Page 449 - Dust Explosions in the Process Industries

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1000
800
600
I 400
7
E
I
>
200
z
w
z
o_
k 100
z
9 80
f 60
I
z
z 40
20
10 I I I I I I I I I
0 5 10
SPARK GAP LENGTH Lmml

Figure 5.21 Influence of electrode gap length on the minimum ignition energy of clouds of four metal
dusts in air at atmospheric pressure. Dust concentration corresponds to equivalence ratio 0.65.
Surfaceholume mean particle size is 40 pm (From Ballal, 1980).

optimum gap length. This is consistent with the general picture for premixed gases, for
which a close correlationbetween quenching distance and minimum ignition energy has
been established.
Norberg, Xu, and Zhang (1988) found that the optimum spark gap length for igniting
clouds in air of various easily ignitable powders was in the range 6-8 mm. The capaci-
tive sparks were of the short-duration type (low series inductance and resistance). The
minimum ignition energies were in the range 1-6 mJ.

5.3.5
THEORIES OF ELECTRIC SPARK IGNITION OF DUST CLOUDS

Smielkow and Rutkowski (1971) derived a semi-empirical equation for the minimum
electric spark ignition energy of dust clouds. Their experiments disclosed the following
empirical relationship:

(5.29)

where Eminis the minimum ignition energy (mJ) and S’ is the spatial laminar flame front
speed (cds) of the dust cloud in question, and A is a constant.
The semi-empirical equation was obtained by inserting a Mallard-le Chatelier-type
expression for S’(see Section 4.2.1 in Chapter 4) into equation (5.29).