Page 64 - Dust Explosions in the Process Industries

Dust Explosions: An Overview 37

& z o I I I I Figure 1.38 The influence of average dust con-
E centration in a 7 m3 IS0 standard vessel on the
0 250 500 750 1000 minimum electric spark ignition engery of ciouds
DUST CONCENTRATION rs~1 of an antioxidant in air (From Bartknecht, 7979).

burned and burning parts of the cloud with the unburned parts, and the cloud becomes
a lkind of three-dimensional laminate of alternating hot burned or burning and cold
unburned zones. Therefore, a turbulent cloud burns much faster than when a single plane
flame sheet propagates through a quiescent cloud.
In the case of ignition of the dust cloud, whether by an electric spark or a hot surface.
the turbulence disturbs the heat transfer by removing heat from the ignition zone by rapid
convection. Therefore, ignition of a turbulent dust cloud generally requires higher energy
or temperature than ignition of quiescent clouds.
In the context of dust explosions, two kmds of turbulence, differing by their origin,
have to be considered. The first is turbulence generated by the industrial process in
which the dust cloud is formed, whether an air jet mill, a mixer, a cyclone, a bag filter,
a pneumatic transport pipe, or a bucket elevator. This lund of turbulence is often called
initial turbulence. The second kind is generated by the explosion itself by expansion-
induced flow of unburned dust cloud ahead of the propagating flame. The level of tur-
bulence generated in this way depends on the speed of the flow and the geometry of the
system. Obstacles, like the buckets in a bucket elevator leg, enhance the turbulence gen-
eration under such conditions.
In long ducts or galleries a positive feedback loop can be established, by which the
flame can accelerate to very high speeds and even transit into a detonation. This is dis-
cussed in Chapter 4.
Figure 1.39 shows a characteristic example of the influence of initial turbulence on the
rate of dust explosions in closed bombs. The dust cloud is generated in a closed vessel
by dispersing a given mass of dust by a short blast of air.
In the early stages of dust dispersion, the dust cloud can be quite turbulent, but the tur-
bulence fades away with time after the dispersion air has ceased to flow. Therefore, if
explosion experiments with the same dust are performed in similar vessels at different
delays between dust dispersion and ignition, they have different initial turbulence. As
Figure 1.39shows, the explosion violence, in terms of the maximum gradient of the pres-
sure rise versus time, decreased markedly, by at least an order of magnitude, as the ini-
tial turbulence faded away. However, Figure 1.39also shows that the maximum explosion