Page 82 - Dust Explosions in the Process Industries

Dust Explosions: An Overview 55

ignition of hybrid mixtures of coal dust/methane/air, in the methane concentration range
0-5 ~01%.
Reeh (1978) conducted a comprehensive investigation of the influence of methane in
the air on the violence of coal dust explosions. He concluded that the influence was
strongest in the initial phase of the explosion. In the fully developed, large-scale, high-
turbulence explosion, it made little difference whether gas or coal dust was the fuel.
Further illustrations of the influence of combustible gas or vapor in the air on the explo-
sion violence are given in Figures 1.65 from Bartknecht (1978) and 1.66 from Dahn
(1986). Dahn studied the influence of small fractions of xylene, toluene, and hexane in
the air, on the maximum rate of pressure rise of explosions of a combustible waste dust
in a 20 liter closed bomb. The waste dust originated from shredded materials, including
paper and plastics. Its moisture content was 20% and the particle size <74 pm. Results
for maize starch of4-5% moisture content in hexane and air are also shown in Figure 1.66.

P Figure 1.65 The influence of small fractions of
methane in the air on maximum explosion pres-
sure and maximum rate of pressure rise in a 1 m3
0 250 500 750 1000
closed vessel, with a 10 k] pyrotechnical igniter
PVC DUST CONCENTRATION [g/rn31 (From Bartknecht, 1978).
Torrent and Fuchs (1989) found that both maximum explosion pressure and maximum
rate of pressure rise of a dry coal dust of 38% volatiles and 10% ash in a closed 20 liter
vessel, increased by 30% when 3 vol% methane was added to the air.There was a signifi-
cant decrease of the dust concentrationsthat gave the most violent explosions,with increas-
ing methane content, from 600-700 g/m3without methane to about 300 g/m3with 3 vol%
methane.This agrees with the trend found by Foniok (1985) for the minimumignitionenergy.

1.3.10
ANERTlNG BY MIXING INERT DUST WITH COMBUSTIBLE DUST

This principle of inerting the dust cloud is of little practical interest apart from in mining.
In coal mines, stone dust has been used extensively for this purpose for a long time.
Comprehensiveinformationconcerningthat specificproblem was provided by Cybulski
(1975). Michelis (1984) indicated that satisfactory protection against propagation of coal
dust explosions in mine galleries cannot be obtained unless the total content of com-
bustible material in the mixture of coal dust and limestone is less than 20 wt%. This is