Page 299 - Dust Explosions in the Process Industries

268 Dust Explosions in the Process Industries

4.2.2
DIFFERENCES BETWEEN FLAMES IN PREMIXED GAS
AND IN DUST CLOUDS

Leuschke (1965) pointed out some characteristicdifferences between a laminar, premixed
gas flame and a laminar dust flame. One important difference is that the reaction zone
in the dust cloud is considerably thicker than in the gas, irrespectiveof the type of dust,
and on the order of at least 10-100 mm. When discussingthis feature of the dust flame,
Cassel(l964) distinguished between two types of flames. The first, the Nusselt type, is
controlledby diffusion of oxygen to the surface of individual, solid particles, where the
heterogeneous chemical reaction takes place. In the second type, the volatile flame, the
rate of gasification,pyrolysis, or devolatilizationis the controlling process and the chem-
ical reaction takes place mainly in the homogeneous gas phase. In Nusselt-type flames,
the greater thickness of the combustionzone, compared with that of premixed gas flames,
results from the slower rate of molecular diffusion, compared to diffusion in premixed,
homogeneous gases. In the volatile flame type, the greater flame thickness is due to the
preheating zone, where volatiles or pyrolysis gases are driven out of the particles ahead
of the flame. When mixed with air, these gases and vapors burn almost as a premixed
gas. The combustion of the remaining solid char particles occurs subsequently at a
slower rate in the tail of the flame, and therefore the volatile flame in clouds of coals and
organic dusts is also in fact coupled to a Nusselt-type flame.
In metals, low melting-pointmaterials may oxidize in the vapor phase, but due to the
oxide film around each particle, this does not result in a homogeneous metal vapor/air
flame. Because of the large heat of combustionper mole of O2for example, of aluminum
and magnesium dust compared with organic dusts, the temperature of the burning par-
ticles is very high and thermal radiation plays a central role in the transfer of heat in the
combustion wave. Radiative heat transfer is also supposed to play a role in coal dust
flames. However, because the thermal radiation is proportional to the fourth power of
the temperature, the role of thermal radiation in coal dust flames is less important than
in, for example, aluminum and magnesium dust flames. Radiative heat transfer in dust
flames is a complex process, and it is of interest to note that Elsner, Koneke, and
Weinspach (1988) investigated the solid particle emissivity in dust clouds as a function
of dust cloud thickness, specific surface area of the particles, dust concentration, and
absorption and scatter coefficients. Experiments were conducted with fluidized bed ash
and quartz sand. Good agreement was found between the experiments and a theoretical
equation.
Leuschke (1965) conducted an illustrative series of experiments demonstrating the
importance of radiative heat transfer in metal dust flames, using the experimental setup
illustrated in Figure 4.8. Two transient dust clouds were generated simultaneouslyon the
two sides of a double-glass window, one being ignited immediately by a gas flame. It
was then observed whether the radiation from the burning cloud was able to ignite the
other cloud.
Table 4.2, summarizing the results, shows that only the flames of Zr, Ti, AI, and Mg
produced sufficient radiation to ignite the other cloud. Ignition of graphite was not
accomplishedat all, in agreement with the inability of graphite dust clouds to propagate
a self-sustained flame in air at normal temperature and pressure. The reason why the gas