Page 458 - Dust Explosions in the Process Industries

Ignition of Dust Clouds and Dust Deposits 425

suggested that it is possible for the ignition temperature of monosized coal particles of
about 50 pm diameter to be minimal even for a limited residence time.
The theory was extended to dust clouds with a distribution of particle sizes. It was
shown that there exists a range of size distributionsfor which the possibility of ignition
is at a maximum. The calculated results were presented as Rosin-Ramanlercharts, indi-
cating the size distributions most sensitive to ignition.
Higuera, Linan, and Trevino (1989) analyzed the heterogeneousignition of a cloud of
sphericalmonodispersecoal particles injected instantaneouslyin the space between two
parallel isothermalwalls. They focused on the range of large gadparticles thermal capac-
ity ratios, for which the temperature difference between the particles and the gas is
important. Radiative heat transfer was accounted for using the Eddington differential
approximation,and heat conductionbetween the particles and the gas was also included
in the model. Heat release was assumed to occur at the surface of the particles through
the heterogeneous reaction C(s) + I/2O2 -+CO, obeying an Arrhenius law with large acti-
vation energy. Critical conditions for ignition were determined on the basis of a quasi-
steady treatment. The effects of the ratio of gas temperature to wall temperature, the
conductiodradiationparameter, and the size of the reacting dust cloud relative to the opti-
cal length was explained.
Tyler (1987) was concerned with the problem of scaling ignition temperatures of
dust clouds from laboratory test apparatus to industrial scale. In particular, he focused
on the Godbert-Greenwaldfurnace (see Chapter 7). As pointed out by Tyler, there seems
to be no single physicalkhemical pattern for ignition of a dust cloud. In substances like
sulfur and polyethylene, the minimum ignition temperatures are high enough to allow
complete evaporationor pyrolysis to form gaseous fuels.At the other extreme are metals
of minimum ignition temperatures,at which neither the metal nor its oxide vaporizes fully.
In the first case, the exothermic oxidationprocess almost certainlytakes place in the gas
phase, whereas in the second it occurs at the surface of or within the particle (see also
Section 4.1 in Chapter4). However, these differencesmay not be importantin the estab-
lishment of the unstable state of ignition that precedes a fully developed flame.
'Tyler developed a Semenov-typemathematical model of the ignition of a dust cloud in
a heated environment (furnace). However, validation of the model was difficult. No reli-
able activation energies were found in the literature that could be definitely attributed to
the heat release reaction that occurs at the ignition temperature, and Tyler pointed out that
the activationenergy could be quite differentfrom that associatedwith a fully fledged flame;
indeed,the dominantmechanism could well be different in the two situations.Nevertheless
useful parametsic studies could be performed. For example, the model predicted compar-
atively large changes of the minimum ignition temperature with furnace diameter. The
Codbert-Greenwaldfurnace has a diameter of 37 111111.For a furnace diameter of 300 mm
and a dust with a Godbert-Greenwaldvalue of 1000 K, the model predicted a minimum
ignition ltemperatureat least 150°Clower than the Godbert-Greenwaldvalue.
owever, few experimental data were traced for the influence of increased furnace
diameter on the minimum ignition temperature except when comparing data from the
new U.S. Bureau of Mines furnace (see Chapter 7) and the Godbert-Greenwaldfurnace.
The ratio of the two furnace diameters is 2.7, and therefore significant differencesin the
minimum ignition temperaturesfrom the two apparatuses are expected. However, the pic-
ture offered by existing data was inconclusive.For some dusts, the experimental Godbert-
Greenwald value was even lower than that from the new furnace.