Health and Explosion Hazards

CHAPTER 14

Health and Explosion Hazards 14.1 Introduction Handling and processing (conveying, metering, mixing, heating, or cooling, etc.) of fine particles—powders and dust—in wide-ranging industrial settings, including agricultural, chemical, cement, coal and minerals, foodstuffs, metals, pharmaceutics, plastics, woodworking, etc., is much more difficult than that of gases and liquids. These difficulties stem from a variety of sources (particle shape and size, free flowing or not, chemical nature like toxicity, explosive, mechanical aspects like hardness, abrasive nature, combustion characteristics, etc.). Thus, the severity of the problem varies both with the type of unit operation and the specific material at hand. While the first aspect related to the mixing, conveying, size reduction or enlargement, liquid–solid separation, etc., have been discussed in detail in Chapters 3–5 of this volume, consideration is given here to the corresponding health, fire, and explosion hazards of powders, which are, indeed, quite acute in a range of situations including granaries, flour mills, sand blasting, grinding, machining, and milling, and a host of other processing operations. Indeed, the so-called dust (comprising solid and liquid constituents) is generated during the course of machining, grinding of particulate minerals and ores, metals, charging and discharging of grain silos, filter bags, flour mills, pouring of powders into empty vessels and those containing liquids, sanding, rapid impact, detonation and decrepitation (by heat) of limestone and nitrates of inorganic and organic substances, woodworking, mining and dredging operations, combustion processes, etc., to name a few. While it is difficult to define precisely the size of the so-called dust, it does clearly span a range of particle size, typically in micron range, and it also depends upon the process of its generation. In order to develop a feel for particle size, it is useful to mention here that a typical cotton fibre is on the order of 15–20 μm in diameter, human hair ranges from 50 μm to 500 μm, and normal red blood cells are in 8–10 μm range. Based on our everyday experience, coupled with the fact that the human eye can see particles as small as 35–40 μm, while we have no problem in seeing a human hair, a microscope is needed to examine the red blood cells. However, the phrase ‘particle size’ itself is ambiguous, as seen in Chapter 2, in the context of a nonspherical shape. The definition of dust varies from one application to another; in the context of industrial hygiene, the National Safety Council (United States) simply defines dust as the ‘fine’ solid or fluid particles generated during the course of handling (charging or discharging of grain silos, pneumatic conveying, powdering, sieving, for instance), grinding, crushing, sanding, etc. Coulson and Richardson’s Chemical Engineering. https://doi.org/10.1016/B978-0-08-101098-3.00015-9 # 2019 Elsevier Ltd. All rights reserved.

739

740 Chapter 14 Notwithstanding this uncertainty, Table 14.1 lists typical particle size (or range) for scores of materials. This encompasses six orders of magnitude, with viruses and atmospheric dust being at the lower end of this spectrum, and beach sand, pollens, etc., lying at the other end. Naturally, smaller the particle, the larger is its specific surface area, thereby resulting in advantages in terms of high rates of heat and mass transfer, chemical reactions, etc. On the other hand, such small particles settle very slowly and, thus, remain suspended in air Table 14.1 Typical particle size1 Substance Anthrax (solid) Asbestos Atmospheric dust Car emissions Bacteria Beach sand Bone dust Burning wood Ca-Zn dust Carbon black dust Cement dust Clay Coal dust Coal flue gas Copier toner Dust mites Face (Talc) powder Fibreglass insulation Fertiliser Flour, milled corn Fly ash Ginger powder Grain dust Ground limestone Insecticides dust Iron dust Lead dust Metallurgical dusts and fumes Mist Mould Mould spores Oil smoke Paints and pigments Pollens Radioactive fallout Rosin smoke Saw dust Smoke from synthetic materials

Particle Size (μm) 1–5 0.7–90 0.001–40 1–150 0.3–60 100–104 3–300 0.2–3 0.7–20 0.2–10 3–100 0.1–50 1–100 0.08–0.2 0.3–15 100–300 0.1–30 1–103 10–103 1–100 1–103 25–40 5–103 10–103 0.5–10 4–20 2 0.1–103 70–350 3–12 10–30 0.03–1 0.1–5 10–103 0.1–10 0.01–1 30–600 1–50

Health and Explosion Hazards 741 Table 14.1 Substance Smouldering or flaming cooking oil Spider web Spores Talcum dust Tea dust Textile dust Viruses Yeast cell

Typical particle size—cont’d Particle Size (μm) 0.03–0.9 2–3 3–40 0.5–50 8–300 6–20 0.005–0.3 1–50

and water for significantly long periods. Depending upon their concentration and nature (toxic, carcinogenic, abrasive, etc.) and the duration of their exposure to such atmospheres, there are varying levels of a range of health hazards. These are discussed in detail in Section 14.2. By the same reasoning, when the particles are prone to ignition, combustion, and burning in the presence of oxygen and/or sources of ignition or at high temperatures, this can lead to fires and explosions with devastating consequences. This is discussed in Section 14.3. Suffice it to add here that excellent books are available on health hazards and industrial hygiene,2–4 as well as on dust fires and explosions,1, 4–7 and reference should be made to these sources for more details and guidelines of regulatory bodies such as the National Safety Council of the United States, the Occupational Safety and Health Administration (OSHA) in the United Kingdom, etc.

14.2 Health Hazards and Risks of Dust and Fine Powders Perhaps the single most important characteristic of fine particles and powders that directly impinge upon human health is their ability to become airborne and to form dust clouds capable of being inhaled through the mouth and nose, although some particles also act as irritants and/or can enter the human body through skin. This is generally less of a problem than that caused by inhalation. In industrial settings, there is a broad spectrum of dusts and airborne particles (varying in concentration, toxicity, hazard index, size) in the environment in which people work and live. For instance, imagine living next to a coal-based thermal power station or a flour mill! The health hazards stemming from a regular exposure over long periods of time often have short-term and long-term ill effects on the well-being of people. It is generally believed that, in addition to their chemical characteristics, the particle size and specific surface area are the other two important factors in determining their potential in inflicting inflammatory injury, oxidative damage, and other biological damage depending upon where the inhaled particles end up inside the human body. For instance, very fine and ultrafine particles can penetrate deep into the air passages of the respiratory tract, and these can even reach alveoli, in which up to 50% of the inhaled particles can be retained in the lung

742 Chapter 14

Fig. 14.1 Schematics of the respiratory tract system in humans.

parenchyma. In this context, the respiratory tract system, shown schematically in Fig. 14.1, can be visualised as a tube progressively bifurcating into smaller and smaller diameter branches. Thus, the so-called coarse (>
10–15 μm) particles are filtered out by nose and upper air passages. The so-called fine (<2.5 μm) and ultrafine (<0.1 μm) particles penetrate deep into the respiratory system, reaching all the way to the alveoli.8, 9 Furthermore, while some dusts exist in the form of primary particles, agglomeration is not uncommon in these systems. For instance, metal fumes start with a base particle size of about
0.01 μm, which almost

Health and Explosion Hazards 743 instantaneously form agglomerates as big as 0.2–0.5 μm in size. In addition to size, of course, chemical composition of particles also plays a significant role in determining the severity of potential health hazard in a given situation.

