Page 33 - Dust Explosions in the Process Industries

6 Dust Explosions in the Process Industries

In some special cases, metal dusts can also react exothermallywith nitrogen or carbon
dioxide, but most often oxidationby oxygen is the heat-generatingprocess in a dust explo-
sion. This means that only materials not already stableoxides can give rise to dust explo-
sions. This excludes substances such as silicates, sulphates, nitrates, carbonates, and
phosphates and therefore dust clouds of Portland cement, sand, limestone, and the like
cannot produce dust explosions.
The materials that can cause dust explosions include
Natural organic materials (grain, linen, sugar, etc.).
Synthetic organic materials (plastics, organic pigments, pesticides, etc.).
Coal and peat.
Metals (aluminum, magnesium, zinc, iron, etc.).

The heat of combustion of the material is an important parameter,because it determines
the amount of heat that can be liberatedin the explosion. However, when comparing the
various materials in terms of their potential hazard, it is useful to relate the heat of com-
bustion to the amount of oxygen consumed.This is because the gas in a given volume of
dust cloud contains a limited amount of oxygen, which determines how much heat can
be released in an explosionper unit volume of dust cloud. Table 1.1lists the heat of com-
bustion of various substances,per mole of oxygen consumed. Ca and Mg top the list, with
A1 closely behind. Si is also fairly high up on the list, with a heat of combustionper mole
of oxygen about twice the value of typical natural and synthetic organic substances and
coals. Table 1.1is in accordance with the experiencethat the temperatures of flames of
dust of metals like A1 and Si are very high compared with those of flames of organic dust
and coal.

Table 1 .I Heats of combustion (oxidation)of various substances per mole O2consumed

Ca CaO I 1270

The equation of state for ideal gases describes the mutual interdependenceof the var-
ious parameters influencing the explosion pressure:

p=- TnR (1.7)
V