierLtd. AllRights Reserved Selection and peer-review under responsibility of the Asian-Oceania Association of Fire Science and Technology

Keyword: Explosion; Methane; Coal dust; Initial pressure

1. Introduction

In coal mine accidents, methane or coal dust explosions have happened frequently and caused disastrous effects in recent years. In practice, methane explosion usually accompanies with coal dust blast, so the explosion and its destructive effects usually deteriorate. Even now, methane-coal dust explosion continues to threaten people's life and properties. So it is necessary to figure out the mechanism of the mixture explosion and the influencing factors.

Gas explosion and dust explosion have been studied for many years, and many influencing factors have been taken into consideration. Deng et al. [1] studied the mixed gases explosion in the goaf area. Some gases which were mainly released by the spontaneous combustion of the coal can boost explosion. Gieras et al. [2] studied the explosion parameters of methane-air mixtures to determine its explosion limits, and they found that the blast range increased with the rise of initial temperature. Pekalski et al. [3] considered both of temperature and pressure as variable conditions. They pointed that the combustion conversion regimes can be divided into three parts, and the soot formation is an important parameter which affects the explosion pressure. Cammarota et al. [4] mainly discussed hydrogen-enriched effects on methane explosion. They considered turbulence effect besides initial pressure. In terms of the dust explosion, Martin et al. [5] carried out a series of tests to study the ignitability and flammability characteristics of pulverized coals and other dusts from the microcosmic view. Di Benedetto et al. [6] applied thermo-kinetic theory to interpret dust explosion. Cashdollar [7] provided an overview of dust explosibility characteristics including both coal dust and non-coal dust, which gave a better explanation to dust explosion. Liu et al. [8] and Ashok G. Dastidar et al. [9] studied the influencing factors consisting of ignition energy and coal dust concentration on explosion, and they also presented methods of suppressing the blast.

* Corresponding author. Tel.: +86 551 6360 6437; fax: +86 551 6360 1669. E-mail address: wxs@ustc.edu.cn.

ELSEVIER

1877-7058 © 2013 International Association for Fire Safety Science. Published by Elsevier Ltd. All Rights Reserved Selection and peer-review under responsibility of the Asian-Oceania Association of Fire Science and Technology doi:10.1016/j.proeng.2013.08.151

In recent years, the researches on exploring the hybrid explosion of combustible gases and dust [10-14] have been studied. The results may provide some experimental support to suppress the occurrence of the gas explosion and provide some directions for designing of explosion mitigation equipments and arresters, though they focus on different aspects.

Most of the above-mentioned researches were related to gas or dust explosion, while only few related to the hybrid explosion, especially to methane-coal dust explosion. The characteristics of methane-coal dust mixture explosion and its mitigation by ultra-fine water mist have been studied by Xu et al. [15], but the effects of initial pressure are still not discussed. Therefore, in this paper, the effects of initial pressure on methane-coal dust hybrid explosion are experimentally studied.

2. Experimental apparatus

The schematic diagram of the experimental apparatus of methane-coal dust mixture explosion is shown in Fig. 1. The whole system consists of five parts, including an explosion propagation vessel, an ignition system, a data acquisition and processing system, a dust emission and gas transportation system and a high-speed video camera. The 99.9% methane and the coal dust which produced by Shanxi Huading Economic and Trade Co. Ltd. were adopted and tested.

The explosion vessel is a rectangular tube sealed with flanges and gaskets. It is about 0.6 m long and has a 0.1 x 0.1 m2 cross-section. Two high-pressure cylinders with 40 L volume are respectively used to store 99.9% methane and compressed air. The D08-3B/ZM mass flow controller is used to adjust methane fraction pumped into the vessel. The propagation vessel consists of three transparent glass plates and a stainless steel plate. High voltage impulse ignition electrode and two PCB pressure transducers are installed on the stainless steel plate. Signals obtained by the transducer are sorted and analyzed through the data acquisition and processing system. Besides, a pressure release hole with a 40 mm diameter is fixed at the port side of the steel plate. During the process of experiment, a GigaView high speed video camera was utilized to record the whole explosion process for later analysis.

Several preparations should be done before the experiment. First, pulverized coal should be well weighted according to different operating modes and be put into the groove of the flange, which was at the bottom of the explosion tube. Second, methane and air mixture is pumped into the cylinder, and at the same time methane is adjusted to needed concentration by the mass flow controller. Then the installation and setting of data acquisition system are confirmed. After these being done, the experiment can be started. Open the outlet valve of the cylinder first, and the compressed gas mixture in the tank will get through the nozzle into the explosion tube. The vortex formed as the gas mixture passing through the nozzle will disperse the pulverized coal uniformly into dust cloud from the bottom to the top of the explosion tube. Then press the pulse ignition button with high-voltage spark being discharged between two electrodes, which will ignite the mixed gas and coal dust cloud. In the meantime, pressure sensors and thermocouples begin to collect data which will be stored into the computer.

cylinder

Fig. 1. Experimental apparatus of methane-coal dust mixture explosion (a) schematic diagram of the experimental apparatus and (b) actual installation of the experimental apparatus.

3. Results and discussion

The methane-coal dust mixture explosion is an intense oxidation reaction between methane, coal dust and oxygen with a certain concentration. According to previous research, the concentration range of methane explosion is 5-15%. The concentration range of coal dust explosion is closely related to the coal dust concentration, coal particle diameter, species of coal dust, environmental conditions, experimental method, etc. So, in most situations, coal dust explosion may occur with a relatively wide concentration range, known as 30 - 1500 g/m3. However, as methane and coal dusts are mixed, the explosion

concentration range changes. For example, when methane is mixed into coal dust, the explosion limit concentration of coal dust reduces. In the following parts, the effects of initial pressures, such as 0.2 MPa, 0.25 MPa, 0.3 MPa and 0.35 MPa on methane-coal dust mixture explosion will be discussed.

