Article pubs.acs.org/IECR
Review of the Explosibility of Nontraditional Dusts S. Morgan Worsfold,† Paul R. Amyotte,*,† Faisal I. Khan,‡ Ashok G. Dastidar,§ and Rolf K. Eckhoff∥ †
Department of Process Engineering and Applied Science, Dalhousie University, Halifax, Nova Scotia, Canada Faculty of Engineering and Applied Science, Memorial University, St. John’s, Newfoundland, Canada § Fauske & Associates, LLC, Burr Ridge, Illinois, United States ∥ Department of Physics and Technology, University of Bergen, Bergen, Norway ‡
ABSTRACT: This paper explores the explosion characteristics of three nontraditional dusts: nanomaterials, flocculent materials, and hybrid mixtures. Nanomaterials have a high likelihood of explosion with minimum ignition energies potentially less than 1 mJ. These low ignition energies may therefore allow nanomaterials to ignite due to electrostatic sparks, collision, or mechanical friction. The severity of nanomaterial explosions is affected by agglomeration and coagulation of the particles. Flocculent materials with a high length-to-diameter ratio exhibit explosion behavior patterns similar to those for spherical dusts. The length of flocculent particles plays a role in explosion likelihood which is not yet fully understood. High voltage discharge during the electrostatic flocking process is a common flocculent ignition hazard. Hybrid mixtures of a combustible dust and a flammable gas/vapor display a higher explosion severity and a lower minimum explosible concentration than that of the dust alone. Violent hybrid explosions may occur even if the dust and the gas/vapor are below their respective lean limit concentrations.
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INTRODUCTION 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 U.S. 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 “nontraditional” dusts. The first such “nontraditional” dust type is nanomaterials, which are particulate matter with dimensions in the nanorange, much smaller than common dusts. The second “nontraditional” dust type to be explored is flocculent materials, which are nonspherical and instead have a more fibrous appearance. The third “nontraditional” 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 behaviors are less well-documented. This review of nontraditional 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 behaviors different from what might be expected if the existing knowledge of traditional dusts were directly applied to these nontraditional types. To date, research is limited in the types of dusts to be discussed. © 2012 American Chemical Society
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
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NANOMATERIALS 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 metals5,6 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 Special Issue: Russo Issue Received: Revised: Accepted: Published: 7651
July 26, 2011 December 26, 2011 January 4, 2012 January 4, 2012 dx.doi.org/10.1021/ie201614b | Ind. Eng.Chem. Res. 2012, 51, 7651−7655
Industrial & Engineering Chemistry Research
Article
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 aluminum 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
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, nonwoven 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 2 m2/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 Minimum ignition temperature was found to decrease with decreasing particle size, increasing the likelihood of explosions with nanosized particles over larger particles.9 Nanosized aluminum has been found to ignite at a rather low ignition temperature of approximately 900 K, which is below aluminum’s melting point, as a result of the oxidation of the aluminum.12−14 Ignition of thermites prepared with nano- and
Table 1. Comparison of Explosibility of Nano- and Micrometer-Sized Aluminum16 particle size
Pmax (bar)
KSt (bar m/s)
MEC (g/m3)
MIE (mJ)
40-μm Al 100-nm Al 35-nm Al
5.9 12.5 7.3
77 296 349
35 50 40
60
