Page 434 - Dust Explosions in the Process Industries

Ignition of Dust Clouds and Dust Deposits 40 7

This model seems to address the case of comparatively high hot-surface temperatures
and thin dust layers. Self-ignition in comparatively thick dust layers resting on hot sur-
faces of quite low temperatures often occurs inside the layer rather than at the hot surface.
Beever (1984) applied the classical self-ignition theory to a dust deposit exposed to a
hot surface at constantheat flux boundary conditions.She adoptedthe step-functionapprox-
imation devisedby Zatwska (1978) and found good agreementbetween values of the crit-
ical Frank-Kamenetzkiiparameter for ignition calculated by the approximate theory and
values obtained analyticallyby Bowes, for self-heatingin a plane dust slab with constant
heat flux on one face. As shown in Section 5.2.2.3, Beever also found good agreement
between the predicted minimum heat flux for ignition and experimental results for cylin-
drical dust deposits heated by an internal concentric cylindrical constant flux heat source.
Leisch et al. (1984) were interested primarily in the propagationof a one-dimensional
smoldering combustionwave in a dust layer. They obtained a numerical solution of the
conservation equations for this process in good agreement with experimental results
(see Section 5.2.2.4). The theoretical model also gave temperature and density profiles
within the combustionwave similar to those observedexperimentally. Further works on
modeling are reviewed in Sections 9.2.3.3 and 9.3.5.2 in Chapter 9.

5.2.4
APPLICATIOhIS TO DIFFERENT POWDEWDUST TYPES:
A BRIEF LITERATURE SURVEY

5.2.4.1
Coal Dust

Elder et al. (1945) studied the relative self-heating tendencies of 46 different coal sam-
ples of particle sizes finer than 6 mm, using an adiabatic calorimeter and a rate-of-
oxygen-consumptionmeter. They found that

0 The self-heating tendency increased with decreasing coal rank.
0 The self-heatingtendency increased with storage temperature.
0 The self-heatingtendency decreased with increasing preoxidation of the coal prior to
the test.
0 The rate of heat generation due to oxidation was proportional to the vol% oxygen in
the air in contact with the coal, raised to the power of 2/3.
0 The rate of heat generation due to oxidation was proportional to the cube root of the
specific surface area of the coal.
0 Increasing the ash content in the coal decreased the self-heatingtendency.
0 An appreciable moisture content in the coal decreased the self-heatingtendency.
Guney and Hodges (1969) reviewed the various experimentalmethods used up to that
time for determining the relative self-heating tendencies of coals. They concluded that
only isothermal and adiabatic methods would give consistent results. Shea and Hsu
(1972)used an adiabaticmethod for studying self-heating of various dried coals and petro-
leum cokes at 70°C in atmospheresof oxygen or nitrogen saturated with water vapor or
in dry oxygen. In a completely dry system, there was no appreciable self-heating,even