Page 227 - Dust Explosions in the Process Industries

hapter 3

Generation of Explosible Dust Clouds

by Reentrainment and Redispersion
of Deposited Dust in Air

3.1
BACKGROUND

The dust concentration range, within which flames can propagate through a cloud of com-
bustible dust in air, spans from the order of 50 g/m3to a few kg/m3.The bulk density of
powders and dusts, when settled in a layer or a heap, range from a few hundred kg/m3
and upward. Therefore, there is a gap of a factor of at least 100 between the maximum
explosible dust concentration and the bulk density in the settled state. Consequently, for
an explosible dust cloud to be formed, the dust must be suspended in the air to the
extent that the concentration of dust per unit volume of cloud drops into the explosible
range.
In dust explosion research, the important role played by this resuspension process has
often been overlooked or underestimated. It is realized that particle size plays a key role
both with respect to the ignition sensitivity and the explosibilityof dust clouds. However,
it has not always been realized that fine, cohesive powders cannot be dispersed in a gas
as individual primary particles unless particle agglomerates are exposed to very high shear
or tensile stresses. This means that the effective particle size in a dust cloud can be much
larger than the size of the primary particles.
It is interesting to note that Professor Weber, one of the pioneers of dust explosion
research, stressed the importance of dust cohesion and dispersibility more than 100years
ago. In his excellent paper on the ignitability and explosibility of flour, Weber (1878)
emphasized that the “cohesion of the flour, which is caused by inter-particle adhesion,
plays an important role with respect to the ability of the flour to disperse into explosible
dust clouds.’’Weber suggested that two large dust explosion disasters, one in Szczecin
(Stettin) and one in Munich, were due mainly to the high dispersibility of the flours. He
also demonstrated, using simple but convincing laboratory experiments, that the dis-
persibility or dustability of a given flour increased with decreasing moisture content in
the flour.
In some special situations, such as in air jet mills, explosible dust clouds may be gen-
erated in situ; that is, the dust particles become suspended in the air as they are produced.
However, in most cases, explosible dust clouds are generated by reentrainment and
redispersion of powders and dusts produced at an earlier stage and allowed to accumu-
late as layers or heaps. Such accumulation may be either intentional, as collectionof pow-
ders and dusts in silos, hoppers, and bag filters, or unintentional, as deposition of dust
on beams, external surfaces of process equipment, or walls and floors of work and stor-
age rooms.