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Experimental investigations of the minimum ignition energy and the minimum ignition temperature of inert and combustible dust cloud mixtures
Journal of Hazardous Materials 307 (2016) 302–311
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Experimental investigations of the minimum ignition energy and the minimum ignition temperature of inert and combustible dust cloud mixtures Emmanuel Kwasi Addai ∗ , Dieter Gabel, Ulrich Krause Otto-von-Guericke-University, Institute of Instrumental and Environmental Technology, Department of Systems Engineering and Plant Safety, Universitätsplatz 2, 39106 Magdeburg, Germany
h i g h l i g h t s • • • •
Ignition sensitivity of a highly flammable dust decreases upon addition of inert dust. Minimum ignition temperature of a highly flammable dust increases when inert concentration increase. Minimum ignition energy of a highly flammable dust increases when inert concentration increase. The permissible range for the inert mixture to minimize the ignition risk lies between 60 to 80%.
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Article history: Received 8 October 2015 Received in revised form 6 January 2016 Accepted 8 January 2016 Available online 12 January 2016 Keywords: Minimum ignition energy Minimum ignition temperature Dust explosion Ignition sensitivity Inert materials
a b s t r a c t The risks associated with dust explosions still exist in industries that either process or handle combustible dust. This explosion risk could be prevented or mitigated by applying the principle of inherent safety (moderation). This is achieved by adding an inert material to a highly combustible material in order to decrease the ignition sensitivity of the combustible dust. The presented paper deals with the experimental investigation of the influence of adding an inert dust on the minimum ignition energy and the minimum ignition temperature of the combustible/inert dust mixtures. The experimental investigation was done in two laboratory scale equipment: the Hartmann apparatus and the Godbert-Greenwald furnace for the minimum ignition energy and the minimum ignition temperature test respectively. This was achieved by mixing various amounts of three inert materials (magnesium oxide, ammonium sulphate and sand) and six combustible dusts (brown coal, lycopodium, toner, niacin, corn starch and high density polyethylene). Generally, increasing the inert materials concentration increases the minimum ignition energy as well as the minimum ignition temperatures until a threshold is reached where no ignition was obtained. The permissible range for the inert mixture to minimize the ignition risk lies between 60 to 80%. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Dust explosions are one of the major concerns in many industries that handle or process combustible bulk materials [1]. Many materials that are flammable in bulk form become explosible if dispersed as a cloud of fine particles in air. Thus, in industries that manufacture, transport, process and/or use combustible, dusts explosions present a real hazard to both staff and equipment. In order to prevent and mitigate the risk associated with these kind of incidences, the explosion parameters, such as maximum explo-
∗ Corresponding author. Fax: +49 391 67 11128. E-mail addresses: [email protected], [email protected] (E.K. Addai). http://dx.doi.org/10.1016/j.jhazmat.2016.01.018 0304-3894/© 2016 Elsevier B.V. All rights reserved.
sion pressure, maximum rate of pressure rise, deflagration index, minimum explosible dust concentration, minimum ignition energy (MIE) and minimum ignition temperature (MIT) have to be determined. Addition of inert substance to highly combustible substance can reduce the risk associated with combustible dust by decreasing the ignition sensitivity such as MIT and MIE. This principle of inerting could be considered as inherent safety (moderation). In order to achieve an acceptable level of safety, the principles of inherent safety have to be kept in mind during the conceptual phase of an installation [2,3]. However, if the potential benefits of inherent safety are generally well recognized, its systematic application remains marginal and sometimes difficult [4,5]. The promotion of this concept must then be done both by industrialists and legislators, especially by the definition of new normative barriers based on these principles [6]. Amyotte et al.
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[7,8] described in detail how the inherent safety principles can be implemented in practice to prevent and mitigate accidental dust explosions in process plants, notably by using the moderation principle. Thus, mixing an inert solid or a less flammable material with a combustible dust can be regarded as a direct application of such principle, as it allows the use of the hazardous material in a less hazardous form. Industrial applications of such mixtures of combustible and inert powders are numerous. For instance, in the pet food industry, minerals are added to organic powders in order to enhance the palatability, to provide nutrients or to modify the appearance of the food. In the plastics or textile industries, mineral loadings are used to obtain thermally stable polymers or to improve the mechanical properties of the fibers, membranes or coatings. Another example is the addition of silicon dioxide as anti-caking agent in dried milk or icing sugar. In these cases, the presence of inert solid materials is required by the process or the consumer and has an influence of the flammability of the products. In the case of solid inerting, for instance, when rock dusts is mixed with coal in the mining industries [9,10] or when flame-retardants are added to foams or textiles, the main objective is to prevent the materials to ignite, but this addition can have significant effects on the fabrication process and the other properties of the final product. It is therefore important to determine safety parameters such as the minimum ignition energy and the minimum ignition temperature of dust clouds and the explosion sensitivity as a function of the inert material and not for the pure flammable dust alone. The MIT is used to evaluate the probability of ignition by hot surfaces. Hot surfaces capable of igniting dust clouds exist in a number of situations in the industry (furnaces, burners and dryers of various kinds). The MIE provides the information to which types of sparks have to be considered as ignition sources. Electric sparks and electrostatic discharges as well as hot surface are considered as one of the common source of ignition for dust explosions occurring in workplaces. Numerous researches have therefore been carried to determine both the MIT and MIE of single substance, but in the case of dust to dust mixtures, only few data is available [11–17]. In this present paper, six highly combustible dusts (toner, lycopodium, niacin, high-density polyethylene, cornstarch and brown coal) were mixed with three inert dusts (ammonium sulphate, magnesium oxide and sand) to determine their ignition sensitivity (MIT and MIE).
