Scholarly article on topic 'An Extensive Discussion on Experimental Test of Dust Minimum Explosible Concentration'

An Extensive Discussion on Experimental Test of Dust Minimum Explosible Concentration Academic research paper on "Earth and related environmental sciences"

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{"Minimum explosibe concentration" / "Ignition energy" / "Calorific value" / "Moisutre content" / "Particle size"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Jingjie Yuan, Weixing Huang, Bing Du, Niansheng Kuai, Zongshan Li, et al.

Abstract An extensive investigation on experimental test of dust minimum explosible concentration (MEC) was carried out by using the Siwek 20 L vessel. Systematic data were reported on the MEC of various dusts and the influences of ignition energy, dust calorific value, moisture content and particle size were taken into account. It is found that the overly low or high ignition energy will result in unrealistic MEC results. To reliably measure MEC, the experimental tests should be preformed under the condition that the test result is independent of ignition energy. For the Siwek 20 L vessel, the 4-6 kJ is the most appropriate energy ranges to determine the MEC of various dusts. The more incombustible component contained in lower calorific value dust acts as a thermal sink in the ignition and heating process of dust cloud, and therefore, the higher MEC can be found in the MEC measurement of lower calorific value dust. Moreover, due to the notable dust agglomeration, to validly measure MEC, the moisture content of test dust should not exceed 10 wt %. When the moisture content is lower than 10wt %, the MEC smoothly increases with the rise of moisture content. With the decrease of particle size, the measured MEC becomes lower, and the MEC has an approximate linear relation with particle size. The results reported in this work provide the experimental basis and data guidance for the prevention and evaluation of dust explosion risk.

Academic research paper on topic "An Extensive Discussion on Experimental Test of Dust Minimum Explosible Concentration"

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SciVerse ScienceDirect Procedia

Engineering

Procedia Engineering 43 (2012) 343 - 347 -

www.elsevier.com/locate/procedia

International Symposium on Safety Science and Engineering in China, 2012

(ISSSE-2012)

An Extensive Discussion on Experimental Test of Dust Minimum

Explosible Concentration

Jingjie Yuana, Weixing Huanga*, Bing Dua, Niansheng Kuaia, Zongshan Lia, Jingyi Tana

aSchool of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, PR China

Abstract

An extensive investigation on experimental test of dust minimum explosible concentration (MEC) was carried out by using the Siwek 20 L vessel. Systematic data were reported on the MEC of various dusts and the influences of ignition energy, dust calorific value, moisture content and particle size were taken into account. It is found that the overly low or high ignition energy will result in unrealistic MEC results. To reliably measure MEC, the experimental tests should be preformed under the condition that the test result is independent of ignition energy. For the Siwek 20 L vessel, the 4-6 kJ is the most appropriate energy ranges to determine the MEC of various dusts. The more incombustible component contained in lower calorific value dust acts as a thermal sink in the ignition and heating process of dust cloud, and therefore, the higher MEC can be found in the MEC measurement of lower calorific value dust. Moreover, due to the notable dust agglomeration, to validly measure MEC, the moisture content of test dust should not exceed 10 wt %. When the moisture content is lower than 10wt %, the MEC smoothly increases with the rise of moisture content. With the decrease of particle size, the measured MEC becomes lower, and the MEC has an approximate linear relation with particle size. The results reported in this work provide the experimental basis and data guidance for the prevention and evaluation of dust explosion risk.

© 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Capital University of Economics and Business, China Academy of Safety Science and Technology.

Keywords: Minimum explosibe concentration; Ignition energy; Calorific value; Moisutre content; Particle size

1. Introduction

For the safe handling of combustible dust, it is imperative to assess the dust explosion risk [1, 2]. Nevertheless, due to the complexity of explosion process, the dust explosion risk can only be represented by measuring explosion parameters, especially by measuring dust minimum explosible concentration. The minimum explosible concentration (MEC) is the lowest concentration of dust dispersed in air that can propagate an explosion [3, 4], and the possibility of dust explosion occurrence strongly depends on the MEC value. To reliably measure the MEC is of great importance for preventing and evaluating dust explosion risk.

