Page 78 - Dust Explosions in the Process Industries

Dust Explosions:An Overview 5 7

The entire vessel A was then replaced by another one of the same size and shape but filled
with a mixture of air and the desired quantity of combustible gas, and the experiment
was repeated. Engler advised the experimenter to protect himself against the flying frag-
ments of glass that could result in the case of a strong hybrid explosion.
Adding small percentages of combustible gas to the air influences the minimum
explosible dust concentration, depending on the type of dust. This is illustrated by the
data of Foniok (1985) for coals of various volatile contents, shown in Figure 1.58. The
effect is particularly pronounced for dusts that have low ignition sensitivity and low com-
bustion rate in pure air. A similar relationship for another combination of dust and gas
is shown in Figure 1.59.

80

60

40

20
NONEXPLOSIBLE

0 1 2 3 L 5
0 1 2 3 4 5
METHANE CONTENT IN THE AIR [vo~YoI
HYDROGEN CONTENT IN THE AIR Ivo~YoI
Figure 1.58 The influence of methane con-
tent in the air on the minimum explosible Figure 1.59 The influence of small percent-
concentration of coal dusts of different ages of hydrogen in the air on the minimum
volatile contents. Average particle size 40 pm explosible concentration of maize starch at
with 700% <71 pm, 4.5 kJ ignition energy normal ambient conditions (From Hertzberg
(From Foniok, 1985). and Cashdollar, 1987).

Nindelt, Lukas, and Junghans (198 1) investigated the limiting concentrations for
flame propagation in various hybrid mixtures of dusts and combustible gases in air. The
dusts and combustible gases were typical of those represented in the flue gases from coal
powder plants.
Reeh (1979) determined the critical minimum contents of volatiles in coals and
methane in the air for self-sustained flame propagation in clouds of coal dust in a 200 m
experimental mine gallery. With no methane in the air, flame propagation was possible