14.2.1 The Respiratory System and Health Effects In its simplest form, our respiratory tract system is shown in Fig. 14.1, which can be visualised as a tube progressively bifurcating into narrow passages. Therefore, when we inhale a particle, the health risk it poses very much depends upon exactly where it ends up in the system. For instance, very fine particles can really penetrate deep, all the way up to the alveoli, and can eventually pass into the blood stream. As noted earlier, the course of action depends principally on the particle size. For instance, the diameter of the main bronchi and terminal bronchioles are
10,000 and 500 μm, respectively. These values together with the particle size influence the deposition and passages of particles entering the lungs. Terminal bronchioles connect to the respiratory bronchioles, ultimately leading to the respiratory space composed of a number of alveoli (
100 μm in diameter). The surface of the alveoli is covered by a thin blanket (
0.5–1 μm) made up of a network of capillaries which facilitates gas exchange involving inflow of oxygen into the blood and that of carbon dioxide in the opposite direction. On the other hand, the cilia—hair-like structures or organelles—covering the surface of trachea and the bronchial capillaries create a mechanism whereby the particles deposited can be dislodged. Beyond the terminal bronchioles, the surface lining of the air passages does not have this action, and, therefore, the particles cannot be dislodged once deposited. For particles entering the respiratory tree through nasal (pharyngeal regions) via the mouth rather than through the nose, breathing being normal for moderately demanding physical work, soluble solids dissolve on a wet surface, or if insoluble in water, they remain as a solid. Such particles can be rejected by the body via spitting or nose blowing, or they may be swallowed, making their way to the digestive tract. Coughing and sneezing also carry the particles (>10 μm) toward the upper parts of the respiratory system. On the other hand, soluble solids may pass into the bloodstream, leading to serious health conditions. Obviously, the inhalation of particles is a problem associated mainly with dusty processes and working environments, but there are other routes through which fine particles can enter the human body. Two such mechanisms are briefly mentioned here. The dust particles deposited on hands, clothes, and other personal articles like cell phones, computers, etc., contribute to the total intake of particles into the body by so-called ingestion. Even with good housekeeping practices in place to control the concentration of airborne dust, a casual working culture can lead to a greater intake of solids by ingestion through smoking (cigarettes, pipes), eating, and drinking activities at the workplace.

744 Chapter 14 Similarly, some industrial dusts directly attack the skin, causing irritation, itching, and even dermatitis. Moreover, some organo-metal and organo-phosphorous compounds may also be absorbed into the body via the intact skin or open wounds, etc. Currently available research results suggest the following health effects caused by the inhalation of particles.4, 8, 9 (1) When the particles are lodged in the pulmonary region, it leads to the malfunction of the lungs. This, in turn, leads to the loss of elasticity of the lung walls, which impedes the diffusion of O2 and CO2. This can be caused by a number of chemical species, free crystalline silica (encountered in mines or grinding wheels, for instance) present in the inhaled dust being the main one. Other potential species leading to pneumoconiosis include asbestos, beryllium, china clay, iron oxides, talc powder, etc. (2) If the toxic particles (Mn, Cd, Pb, etc.) penetrate into the circulation network and internal organs after dissolution, the human body fails through so-called system poisoning. (3) The relationship between long-term exposure to working atmospheres with arsenic, chromate, nickel, and radioactive particles, and lung cancer has been studied extensively for a long time.9 While the debate continues, soluble cancer-causing agents are regarded as a risk to the lungs as well as to the other organs. (4) Less severe hazards include irritation and allergies. The presence of particles like fumes of acids, cadmium, beryllium, vanadium, plastics, pesticides, acid mists, and fluorides in the work atmosphere is a common source of irritation, which may manifest in the form of bronchitis, pneumonitis, and pulmonary oedema, etc. On the other hand, vegetable dusts from bagasse, corn, cotton, flour, sawdust, pollens, metallic dusts of nickel and chromium, etc., result in allergic reactions in the form of asthma, hay fever, metal fume fever, etc. Also, airborne particles carrying fungal, viral, or bacterial pathogens result in the transmission and spread of various types of infections. More details concerning other health hazards, threshold limiting values, and underlying mechanisms leading to the aforementioned conditions are available in the literature.4, 8, 9 Some remedial measures in the form of respiratory protective devices have been described by Tanaka and Hori.10

14.3 Dust Explosions A dust explosion is caused by a quick release of a large amount of energy due to the combustion of a particulate material of small size. Given the fact that >50% of the solids processed in chemical and allied process engineering applications are combustible to some extent, accompanied by a rapid increase in pressure due to confinement, dust explosions pose a serious process safety hazard in a wide range of settings entailing a broad range of obvious and not so obvious materials. For instance, coal dust, flour, sulphur, etc., immediately come to mind as

Health and Explosion Hazards 745

Fig. 14.2 Dust explosion pentagon.

posing a risk of dust explosion, whereas one would normally not consider metallic dust (aluminium, copper, iron, etc.) as prone to dust explosions. Generally, a dust explosion occurs when the following five conditions are met: fuel (combustible particulate), mixing (turbulence), oxidising medium (generally air), ignition source (hot surfaces, open flames, sparks, electrostatic charging), and confinement (partial or complete). These are often shown schematically in the form of the so-called dust explosion pentagon; a term introduced by Kauffman possibly for the first time,11 shown schematically in Fig. 14.2. The process can simply be described as follows: when a combustible particulate material forms a dust cloud of certain concentration in air when ignited, the combustion process initiates, releasing a large amount of heat which increases the pressure in a confined space very rapidly, eventually leading to the so-called dust explosion. Indeed, in their extensive review, Abbasi and Abbasi12 have documented the available historical data on dust explosion dating back to the 18th century. A quick glance at this review reveals the following three aspects of dust explosions: (a) Diversity of materials Dust explosions have occurred in the handling and processing of wheat flour, grains and grain dust, corn starch powder, sulphide dust, silicone, aluminium, magnesium-aluminium alloys, fish meat, coal, rape seed flour pellets, textile dust, potassium chlorate, cotton waste, rubber waste, resins and gums, polymeric materials, agricultural dusts, dyes, pesticides, sugar dust, etc. Undoubtedly, this range of materials posing the risk of a dust explosion is beyond the imagination of an average process engineer! (b) Processing operations and equipment Bearing in mind that one side of the explosion pentagon is confinement, Abbasi and Abbasi12 have also included the setting of dust explosions. These include bakeries, ships, storage and shipping bins, hammer mills, elevators, flour mills, grain silos, milling

