Page 67 - Dust Explosions in the Process Industries

40 Dust Explosions in the Process Industries

*/

0 500 1000 1500

100

-
In
2
m
-0
- x 50
a
z
c
-0
\
a
E
0
0 500 1000 1500
OUST CONCENTRATION Ig/m31

Figure 1.41 The influence of oxygen content in the gas on the maximum explosion pressure and
maximum rate ofpressure rise of brown coal dust concentrations. Nitrogen as an inert gas is in a 1 m3
IS0 standard explosion vessel at 750°C and atm pressure (From Wiemann, 1984).

explosible concentration. However, as the particle size increased, the influence of reduc-
ing the oxygen content became pronounced. At a mean particle size of 50 pm, 15.5%
oxygen was sufficiently low to prevent flame propagation. It seems probable that the par-
ticle size fractions used by Hertzberg and Cashdollar (1987) were quite narrow. This can
explain why particles of larger mean diameters than 100 pm did not produce explosions
in air at all, irrespective of dust concentration. In practice, most powders and dusts
involved in dust explosions have comparatively wide particle size distributions, and
characterizing their fineness by only a mean particle size can be misleading in the con-
text of dust explosibility assessment. It would be expected that many coal dusts of mean
particle diameter larger than 100 pm would be explosible in air if they contain a signifi-
cant “tail” of fine particles.