However, the determination of MEC is not only related to dust properties, but is also affected by test conditions [5]. Therefore, to reasonably prevent and evaluate dust explosion risk should comprehensively investigate the influences of test conditions and dust properties on MEC. On this field, the previous studies mostly focused on the effects of test vessel size and turbulence on MEC [6-9]. Unfortunately, the influences of ignition energy, dust calorific value, moisture content and

* Corresponding author. Tel.: + 86 28 85408126; fax: +86 28 85403397. E-mail address: hwx@scu.edu.cn

ELSEVIER

1877-7058 © 2012 Published by Elsevier Ltd. doi:10.1016/j.proeng.2012.08.059

particle size are seldom mentioned systematically.

Therefore, the purpose of this paper is to present MEC results of different dusts under various conditions. The influences of ignition energy, dust calorific value, moisture content and particle size on MEC were analyzed in detail.

2. Experimental

At present, most MEC measurements are made in either a 20 L vessel or a 1 m3 vessel[3]. In this paper, the Siwek 20 L chamber recommended by IEC standard [10] and Chinese standard GB/T16425 [11] is selected as the test vessel (Fig. 1).

Fig. 1. Schematic diagram of the 20 L sphere

1-End Closure 2-Jacket 3-Sphere Shell 4-Vacuum Gauge 5-Circulating Water Inlet 6-Two-way Mechanical Valve 7-Base 8-View Port 9-To Vacuum Pump 10-Dispersion Nozzle 11-Dust Container 12-Air Pressure Gauge 13-Piezoelectric Pressure Sensor 14-Circulating Water Outlet 15-Safety Limit Switch 16-Ignition Electrode

Table 1. Composition analysis and calorific value of test coal dust

Composition Fixed carbon Volatile Ash Moisture Calori^c , _value (kJ/g)

Bituminous 37.01 20.12 41.76 1.11 21.698

coal (wt%)

Qeaned rod 77 3 30.939

(wt%)_

Table 2. Size distributions of samples

Bituminous coal dust Cleaned Magnesium

Sample - , , Flour ,

No. 1 No. 2 No. 3 coal dust powder

Size distentions 125-550 75-125 43-75 ^ ^ 75-125

Before the experiment, the test chamber was first evacuated to a pressure of 0.04 MPa (absolute), and a pre-weighed amount of dust was placed in the dust container which was pressurized up to 2 MPa (gage) by compressed air. Then, the dust was discharged into the vessel via the nozzle to form dust cloud. Finally, after a time delay of 60 ms, the dust cloud was ignited by the centrally mounted ignitor.

During an explosion, the pressure development inside the vessel was measured by the piezoelectric sensors mounted in the chamber wall. A computer was used to record the resultant pressure-time curve. The maximum explosion pressure of this curve was considered as the criterion to judge whether dust explosion occurred. The IEC standard [10] and Chinese Standard GB/T16425 [11] propose that dust explosion occurs when the maximum pressure rise is not lower than 0.04 MPa excluding ignitor effects. Obviously, the MEC is the concentration boundary above which a dust explosion occurs.

The test samples used in this work mainly include bituminous coal dust, cleaned coal dust, flour and magnesium powder. Their compositions, heat value and size distributions are illustrated in Table 1 and 2.

3. Results and discussion

3.1. Influence of ignition energy on MEC

To investigate the effect of ignition energy, the MEC values of No. 3 bituminous coal dust and magnesium powder were measured under five ignition energies of 1, 2, 4, 6 and 10 kJ. For various powders, evolution of MEC was plotted as a function of ignition energy shown in Fig. 2. It can be seen that ignition energy has a significant influence on dust MEC, and it is essential to point out that the MEC values of various powders gradually become stabilization when ignition energy is high enough.