746 Chapter 14 stations, mixing tanks, mines, cement plants, bucket elevators, coal dust burners, dust removal and collection systems, power houses, ovens, etc. Additional possibilities of dust explosions also exist in cyclones, scrubbers, electrostatic precipitators, different types of dryers, packaging facilities, etc. This list reinforces the fact that even a partial confinement can lead to a dust explosion. (c) Consequences The number of workers injured and/or deaths resulting from such explosions, not to mention the financial loss and down time, are also troubling and discomforting. Therefore, this chapter aims to provide a brief overview of dust explosions including the identification and characterisation of factors contributing to such a risk, and its mitigation. However, more detailed discussions can be found in excellent books1, 5–7 and other publications.11–15

Types of Dust It is rather difficult to offer a precise definition of dust itself. For instance, according to the National Fire Protection Association (of the United States),16 any finely divided solid of diameter 420 μm is classified as dust. In contrast, according to the British Standards,17 any fine particulate material of particle size smaller than 76 μm (200 BS mesh size) is regarded as dust. Notwithstanding the 6-fold range, it is perhaps justified to accept the conservative proposal of Palmer,18 which does not exclude particles as big as 1 mm from such a discussion. Because one of the key factors in initiating and sustaining a dust explosion is the amount of energy released by the combustion reaction, the particle size, together with the heat of combustion of the dust material, are important factors in this regard. As a particle progressively becomes smaller, its specific surface area increases, which accelerates its combustion, and hence, the quantum of energy liberated. Second, it is much easier to form a cloud of fine particles with air, and this also facilitates their combustion and flame propagation. Representative values of heat of combustion (for complete oxidation) of a few common dusts are listed in Table 14.2. Evidently, the dust explosion of metallic dusts can be much more violent and energetic than that of nonmetallic dusts. Because combustion reactions lead to the formation of stable oxides, dusts comprising such oxides (like silicates and carbonates, for instance) are generally not explosive. Instead, such substances are used as suppressants for dust explosions, e.g. the use of limestone in coal mines. Such inert materials act as heat sinks thereby regulating the temperature rise. Similarly, not all combustible dusts explode, but all explosible dusts must be combustible in nature. For instance, both graphite and anthracite have very high values of heat of combustion but are not easily explosible. Similarly, fly ash, a product of combustion reaction, is regarded to be nonexplosible. If, however, if it becomes contaminated with unburned fuel, (such as pulverised coal or petroleum coke) or the fly ash is a reaction

Health and Explosion Hazards 747 Table 14.2 Typical values of heat of combustion Dust

Heat of Combustion (kJ/mol Oxygen)

Aluminium Calcium Carbon Chromium Coal Iron Magnesium Polyethylene Silicon Starch Sucrose Sulphur

1100 1270 400 750 400 530 1240 390 830 470 470 300

Modified from Abbasi T, Abbasi SA. Dust explosions—cases, causes, consequences, and control. J Hazard Mater 2007;140:7–44.

product of incomplete combustion of coal, nonexplosible fly ash can transition to an explosible mixture. Numerous criteria are used to characterise a dust for its explosion potential. These include the so-called Group A (capable of sustaining flame propagation following its ignition) or Group B (flame does not propagate and thus explosion does not occur).19 However, this rather crude classification is based on the observations when the ignition occurs about 25°C. Thus, it is quite possible that a dust classed as Group B can migrate to Group A if it is ignited at temperatures >25°C. The second approach is based on the nature of combustion20, 21 and this scheme classifies dusts into six types, simply labelled as CC1 to CC6. The salient characteristics of each type are summarised in Table 14.3. This classification is, however, based on the behaviour of a well defined dust heap when exposed to a gas flame or hot platinum wire as the source of ignition. Thus, it is more of a measure of the ignitability of a dust layer and intensity of burning of a dust layer. This is of Table 14.3 Combustion-based classification of dusts21

# Increasing Explosibility

Type

Key Characteristics

CC1 CC2

No ignition and no self-sustained combustion Short ignition, quick extinguishing; local combustion of short duration Localised burning and glowing, but without spreading and propagation. Spreading of a glowing fire and propagation of smouldering combustion Spreading of an open fire; propagating open flame Explosive burning and combustion

CC3 CC4 CC5 CC6

748 Chapter 14 direct relevance to the applications wherein powders or dusts are accumulated on hot surfaces such as that on dryers, kilns, heat exchangers, etc. Both preceding schemes are direct measures of the intrinsic fire and explosion risk associated with a given dust. Returning to Fig. 14.2, confinement is a key element for a dust explosion to occur. This is related to the maximum pressure and the rate of pressure rise in the confined region or inside process equipment. Neither of the preceding classification schemes account for this aspect, and this deficiency is rectified in the scheme proposed by the US Bureau of Mines,22 which categorises the hazard potential of a material using the ignition and explosion characteristics of Pittsburgh seam coal (PC) as the reference. This approach relies on the use of two individual parameters, both of which are dimensionless: ignition sensitivity (IS) and explosion severity (ES), defined as follows: IS ¼

fMIT  MIE  MECgPC fMIT  MIE  MECgdust sample

(14.1)

where MIT ¼ minimum ignition temperature, MIE ¼ minimum ignition energy, MEC ¼ minimum explosive concentration ES ¼

fMEP  MRPRgdust sample fMEP  MRPRgPC

(14.2)

where MEP ¼ maximum explosion pressure and MRPR ¼ maximum rate of pressure rise. And finally, the index of explosibility (IE) is given simply by the product of these two parameters, i.e. IE ¼ IS  ES

(14.3)