2. Materials and experimental work 2.1. Materials Six combustible dusts namely cornstarch, lycopodium, toner, niacin, PE-HD and brown coal were mixed with three inert materials namely; ammonium sulphate, magnesium oxide and sand to determine both the MIT and MIE. The particle size distribution is one of the important properties that affect the ignition sensitivity of dust clouds. As a results of that, the particle size distribution of all the dust used was examined using a standard equipment (CAMSIZER). The equipment measures particle size distribution by laser diffraction. Figs. 1 and 3 show the particle size distributions for both combustible and inert dusts used. Furthermore, in order to reveal the surface structure of the particles, Scanning Electron Microscopy (SEM) images for all the dusts used were taken with different magnifications. The images provide the various shapes and pore size of each dust sample. For example, it could be seen that starch particle agglomerates together thereby increasing the individual particle size which could affect the global settling velocity. Figs. 2 and 4 also show the Scanning Electron Microscopy (SEM) images for both the combustible and inert dust respectively.
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Table 1 summarizes data for various preparatory analysis and properties of the combustible dust. The parameters considered are the elemental analysis, median particle sizes, moisture content, heat of combustion and molecular formula of the dusts that were calculated from elemental analysis. Table 2 also lists the various preparatory analysis of the inert materials. 2.2. Sample preparation A 250 ml glass bottle (transparent) was used for mixing the test samples. For every mixture combination, both the combustible dust and inert dust were weighed separately and then placed in a glass bottle. The bottle was shaken vigorously in all possible directions to get homogenous mixing, it was then left for some time for dust to settle down as a flying dusts. To prevent segregation of the finer dust from coarser one, a continuous slow tumbling of the bottle was further done to ensure complete homogeneity. 2.2.1. Measurement of the minimum ignition temperature In this present paper a modified Godbert-Greenwald furnace was used. It mainly consists of a steel furnace tube, an air reservoir, a pressure regulator. The furnace tube is 42 cm long which is twice the length of the standard as described in EN50281 [19] and 3.5 cm inside diameter. It is heated externally by an electric coil of chrome wire. The furnace tube is mounted vertically in a mild steel case lined with glass wool and filled up with bulk wool to act as a thermal insulation. The furnace is vertical with an opening at the bottom. The upper end is connected to the dust holder by means of an adaptor. The dust is injected into the furnace by an air pulse, which is obtained by opening a solenoid valve to discharge the air stored in a reservoir. A mirror is placed at the bottom of the furnace which allows the operator to observe the inside of the furnace. A thermocouple is placed closed to the inner wall of the furnace that is connected to a PID temperature controller. Fig. 5 shows the experimental setup for MIT of dust mixtures test. In order to test for the MIT of combustible and inert dust mixtures, the furnace tube was heated and fixed to the desired temperature (the maximum allowable temperature of the furnace used was 700 ◦ C) and the amount of dust weighed beforehand was placed in the dust chamber. The air reservoir was filled with air up to the desired dispersion pressure and the dust sample was then dispersed through the furnace tube by the blast of air. The criterion for an explosion was an observation of a flame at the bottom open mouth of the furnace or within (with the help of the mirror). Both the pressure in the air reservoir (0.1–0.5 bar above atmospheric pressure) and the mass of dust (0.1–0.5 g) were varied until a vigorous explosion was obtained. The condition at which the vigorous explosion occurs was taken as the “best” explosion condition. This condition was maintained, the furnace temperature was lowered and testing continued until no flame was observed in for ten successive attempts as shown in Fig. 6. The difference in temperatures between explosion and no explosion was 5 ◦ C. The lowest temperature at which ignition with flame occurred was taken as the minimum ignition temperature. As soon as the MIT was obtained, further tests were performed at a furnace temperature 5 ◦ C below the MIT by varying both pressure and mass of dust mixtures to confirm the non-ignition state. 2.3. Measurement of the minimum ignition energy The minimum ignition energy was measured using an electric spark igniter (Hartmann apparatus MIE III by the courtesy of Chilworth—DEKRA Company) according to a protocol similar to that defined in EN 13821 standards [20]. The combustion chamber is a glass tube with a volume of 1.2 l and is provided with a mushroom-shaped dust dispersion system. Dust dispersion was
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Fig. 1. Particle size distribution of combustible dust.
Fig. 2. SEM images for the combustible dust with different magnification and length scale.
triggered by a compressed air blast at 7 bar. The air blast generates considerable turbulence and results in the creation of a dust cloud. A spark was drawn between two electrodes. According to the standard, the spark gap was between 3–6 mm. The ignition criterion was visual propagation of flame within 20 repeated attempt as shown in Fig. 7. The minimum ignition energy lies between the highest energy at which ignition fails to occur (E1) for ten successive attempts to ignite the dust/air mixture and the lowest energy
at which ignition occurs (E2) within up to ten successive attempts. For the purpose of comparison between different combustible/inert mixtures, instead of choosing a specific energy range, only one single value estimated by the use of the probability of ignition as specified in EN 13821 standards was used: lgMIE = lgE2 − I [E2] ×
(lgE2 − lgE1) (NI + I) × [E2] + 1
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Fig. 3. Particle size distribution for the inert materials.
