Page 34 - Dust Explosions in the Process Industries

Dusr Explosions: An Overview 7

Here P is the pressure of the gas, Tits temperature, Vthe volume in question,n the number
of gas molecules in this volume, and R the universal gas constant. For constant volume,
P is proportionalto T and n. Normally, the increase of T due to the heat developed in the
burning dust cloud has the deciding influence on P, whereas the change in n plays only
a minor role.
Combustion of metal dust can cause the maximum possible relative reduction of n,
by consuming all the oxygen in the formation of condensed metal oxides. If the gas is
ab-and all the oxygen is consumed and all the nitrogen is left, n is reduced by about 20%.
In the case of organic dust and coal, assuming that CO, (gas) and H,O (gas) are the
reaction products, the number of gas molecules per unit mass of dust cloud increases
somewhat during combustion.This is because two H20molecules are generated per 0,
molecule consumed.Furthermore, in organic substancescontaining oxygen, some H,O
and CO, are generated by decompositionof the solid material itself, without a supply of
oxygen from the air.
Consider as an example a starch of composition (C6H1005)*suspended in air at the dust
concentration that just consumes all the oxygen in the air to be completely transformed
to GO2and H,O (= stoichiometric concentration); 1 m3of air at normal ambient condi-
tions contains about 8.7 moles of 0,and 32.9 moles of N2. When the starch is oxidized,
all the 0, is spent on transforming the carbon to CO,, whereas the hydrogen and the
oxygen in the starch are in just the right proportions to form H20by themselves. The
8.7 moles of 0, is then capable of oxidizing 8.7/6 = 1.45 moles of (C6HI0Oj),that is,
ablout 235 g, which is the stoichiometric dust mass per m3 of air at normal conditions.
The reaction products then are 8.7 moles of CO, and 7.3 moles of H,O. The total number
of 41.6 moles of gas in the original 1 m3 of dust cloud has therefore been transformed
to 48.9 moles, that is, an increase by 17.5%. In an explosion, this contributes to increas-
ing the adiabatic constant-volumeexplosion pressure correspondingly.
13must be emphasized,however, that this formal considerationis not fully valid if the
combustion of the organic particles also results in the formation of CO and char parti-
cles. This is discussed in greater detail in Chapter 4.

1.I .3
EXPLQSIBLE RANGE OF DUST CONCENTRATIONS-
RJMARY AND SECONDARY EXPLOSIONS

Thle explosive combustion of dust clouds, illustrated in Figure l.l(c), cannot take place
unless the dust concentration (i.e., the mass of dust per unit volume of dust clouds) is
within certain limits. This is analogous to combustion of homogeneous mixtures of
gas,eousfuels and air, for which the upper and lower flammability limits are well estab-
lished. Figure 1.3 shows the explosiblerange for a typical natural organic material, such
as corn starch, in air at normal temperature and atmosphericpressure.
The explosible range is quite narrow, extending over less than two orders of magni-
tude, from 50-100 g/m3on the lean side to 2-3 kg/m3on the rich one. As discussed in
greater detail in Chapter 4, the explosibility limits differ somewhat for the various dust
materials. For example, zinc powder has a minimum explosible concentration in air of
about 500 g/m3.Explosible dust clouds have a high optical density, even at the lower