Page 349 - Dust Explosions in the Process Industries

3 18 Dust Explosions in the Process Industries

Reliable experimental data for metal dusts are scarce. However, Schlapfer (1951)
found a value of 90 g/m3for fine aluminum flakes, which indicates that both equations
underestimate the minimum explosible concentration considerably, equation (4.72) by
a factor of nearly 4 and (4.79) by a factor of nearly 2. A main reason for this is proba-
bly the use of the ignition temperature Tias a key parameter.
Mitsui and Tanaka (1973) derived a theory for the minimum explosible concentration
using the same basic discrete microscopic approach as adopted later by Nomura and
Tanaka (1978) to model laminar flame propagation in dust clouds, and discussed in
Section 4.2.4.4. Working with spherical flame propagation, they defined the minimum
explosible dust concentration in terms of the time needed from the moment of ignition
of one particle shellto the moment when the air surroundingthe particles in the next shell
has been heated to the ignition temperature of the particles. If this time exceeds the total
burning time of a particle, the next shell never reaches the ignition temperature.Because
this heat transfer time increases with the mean interparticle distance, it increases with
decreasing dust concentration. By using some empirical constants, the theory repro-
duced the trend of experimental data for the increase of the minimum explosible dust
concentration of some synthetic organic materials with mean particle size in the coarse
size range from 100-500 pm particle diameter.
Nomura, Torimoto, and Tanaka (1984) used a similar discrete theoretical approach to
predict the maximum explosible dust concentration. They defined this upper limit as the
dust concentration that just consumed all available oxygen during combustion, assuming
that a finite limited quantity of oxygen, much less than required for complete combustion,
was allocated for partial combustionof each particle.Assuming that oxygen diffusionwas
the rate-controlling factor, they calculated the total burning time of a particle in terms of
the time taken for all the oxygen allocated to the particle to diffuse to the particle surface.
For the flame to be transmitted to the next particle shell, the particle burning time has to
exceed the heat transfer time for heating the gas surrounding the next particle shell to the
ignition temperature. Equating these two times defines the maximum explosible dust con-
centration.Two calculatedvalues were given, 1400g/m3for terephthalic acid of 40 pm par-
ticle diameter and 4300 g/m3 for aluminum of 30 pm particle diameter. The ignition
temperatures for the two particle types were taken as 950 K and 1000 K, respectively.
Bradley et al. (1989) proposed a chemical kinetic theoretical model for propagation
of flames of fine coal dust near the minimum explosible dust concentration. It was
assumed that the combustion occurred in premixed volatiles (essentially methane) and
oxidizing gas, the char particles being essentially chemically passive. The predicted
minimum explosive concentrations were in good agreement with experimental values
(about 100 g/m3for 40% volatile coal, and 500 g/m3for 10-15% volatiles).

4.3
NONLAMINAR DUST FLAME PROPAGATION
PHENOMENA IN VERTICAL DUCTS

This section treats some transitional phenomena observed under conditions where lam-
inar flames could be expected. This does not include fully turbulent combustion, which
is discussed in Section 4.4.