Page 31 - Dust Explosions in the Process Industries

4 Dust Explosions in the Process Industries

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Factors Influencing Ignition Sensitivity and ExplosionViolence of Dust Clouds

Particle sizehpecific surface area of the dust is a central factor. However, there are other
important factors too, and the comprehensive list may look as follows:

1. Chemical composition of the dust, including its moisture content.
2. Chemical composition and initial pressure and temperature of the gas phase.
3. Distributions of particle sizes and shapes in the dust, determiningthe specificsurface
area of the dust in the fully dispersed state.
4. Degree of dispersion, or agglomeration, of dust particles, determining the effective
specific surface area availableto the combustionprocess in the dust cloud in an actual
industrial situation.
5. Distribution of dust concentration in an actual cloud.
6. Distribution of initial turbulence in an actual cloud.
7. Possibility of generation of explosion-induced turbulence in the still unburned part
of the cloud. (Location of ignition source important parameter.)
8. Possibility of flame front distortion by mechanisms other than turbulence.
9. Possibility of significant radiative heat transfer (highly dependent on flame tempera-
ture, which in turn depends on particle chemistry).
Factors 3 and 4 have already been mentioned. These and other factors are discussed
in more detail in the subsequent sections. Factors 1,2,3, and 9 can be regarded as basic
parameters of the explosible dust cloud. Factors 4 to 8 are, however, influenced by the
actual industrial dust cloud generationprocess and explosion development.These, in turn,
depend on the nature of the industrial process (flow rates, etc.) and geometry of the
systemin which the dust cloud bums. The location of the ignition point is another param-
eter that can play an important role in deciding the course of the explosion.
In view of the wide spectrum of dust cloud concentrations,degrees of dust dispersion and
turbulence, and locations of potential ignition sources in industry, a correspondingly wide
spectrumof possible dust cloud ignition sensitivitiesand combustionrates must be expected.
This complex reality of the process industry is also shared by laboratory experimen-
tation and represents a constant challenge in the design of adequateexperiments and inter-
pretation of experimental results.

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Previous Books on the Dust Explosion Problem

Over the years, several textbooks on the dust explosionhazard have been produced. One
of the first ones, by Beyersdorfer (1925),was published in Germany; he mentioned that
his motivation for writing the book arose from three questions. The first, asked by most
people, was, “Are dust explosions really existing?” The second question, asked by the
plant engineer, was, “Why are we having so many dust explosions?”The final question
was asked by the researcher, “Why are we not having many more dust explosions?”
Although out of date on some points, Beyersdorfer’s pioneering book is still fascinat-
ing reading.
Almosthalf a century elapsed from the publication of Beyersdorfer’stext until the next
comprehensive book on dust explosions appeared. It should be mentioned though, that