Fig. 4. SEM image for the inert materials with different magnification and length scale.
Table 1 Preparatory analysis of the combustible dust used. Properties
Toner
Lycopodium
Starch
Niacin
Brown coal
PE–HD
Molecular formula Median diameter (mm) Volatile content (%, mass) Density (g/cm3 ) Moisture content (%, mass) Heat of combustion (kJ/kg) C H O S N
C7.17 H7.75 O0.33 0.013 90.18 0.45 0.92 35792 86.05 7.72 5.23 1 0
C5.77 H9.59 O1.23 S0.001 N0.08 0.031 91.06 0.38 0.35 28447 69.26 9.59 19.62 0.38 1.15
C3.69 H6.34 O3.06 S0.01 0.029 93.77 0.73 0.5 15302 44.34 6.34 48.94 0.38 0
C6 H5 NO2 0.028 99.85 0.45 1.56 20538 58.31 4.31 25.8 0.30 11.32
C4 .06 H4 .45 OS0 .01 N0.07 0.095 89.5 0.61 1.93 17148 69 6.3 22.7 0.6 1.4
C7.07 H13 .74 O0.33 S0.03 0.054 99.78 0.63 0.16 42740 84.81 13.74 1.37 0.09 0
I [E2] is the number of tests with successful ignition at energy level E2 and (NI + I) [E2] stands for the total number of tests at the energy level of E2. The values obtained using the above formula has a maximum deviation of 1 mJ. Fig. 8 also explains how the MIE of each test or point presented in the results were obtained.
3. Results and discussion The ignition sensitivity (minimum ignition temperature and minimum ignition energy) of mixtures of three inert materials and six combustible dusts was investigated. Globally, addition of inert dust to combustible dust decreases the ignition sensitivity of the combustible dust. This effect could be attributed to the following
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Fig. 5. Experimental setup for MIT test.
Fig. 6. A representation of how the MIT of both single dust and inert mixtures was obtained.
mechanisms [16,21,22,23]; cooling effect, dilution effect, radiation absorption, turbulence modification and limitation of oxygen. The cooling effects are related to endothermic physical processes (vaporization for instance) or to the high specific heat of the additive, which creates a thermal sink. In the case of dilution, it is simply due to the reduction of combustible content, in solid phase by addition of noncombustible substance. The absorption of radiation is obtained when fine inert particles are dispersed ahead of the propagation flame. This absorption reduces the preheating zone and then modifies the flame speed. The addition of solid inert materials can also modify the homogeneity of the dust cloud (especially the settling velocity) and alter the initial turbulence, which has a strong effect on the mixture flammability. Finally, the limitation of oxygen diffusion at the surface of the combustible dust is ensured by the formation of a protective layer of flame retardant.
3.1. Effect of inert materials on the MIT of combustible dusts The minimum ignition temperature is the lowest temperature of a heated surface which can ignite a fuel oxidizer mixture within the
Table 2 Properties of the inert materials used [18]. Properties
Ammonium sulphate
Magnesium oxide
Sand
Molecular formula Molar mass (g/mol) Median diameter (mm) Density (g/cm3 ) Moisture content (mass%) Melting point (◦ C) Heat capacity (Cal/mol K)
(NH4 )2 SO4 132.14 0.051 1.77 0.33 235 51.6
MgO 40.30 0.031 2.58 0.18 3600 10.87
SiO2 60.08 0.097 3.65 2.1 1600 10.86
explosible range. The MIT of combustible dust could be increased by adding inert materials. Fig. 9a and b demonstrates the effect of mixing different inert materials with brown coal and lycopodium. As expected, the MIT of the combustible dust is increased when the inert dust concentration increases until a threshold value is reached where ignition was no longer obtained. Considering the effect of inert material on brown coal and lycopodium, both ammonium sulphate and magnesium oxide showed a similar trend on the MIT of which an exponential increase was observed until 60% and 70% for brown coal and lycopodium respectively where no ignition
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Fig. 7. Image of explosion development in the Hartmann apparatus.
Fig. 8. A representation of how the MIE of both single and mixtures was obtained.
was obtained. In contrast to the inerting behavior of ammonium sulphate and magnesium oxide, sand behaved differently with the last ignition obtained at 80% for both brown coal and lycopodium. The result for the effect of inert materials on the MIT of starch is similar to that of lycopodium as presented in Fig. 10a. There was not much effect on the MIT of starch up to 30% for the three inert materials but a prominent increase was then observed above 40%. Magnesium oxide and ammonium sulphate exhibited good inerting effects as no explosion was observed above 60% and 70% inert concentration respectively. In case of sand an explosion was observed
even at 80% which clearly shows a less influence of sand on starch as compare to the other two inert materials. A similar behavior as explained in the effect of inert materials on starch was also seen in niacin as shown in Fig. 10b. Fig. 11a and b also presents the results for the effect of the various inert materials on the MIT of both toner and high density polyethylene respectively. It could be seen in Fig. 11a that magnesium oxide had good inerting effect on toner comparing to the other two inert materials. A rapid increase in the ignition temperature was seen by increasing the concentration of magnesium
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Fig. 9. (a) Effect of three inert materials on the MIT of brown coal. (b) Effect of three inert materials on the MIT of lycopodium.
