Page 39 - Dust Explosions in the Process Industries

12 Dust fxplosions in the Process Industries

The initial oxidation inside the deposit may sometimes be due to the dust or powder
being deposited having a higher temperaturethan planned. However, some natural veg-
etablematerials may developinitial spontaneouscombustioneven at normal ambienttem-
peratures due to biochemical activity, if the content of fat or moisture is high.
In other cases, the dust deposit or layer rests on a heated surface, which supplies the
heat needed to trigger self-ignition in the dust. Such surfaces can be overheated bear-
ings, heaters in workrooms, lightbulbs, walls in dryers, and the like. If the surface is not
intended to be covered with dust, the dust deposit may prevent normal coolingby forming
an insulating layer. This may give rise to an undesirable temperature rise in the surface,
which further increases the probability of ignition of the dust. In general, the minimum
temperature of the hot surface for the dust layer to self-ignite decreases with increasing
thickness of the dust layer.
Figures 1.8, 1.9, and 1.10 illustrate various ways in which smoldering combustion in
dust deposits can initiate dust explosions. The critical conditions for the generation of
smoldering nests are discussed in Chapter 5, and test methods assessing the self-heating
tendency of various dusts are described in Chapter 7.

DUST DEPOSIT

SMOLDERING
NEST
GROWING
SMOLDERING
NEST

Figure 1.9 Complex ignition sequence via gas
explosion: Due to limited supply of oxygen, the
smoldering nest develops CO and other com-
bustible gases and creates an explosible mixture
above the dust deposit. When the edge of the
smoldering nest penetrates the top surface of
Figure 1.8 A smoldering nest in a dust or the dust deposit, the gas ignites and the gas
powder deposit in a silo can initiate a dust explosion blows up the silo roof, Dust deposits
explosion if the nest is discharged into an in the room above the silo are dispersed and a
explosible dust cloud. major secondary dust explosion results.