Page 282 - Dust Explosions in the Process Industries

apter 4

Propagation of Flames in Dust Clouds

4.1
IGNITION AND COMBUSTION OF SINGLE PARTICLES

4.1 .I
ALUMINUM

Friedman and Macek (1962, 1963) studiedthe ignition and combustion of aluminumpar-
ticles in hot gases of varying oxygen content. They concluded that ignition occurred only
after melting of the oxide layer (melting point 2300 K) that coats the particle. The
process of ignition did not appear to be affected by the moisture content of the hot ambi-
ent gas and was only slightly influenced by the oxygen content. At an oxygen content
of only 2-3 mole percent (mol%), ignition occurred at 2300 K, whereas at 35 mol%
oxygen,it occurred at 2200 K. On the other hand, the concentrationsof oxygen and water
vapor had significant influence on the combustion of the metal. Oxygen promoted vig-
orous combustion and, if its concentration was sufficiently high, fragmentation of par-
ticles. In the absence of moisture, diffusion and combustiontook place freely in the gas
phase, whereas in the presence of moisture, the process was impeded and confined to a
small region, because the reactants had to diffuse through a condensed oxide layer on
the surface of the molten particle.
Cassel(1964) injected single 60 pm diameter aluminumparticles into the center of a
laminar aluminum dust flame of known spatial temperature distribution.Ignition of the
particles occurred at 2570 K, but this was probably higher than the minimum tempera-
ture required for ignition, because the residence time of the particle in the hot environ-
ment was no more than 2 ms. This is shorter than the induction period required for
self-heatingof the particle from its minimum ignition temperatureto the minimum tem-
perature for self-sustainedoxidation.
Cassel further observed that, within 2 rns after ignition, a concentricburning zone, of
diameter about nine times the original particle diameter, developed around the particle.
After 3 ms, a detached envelope appeared, which at first surrounded the particle con-
centrically but then became elongated and gradually developedinto a cylinder of length
more than 10times its diameter.This expanding oxide envelope, being in the liquid state,
followed the relative motion of the ambient atmosphere.
Burning times of 60 pm aluminumparticles located between the lobes of the aluminum-
dust flame were found to be on the order of 10.5ms (about 4.5 times longer than for mag-
nesium particles burning under the same conditions).Cassel attributedthis to the greater
oxygen requirement for the oxidation of aluminum.
Prentice (1970) studied the ignition and combustion of single 300-500 pm aluminum
particles in dry air, following initial heating and melting by a light flash from either a
pulsed Nd-glass laser or a xenon-flash discharge lamp. In air (as opposed to in Ar/02),