Fig. 10. (a) Effect of three inert materials on the MIT of starch. (b) Effect of three inert materials on the MIT of Niacin.
oxide up to 50%, eventually, leading to the temperature of 680 ◦ C with 70% inert concentration. On the other hand, the trends for ammonium sulphate and sand portray similar behavior up to 70% and 80% respectively. In the case of HDPE a similar behavior as explained above was notice, ammonium sulphate and magnesium oxide exhibited similar inerting behavior with the same threshold limit of 70% of inert materials concentration while sand behaved differently with threshold limit of 80%.
3.2. Effect of inert materials on the MIE of combustible dusts The minimum ignition energy (MIE) is defined as the lowest energy value of a high-voltage capacitor discharge that is capable of igniting an ignitable mixture. In the process of dust explosion prevention, it has been noticed that, adding some amount of inert materials to a highly combustible dust could increase the MIE of the combustible dust. Fig. 12a and b presents the results for the influence of the above mentioned inert materials on brown coal
and lycopodium. There was a rapid rise in the MIE beyond 30% and no ignition was observed after 60% concentration of inert (magnesium oxide, ammonium sulphate) for brown coal and 60% and 70% for lycopodium respectively. Moreover, sand tends to be less effective compared to the other two inert materials where last ignition was obtained at 70% and 80% for brown coal and lycopodium respectively. Furthermore, Fig. 13a and b explains the results of the influence of the three inert materials on the MIE of corn starch and niacin. A similar effect as explained above was noticed, the influence of both ammonium sulphate and magnesium oxide are more pronounce as compared to sand. Up to 30% of inert materials concentration, a slight increase in the MIE for both starch and niacin was obtained. A sharp increase was later observed above 30% of inert concentration until 60% where last ignition was obtained for ammonium sulphate and magnesium oxide as well as 80% for sand. Fig. 14a demonstrates the progression of minimum ignition energy of toner–inert material mixtures in relation to the inert
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Fig. 11. (a) Effect of three inert materials on the MIT of toner. (b) Effect of three inert materials on the MIT of HDPE.
Fig. 12. (a) Effect of three inert materials on the MIE of brown coal. (b) Effect of three inert materials on the MIE of lycopodium.
concentration. It can be established that the value of MIE experienced a slow rise, until an inert material concentration of 50% is reached. A rapid increase was then seen above 50% inert until 80% where the last ignition was obtained. Contrary to the other combustible dust inert mixtures, all the three inert materials has similar behavior on the MIE of toner, this could be as a result of very low particle size of toner compare to the inert materials. Large difference of particle size between the combustible dust and the inert material is likely to lead to segregation of the particles and difficulties to get a homogenous mixture when dispersed. With respect to high density polyethylene and the various inert materials, magnesium oxide showed high inerting effect follow by ammonium sulphate and sand with last ignition obtained at 60%, 70% and 80% respectively.
the other two inert materials. This could be due to its high thermal stability with a melting point of 3600 ◦ C. Since it does not decompose within the test temperature range used in this work, its effect could be due to dilution. As explained by Agnès et al. [16], dilution effect is the reduction of combustible content in solid phase by addition of noncombustible substance. Furthermore, magnesium oxide has peculiar properties which make it an excellent inerting material. For example in the ceramic industries, magnesium oxide is used as an inert material which exhibits excellent resistance to attack by metals such as sodium, nickel based super alloys and plutonium/uranium systems, fluxes and superconductor compounds. Secondly, ammonium sulphate also has a good inerting effect on the combustible materials since it is unstable to heat. When heated, ammonium sulphate decomposes according to the chemical equation below.
3.3. Comparison between the three inert materials
2(NH4 )2 SO4 → (NH4 )2 S2 O7 (s) + 2NH3 + H2 O
It was generally noticed that magnesium oxide had a higher effect on the MIT or MIE of the combustible dust tested compared to
The heat balance of fuel–inert mixtures explosion addressed by Chatrathi and Going [24] gives the idea that the decomposition
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Fig. 13. (a) Effect of three inert materials on the MIE of Niacin. (b) Effect of three inert materials on the MIE of starch.
Fig. 14. (a) Effect of three inert materials on the MIE of toner. (b) Effect of three inert materials on the MIE of PE–HD.
plays a key role in the inerting effectiveness that is consistent with Abbasi and Abbasi [25]. The thermal decomposition of ammonium phosphate starts at 235 ◦ C [26] which is significantly below the ignition temperature of the combustible dusts used. Hence the efficient decomposition of ammonium sulphate results in the excellent thermal sink. Furthermore, the free ammonia decomposed from ammonium sulphate has a particular efficacy in flame extinguishing [27]. Finally, sand had a less inerting effect on the combustible dust compared to magnesium oxide and ammonium sulphate. This could be attributed to its high bulk density and particle size. Large difference of particle size between the combustible dust and the solid inert is likely to lead to segregation of the particles and hence difficult to get a homogenous mixture when dispersed. It is also known that finer sizes of a given solid inert permit a more efficient inerting of a combustible dust than coarser size fractions. This statement is in accordance with works performed by other authors [28,29].
4. Conclusions This study was carried out to investigate the effects of ignition sensitivity of three inerting materials mixed with six combustible dust. For this purpose, series of experiments were performed for different mixtures of combustible dust samples and inert dusts. The new insight presented in this experimental investigation has provided a good opportunity that is applicable to the inherent safety principle of moderation in process design. There exists a certain threshold value for the concentration of inert material, above which the ignition sensitivity of the dust mixture decreases thereby increasing the minimum ignition energy and minimum ignition temperature. It was generally noticed that the permissible range for the inert mixture to minimize the ignition risk lies between 60% to 80%.
