Flammability of Metals and Other Elemental Dust Clouds - Summary

Flammability of Metals and Other Elemental Dust Clouds – Summary

1-Sentence-Summary: Flammability limits and explosion severity of elemental dusts are characterized by adiabatic flame temperature, ease of phase transition, and particle size.

Authors: K.L. Cashdollar

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Flammability of Metals and Other Elemental Dust Clouds - Summary

This author presents experiential data for explosion of elemental dust clouds. 15 metal dusts and 4 nonmetal dusts are explored. Tests are completed in the U.S Bureau of Mines 20-L chamber using 2.5 and 5.0 kJ ignitors. An increased ignition delay time over the ASTM standards is used. Therefore, the maximum rate of pressure rise results should be used for qualitative comparison only.

The dusts explored include boron, carbon, magnesium, aluminum, silicon, sulfur, titanium, chromium, iron, nickel, copper, zinc, niobium, molybdenum, tin, hafnium, tantalum, tungsten, and lead. Minimum explosible concentration (MEC), maximum pressure (Pmax), and maximum rate of pressure rise ((dp/dt)max) are presented for each. The results are compared based on the theoretical maximum adiabatic flame temperature, the particle size, and the stoichiometric concentration.

Three of the main findings from this paper are:

  1. Nickle, copper, molybdenum, and lead could not be ignited using 2.5 or 5.0 kJ ignitors.
  2. Explosion severity is indicated first by adiabatic flame temperature followed by ease of vaporization.
  3. Explosion severity increases with decreasing particle size, while MEC decreases until a limiting particle size below which it is constant.

The following sections outline the main findings in more detail. The interested reader is encouraged to view the complete article at the link provided below.

Finding #1: Ni, Cu, Mo, and Pb could not be ignited using 2.5 or 5.0 kJ ignitors

The author found that nickle, copper, molybdenum, and lead dusts could not be ignited, even using the higher strength ignitors. He attributes this to low adiabatic flame temperatures and high stoichiometric concentrations. All of these dusts but nickle have adiabatic temperatures below 2000 K and the stoichiometric concentrations are calculated as 1020, 1100, 560, and 3600 g/m3 for Ni, Cu, Mo, and Pb, respectively.

Finding #2: High flame temperature and ease of vaporization govern explosion severity

The author found that elemental dusts with the highest maximum pressure and maximum rate of pressure rise (B, Mg, Al, and Si), also had the highest adiabatic flame temperature, with values above 2800 K. Interestingly, some other dusts with adiabatic flame temperatures above 2800 K had very low Pmax and (dp/dt)max (e.g., Chromium, Niobium, and Tantalum). The author attributes this to difficulties vaporizing the dust or low intrinsic reaction rates.

Finding #3: Below a Critical Particle Diameter MEC is Constant

The author presented MEC results for all of the elemental dusts studied. In general the 5 kJ ignitor results were 10-35% lower than using the 2.5 kJ ignitor.

The effect of particle size on MEC was further explored for aluminum and iron dust samples. Both showed a maximum particle size above which the dust could not be ignited. This limit was approximately 60 µm and 80 µm for the aluminum and iron, respectively. The author notes that the dusts used in these tests were sieved between upper and lower screen sizes. He also notes that wider size distributions with the presence of fines may ignite with a higher mean diameter.

A lower limit was also seen for the dusts, below which the MEC was independent of particle diameter. For aluminum dust the MEC was constant around 90 g/m3 below a particle diameter of 20 µm. For iron dust the MEC was constant around 205 g/m3 below a particle diameter of 10 µm.

My Personal Take-Aways From
“Flammability of Metals and Other Elemental Dust Clouds”

This paper presents a significant amount of testing results and should be reviewed by anyone studying the explosibility of metals or elemental dusts. Several of the originating references for early dust explosion literature are included such as the textbooks by Bartknecht, 1981, Field, 1982, Nagy and Verakis, 1983, and Eckhoff, 2003. Other useful papers by Hertzberg, 1988, Cashdollar and Chatrathi, 1993, and Ogle, et al.1988 are also discussed.

The main findings on the effect of adiabatic flame temperature and ease of vaporization are important and ongoing research areas for dust flame propagation and explosion. The literature has shown important links between the flame temperature and the melting and boiling temperature for metal dusts. These results are discussed and summarized in the works of Gordon, 1960, and Broumand and Bidabadi, 2013, the textbook of Glassman, 2013, and the thesis of Jones, 2011.

The characteristic particle diameter below which the MEC is constant is another interesting feature. This may indicate a transition from diffusion limited reaction to kinetic limited. This area has been studied extensively for aluminum dusts (e.g., see Zhang et al., 2001 and Zhang, 2009), and further research for other processing dusts would be an important contribution to this area.

Full Citation:

  • K. Cashdollar, “Flammability of metals and other elemental dust clouds,” Process safety progress, vol. 13, p. 139—-145, 1994.
    title={Flammability of Metals and Other Elemental Dust Clouds},
    author={Cashdollar, K.},
    journal={Process Safety Progress},
    link ={http://onlinelibrary.wiley.com/doi/10.1002/prs.680130306/abstract},

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