In fact, whether a dust explosion occurs depends on the volatile yields in the heating process to a great extent. (Here the accepted ignition mechanism of dust cloud ignition by combustion of volatiles is being used implicitly.) Obviously, the strength of ignition sources will significantly affect the volatile yields of dusts [4]. When the ignition source is too weak, the volatile yields of dust are subject to the ignition energy, which means the particles cannot yield the sufficient volatiles effectively so that the measured results are based more on ignitability than flammability. In this situation, the measured

MEC will be higher than the true value, and this deviation of MEC will enhance with the reduction of dust ignitability.

Ignition energy (kJ) Tgnition

energy (kj)

Fig. 2. Evolutions of various dust MEC with ignition energy Fig. 3. Evolution of flour MEC with ignition energy

Therefore, to obtain true MEC, the ignition energy should be enough to make the particle release volatiles sufficiently. The results obtained by Going [3], Cashdollar [6], and Hertzberg [12] indicated that the ignition energy should be increased until the MEC measurements are independent of energy. For No. 3 bituminous coal dust and magnesium powder, as shown in Fig. 2, it can be found that the MEC measurements are independent of energy when the ignition energy increases to 2 kJ and 4 kJ, respectively.

Of course, an overly strong energy is also unfavourable for the MEC measurement, especially for that of dusts with worse ignitability. For instance, Fig. 3 is the MEC evolution of flour with ignition energy. (Compared with No.3 bituminous coal dust and magnesium powder, the flour has a worse ignitability.) As expected, the measured MEC of flour becomes stabilization when ignition energy ranges from 4 to 6 kJ. Nevertheless, when the energy of ignitor arises to 10 kJ, a sudden decline of MEC appears due to the overly strong ignitor. On the one hand, when an overly strong ignitor is used in the MEC tests, the energy contributed by the ignitor is sufficient to combust enough dust so that a nonexplosible system may appear to be explosible. On the other hand, due to the overly strong ignition energy, the initial test temperature of system will markedly raise, which in turn will lower the measured MEC further.

In practice, the most appropriate ignition energy in the MEC measurements is related to the size of test chamber. For laboratory-scale vessels, the 10 kJ energy may yield overly low values of MEC [1, 6, 13], and based on Fig. 2 and Fig. 3, the energy ranges of 4-6 kJ is the most appropriate for the Siwek 20 L vessel in the MEC measurements of various dusts.

3.2. Influence of dust calorific value on MEC

Rale of calorific value Race of combustible content MEC Moisture content (wt%)

Fig. 4. Comparisons of calorific value and MEC of coal dusts with Fig. 5. Developments of MEC with dust moisture content different compositions

The calorific value is the heat release amount of unit mass dust burning completely at given pressure and temperature. Hence, to some extent, it determines the dust explosion severity. At given size distribution, Fig. 4 compares the lower calorific value and MEC of coal dusts with different compositions. It can be found that the lower calorific value of dust is

proportional to the combustible composition (mainly including fixed carbon and volatiles) content, and for various coal dusts, the rate of lower calorific value is approximately equal to that of combustible composition content. Moreover, the MEC of lower calorific value coal dust is markedly higher than that of higher calorific value coal dust. This is because lower calorific value coal dust usually has more ash content and the incombustible component can act as a thermal sink to absorb heat in the ignition and heating process of dust cloud.

3.3. Influence of moisture content on MEC

As a crucial physical property, the moisture content (or humidity) will inevitably affect the determination of MEC by varying ignition and dispersion behaviours of dust particles. In order to study the influence of moisture content, Fig. 5 gives the MEC values of flour and No. 3 bituminous coal dust at the ignition energy of 5 kJ in an extensive moisture content range.

As presented in Fig. 5, the moisture content has a stronger influence on the MEC measurements of dust with a worse ignitability, and with the rise of moisture content, the enhancement of MEC includes two distinct stages. When moisture content is lower than 10 wt%, the variation of MEC is smooth, whereas that is sharp when moisture content exceeds 10 wt%.