Because IE is a relative measure, it is less dependent on the experimental methods employed to measure the values of MIT, MIE, MEC, MEP, and MRPR, etc., as long as the same procedure is employed for both materials. Its main disadvantage, however, is the fact that the evaluation of IE necessitates the full range of tests. Based on extensive experimental data for scores of powders of pure metals, alloys, copper, nickel, and lead-bearing ores, Jacobsen et al.22 reported values of IE ranging from IE
0 to IE > 10. Based on the values of IE, they also classified various dusts posing no (
0), weak (IE < 0.1), moderate (0.1  IE  1), strong (1  IE  10), and severe (IE > 10) risks of dust explosion. To put this classification in context, atomised aluminium dust poses a severe risk, as do the magnesium, uranium, and thorium powders. This fits in qualitatively with the fact that their heats of combustion are rather high, Table 14.2. Next, the hydrides of titanium and zirconium, for instance, fall within the strong risk category. Obviously, some of the individual quantities appearing in Eqs (14.1) and (14.2) are strongly influenced by the particle size and cloud concentration, source of ignition, size of apparatus, etc.; these aspects are discussed in a later section in this chapter.

Health and Explosion Hazards 749 Finally, perhaps the most widely used classification scheme is based on the so-called Kst value, which is a measure of the MRPR (maximum rate of pressure rise) in 1 m3 vessel upon the ignition of the dust. This concept was initially introduced by Bartknecht [as cited in Ref. 6] in the following form:    1 dP  V 3 ¼ Kst ðconstant for a dustÞ (14.4) dt max Eq. (14.4) has been shown to be applicable for test vessel volumes greater than
0.04 m3. In order to make a sensible comparison between the Kst values from various sources, it is tacitly assumed that (i) geometrically similar test vessels yield geometrically similar flame characteristics, (ii) the flame thickness is negligible compared to the radius of the vessel so that the lateral spreading of flame is not influenced, and (iii) the burning velocity as a function of temperature and pressure is identical in vessels of different volumes. Deviations from these assumptions can lead to wide-ranging values of Kst for a given dust. For instance, maize starch dust clouds tested in the range of 0.0012  V  13.4 m3 vessels, the reported values of Kst vary from
3 to
209 bar m/s! While some of the deviations can be ascribed to the differences in the dust samples themselves (such as particle size and moisture content, for instance), both degree of turbulence in the dust clouds and flame characteristics (especially flame thickness) are regarded to be the main contributing factors here,7 both of which are difficult to control and quantify. This indicates a degree of arbitrariness in the value of Kst obtained from nonstandard tests.23 On the other hand, the values of Kst obtained using the standardised protocols seldom vary by >10%–15%. Generally, materials with Kst
0 are classified as nonexplosible, Kst < 200 bar m/s as being weakly explosible, and 200 < Kst < 300 as very strongly (severely) explosible.

14.3.2 Experimental Methods In essence, two systems—the Hartmann vertical tube17 and the so-called 20-L sphere – have been used extensively in the literature. Most of the experimental results prior to the 1980s were obtained using the Hartmann vertical tube method, shown schematically in Fig. 14.3. It consists of a 300 mm long and 64 mm inner diameter tube (approximate volume of
1.2 L) in which dust is dispersed in the form of a cloud using air blast, which is then ignited using a hot wire or a spark device. Flame propagation is monitored as a function of particle size and shape, concentration, ignition energy, and temperature, all of which are known to influence the degree of potential risk, irrespective of the classification one uses. Notwithstanding the fact that it is virtually impossible to achieve a uniform dust cloud and/or to regulate the level of turbulence, additional effects arise from confinement (wall effects) and from the fact that the flame travels in both directions—upward and downward – following its initial spherical expansion. Naturally, all these factors influence (generally tend to reduce) the rate of combustion, and hence, the value of MRPR. Therefore, these values must be

750 Chapter 14

Fig. 14.3 Schematics of the Hartmann vertical tube.

treated with reserve and used for indicative purposes only, rather than using them as a basis for designing appropriate mitigation systems. Over the years, many improved versions of this apparatus have been developed to overcome some of these deficiencies. Perhaps the best of all such designs is the one which employs a large spherical vessel rather than a vertical tube. Two designs – the so-called 20-L vessel24–26 and the ISO standard 1 m3 vessel23 – have gained wide acceptance in the literature. In this method, the dust is introduced via a pressurised container or through a “ring sparger” type device, as shown schematically in Fig. 14.4, for both 20 L and 1 m3 versions. The source of ignition is located at the centre of the sphere. Detailed experimental protocols for the 1 m3 ISO vessel are available in the relevant ASTM standard.6 While the results obtained in the 20 L and 1 m3 vessels are not always in perfect agreement, one important factor to bridge this gap between the two values is to use an ignition source smaller than 5 kJ in a 20-L vessel, whereas it is fixed at a value of 10 kJ in the case of the 1 m3 vessel. Of course, the mechanical details of the methods of dust injection and dispersion also play a role in the case of the 20-L apparatus.

14.3.3 Influencing Factors From the foregoing discussion, it is abundantly clear that the severity and consequences of a dust explosion (irrespective of what classification is used) are determined by an intricate interplay between numerous factors. Probably, the most significant of these are particle size, dust concentration, oxidant concentration, ignition energy and temperature, turbulence level in the cloud, value of MRPR, etc. These are summarised in Table 14.4 for a quick reference.

Health and Explosion Hazards 751

Fig. 14.4 Schematics of spherical test vessels for measuring the maximum pressure (Pmax) and Kst values (A) 20 L vessel (B) 1 m3 (150) vessel.

