Page 59 - Dust Explosions in the Process Industries

32 Dust Explosions in the Process Industries

Figure 1.29 Influence of mean particle diame-
te; on minimum explosibleconcentrationfor three
0 20 40 60 80 100 120 140 different dusts in a 20 liter USBM vessel (From
MEAN PARTICLE DIAMETER [pml Hertzberg and Cashdollar, 1987).

(1987) interpreted the data in Figure 1.29 in terms of the existence of a critical particle
size, above which the devolatilization process becomes the critical factor in the flame
propagation process. Below this size, devolatilization is so fast that the combustion is
controlled by gas mixing and gas combustion only. Note that the limiting particle size
at the minimum explosible dust concentration is not necessarily the same as at higher
concentrations, where the explosions are more violent.
Figure 1.30 shows how particle size influencesthe minimum ignition energy for three
differentdusts. The vertical scale is logarithmic,and it is seen that the effect is very strong.
Kalkert and Schecker (1979) developed a theory indicating that the MIE is proportional
to the cube of the particle diameter, as illustrated in Figure 1.30by their theoretical pre-
diction of the relationship for polyethylene.
Investigations at the Chr. Michelsen Institute (CMI) showed that a 50-150 pm frac-
tion of atomized aluminum powder could not be ignited as a cloud in air, even with a
welding torch. This contradicts somewhat with the data in Figure 1.30.The discrepancy
could be due to the presence of a fine particle size fraction in the powders used by
Bartknecht (1978). This emphasizesthe need for consideringthe entire size distribution
rather than just a mean size.
Figure 1.31 gives some independent experimentalresults for MIE as a function of par-
ticle size for methyl cellulose, confirming the trends in Figure 1.30.

1.3.3
DEGREE OF DUST DISPERSION EFFECTIVE PARTICLE SIZE

In his experimental studies of burning times of pulverized fuels, Bryant (1973) found
that persistent agglomeration was the reason for comparatively long burning times for
apparently small particles. The situation is illustrated in Figure 1.32.A stable agglom-
erate behaves as a large single particle of the size of the agglomerate.