Page 77 - Dust Explosions in the Process Industries

50 Dust Explosions in the Process Industries

devolatilization of polyethylene in the region of the minimum explosible concentration.
In the case of coal, only the volatiles contribute significantly to flame propagationin this
concentration range. A more detailed discussion of these aspects is given in Chapter 4.

1.3.9
COMBUSTIBLE GAS ORVAPOR MIXED WITH A DUST CLOUD
("HYBRID" MIXTURES)

It is not clear who was the first researcher to study the influence of comparatively small
amounts of combustible gas or vapor on the ignitability and explosibility of dust
clouds. However, more than a century ago, Engler (1885) conducted experiments in a
wooden explosion box of 0.25 m2cross section, 0.5 m height, and essentially open at
the bottom. The box was filled with a mixture of air and marsh gas (methane) of the
desired concentration, and a cloud of fine charcoal dust, which was unable to produce
dust explosions in pure air, was introduced at the container top by a vibratory feeder.
Engler made the interesting observationthat methane concentrationsas low as 2.5 vol%
made clouds of the charcoal dust explosible, whereas the methane and air alone, with-
out the dust, did not burn unless the gas content was raised to 5.5-6 ~01%.One gen-
eration later, Engler (1907) described a simple laboratory-scale experiment by which
the hybrid effect could be demonstrated. The original sketch of the apparatus is repro-
duced in Figure 1.57.

Figure 1.57 Apparatus for demonstrating the hybrid
interaction of combustible dust andgas: A is a glass explo-
sion vesselof volume 250-500 cm3,B is a glass dust reser-
voir connected to A via a flexible hose, b is the inlet tube
for the dispersing air, and a is the gap for the spark igni-
tion source (From Engler, 1907).

The experimental procedure was first to raise reservoir B to allow an appropriate
quantity of dust (unable to propagate a flame in pure air) to drop into vessel A. A con-
tinuous train of strong inductive sparks was then passed across the spark gap a, while a
short blast of air was injected via b by pressing a rubber bulb, to generate a dust cloud
in the region of the spark gap. With only air as the gaseousphase, no ignition took place.