Page 633 - Dust Explosions in the Process Industries

600 Dust Explosions in the Process Industries

They found that, for some metals, flamepropagation appears to occw in a mixture of metal
vapor and air, similarto the gas phase flame propagationmechanism in clouds of organic
dusts.
It is well known that pulverized coal and coal dust in mines do not represent a dust
explosion hazard unless the content of volatiles exceeds 7-8%. However, this does not
apply to carbon dusts of specific surface areas exceeding the order of 100 m2/g (N,
adsorption).Wiemann (1992) showed that dusts of such materials (active carbodactive
coke) of considerably lower volatile content than 74% could produce fully developed
dust explosions in the standard 1m3IS0 vessel.
The influence of particle size on the flammability limits of clouds of stearic acid in air
was investigated by Ju et al. (1998b). They found that the lower flammabilitylimit was
defined mainly by the mass concentration of particles of diameters smaller than 60 pm.
Horstmann et al. (1996) found that the minimum air pressure, below which clouds of
a given dust can no longer propagate a flame, decreases with increasing volume of the
test apparatus.The underlying reason for this is that the quenching distance of clouds of
a given dust increases with decreasing air pressure.

9.2.4.4
Turbulent Flame Propagation in Dust Clouds

This important topic has been studied experimentally and theoretically by a number of
investigators.Work up to 1990 is discussed in Section 4.4 in Chapter 4. Eckhoff (1992)
summarizedsome work on the influence of initial and explosion-inducedturbulence, on
dust explosions in closed and vented vessels. We1 et al. (1992, 1993) reemphasized the
important role played by turbulence in dust explosion propagation in closed vessels.
Kauffman et al. (1992) and Austin et al. (1993) summarizedtheir quite extensive research
on turbulent combustionof dust clouds, whereas Tamanini and Ural(l992) outlined their
work on the effect of the initial turbulence of the dust cloud on the flame propagation in
closed and vented systems.
Scheuermann(1 994) also investigated the influence of the initial dust cloud turbulence
on the developmentof dust explosions in vented enclosures.Rzal-Rebikre and Veyssikre
(1992) addressed some central basic aspects of turbulent dust flames. Veyssikre (1992)
summarized all the fundamental studies on flame propagation in dust clouds conducted
at LED in Poitiers, comprisinglaminar flame propagation in dust clouds, the role of tur-
bulence in flame acceleration, and the conditions for propagation of detonationlike, but
nonideal, combustion waves. Rzal-Rebibre and Veyssibre (1994) in their basic studies
investigated the interaction of a laminar maize starcwair flame with an obstacle: a
sphere,a disk, or a vortex ring. With the ring, flame quenchingphenomena were observed,
which were attributed to centrifugal separation of dust particles and air in the turbulent
eddies. This is a very important observation, indicating that the burning rate of a dust
cloud may not respond to turbulence in the same way as the burning rate of a premixed
gas.
Further work toward improved understanding of the relation between the dynamic
state of a dust cloud and its combustion rate is needed. The basic microscopic turbu-
lence mechanisms that promote the combustion process must be identified. The inves-
tigation by Mitgau (1996) and Mitgau, Wagner, and Klemens (1997) seem to be
highly significant in this respect. These workers determined correlations between