Obviously, this distinct variation of MEC is attributed to the difference of dust humidity. Under the lower moisture content (lower than 10 wt%), the dust particle can still maintain its good dispersion, and the moisture mainly acts as a thermal sink to absorb heat in the ignition process of dust cloud. However, due to the limited amount, the heat absorption of moisture is not obvious so that the variation of measured MEC is smooth with the rise of moisture content. To the opposite, when the moisture content is higher than 10 wt%, dust agglomeration begins to appear, and then enhances with the increase of moisture content further. This notable dust agglomeration will hinder the dispersion of dust particles significantly in the MEC measurements, and result in an overly high MEC.

Consequently, based on the data in Fig. 5, to validly measure MEC, the moisture content of test dust should not exceed 10 wt%. This moisture content limit of 10 wt% obtained in this paper is in agreement with the requirement of IEC standard and Chinese standard on test dust of MEC.

3.4. Influence of particle size on MEC

With respect to the effect of particle size, a number of tests were carried out at 5 kJ ignition energy, with moisture content ranging from 0 (dry dust) to 8 wt%. The MEC of bituminous coal dusts with three different size distributions are plotted in Fig. 6. As can be seen, for given moisture content, the MEC arises with the increase of particle size. This is because the flammability and the chemical activity of dust are related to the particle size. The rise of particle size leads to the reduction of particle specific surface area. It means that the effective heating and reaction area of particles decreases. Therefore, the fine dusts have a better flammability and chemical activity so that the measured MEC becomes lower.

Fig. 6. Influence of particle size on MEC

Fig. 7. MEC and particle size data in the previous papers of other researchers

Furthermore, according to the data in Fig. 6, it seems that the MEC has a linear relation with particle size. This conclusion is in accordance with the model of dust explosion derived by Zhao [14] using the thermal explosion theory. Fig. 7 summarizes the MEC data derived from the previous papers of other researchers [15, 16]. It also can be found that the measured MEC of various dusts varies linearly with the increase of particle size.

4. Conclusions

The extensive investigation on experimental test of MEC has been performed by using the Siwek 20 L vessel. The influences of ignition energy, dust calorific value, moisture content and particle size were studied. A lot of results of MEC in various situations were reported to provide the experimental basis and data guidance for the prevention and evaluation of dust explosion risk.

As a consequence, the data in this paper show that the strength of ignition source has an obvious influence on the determination of MEC. The overly low ignition energy will lead to the unrealistic volatile yields in the heating and ignition process of dust cloud, and the measured MEC is based more on ignitability than flammability. The overly high ignition energy may change the initial test temperature and overdrive the combustion of dust cloud so that the measured MEC is underestimated. The reliable MEC should be determined under the condition that the test result is independent of ignition energy. For the Siwek 20 L vessel, the energy ranges of 4-6 kJ is the most appropriate in the MEC measurements of various

A comparison between the MEC of various coal dusts shows that the more incombustible component contained in lower calorific value dust acts as a thermal sink to absorb heat in the ignition and heating process of dust cloud, and therefore the lower calorific value dust has a higher MEC.

Furthermore, it emerges from this study that the determination of MEC is also affected by the moisture content of dust. When the moisture content is lower than 10 wt%, the MEC will smoothly increase with the rise of moisture content. Once the moisture content exceeds 10 wt%, the notable dust agglomeration will result in an overly high MEC. Therefore, to validly measure MEC, the moisture content of test dust should not exceed 10 wt%. This result is consistent with the requirement of IEC standard and Chinese standard on test dust of MEC.

The MEC has an approximate linear relation with particle size. Due to the increase of particle specific surface area, the fine dust has a better flammability and chemical activity, and the measured MEC becomes lower.

Acknowledgements

The authors are grateful to the Ministry of Education of P. R. China for its financial support under "211" Project.

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