752 Chapter 14 Table 14.4 Parameters used to characterise dust explosion risk7 Factor

Description

Pmax MRPR Kst

Maximum explosion pressure in a constant volume device Maximum rate of pressure rise in a constant volume explosion The value of MRPR in a 1 m3 constant volume explosion and accounts for other volumes via Eq. (14.4) Minimum explosible dust concentration Minimum energy needed to ignite a dust cloud Minimum ignition temperature of a dust cloud Minimum ignition temperature of a deposit/layer of dust Limiting oxygen concentration in the atmosphere for flame propagation in a dust cloud

MEC MIE MIT LIT LOC

Because the key underlying processes in dust explosion are the combustion and flame propagation, both of which are also influenced by these very factors. In this section, the role of each of these factors is discussed in brief, and more detailed discussions can be found in numerous references, e.g. see Refs. 1, 6, 7. Table 14.5 provides a quick guide to the variation of these factors with the dust characteristics, and these are described in brief in the following section. (a) Particle Size As mentioned earlier, the specific surface area of a particle available for combustion increases with the decreasing particle size, and, thus, the smaller the particle size, the greater is the hazard associated with it. On the other hand, some fine particles tend to agglomerate, and if the resulting lumps are >
500 μm, the substance may even become nonexplosible. Fig. 14.5 shows the effect of particle size on the value of Kst reported for HDPE when burned with pure air and blended with various hydrocarbons.27 Clearly, the severity of the dust explosion increases (as reflected by the increasing values of Kst) with the decreasing particle size and/or the addition of hydrocarbons to air. (b) Particle Concentration in dust cloud Naturally, a dust cloud explodes only within a certain range of concentrations. Typical values of the so-called minimum concentration required to initiate an explosion are 50–100 g/m3, and that of the maximum concentration are 2–3 kg/m3. For many combustible dusts, a dust concentration of
500 g/m3 produces the most devastating overpressures and the values of MRPR. The lower limit of the concentration is set by the fact that the combustion should lead to moderate values of Kst to pose a threat. On the other hand, the maximum concentration is limited by the availability of sufficient oxygen required for a sustained combustion, which may not always be possible. Also, flame propagation is somewhat impeded at high concentration thereby lowering the probability of dust explosion. Thus, it is virtually impossible to disperse and efficiently burn excessively thick dust deposits.

Health and Explosion Hazards 753 Table 14.5 Influence of dust properties/characteristics on dust explosion parameters12, 17 Parameter

Increases With

Explosibility of the dust

1. Lower explosible concentration 2. Minimum ignition temperature 3. Lower minimum ignition energy 4. Burning velocity 5. Maximum rate of pressure increase 6. Presence of chemical groups such as COOH, OH, NH2, NO2, C^N, C]N, N]N 7. Presence of volatile matter in the dust at levels above 10% 8. Relatively small proportion of fines 9. Increasing oxygen concentration

Effect of particle size on the likelihood of explosion initiation Minimum explosive concentration

Minimum ignition temperature

Decreases With 1. 2.

3.

50–70 μm < particles size (μm) <500 μm (with decreasing particle size) 1. Increasing moisture content 2. Increasing concentration of inertant

500 μm < particles size (μm) <50–70 μm (with increasing particle size) 1. Decreasing particle size 2. Increasing volatile matter 3. Increasing oxygen concentration

1. Increasing moisture content 2. Increasing concentration of inertant

1. 2. 3. 4.

Maximum permissible oxygen concentration Maximum explosion pressure Maximum rate of pressure rise

Presence of chemical groups such as Cl, Br, F Presence of inert material at concentrations above 10%–20% Dust moisture content above 30%

Decreasing particle size Increasing volatile matter content Increasing oxygen concentration Increasing thickness of the dust layer

Decreasing dust temperature

Increasing dust temperature

Decreasing particle size, though weakly 1. Decreasing particle size 2. Increasing volatile matter content 3. Increasing oxygen concentration

– 1. 2.

Increasing moisture content Increasing concentration of inertant

754 Chapter 14

Fig. 14.5 Dependence of Kst on particle size and type of fuel for polyethylene obtained using 20 L test vessel.27

From another vantage point, one can appreciate the role of dust layer thickness (h) through the following equation:   h C ¼ ρbulk (14.5) H Where ρbulk is the bulk density of dust layer (typical values for coal dust, corn flour, and iron powder are 560 kg/m3, 820 kg/m3, and 2800 kg/m3, respectively). In Eq. (14.5), H is the height of the dust cloud and h is the thickness of dust layer. For the purpose of illustration, here, let ρbulk ¼ 500 kg/m3 and h ¼ 1 mm. In a 5 m high room, this dust layer will create a dust cloud of 100 g/m3 concentration, which will rise to 500 g/m3 if the height of the room is reduced to 1 m. Thus, it stands to reason that even a small amount of dust, under suitable conditions, can lead to explosions. Finally, dust concentration influences both the value of Pmax and MRPR, as shown in Fig. 14.6 for two coal dust samples. Typical values of the minimum explosive concentration are listed in Table 14.6. (c) Concentration of Oxidising Medium Most common oxidising medium is oxygen present in the atmospheric air. Naturally, artificially oxygen enriched air increases the rate of combustion, and vice versa. Therefore, the air participating in combustion is progressively depleted in oxygen, thereby slowing down the burning rate with the passage of time. Under such conditions, either the combustion may cease to occur, or if an explosion occurs, it may not be very severe.

Health and Explosion Hazards 755 700

20 18

600

500

14 12

400

10 300

8 6

200 Datong coal dust (left Y-axis) Colombian coal dust (left Y-axis) Datong coal dust (right Y-axis) Colombian coal dust (right Y-axis)

100

0

0

200

400

600

800

1000

1200

Explosion pressure [bar(g)]

Rate of pressure rise (bar/s)

16

4 2

0 1400

Dust concentration (g/m3)

Fig. 14.6 Maximum pressure (right scale, : Datong coal dust, □: Colombian coal dust) and rate of pressure rise (left scale, Datong coal dust, Colombian coal dust) in two coal dust. Repotted from Amyotte PR, An introduction to dust explosions. Oxford, UK: Elsevier; 2013.

▪

Generally, fires cannot be sustained when the oxygen concentration in the air falls below 10%. In this context, it is customary to introduce the so-called limiting oxygen concentration (LOC). It is defined as the highest O2 concentration in the dust/air/inert gas mixture which does not lead to an explosion. Naturally, such values vary from one inertant to another. Typical LOC values of various dusts with N2 as the inertant are presented in Table 14.7. These values are also influenced by the temperature and pressure conditions. (d) Sources of Ignition and Minimum Ignition Energy (MIE) One of the pillars of Fig. 14.2 is the source of ignition. Both Abbasi and Abbasi12 and Taveau29 offer long and somewhat similar lists of potential sources of ignition in processing industries. These include open flames and direct heat, mechanical sparks, welding and cutting, self-heating, electrostatic discharges, self-ignition, hot surfaces, fire, shock waves, lighting, etc. Indeed, dusts can be ignited by low-energy as well as high-energy sources. Over the years, standardised equipment and procedures have been developed,7, 12 namely, the so-called MIKE 3 apparatus (Kuhner A.G., Basel, Switzerland) and the ASTM