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Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Experimental investigations of the minimum ignition energy and the minimum ignition temperature of inert and combustible dust cloud mixtures Emmanuel Kwasi Addai ∗ , Dieter Gabel, Ulrich Krause Otto-von-Guericke-University, Institute of Instrumental and Environmental Technology, Department of Systems Engineering and Plant Safety, Universitätsplatz 2, 39106 Magdeburg, Germany
h i g h l i g h t s • • • •
Ignition sensitivity of a highly flammable dust decreases upon addition of inert dust. Minimum ignition temperature of a highly flammable dust increases when inert concentration increase. Minimum ignition energy of a highly flammable dust increases when inert concentration increase. The permissible range for the inert mixture to minimize the ignition risk lies between 60 to 80%.
a r t i c l e
i n f o
Article history: Received 8 October 2015 Received in revised form 6 January 2016 Accepted 8 January 2016 Available online 12 January 2016 Keywords: Minimum ignition energy Minimum ignition temperature Dust explosion Ignition sensitivity Inert materials
a b s t r a c t The risks associated with dust explosions still exist in industries that either process or handle combustible dust. This explosion risk could be prevented or mitigated by applying the principle of inherent safety (moderation). This is achieved by adding an inert material to a highly combustible material in order to decrease the ignition sensitivity of the combustible dust. The presented paper deals with the experimental investigation of the influence of adding an inert dust on the minimum ignition energy and the minimum ignition temperature of the combustible/inert dust mixtures. The experimental investigation was done in two laboratory scale equipment: the Hartmann apparatus and the Godbert-Greenwald furnace for the minimum ignition energy and the minimum ignition temperature test respectively. This was achieved by mixing various amounts of three inert materials (magnesium oxide, ammonium sulphate and sand) and six combustible dusts (brown coal, lycopodium, toner, niacin, corn starch and high density polyethylene). Generally, increasing the inert materials concentration increases the minimum ignition energy as well as the minimum ignition temperatures until a threshold is reached where no ignition was obtained. The permissible range for the inert mixture to minimize the ignition risk lies between 60 to 80%. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Dust explosions are one of the major concerns in many industries that handle or process combustible bulk materials [1]. Many materials that are flammable in bulk form become explosible if dispersed as a cloud of fine particles in air. Thus, in industries that manufacture, transport, process and/or use combustible, dusts explosions present a real hazard to both staff and equipment. In order to prevent and mitigate the risk associated with these kind of incidences, the explosion parameters, such as maximum explo-
∗ Corresponding author. Fax: +49 391 67 11128. E-mail addresses: [email protected], [email protected] (E.K. Addai). http://dx.doi.org/10.1016/j.jhazmat.2016.01.018 0304-3894/© 2016 Elsevier B.V. All rights reserved.
sion pressure, maximum rate of pressure rise, deflagration index, minimum explosible dust concentration, minimum ignition energy (MIE) and minimum ignition temperature (MIT) have to be determined. Addition of inert substance to highly combustible substance can reduce the risk associated with combustible dust by decreasing the ignition sensitivity such as MIT and MIE. This principle of inerting could be considered as inherent safety (moderation). In order to achieve an acceptable level of safety, the principles of inherent safety have to be kept in mind during the conceptual phase of an installation [2,3]. However, if the potential benefits of inherent safety are generally well recognized, its systematic application remains marginal and sometimes difficult [4,5]. The promotion of this concept must then be done both by industrialists and legislators, especially by the definition of new normative barriers based on these principles [6]. Amyotte et al.
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[7,8] described in detail how the inherent safety principles can be implemented in practice to prevent and mitigate accidental dust explosions in process plants, notably by using the moderation principle. Thus, mixing an inert solid or a less flammable material with a combustible dust can be regarded as a direct application of such principle, as it allows the use of the hazardous material in a less hazardous form. Industrial applications of such mixtures of combustible and inert powders are numerous. For instance, in the pet food industry, minerals are added to organic powders in order to enhance the palatability, to provide nutrients or to modify the appearance of the food. In the plastics or textile industries, mineral loadings are used to obtain thermally stable polymers or to improve the mechanical properties of the fibers, membranes or coatings. Another example is the addition of silicon dioxide as anti-caking agent in dried milk or icing sugar. In these cases, the presence of inert solid materials is required by the process or the consumer and has an influence of the flammability of the products. In the case of solid inerting, for instance, when rock dusts is mixed with coal in the mining industries [9,10] or when flame-retardants are added to foams or textiles, the main objective is to prevent the materials to ignite, but this addition can have significant effects on the fabrication process and the other properties of the final product. It is therefore important to determine safety parameters such as the minimum ignition energy and the minimum ignition temperature of dust clouds and the explosion sensitivity as a function of the inert material and not for the pure flammable dust alone. The MIT is used to evaluate the probability of ignition by hot surfaces. Hot surfaces capable of igniting dust clouds exist in a number of situations in the industry (furnaces, burners and dryers of various kinds). The MIE provides the information to which types of sparks have to be considered as ignition sources. Electric sparks and electrostatic discharges as well as hot surface are considered as one of the common source of ignition for dust explosions occurring in workplaces. Numerous researches have therefore been carried to determine both the MIT and MIE of single substance, but in the case of dust to dust mixtures, only few data is available [11–17]. In this present paper, six highly combustible dusts (toner, lycopodium, niacin, high-density polyethylene, cornstarch and brown coal) were mixed with three inert dusts (ammonium sulphate, magnesium oxide and sand) to determine their ignition sensitivity (MIT and MIE).
