Page 56 - Dust Explosions in the Process Industries

Dust Explosions: An Overview 29

the preheating zone of the combustion wave and makes the gas mixture less reactive.
Third, moisture increases the interparticle cohesion of the dust and prevents dispersion
into primary particles (see Chapter 3).
More detailed analyses of flame propagation in dust clouds of various materials are
given in Chapter 4.

1.3.2
PARTICLE SIZE OR SPECIFIC SURFACE AREA

Figure 1.17, in addition to illustrating the influence of dust chemistry on the dust cloud
combustion rate, shows a clear dependence on particle size or specific surface area for
both materials. This is a general trend for most dusts. However, as discussed in detail in
Chapter 4 for coal, this trend does not continue indefinitely as the particles get smaller.
In the case of coal and organic materials, pyrolysis or devolatilization always precedes
combustion, which primarily occurs in the homogeneous gas phase. The limiting parti-
cle size, below which the combustion rate of the dust cloud ceases to increase, depends
on the ratios between the time constants of the three consecutive processes: devolatiliza-
tion, gas-phase mixing, and gas-phase combustion. Particle size primarily influences the
devolatilization rate. Therefore, if the gas-phase combustion is the slowest of the three
steps, increasing the devolatilization rate by decreasing the particle size does not increase
the overall combustion rate. For coals, it was found that the limiting particle diameter is
on the order of 50 pm. However, for materials yielding gaseous pyrolysis products that
are more reactive than volatiles from coal, e.g., due to unsaturated gaseous compounds,
one would expect the limiting particle size to be smaller than for coal. For natural organic
compounds, such as starch and protein, the limiting particle diameter is probably not much
smaller than about 10 pm; whereas for reactive dusts, such as some organic dyes, it may
well be considerably smaller.
Figures 1.21 and 1.22 show scanning electron microscope pictures of two typical nat-
ural organic dusts, a wood dust containing very irregular particle shapes and maize
starch having well-defined, nearly monosized, spherical particles.

Figure 1.21 Scanning electron microscope Figure 1.22 Scanning electron microscope
picture of wood dust (Courtesy of W. C. picture of native maize starch; typical particle
Wedberg). size 10- 15 pn (Courtesy of W C. Wedberg).