Review of the Explosibility of Non-traditional Dusts
04 July 2013
This paper explores the explosion characteristics of three non-traditional dusts. Nanomaterials have a high likelihood of explosion with minimum ignition energies potentially less than 1 mJ. Flocculent materials with a high length-to-diameter ratio exhibit explosion behaviour patterns similar to those for spherical dusts, while hybrid mixtures of a combustible dust and a flammable gas display a higher explosion severity and a lower minimum explosible concentration than dust alone.
A dust explosion may occur as the result of dust particles suspended in the air under confinement and exposed to an ignition source. The severity of the incident is comparable to a gas explosion event.(1) Among the earliest records of the cause of an industrial accident being attributed to a dust explosion was the account of an explosion in a flour warehouse in Turin, Italy in 1785.(2,3) Despite significant research, the risks of dust explosions are still not well-known in industry, and dust explosions continue to occur.
According to the US National Fire Protection Association (NFPA), a dust is defined as a finely divided solid with a diameter of less than 420 µm (0.017 in.). A dust will pass through a US No. 40 standard sieve.(4) The current paper explores the explosibility of three different types of dust which do not necessarily follow this definition and which may therefore be considered “non-traditional” dusts. The first such non-traditional dust type is nanomaterials, which are particulate matter with dimensions in the nano range, much smaller than common dusts. The second non-traditional dust type to be explored is flocculent materials, which are non-spherical and instead have a more fibrous appearance. The third non-traditional dust type to be explored in this review is hybrid mixtures, which can be any dust that also has an admixed gas or is wetted with an organic solvent. These three categories of dust are less frequently the topic of dust explosion research, and so their explosibility behaviours are less well-documented.
This review of non-traditional dust types is important because each of the three dust types to be discussed has characteristics that are different from the traditional dusts typically studied. These characteristics may result in behaviours different from what might be expected if the existing knowledge of traditional dusts were directly applied to these non-traditional types. To date, research is limited in the types of dusts to be discussed. This review will aid in determining areas of existing research focus, where gaps in knowledge exist and where future research should be focused.
The following discussion makes use of several dust explosibility parameters. Pmax is the maximum explosion pressure in a constant volume explosion. (dP/dt) max is the maximum rate of pressure rise in a constant volume explosion.
The value of (dP/dt)max is dependent upon the explosion chamber volume and should be scaled for better comparison of data. KSt is the volume-normalized maximum rate of pressure rise, which is determined by multiplying the (dP/dt)max found experimentally, by the cube-root of the volume of the explosion chamber; the acquisition, use, and limitations of KSt data have been discussed by Amyotte and Eckhoff. (1) Pmax , (dP/dt)max , and KSt are all measures of explosion consequence severity. MEC is the minimum explosible concentration of a dust. MIE is the minimum ignition energy of a dust cloud. MIT is the minimum ignition temperature of a dust cloud. MEC, MIE, and MIT are all measures of the likelihood of explosion occurrence. (1)
A nanoparticle is a particulate with lengths between 1 and 100 nm in at least two of three dimensions. (5) Nanomaterials can be composed of organic materials, either natural or synthetic, or metals (5,6) and can come in a variety of shapes including nanotubes, nanowires, and crystalline structures.(5) Nanomaterials have a large specific surface area as compared to micrometer-sized materials, and as a larger number of atoms occur on the surface of nanoparticles, they often have very different properties such as a greater reactivity, strength, fluorescence, and conduction.(5) Due to their extremely small size, nanomaterials are respirable, and it is believed that their inhalation may result in adverse respiratory and cardiovascular effects.(5) Therefore, when handling nanomaterials special precautions should be taken, such as the use of nitrile gloves, airline hood, non-woven coveralls, and HEPA/ULPA vacuums.(7) Particles of nm size may remain suspended in the air for days or even weeks.(5)
In general as particle size decreases (and the specific surface area increases), it has been found that explosion severity will increase. Following this logic, it would be expected that nanomaterials would exhibit very high values of KSt. However as particle size approaches the nanometer range it is expected that the increased explosion hazard due to reduced particle size would be limited to some degree.(6) In an experiment with an aluminum dust, as specific surface area increased/particle size decreased, explosion severity began to decrease when the specific surface area of the aluminum particles was approximately 2m2/g and particle size was 1 µm.(8)
There are two physical processes that are believed would reduce explosion severity with nanosized particles: limited dispersibility and high coagulation rates. Nanopowders naturally tend to agglomerate.(9) Dispersion of fine, cohesive powders into a cloud of individual particles is not possible without significant stresses to break interparticle bonds of agglomerates. After the incomplete dispersion, agglomerates will continue to form as a result of collision between particles.