2. Materials and experimental work 2.1. Materials Six combustible dusts namely cornstarch, lycopodium, toner, niacin, PE-HD and brown coal were mixed with three inert materials namely; ammonium sulphate, magnesium oxide and sand to determine both the MIT and MIE. The particle size distribution is one of the important properties that affect the ignition sensitivity of dust clouds. As a results of that, the particle size distribution of all the dust used was examined using a standard equipment (CAMSIZER). The equipment measures particle size distribution by laser diffraction. Figs. 1 and 3 show the particle size distributions for both combustible and inert dusts used. Furthermore, in order to reveal the surface structure of the particles, Scanning Electron Microscopy (SEM) images for all the dusts used were taken with different magnifications. The images provide the various shapes and pore size of each dust sample. For example, it could be seen that starch particle agglomerates together thereby increasing the individual particle size which could affect the global settling velocity. Figs. 2 and 4 also show the Scanning Electron Microscopy (SEM) images for both the combustible and inert dust respectively.
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Table 1 summarizes data for various preparatory analysis and properties of the combustible dust. The parameters considered are the elemental analysis, median particle sizes, moisture content, heat of combustion and molecular formula of the dusts that were calculated from elemental analysis. Table 2 also lists the various preparatory analysis of the inert materials. 2.2. Sample preparation A 250 ml glass bottle (transparent) was used for mixing the test samples. For every mixture combination, both the combustible dust and inert dust were weighed separately and then placed in a glass bottle. The bottle was shaken vigorously in all possible directions to get homogenous mixing, it was then left for some time for dust to settle down as a flying dusts. To prevent segregation of the finer dust from coarser one, a continuous slow tumbling of the bottle was further done to ensure complete homogeneity. 2.2.1. Measurement of the minimum ignition temperature In this present paper a modified Godbert-Greenwald furnace was used. It mainly consists of a steel furnace tube, an air reservoir, a pressure regulator. The furnace tube is 42 cm long which is twice the length of the standard as described in EN50281 [19] and 3.5 cm inside diameter. It is heated externally by an electric coil of chrome wire. The furnace tube is mounted vertically in a mild steel case lined with glass wool and filled up with bulk wool to act as a thermal insulation. The furnace is vertical with an opening at the bottom. The upper end is connected to the dust holder by means of an adaptor. The dust is injected into the furnace by an air pulse, which is obtained by opening a solenoid valve to discharge the air stored in a reservoir. A mirror is placed at the bottom of the furnace which allows the operator to observe the inside of the furnace. A thermocouple is placed closed to the inner wall of the furnace that is connected to a PID temperature controller. Fig. 5 shows the experimental setup for MIT of dust mixtures test. In order to test for the MIT of combustible and inert dust mixtures, the furnace tube was heated and fixed to the desired temperature (the maximum allowable temperature of the furnace used was 700 ◦ C) and the amount of dust weighed beforehand was placed in the dust chamber. The air reservoir was filled with air up to the desired dispersion pressure and the dust sample was then dispersed through the furnace tube by the blast of air. The criterion for an explosion was an observation of a flame at the bottom open mouth of the furnace or within (with the help of the mirror). Both the pressure in the air reservoir (0.1–0.5 bar above atmospheric pressure) and the mass of dust (0.1–0.5 g) were varied until a vigorous explosion was obtained. The condition at which the vigorous explosion occurs was taken as the “best” explosion condition. This condition was maintained, the furnace temperature was lowered and testing continued until no flame was observed in for ten successive attempts as shown in Fig. 6. The difference in temperatures between explosion and no explosion was 5 ◦ C. The lowest temperature at which ignition with flame occurred was taken as the minimum ignition temperature. As soon as the MIT was obtained, further tests were performed at a furnace temperature 5 ◦ C below the MIT by varying both pressure and mass of dust mixtures to confirm the non-ignition state. 2.3. Measurement of the minimum ignition energy The minimum ignition energy was measured using an electric spark igniter (Hartmann apparatus MIE III by the courtesy of Chilworth—DEKRA Company) according to a protocol similar to that defined in EN 13821 standards [20]. The combustion chamber is a glass tube with a volume of 1.2 l and is provided with a mushroom-shaped dust dispersion system. Dust dispersion was
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Fig. 1. Particle size distribution of combustible dust.
Fig. 2. SEM images for the combustible dust with different magnification and length scale.
triggered by a compressed air blast at 7 bar. The air blast generates considerable turbulence and results in the creation of a dust cloud. A spark was drawn between two electrodes. According to the standard, the spark gap was between 3–6 mm. The ignition criterion was visual propagation of flame within 20 repeated attempt as shown in Fig. 7. The minimum ignition energy lies between the highest energy at which ignition fails to occur (E1) for ten successive attempts to ignite the dust/air mixture and the lowest energy
at which ignition occurs (E2) within up to ten successive attempts. For the purpose of comparison between different combustible/inert mixtures, instead of choosing a specific energy range, only one single value estimated by the use of the probability of ignition as specified in EN 13821 standards was used: lgMIE = lgE2 − I [E2] ×
(lgE2 − lgE1) (NI + I) × [E2] + 1
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Fig. 3. Particle size distribution for the inert materials.
Fig. 4. SEM image for the inert materials with different magnification and length scale.