756 Chapter 14 Table 14.6 Typical MEC and MIT values1 Dust Aluminium Al-Mg alloy Chromium Coal Copper Corn starch Epoxy resin Flax shive Grain dust Iron dust Wheat flour Wheat straw Rye dust Magnesium dust Rice Silicone Soy flour Tin Titanium Zinc

MEC (g/m3)

MIT (°C)

45 – 230 55 – – 20 – – 100 – – – 20 – 110 – 190 45 480

650 550 – 610 900 430 530 430 430 420 430 470 430–500 520 440 – 540 630 460 600

Table 14.7 Typical LOC (with N2 as inert medium) values28 Type of Dust Aluminium Cadmium stearate High density polyethylene Organic pigment Pea flour Sulphur Wheat flour

LOC (%v/v) 5 12 10 12 15.5 7 11

standard30 for measuring the minimum ignition energy for a given dust. The MIKE 3 setup consists of a 1.2 L cylindrical glass chamber, similar to the Hartmann tube shown in Fig. 14.3, into which dust is dispersed and ignited by an electrical spark (1 J), and the ignition criterion is that the resulting flame must propagate at least up to 60 mm away from the electrode. Because the MIKE3 device uses 1 mH inductance, it results in a longer duration spark, and, thus, one tends to measure a somewhat smaller value of MIE than what would be obtained in the absence of any inductance in the spark circuit. The available practical experience indicates that most, if not all, electrostatic discharges in chemical and

Health and Explosion Hazards 757 Table 14.8 Typical values of MIE31 MIE (mJ) Material Aluminium granulate Benzanthron Epoxy coating powder Epoxy polyester coating powder Flock Lycopodium Magnesium granulate Organic stabiliser Polyamide coating powder Polyester coating powder Polyurethane coating powder

With (1 mH) Inductance

Without Inductance

50 0.9 1.7 2.3
70–100 5 25 0.4 4 2.9 2

500 1.0 2.5 9.0 1300–1600 50 200 0.4 19 15 8

processing plants entail negligible inductance. Table 14.8 presents representative values of MIE (with and without) inductance.31 The values of MIE with inductance are seen to be appreciably lower than that in purely capacitive cases. In either event, these values are of the order of 1.5 J only. However, the values of MIE (with inductance) generally decrease with the decreasing particle size, as does the corresponding minimum ignition temperature.32 (e) Ignition Temperature When an ignitable dust and air mixture is heated, at some temperature, it catches fire. The lowest temperature at which such a mixture ignites is called the minimum ignition temperature (MIT). However, this is generally different form the ignition point of a dust layer (LIT). For a given dust, MIT decreases with the decreasing particle size (due to increased surface area) and increasing proportion of volatile matter present in the dust cloud and/or in oxygen-rich atmosphere. On the other hand, concentration of water (moisture) and other inert molecules (like carbonates, slilicates) raise the minimum ignition temperature. The so-called BAM oven along with ASTM E1491 standard7, 33 is used to experimentally measure the minimum ignition temperature of a dust cloud. In this method, a dust cloud is generated by squeezing a rubber bulb (in which the sample to be tested is loaded), and the resulting cloud impinges on a circular concave metal plate heated to a known temperature (600°C). The ignition is ascertained by visual observation of the flame exiting from the rear of the oven covered by a metal flap. Typical ignition temperatures of common dusts in air are summarised in Table 14.6. (f ) Effect of Turbulence While it is virtually impossible to measure and quantify the level of turbulence in the context of dust explosions, its signatures can be seen in terms of the high rate of mixing, heat and mass transfer, and the extent of chemical reaction, all of which facilitate the

758 Chapter 14 release of a large amount of energy and the temperature and pressure rise in a confined system. Generally, it is customary to distinguish between so-called pre- and post-ignition turbulence. Pre-ignition turbulence is related to the process of converting a heap or layer of dust into a dust cloud. Thus, it is influenced by the type of unit operation, equipment, etc., used to create a dust cloud. This turbulence seems to have a much greater effect on the rate of pressure rise than on over-pressurisation due to the explosion. As the name implies, the post-ignition turbulence is caused by flame propagation in the unburned part of the dust cloud. Because the rate of heat removal from the ignition zone under turbulent conditions is intensified, the ignition temperature (MIT) and ignition energy (MIE) requirements generally increase with the post-ignition turbulence levels. It has been argued in the literature that the degree of mixing is increased, leading to the formation of dynamic, three-dimensional structures comprising burned, burning, and unburned fuel particles. This, in turn, further creates combustion sites, thus, leading to rapid combustion and pressure rise at higher turbulence levels. It must be borne in mind that the values of MIT included in Table 14.6 are only indicative because some of these dusts (grain and flour dusts) can be ignited (in the form of smouldering) even at temperatures as low as 200 °C under appropriate conditions, e.g. fine particles, high turbulence and low relative humidity, etc.

14.4 Prevention of Dust Explosions Recalling the explosion pentagon (Fig. 14.2), an explosion will not occur if one of the five elements is missing in a given application. Therefore, most of the tools developed to minimise the risk of an explosion can be traced back to Fig. 14.2. The available explosion prevention strategies fall into the following four main categories: process modification, elimination of the generation of dust cloud, elimination or minimisation of the occurrence of ignition sources, and inertion of dust and/or of the oxidising medium. Each of these are discussed here in some detail.

14.4.1 Process Modifications This idea is similar to the philosophy of inherently safer process design and process intensification. While it is not always possible or feasible to replace the existing processes by new ones eliminating the presence of combustible dusts, one can always modify the process (or part thereof ) so that the tendency for the formation of dust cloud is reduced, e.g. using mass flow silos and hoppers rather than the common practice of funnel flow to charge a vessel. Similarly, the use of nitrogen as the transporting medium and as the sealing medium in silos as opposed to air will lower the risk. Other possibilities, including maintaining the dust concentration below the MEC value, or controlling moisture in pipes and storage silos, and installing isolation valves between silos to minimise the possibility of secondary or sequential

Health and Explosion Hazards 759 explosions, etc., have been discussed in detail by Amyotte et al.32 and others.1, 12 In line with the idea of inherently safer design, the use of smaller amounts of hazardous materials, or replacement with less hazardous ones, or use of a material in its least hazardous form by suitable modifications to the process and process conditions, etc., are some of the tools employed in industry to reduce the risk of a potential hazard.