3.1. Maximum explosion overpressure

The maximum explosion overpressure is a very important parameter for characterizing an explosion. In order to investigate the effects of initial pressure on methane-coal dust mixture explosion overpressure, a series of experimental tests were conducted. Four initial pressures of 0.2 MPa, 0.25 MPa, 0.3 MPa and 0.35 MPa, two coal dust concentrations of 200 g/m3 and 300 g/m3 were considered in the experiments, while the methane concentration was about 7% and the coal particle diameter was 38.5 ^m.

The maximum explosion overpressure of the mixture explosion under different initial pressures is shown in Fig. 2. The results show that the maximum explosion overpressure increases when the initial pressure rises. The increase of the maximum explosion overpressure with the initial pressure corresponds to the increase of the maximum adiabatic pressure, which may lead to negligible heat loss rate compared to heat production rate. Differences are also observed on increasing the coal dust content.

- 200 g/m

- 300 g/m'

0.24 0.27 0.30 initial pressure /MPa

-â 900-,

S 700-1

§ 500-1

1 400 g 300-

initial pressure /MPa

0.18 0.21

0.33 0.36

Fig. 2. Maximum explosion overpressure of methane-coal dust mixture Fig. 3. Maximum rate of overpressure rise of methane-coal dust mixture

explosion with different initial pressures. explosion with different initial pressures.

Meanwhile, with the increase of coal dust concentration, maximum explosion overpressure increased. This implies that, with a certain initial pressure, the maximum explosion overpressure of the case with higher coal dust concentration should be larger than that with lower one.

The coal dust quantity is directly proportional to the coal dust concentration within a certain volume. Therefore, the quantity of coal particle will increase when the coal dust concentration increases. As the concentration of coal dust increases, the quantity of coal particle participating in the mixtures must increase. Thus, the power of the mixtures explosion would be enhanced and the maximum explosion overpressure would be higher.

3.2. Maximum rate of overpressure rise

The maximum rate of overpressure rise is another very important parameter for characterizing the explosion, which is closely related to the maximum explosion overpressure. Fig. 3 gives the maximum rate of overpressure rise of the methane-coal dust mixture explosion. The experimental test conditions are the same as those for explosion overpressure determination, i.e., initial pressures of 0.2 MPa, 0.25 MPa, 0.3 MPa and 0.35 MPa were considered, while the methane concentration was set to 7% and the coal dust with a 38.5 ^m diameter was tested.

The figure implies that when initial pressure increases, the maximum rate of overpressure rise increases, and when coal dust concentration increases from 200 g/m3 to 300 g/m3, the maximum rate of overpressure rise increases. To the cases with higher initial pressures, this phenomenon is more obvious. This result fits well with the result of maximum explosion overpressure, and the reason may be similar as discussed above. However, it also indicates that the effects of coal dust concentration on the maximum rate of overpressure rise tend to weaken. The reason is that the explosion cannot always be enhanced by increasing the coal dust concentration. For example, to a certain type of coal dust and initial pressure condition, there exists a certain concentration of the coal dust where the strongest explosion occurs. The related detail discussions can be seen elsewhere [15].

3.3. Explosion visualization

The sequential pictures of methane-coal dust mixtures explosion process is shown in Fig. 4. The frequency of the high speed video camera is 1000 fps. The time when the blast begins is regarded as the initial time, i.e., 0 ms, and other pictures are selected with an interval of 40 ms. Pictures ranked in each column represent the development of the explosion with a certain initial pressure. It shows that the flame front is relatively smooth as the initial pressure is 0.2 MPa, while wrinkles can be seen when the initial pressure increases to 0.25 MPa. When the initial pressure increases to 0.3 MPa or 0.35 MPa, the wrinkle can be seen more obviously. The flame produced under higher initial pressures propagates a little faster than that under lower initial pressures. For instance, at an initial pressure of 0.35 MPa, the flame reaches to the top end of the vessel at about 160 ms after the explosion occurs, but when it comes to the case with an initial pressure of 0.2 MPa, it needs 200 ms at least.

120 ms

160 ms

200 ms

240 ms

280 ms

0.20 MPa 0.25 MPa 0.30 MPa 0.35 MPa

Fig. 4. The dynamic process of mixture explosion with different initial pressures.

4. Conclusions

Experimental study of methane-coal dust mixtures explosion under different initial pressures, such as 0.2 MPa, 0.25 MPa, 0.3 MPa and 0.35 MPa, have been conducted with 7% methane concentration and 38.5 ^m diameter of the coal particles, while the coal dust concentrations of 200 g/m3 and 300 g/m3 were considered. The results show that both of the maximum explosion overpressure and maximum rate of overpressure rise increase with the increasing of the initial pressures and coal dust concentrations. That's because on increasing the initial pressure, the initial turbulence level increases, and its effects become more significant. In coal mine tunnel, instantaneous pressure, namely, initial pressure forms at the moment of coal and gas outburst. The pressure can kick up coal dust deposited in the roadway, which makes the dust well mixed with methane. Once a spark exists, it can make an intense explosion immediately. So the study of initial pressure has remarkable significance to coal mine safety. The future study can focus on influences of initial pressure on methane-coal dust mixtures explosion mitigation, known as explosion mitigation with fine water mist.

Acknowledgements

The authors appreciate the support of the China National Key Basic Research Special Funds project (Grant No. 2012CB719704) and the National Key Technology R&D Program (Grant No. 2011BAK03B02).

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