Table 1 Preparatory analysis of the combustible dust used. Properties
Toner
Lycopodium
Starch
Niacin
Brown coal
PE–HD
Molecular formula Median diameter (mm) Volatile content (%, mass) Density (g/cm3 ) Moisture content (%, mass) Heat of combustion (kJ/kg) C H O S N
C7.17 H7.75 O0.33 0.013 90.18 0.45 0.92 35792 86.05 7.72 5.23 1 0
C5.77 H9.59 O1.23 S0.001 N0.08 0.031 91.06 0.38 0.35 28447 69.26 9.59 19.62 0.38 1.15
C3.69 H6.34 O3.06 S0.01 0.029 93.77 0.73 0.5 15302 44.34 6.34 48.94 0.38 0
C6 H5 NO2 0.028 99.85 0.45 1.56 20538 58.31 4.31 25.8 0.30 11.32
C4 .06 H4 .45 OS0 .01 N0.07 0.095 89.5 0.61 1.93 17148 69 6.3 22.7 0.6 1.4
C7.07 H13 .74 O0.33 S0.03 0.054 99.78 0.63 0.16 42740 84.81 13.74 1.37 0.09 0
I [E2] is the number of tests with successful ignition at energy level E2 and (NI + I) [E2] stands for the total number of tests at the energy level of E2. The values obtained using the above formula has a maximum deviation of 1 mJ. Fig. 8 also explains how the MIE of each test or point presented in the results were obtained.
3. Results and discussion The ignition sensitivity (minimum ignition temperature and minimum ignition energy) of mixtures of three inert materials and six combustible dusts was investigated. Globally, addition of inert dust to combustible dust decreases the ignition sensitivity of the combustible dust. This effect could be attributed to the following
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Fig. 5. Experimental setup for MIT test.
Fig. 6. A representation of how the MIT of both single dust and inert mixtures was obtained.
mechanisms [16,21,22,23]; cooling effect, dilution effect, radiation absorption, turbulence modification and limitation of oxygen. The cooling effects are related to endothermic physical processes (vaporization for instance) or to the high specific heat of the additive, which creates a thermal sink. In the case of dilution, it is simply due to the reduction of combustible content, in solid phase by addition of noncombustible substance. The absorption of radiation is obtained when fine inert particles are dispersed ahead of the propagation flame. This absorption reduces the preheating zone and then modifies the flame speed. The addition of solid inert materials can also modify the homogeneity of the dust cloud (especially the settling velocity) and alter the initial turbulence, which has a strong effect on the mixture flammability. Finally, the limitation of oxygen diffusion at the surface of the combustible dust is ensured by the formation of a protective layer of flame retardant.
3.1. Effect of inert materials on the MIT of combustible dusts The minimum ignition temperature is the lowest temperature of a heated surface which can ignite a fuel oxidizer mixture within the
Table 2 Properties of the inert materials used [18]. Properties
Ammonium sulphate
Magnesium oxide
Sand
Molecular formula Molar mass (g/mol) Median diameter (mm) Density (g/cm3 ) Moisture content (mass%) Melting point (◦ C) Heat capacity (Cal/mol K)
(NH4 )2 SO4 132.14 0.051 1.77 0.33 235 51.6
MgO 40.30 0.031 2.58 0.18 3600 10.87
SiO2 60.08 0.097 3.65 2.1 1600 10.86
explosible range. The MIT of combustible dust could be increased by adding inert materials. Fig. 9a and b demonstrates the effect of mixing different inert materials with brown coal and lycopodium. As expected, the MIT of the combustible dust is increased when the inert dust concentration increases until a threshold value is reached where ignition was no longer obtained. Considering the effect of inert material on brown coal and lycopodium, both ammonium sulphate and magnesium oxide showed a similar trend on the MIT of which an exponential increase was observed until 60% and 70% for brown coal and lycopodium respectively where no ignition
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Fig. 7. Image of explosion development in the Hartmann apparatus.
Fig. 8. A representation of how the MIE of both single and mixtures was obtained.
was obtained. In contrast to the inerting behavior of ammonium sulphate and magnesium oxide, sand behaved differently with the last ignition obtained at 80% for both brown coal and lycopodium. The result for the effect of inert materials on the MIT of starch is similar to that of lycopodium as presented in Fig. 10a. There was not much effect on the MIT of starch up to 30% for the three inert materials but a prominent increase was then observed above 40%. Magnesium oxide and ammonium sulphate exhibited good inerting effects as no explosion was observed above 60% and 70% inert concentration respectively. In case of sand an explosion was observed
even at 80% which clearly shows a less influence of sand on starch as compare to the other two inert materials. A similar behavior as explained in the effect of inert materials on starch was also seen in niacin as shown in Fig. 10b. Fig. 11a and b also presents the results for the effect of the various inert materials on the MIT of both toner and high density polyethylene respectively. It could be seen in Fig. 11a that magnesium oxide had good inerting effect on toner comparing to the other two inert materials. A rapid increase in the ignition temperature was seen by increasing the concentration of magnesium
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Fig. 9. (a) Effect of three inert materials on the MIT of brown coal. (b) Effect of three inert materials on the MIT of lycopodium.
