Page 441 - Dust Explosions in the Process Industries

408 Dust Explosions in the Process Industries

energy for ignition to only 1%, or perhaps even less, of the energy required without addi-
tional series resistance.
In a later investigation, Line, Rhodes, and Gilmer (1959) ignited steady-state wall-free
and wall-confined 25 mm and 50 mm diameter columns of lycopodium spores in air by
electric sparks. Some results for 25 mm columns are shown in Figure 5.14.

100
-
5 80
z
0
5 60
‘3
LL
0
t 40
B
3
20
n
LL
0
10-3 10-2 10 -1 1 10
lAPAClTOR ENERGY 1/2 CV2 [JI
Figure 5.14 Frequency of ignition of a 25 mm diameter stationary column of 80 g/m3of lycopodium
in air, as a function of capacitor energy, showing the effect of wall confinement and additional series
resistance in the discharge circuit (Data from Line et al., 1959).

As can be seen both for wall-free and confined dust columns, the stored capacitor ener-
gies required for producing a given probability of ignition decreased roughly by a factor
of 10 if a series resistance of lo5Q was included in the discharge circuit. Both the order
of magnitude of the decrease and the order of magnitude of the series resistance giving
this maximum decrease agree with the corresponding figures found by Boyle and
Llewellyn for other powders.
Line et al. attributed the dramatic influence of spark discharge time to decreasing the
disturbance of the dust cloud by the blast wave from the spark discharge as the discharge
time increased and the spark energies decreased. In the case of high stored capacitor ener-
gies and short discharge times, using high-speed filming, they were able to observe the
formation of a dust-free zone around the spark before ignition got under way.
Smielkow and Rutkowski (1971) conducted an independent study of the influence of
spark discharge duration on the minimum ignition energy of dust clouds. As in the work
of Line et al. (1959), the spark discharge duration was increased by either adding a very
large series inductance (0.1-1.0 H) or a large series resistance (0.45-0.90 MQ).
Reductions in the minimum ignition energies (1/2CV2)on the order of a factor of 10 was
observed, as by Line et al.
Eckhoff (1970) conducted further studies of the ignition of clouds of lycopodium
spores in air by capacitor sparks of comparatively high energies and short discharge times.
Some results are given in Figure 5.15.
The results in Figure 5.15 are in accordance with those for no wall and no series resist-
ance in Figure 5.14. Even with spark energies of nearly 10 J, the frequency of ignition
is lower than 100%.The most probable reason for this is that there is a “knife edge” com-
petition between the heat transfer from the spark to the surrounding dust, which promotes