The initial coagulation rate will be greater for dust clouds with smaller initial particle sizes.6 As a result of the incomplete dispersion and further coagulation, the effective size of particles will be greater than the particles’ primary nanometer size. These agglomerations of nanoparticles have been found in ranges of 10-200 µm.(7) Multiwalled carbon nanotubes, which have a very high specific surface area when compared to carbon black (Corax, Printex, and Thermal Blacks brands were tested), were found to have agglomerates of approximately 200 µm, and their explosion severity was lower than the carbon blacks.(9) Additionally, 100-nm aluminum particles were found to explode less violently than aluminum particles with a diameter of 200 nm. This may be due to a greater impact through agglomeration for the 100-nm sample.(9)
It was found that the minimum explosion concentration did not change significantly with reduced particle size, and a theoretical plateau was observed.(9) Minimum ignition energy decreased with decreasing particle size.(6,9) Experimentation with metallic nanopowders has shown that they can explode with energies less than 1 mJ,(6,10,11) which is the lowest energy that can be tested using a MIKE3 apparatus (a common test apparatus for MIE values which is manufactured by Kuhner AG, Switzerland). This low MIE puts these nanopowders at a higher ignition risk than similar micrometer-sized dusts. The nanopowders could ignite as a result of electrostatic spark, collision or mechanical friction, and precautions should be taken to prevent such events.(6,10)
Ignition and flame propagation of nylon flock in a MIKE3 apparatus
Minimum ignition temperature was found to decrease with decreasing particle size, increasing the likelihood of explosions with nanosized particles over larger particles.(9) Nanosized aluminium has been found to ignite at a rather low ignition temperature of approximately 900K, which is below aluminium’s melting point, as a result of the oxidation of the aluminium.(12-14) Ignition of thermites prepared with nano- and micrometer-sized particles was compared. Micron powder ignited at 610°C, while the nanopowders ignited at 100°C.(15)
Nanomaterials have different properties than their respective micrometer-sized counterparts as a result of very high specific surface areas and high reactivities. These changes result in lower ignition and melting temperatures and faster burning rates. For aluminium nanoparticles, these changes become more significant at a particle size less than 10 nm.(13-15) Changes in the oxide shell at this size range, which are often overlooked, may also have an impact on particle combustion and ignition.(13,15) Both the particle fuel and oxide size affect reaction, and decreasing either increases the reaction rate.(13)
The combustion reaction of micrometer-sized particles is controlled by diffusion, whereas for nanosized particles the reaction is kinetically controlled.(9,13,14) The severity of nanomaterial explosions will not be controlled by the particle size but rather by the combustion of the pyrolysis gas/air mixture.(6) For most organic materials, this transition from diffusion to kinetically controlled reactions occurs at approximately 10 µm.(6,9,13)
It has been found that carbon nanomaterials (i.e., dust clouds in air of nm-size carbon particles) are typically not as reactive from an explosibility perspective as metallic nanomaterials, which are quite reactive.(7,9)
Table 1 illustrates the reactivity of metallic nanopowders by giving an example of explosibility results comparing nano- and micrometer-sized aluminium powders. The maximum explosion pressure appears to increase as it transitions from the micrometer to nanorange. However, once within the nanorange, the explosion pressure decreases. The maximum rate of pressure rise also shows an increase with decreasing particle size. This trend continues into the nanoparticle range. The minimum explosible concentration does not change significantly over the range of samples tested. Once within the nanorange, the minimum ignition energy is quite small, at less than 1 mJ for both nanosizes tested.
Nanomaterials present a dust explosion hazard, with metallic nanoparticles being particularly reactive. Nanomaterials have been shown to display lower ignition energy and temperature requirements than larger particles. Due to this high sensitivity, explosion hazards may exist for many processes including, but not limited to, mixing, grinding, drilling, sanding, and cleaning.(10,17)
Experimental data for nanomaterials explosibility are still limited. More research is needed to gain a better understanding of ignition behaviour of nanomaterials, as well as the effects of agglomeration on combustion (particularly with respect to Pmax and KSt). It is encouraging to see the recent work undertaken in this regard by the UK Health and Safety Executive.(18,19)
Flocculent materials are nonspherical and have a fibrous (fluffy) shape. Flocculent fibers cannot be well characterized in terms of a diameter but rather are better described in terms of a length-to-diameter ratio.(20) The SEM micrograph images in Figure 1 compare the shapes of flocculent and spherical polyethylene. Such fibers can be composed of a multitude of materials, including cotton, acrylic, and polyester, and have a multitude of uses such as upholstery, carpeting, toys, paper, noise reduction, and insulation materials.(21) Flocculent materials would not be considered as dusts according to the NFPA definition of dust described earlier in this review. However, flocculent materials may be considered dusts, though “non-traditional”, given the NFPA definition of a combustible dust. Under this definition, a combustible dust is any combustible particulate solid that when suspended in air poses a risk of fire or deflagration. This definition gives no restriction on the shape or size of the particle.(4)