Fig. 10. (a) Effect of three inert materials on the MIT of starch. (b) Effect of three inert materials on the MIT of Niacin.
oxide up to 50%, eventually, leading to the temperature of 680 ◦ C with 70% inert concentration. On the other hand, the trends for ammonium sulphate and sand portray similar behavior up to 70% and 80% respectively. In the case of HDPE a similar behavior as explained above was notice, ammonium sulphate and magnesium oxide exhibited similar inerting behavior with the same threshold limit of 70% of inert materials concentration while sand behaved differently with threshold limit of 80%.
3.2. Effect of inert materials on the MIE of combustible dusts The minimum ignition energy (MIE) is defined as the lowest energy value of a high-voltage capacitor discharge that is capable of igniting an ignitable mixture. In the process of dust explosion prevention, it has been noticed that, adding some amount of inert materials to a highly combustible dust could increase the MIE of the combustible dust. Fig. 12a and b presents the results for the influence of the above mentioned inert materials on brown coal
and lycopodium. There was a rapid rise in the MIE beyond 30% and no ignition was observed after 60% concentration of inert (magnesium oxide, ammonium sulphate) for brown coal and 60% and 70% for lycopodium respectively. Moreover, sand tends to be less effective compared to the other two inert materials where last ignition was obtained at 70% and 80% for brown coal and lycopodium respectively. Furthermore, Fig. 13a and b explains the results of the influence of the three inert materials on the MIE of corn starch and niacin. A similar effect as explained above was noticed, the influence of both ammonium sulphate and magnesium oxide are more pronounce as compared to sand. Up to 30% of inert materials concentration, a slight increase in the MIE for both starch and niacin was obtained. A sharp increase was later observed above 30% of inert concentration until 60% where last ignition was obtained for ammonium sulphate and magnesium oxide as well as 80% for sand. Fig. 14a demonstrates the progression of minimum ignition energy of toner–inert material mixtures in relation to the inert
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Fig. 11. (a) Effect of three inert materials on the MIT of toner. (b) Effect of three inert materials on the MIT of HDPE.
Fig. 12. (a) Effect of three inert materials on the MIE of brown coal. (b) Effect of three inert materials on the MIE of lycopodium.
concentration. It can be established that the value of MIE experienced a slow rise, until an inert material concentration of 50% is reached. A rapid increase was then seen above 50% inert until 80% where the last ignition was obtained. Contrary to the other combustible dust inert mixtures, all the three inert materials has similar behavior on the MIE of toner, this could be as a result of very low particle size of toner compare to the inert materials. Large difference of particle size between the combustible dust and the inert material is likely to lead to segregation of the particles and difficulties to get a homogenous mixture when dispersed. With respect to high density polyethylene and the various inert materials, magnesium oxide showed high inerting effect follow by ammonium sulphate and sand with last ignition obtained at 60%, 70% and 80% respectively.
the other two inert materials. This could be due to its high thermal stability with a melting point of 3600 ◦ C. Since it does not decompose within the test temperature range used in this work, its effect could be due to dilution. As explained by Agnès et al. [16], dilution effect is the reduction of combustible content in solid phase by addition of noncombustible substance. Furthermore, magnesium oxide has peculiar properties which make it an excellent inerting material. For example in the ceramic industries, magnesium oxide is used as an inert material which exhibits excellent resistance to attack by metals such as sodium, nickel based super alloys and plutonium/uranium systems, fluxes and superconductor compounds. Secondly, ammonium sulphate also has a good inerting effect on the combustible materials since it is unstable to heat. When heated, ammonium sulphate decomposes according to the chemical equation below.
3.3. Comparison between the three inert materials
2(NH4 )2 SO4 → (NH4 )2 S2 O7 (s) + 2NH3 + H2 O
It was generally noticed that magnesium oxide had a higher effect on the MIT or MIE of the combustible dust tested compared to
The heat balance of fuel–inert mixtures explosion addressed by Chatrathi and Going [24] gives the idea that the decomposition
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Fig. 13. (a) Effect of three inert materials on the MIE of Niacin. (b) Effect of three inert materials on the MIE of starch.
Fig. 14. (a) Effect of three inert materials on the MIE of toner. (b) Effect of three inert materials on the MIE of PE–HD.
plays a key role in the inerting effectiveness that is consistent with Abbasi and Abbasi [25]. The thermal decomposition of ammonium phosphate starts at 235 ◦ C [26] which is significantly below the ignition temperature of the combustible dusts used. Hence the efficient decomposition of ammonium sulphate results in the excellent thermal sink. Furthermore, the free ammonia decomposed from ammonium sulphate has a particular efficacy in flame extinguishing [27]. Finally, sand had a less inerting effect on the combustible dust compared to magnesium oxide and ammonium sulphate. This could be attributed to its high bulk density and particle size. Large difference of particle size between the combustible dust and the solid inert is likely to lead to segregation of the particles and hence difficult to get a homogenous mixture when dispersed. It is also known that finer sizes of a given solid inert permit a more efficient inerting of a combustible dust than coarser size fractions. This statement is in accordance with works performed by other authors [28,29].
4. Conclusions This study was carried out to investigate the effects of ignition sensitivity of three inerting materials mixed with six combustible dust. For this purpose, series of experiments were performed for different mixtures of combustible dust samples and inert dusts. The new insight presented in this experimental investigation has provided a good opportunity that is applicable to the inherent safety principle of moderation in process design. There exists a certain threshold value for the concentration of inert material, above which the ignition sensitivity of the dust mixture decreases thereby increasing the minimum ignition energy and minimum ignition temperature. It was generally noticed that the permissible range for the inert mixture to minimize the ignition risk lies between 60% to 80%.
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