14.4.2 Formation of Explosible Dust Clouds While it is desirable to keep the dust concentration below MEC, it is always not possible due to process requirements and operating conditions which are often dictated by economic and other considerations. Notwithstanding this aspect, the following preventive steps may be helpful in reducing the possibilities of dust formation: (i) Free fall of dust should be avoided as much as possible. (ii) Work with smaller piles and heaps of dust rather than with one large heap or layer. (iii) Dust removal from a gaseous process stream may be carried out at an early stage if the process permits it. (iv) Good housekeeping practices like proper ventilation and cleaning help reduce the accumulation of dust on the floor, external surfaces, etc. For instance, NFPA standard34 recommends that dust layers of a thickness of 0.8 mm warrant immediate cleaning of the region; a dust layer this thick is prone to create hazardous conditions if it covers >5% of the floor area or
90 m2, whichever is smaller; such calculations should also include the dust on walls, overhead beams, ductwork, conduit cabling, piping, etc. Similarly, emission of dusts should also be reduced by proper design of process equipment. On the other hand, housekeeping practices must be selected based on the explosion characteristics of a dust, e.g. vigorous sweeping or the use of compressed air to blow down equipment in dusty areas may themselves lead to the formation of explosible dust clouds. Thus, it may be desirable to use a suitable vacuum cleaner first before sweeping or blowing down the plant area.

14.4.3 Management of Ignition Sources While it is generally not possible to precisely identify the source of ignition in a dust explosion, some general observations are made here. There are two types of ignition sources present in a plant: the types which originate from routine conditions and/or working styles of individuals such as smoking, unprotected light bulbs, open flames, welding, cutting and grinding, etc. These can be addressed by staff training and good working practices and discipline. However, more serious are those which are inherent to the processing operations itself. Typical examples include hot surfaces (dryers, kilns), open flames, smouldering nests and exothermic

760 Chapter 14 decompositions and oxidation reactions, electric sparks, electrostatic discharges, etc. In this case, the hazard cannot be completely eliminated, but it can certainly be reduced by periodic removal of the dust layers deposited on such hot surfaces, or by proper earthing of equipment and paying attention to the unusual operation of the equipment.

14.4.4 Inertion This refers to the ways and means of lowering the oxygen concentration in a plant area or in a process equipment by adding an inert gas to the extent that the dust cloud can no longer sustain a propagating flame upon ignition, i.e. the fuel is deprived of the oxygen it requires to burn. Inertion is also used in another way by adulterating the dust cloud using a noncombustible entity. This is, however, not used often because it is not always possible to add an impurity to a product. Typical inerting gases include nitrogen, carbon dioxide, water vapor, and rare gases. However, the final choice of a particular gas depends upon its chemical affinity with the dust. For example, carbon dioxide is an excellent inerting gas in many situations, but it cannot be used with aluminium dust due to its severe reactivity in this case. Similarly, nitrogen, a very effective inertant for many combustible dusts, reacts with magnesium dust, thereby rendering it unsuitable for such an application. In addition, economic considerations also influence the final choice of an inerting material in a given situation. In view of the cost and problems in achieving complete inertion, lowering the risk of explosion through partial inertion is quite common in practical situations. Admittedly, numerous studies are available in the literature, e.g. see,12 but it is not yet possible to put forward definitive guidelines in this regard. For instance, it is generally believed that the value of Kst varies linearly with the percentage of oxygen in the gas phase. Another factor to be borne in mind is that the values of the maximum permissible oxygen concentration to prevent ignition are generally measured at ambient temperatures. Therefore, caution must be exercised in using such values at temperatures above 100°C or so. Thus, additional tests must be carried out in such cases or in cases of hybrid vapor-dust mixtures. It is thus not uncommon to use 2% as a safety margin, i.e. if the maximum permissible oxygen concentration is known to be 11%, the oxygen concentration must be maintained below 9%. Of course, reducing the risk of explosion by inertion increases the risk of suffocation. Liquid/solid inerting materials are used with two objectives: prevention or mitigation/control of a dust explosion. As noted earlier, in the context of the first objective, such inertants are added to an otherwise explosible dust in sufficient amounts to render it nonexplosible. One outstanding example of this approach is to spray rock dust (mainly CaCO3 with or without MgCO3) in coal mine galleries. The rock dust simply acts as a heat sink and absorbs heat from the flame front, thereby inhibiting its propagation. Naturally, smaller particles are more effective than coarse ones due to their higher specific surface areas, but very fine particles lose

Health and Explosion Hazards 761

Blast waves from primary explosion

Layer of dust

Dust cloud formed

Another layer of dust

Blast waves from secondary explosion

Dust cloud ignited by primary explosion, explodes

Fig. 14.7 Domino effect leading to secondary explosion. Replotted from Abbasi T, Abbasi SA, Dust explosions—cases, causes, consequences, and control. J Hazard Mater 2007;140:7–44.

some of this effectiveness due to their tendency to form agglomerates. This approach is, however, generally limited to coal mines only. The second idea behind the use of solid inertants is to control an explosion by dispersing such a solid in adequate quantity to the vessel on fire. The success of this approach also hinges on quenching the flame or inhibiting flame propagation. The discussion in this chapter has, thus far, been limited to isolated dust explosions. In practice, however, a dust explosion can lead to several secondary explosions, depending upon the proximity of dust heaps to the site of the primary explosion. This is known as the ‘domino effect’ and is shown schematically in Fig. 14.7. Similarly, when a layer of dust is ignited, the celerity of the flame depends upon the heat of combustion. Such a fire in an unconfined environment generally does not lead to an explosion because the heat is dissipated to the atmosphere by convection and radiation, thereby raising the temperature of atmospheric air in a small region of flame (Fig. 14.8). Also, the resulting flame can act as an ignition source directly or indirectly by heating nearby process equipment. Many other issues relating to dust explosions, including mitigation, relevant standards, strategies for inherently safe design to reduce the potential risk, expert systems, etc., are discussed in several excellent reviews12 and books.1, 6, 7 However, this chapter is expected to provide a useful starting point to the vast field of dust hazards and explosions.

762 Chapter 14

Fig. 14.8 Dust layer fire: (A) dust layer with various particle sizes; (B) air ingress and initial self-heating; (C) rate of removal of heat by convection and radiation lower than the rate of heat generation; (D) self-ignition at some critical temperature. Redrawn from Amyotte PR, An introduction to dust explosions. Oxford, UK: Elsevier; 2013.