Much like the pattern seen for spherical particles, as flocculent particle diameter (determined by sieve analysis) decreases, explosion severity is increased. The magnitude of rate of pressure rise is impacted more than the explosion pressure. Additionally, Pmax and KSt are achieved at lower concentrations for particles with smaller diameters.(20) Table 2 shows explosion severity results for fibrous polyethylene and wood samples. Corresponding data for spherical polyethylene dust are given by Amyotte et al.(20)
Explosion likelihood also increases with a diminishing particle size. Minimum explosible concentration, minimum ignition energy, and minimum ignition temperature have all been shown to decrease with reduced particle size.(20,22) In MIE experimentation for a variety of textile fibers it was found that samples with the smallest diameters and shortest lengths were the most easily ignitable, with values of MIE below 100 mJ for some samples. Also, since flocculent materials generally have larger dimensions as a result of the length, they were found to have higher MIE values than traditional dusts.(22) While MIE is reduced when either the diameter or length, or both, are reduced, the diameter is the rate-controlling parameter as it is, at minimum, an order in magnitude smaller than the length.(20,23)
While length does have a lesser impact on the minimum ignition energy than diameter, it still has some influence. There are two possible causes for the effect of length on the MIE. The first is the melting process, which causes the fiber to shorten and form a more spherical shape, thereby increasing its diameter. The melting temperature of nylon fiber is lower than its ignition temperature, and so the material would likely begin to melt and deform before ignition. The second possible reason could be that as fiber length increases there is a greater tendency to coagulate and form spherical aggregates, which would reduce dispersion and increase MIE.(23)
Nylon fibers were found to have similar explosion properties to organic dusts, with explosion severity generally falling into the St1 dust explosion class, MIE as low as 145 mJ, and MEC in the range of 70-85 g/m3.(21,23) Flocculent material explosibility follows the same general trends as spherical dusts, and there is no reason to assume such particles would be less explosible simply because they are nonspherical.(20)
Flocculent materials therefore present explosion hazards similar to spherical dusts and should be considered as combustible dusts. Nylon flock ignition has been determined as the cause of at least one explosion in flock manufacturing plants in recent years.(21) As flocculent materials are often manufactured using an electrostatic flocking process, the hazard of ignition from a high voltage discharge during a process breakdown must not be ignored.(22)
To date, flocculent dusts have not been extensively studied - particularly at the scale of the standard 20-L explosion chamber.(24) As research data are limited, much more experimentation must be conducted to gain a proper understanding of flocculent explosion behaviour. With the increasing use of flameless venting techniques, additional work is required to properly consider the mechanical blockage of mesh-devices by flocculent material.
A hybrid mixture is the result of a combination of a combustible dust and a flammable gas or solvent vapour. (In the case of an admixed flammable gas, it is not really the dust itself that is “non-traditional” but rather the composite fuel-air system in its entirety.) In a hybrid mixture, the dust may be present below its minimum explosible concentration, and the gas may be present below its lower flammable limit, the combination becoming an explosible mixture. Once a gas concentration is above its lower flammable limit, an explosion of the dust/gas mixture would no longer be considered a hybrid explosion but rather a gas explosion.(1)
The addition of an admixed gas to a dust results in a greater Pmax and KSt than for the dust alone. For the maximum explosion pressure, which is a thermodynamic parameter, the gas and the dust each bring their own combustion enthalpy to the mixture. However, there are more than additive effects for the kinetic reaction reflected in the maximum rate of pressure rise. This reaction, which depends on the fuel-air ratio, can be more difficult to predict. Both Pmax and KSt show a promotion effect in that the results are greater than for either the dust or gas alone, although the influence is more apparent for KSt.(25-28)
Flammable gas admixture thus has a greater influence on the maximum rate of pressure rise than on the maximum explosion pressure.(28) Reactions can occur in hybrid mixtures below the minimum explosible concentration of the dust alone. At concentrations at or below the MEC, the explosion pressure is somewhat dependent upon the solvent concentration (partial pressure of the vapour); however, at higher concentrations, the explosion pressure becomes independent of the solvent and instead depends upon the oxygen content.(27,29,30) However, the rate of pressure rise is significantly affected by the solvent concentration, even above the MEC; the influence of the solvent does decrease as dust concentration increases above the MEC.(27) Admixture of a flammable gas has the greatest effect on Pmax and (dP/dt)max at leaner dust concentrations.(25,31)
Generally, the maximum results are achieved with a low dust concentration and a high solvent content, as the controlling variable in the combustion kinetics is the solvent content.(26)
