Page 348 - Dust Explosions in the Process Industries

Propagation of Flames in Dust Clouds 3 77

for other types of fuel. The results for bituminous coal and the metals also reflect this
deficiency.
Bukswwicz and Wolanski (1983) postulated that, at the minimum explosible concen-
tration, flames of organic dusts have the same temperature as lower limit flames of pre-
mixed hydrocarbon gadair. They then proposed the following simple, semi-empirical
correlation between the heat of combustion (calorificvalue) Q (Hkg) of the dust, and the
minimum explosible concentration C1(g/m3)in air at normal pressure and temperature:

Cl = 1.55.1O7 Q-I2l (4.78)
The assumptionsimplied confine the applicability of this equation to the same dusts to
which Zeh’s equations (4.75) and (4.76) apply. For starch, equation (4.78) gives C, =
114g/m3,which is somewhathigher than the value of 70 g/m3found experimentallyby
roust and Veyssiere (1988) but close to that calculated by Zehr for constant pressure.
or polyethylene, equation (4.78) gives 36 g/m3,in close agreement with both experi-
ments and Zehr’s calculations.
Lunn (1988) also investigated this group of materials and obtained further support for
the hypothesis that the minimum explosible concentrationof organic dusts that bum more
OF less completely in the propagating flame is primarily a function of the heat of com-
bustion of the dust.
Shevchuket al. (1979),being concernedprimarily with metal dusts, advocated the view
that a discrete approach,consideringthe behavior and interaction of individualp
is necessary for producing an adequate theory for the minimum explosible dust con-
centration.They analyzed the distributionof a heat wave in a dilute suspension of mono-
sized solid fuel particles in a gas, assumingno relative movement between particles and
gas, no radiative heat transfer, and that the rate of heat production qpduring combustion
of a single particle of mass rnp was constant during the entire burning lifetime tbof the
particle and equal to qp=Qrn&,, where Q is the heat of combustionof the particle mate-
rial. The resulting equation for the minimum explosible dust concentration,assumingthat
the average flame temperatureequals the ignitiontemperature T,of the dust cloud as deter-
mined in a heated-wall furnace, is
Cl = 67i lcgpg LFI)- ‘d<T - TO )I (4.79)

Here, Tois the ambient temperature, cg and cd are the heat capacities of gas and dust
material, pgis the gas density, and F is a special particle distribution factor resulting
from this particular analysis; and the last term causes equation (4.79) to differ from
Jaeckel’s equation (4.72). Using T, data from Jacobson, Cooper, and Nagy (19641,
Shevcfiuk et al. compared equations (4.72) and (4.79), as shown in Table 4.12.
Table 4.12 Minimum explosible concentrations of metal powders in air

Source: Shevchuk et al.. 1979.