Health and Explosion Hazards 763

14.5 Nomenclature C h H ES IE IS Kst LIT LOC MEC MEP MIE MIT MRPR P t V ρbulk

Concentration Height of dust layer Height of room Explosion severity Index of explosibility Ignition sensivity Parameter, Eq. (14.4) Minimum ignition temperature for a dust layer Limiting oxygen concentration (volume % basis) Minimum explosion concentration on volume basis Maximum explosion pressure Minimum ignition energy Minimum ignition temperature of a dust cloud Maximum rate of pressure rise Pressure Time Volume Bulk density of solid

SI Unit

Dimensions

g/m3 m m – – – bar.m/s K – m3/m3 Pa J K Pa/s Pa s m3 kg/m3

ML23 L L M0L0T0 M0L0T0 M0L0T0 MT23 θ – M0L0T0 ML21 T22 ML22 T22 θ ML21 T23 ML21 T22 M0L0T M0L3T0 ML23 T0

References 1. Cheremisinoff NP. Dust explosion and fire prevention handbook: a guide to good industry practices. Beverly, MA: Scrivener/Wiley; 2014. 2. Plog BA, Quinlan PJ. Fundamentals of industrial hygiene. 6th ed. Washington, DC: National Safety Council; 2012. 3. Fuller TP. Essentials of industrial hygiene. Washington, DC: National Safety Council; 2015. 4. Masuda H, Higashitani K, Yoshida H, editors. Powder technology handbook. 3rd ed. Boca Raton, FL: Taylor and Francis; 2006. 5. Baker WE, Tang MJ. Gas, dust and hybrid explosions. Amsterdam: Elsevier; 1991. 6. Eckhoff RK. Dust explosion in the process industries. 3rd ed. New York: Gulf Publishing/Elsevier; 2003. 7. Amyotte PR. An introduction to dust explosions. Oxford, UK: Elsevier; 2013. 8. Anderson JO, Thundiyil JG, Stolbach A. Clearing the air: a review of the effects of particulate matter air pollution on human health. J Med Toxicol 2005;8:166–75. 9. Valavanidis A, Fiotakis K, Vlachogianni T. Airborne particulate matter and human health: toxicological assessment and importance of size and composition of particles for oxidative damage and carcinogenic mechanisms. J Environ Sci Health 2008;26C:339–62. 10. Tanaka I, Hori H. Respiratory protective devices for particulate matter. In: Powder technology handbook. 3rd ed. Boca Raton, FL: CRC Press; 2006. 11. Kauffman CW. Agricultural dust explosions in grain handling facilities. In: Lee JHS, Guirao CM, editors. Fuelair explosions. Waterloo, ON, Canada: University of Waterloo Press; 1982. p. 305–47.

764 Chapter 14 12. Abbasi T, Abbasi SA. Dust explosions—cases, causes, consequences, and control. J Hazard Mater 2007;140:7–44. 13. Cartwright P, Pilkington G. Explosions–Part 1, The Chemical Engineer (U.K.); 24 November 1994. p. 15–7. Part 2, ibid, 12 January 1995, pp. 16–18; Part 3, ibid, 23 February, 1995, pp. 17–18. 14. Agarwal A. Dust explosions, prevention and protection. Chem Eng 2012;115:26–30. 15. Glor M. A synopsis of explosion hazards during the transfer of powders into flammable solvents and explosion preventative measures. Pharm Eng 2010;30(1):1–8. 16. National Fire Protection Association, NFPA-68. Guide for venting of deflagrations, Quincy, MA, 2002. 17. Lees FP. Lees’ loss prevention in the process industries: hazard identification, assessment and control. 3rd ed. vols. 1–3. Oxford: Butterworth–Heinemann; 2005. 18. Palmer KN. Dust explosions and fire. London, UK: Chapman and Hall; 1973. 19. Amyotte PR, Basu A, Khan FI. Dust explosion hazard of pulverized fuel carry-over. J Hazard Mater 2005;122:23–30. 20. Gummer J, Lunn GA. Ignitions of explosive dust clouds by smouldering and flaming agglomerates. J Loss Prev Process Ind 2003;16:27–32. 21. ISSA. Determination of the combustion and explosion characteristics of dusts. Mannheim, Germany: International Social Security Agency; 1998. 22. Jacobsen M, Cooper AR, Nagy J. Explosibility of metal powders. New York: Bureau of Mines Report # 6516; 1964. 23. International Standards Organization. Explosion protection systems part 1. Determination of explosion indices of combustible dusts in air, ISO 6184/1. Geneva: ISO; 1985. 24. Siwek R. 20-L laboratory apparatus for the determination of the explosion characteristics of flammable dusts. [Dissertation], Winterthur, Switzerland: Winterthur Engineering College; 1977. p. 109. 25. Cashdollar KL. Flammability of metals and other elemental dust cloud. Process Saf Prog 1994;13:139–45. 26. Denkevits A, Dorofeev S. Explosibility of fine graphite and tungsten dusts and their mixtures. J Loss Prev Process Ind 2005;19:174–80. 27. Amyotte PR, Lindsay M, Domaratzki R, Marchand N, Di Benedetto A, Russo P. Prevention and mitigation of dust and hybrid mixture explosions. Process Saf Prog 2010;29:17–21. 28. Hoppe T, Jaeger N. Reliable and effective inerting methods to prevent explosions. Process Saf Prog 2005;24:266–72. 29. Taveau J. Secondary dust explosions: how to prevent them or mitigate their effects? Process Saf Prog 2012;31:36–50. 30. ASTM E 2019-03. Standard test method for minimum ignition energy of a dust cloud in air. West Conshohocken, PA, 2003. 31. von Pidoll U. The ignition of clouds of sprays, powders and fibers by flames and electric sparks. J Loss Prev Process Ind 2001;14:103–9. 32. Amyotte PR, Khan F, Boilard S, Iarossi I, Cloney C, Dastidar A, Eckhoff R, Marmo L, Ripley R. In: Explosibility of non- traditional dusts: experimental and modelling challenges. Hazards–XXIII, I Chem E Symp. Ser No. 158, November 13–15; 2012. p. 83–90. 33. ASTM E1491-06. Standard test method for minimum autoignition temperature of dust clouds. West Conshohocken, PA, 2006. 34. NFPA 654. Standard for the prevention of fire and dust explosions from the manufacturing, processing and handling of combustible particulate solids; 2000.

Further Reading Barton J, editor. Dust explosion prevention and protection: a practical guide. IChem E (U.K.): Warwickshire; 2002.