Page 55 - Dust Explosions in the Process Industries

28 Dust Explosions in the Process Industries

the rate of heat release as the median particle size increases beyond about 20 pm. Due
to the very slow combustion,the P,, for PVC also drops much faster as the particle size
increases than for polyethylene. The retarding influence of chlorine on the combustion
process most probably is of the same nature as that of the halogens in the halons, which
were extensively used for explosion and fire suppression before the negative influence
of such materials on the global environment was fully realized.
Moisture in the dust reduces both the ignition sensitivity and explosion violence of
dust clouds. Figure 1.19 illustrates the influence of dust moisture on the minimum elec-
tric spark ignition energy. The vertical axis is logarithmic, and it is seen that the effect
is quite significant. If safety measures against electric spark ignition are based on MIE
data for a finite dust moisture content, it is essential that this moisture content is not sub-
sided in practice. The influence of dust moisture on the minimum ignition temperature
of dust clouds is less marked. For example, van Laar and Zeeuwen (1985) reported that
flour of 14% moisture had a minimum ignition temperature of 470°C, whereas dry
flour had 440°C. For starch, the values were 400°C for the dry powder and 460°C with
13% moisture.
Figure 1.20 illustrates how the explosion violence is systematically reduced with
increasing dust moisture content.The ignition delay characterizesthe state of turbulence
of the dust cloud at the moment of ignition in the sense that the turbulence intensity
decreases as the ignition delay increases (see Chapter 4).

200 I

n I I I I I
0 5 10 15 20 25 30
0 5 10 15 20 MOISTURE CONTENT IN STARCH [wt. %I
MOISTURE CONTENT IN DUST [wt %I
Figure 1.20 Influence of moisture content in
Figure 1.19 lnfluence of dust moisture maize starch on maximum rate of pressure rise
content on minimum electric spark igni- in Hartmann bomb for various ignition delays
tion energy for three dusts (From van Laar (time from dust dispersion to ignition) (From
and Zeeuwen, 1985). Eckhoff and Mathisen, 7 977/1978).

The specific role of moisture in reducing both the ignition sensitivity and explosion
violence of clouds of organic dusts is complex. First, evaporation and heating of water
represents an inert heat sink. Second, the water vapor mixes with the pyrolysis gases in