When a solvent is added to a dust mixture, ignition can occur at a leaner dust concentration than if the dust were alone, effectively reducing the minimum explosible concentration.(26,29-31) Likewise, the lower flammable limit of the solvent will effectively be reduced with addition of a small amount of dust. For example, the MEC of an antibiotic powder was found to be 500 g/m3, but when a small amount of solvent was introduced, the dust was now able to explode at a concentration of 200 g/m3. Additionally, the lower explosive limit of toluene is 8%, but it can explode at a concentration of 4% with the addition of combustible dust.(26) A hybrid explosion can occur if the dust is below its minimum explosible concentration and/or the solvent is below its lower flammable limit, and these explosions can still be violent.(25-27,30)
There appears to be a correlation between the higher burning rates of the gas and violent hybrid explosions.(28,29) The extent of the increased explosion risk is dependent upon the burning velocity of the gas, among other factors. For example, when testing polyethylene dust samples, the values of KSt were similar for the addition of either hexane (C6H14) or propane (C3H8), which both have a burning velocity of 46 cm/s. A proportionally higher value of KSt was achieved with the addition of ethylene (C2H4), which has a burning velocity of 80 cm/s. These phenomena are displayed in Figure 2.(28)
It should be noted that not all gases show the same patterns for hybrid explosions. For example, while the addition of cyclopentane to polyurethane dust did result in an expected reduction in MEC and MIE, it did not increase either Pmax or KSt.(32) Also, due to methane’s unique oxidation characteristics, it has been found that ignition is more difficult in methane/air mixtures than for other hydrocarbons, although increased explosion risks in methane/dust mixtures were still observed.(30,31) The difficulty in igniting methane is due to the high level of energy required to break the C-H bond of the methane gas.(30) This is not to say that a hybrid mixture with methane should not be of concern, as these explosions can still be quite severe. One such example of the severity of methane/dust explosions is methane/coal dust hybrid explosions in coal mines.
The addition of a flammable gas results in hybrid mixtures being a greater dust explosion hazard. Admixture of flammable gases to a dust mixture increases the explosion pressure and rate of pressure rise, as well as reducing the minimum explosible dust concentration. These effects can be observed even if the gas concentration is below its lower flammable limit and the dust is below its minimum explosible concentration.
As hybrid mixtures are a special class of dust explosions, existing research on the relevant dust without the admixture of gas is insufficient for predicting hybrid mixture effects. Further research would aid in better identifying the explosion behaviour of hybrid mixtures. Quantitative consideration of the additive effect of the gas burning velocity would appear to be a fruitful line of pursuit.
Figure 1. SEM micrographs of polyethylene fibrous sample (A) and spherical sample (B).(20)
Nanomaterials (dusts/powders having nm-size particles) exhibit an explosion severity which is not disproportionate to micrometer-sized materials, but the likelihood of explosion is quite high due to very low ignition energies and temperatures. As a result of the extremely low ignition energy, many explosion hazards exist during the processing and handling of nanomaterials, such as the result of friction, collision, mixing, grinding, drilling, sanding, and cleaning. More research is required to gain a better understanding of nanomaterial ignition behaviour, in particular, the effects of agglomeration on ignition and subsequent explosion development.
Flocculent materials (having large length-to-diameter ratios) appear to have similar explosion characteristics to spherical dusts. A significant risk of ignition of flocculent materials is the event of a high voltage discharge during the electrostatic flocking process frequently used in flock manufacturing. Research on the explosibility of flocculent material is very limited, and more experimentation is needed. In particular, a better understanding of the impact of the length on the likelihood of explosion is needed, as well as further work on the explosion severity of these materials.
The addition of a flammable gas to a dust (or wetting with an organic solvent), thus creating a hybrid mixture, results in an increase in the severity of the explosion over that of the dust alone. The minimum explosible dust concentration is reduced for hybrid mixtures. Violent explosions can occur even if both the dust and gas are below their minimum explosible concentration/lower flammable limit. Continued research on hybrid explosions is needed; hybrid mixtures are a special class of explosible fuel, and their explosion behaviour cannot be predicted from knowledge of the dust alone.
S. Morgan Worsfold and Paul R. Amyotte: Department of Process Engineering and Applied Science, Dalhousie University, Halifax, Nova Scotia, Canada
Faisal I. Khan: Faculty of Engineering and Applied Science, Memorial University, St. John’s, Newfoundland, Canada
Ashok G. Dastidar: Fauske & Associates, LLC, Burr Ridge, Illinois, United States
Rolf K. Eckhoff: Department of Physics and Technology, University of Bergen, Bergen, Norway
Paul Amyotte - Phone: (+1) 902-494-3976. E-mail: email@example.com.
The authors gratefully acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada in the form of a strategic grant.
This article first appeared in Industrial & Engineering Chemistry Research 2012 - American Chemical Society. A version was presented at the IChemE Hazards XXIII symposium in November 2012.
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SEM view of flock
Figure 2. Influence of admixed gas burning velocity (and particle diameter) on volume-normalised maximum rate of pressure rise.(28)