Page 66 - Dust Explosions in the Process Industries

Dust Explosions: An Overview 39

w
E
2 lo3 -- L 'A '?.
u-
m "TEA FLOUR
w
z
W
z h. \ O\O.
0 102 -
k
5
r
3 10 -- LYCOPODIUM
E
z Figure 1.40 The influence of initial turbulence
on the minimum electric spark ignition of dust
1- clouds. Experiments with various dusts in a 20
liter spherical explosion bomb (From Clarner,
1984).
Chapter 5 gives some further information. Further analysis of the role of turbulence on
propagation of dust flames is given in Section 4.4 in Chapter 4 and Section 9.2.4.4 in
Chapter 9.

1.3.6
OXYGEN CONTENT OF OXIDIZER GAS

As one would intuitively expect, both the explosion violence and ignition sensitivity of
dust clouds decrease with decreasing oxygen content of the gas in which the dust is sus-
pended. Wiemann (1984) investigated the influence of the oxygen content of the gas (air+
nitrogen) on the maximum pressure and maximum rate of pressure rise of coal dust
explosions in a 1m3closed vessel. The results, illustrated in Figure 1.41, show that both
the explosion pressure and the rate of pressure rise decreased with decreasing oxygen
content. Furthermore, the explosible dust concentration range was narrowed, in partic-
ular on the fuel-rich side. It is worth noting that a reduction of the oxygen content from
that of airto 115%caused a reduction of the maximum rate of pressure rise by a factor of
10 or more, whereas the maximum pressure was reduced by less than a factor of 2. This
illustrates the strong influence of the oxygen content on the kinetics of the combustion
process. The reduction of the maximum pressure is approximately proportional to the
reduction of the oxygen content, as would be expected from thermodynamic considerations.
Figure 1.42 shows some earlier results from the work of Hartmann (1948). The trend
is similar to that of Wiemann's results in Figure 1.41. The maximum explosion pressure
is approximately proportional to the oxygen content down to 16-17%, whereas the
maximum rate of pressure rise falls much more sharply. For example, at 15% oxygen
tie., 71% of that in air), (dP/dt),, is only 13% of the value in air.
The influence of the oxygen content in the oxidizing gas on the minimum explosi-
ble dust concentration was studied in detail by Hertzberg and Cashdollar (1987).
Some iresults for a high-volatile-content coal dust are shown in Figure 1.43. For par-
ticles smaller than about 10pm, a reduction of the oxygen content from that of air to
15.5% caused only a moderate increase, from 130g/m3 to 160 g/m3, of the minimum