Page 294 - Dust Explosions in the Process Industries

Propagation of Flames in Dust Clouds 263

radius increased with increasing volatile content. This trend was interpreted in terms of
the volatiles burning more rapidly than the char, in agreement with the general under-
standing of the combustion of coal particles.
In a further series of experiments,Gieras et al. (1985) studiedthe propagationof com-
bustion through static linear chains of consecutive coal particles separated by a given
optimal center-to-center distance Doptdepending on the volatile content. It was con-
firmed that the velocity of the “one-dimensional”flame propagation increased (approx-
imately proportionally) with the volatile content of the coal.
When similarinterparticleflame transfer experiments were conducted at normal grav-
ity conditions, buoyancy played an important role (Gieras et al., 1986).The maximum
interparticledistance for upward flame transfer was then significantlylarger than for hor-
izontal transfer. This has important implications in dust explosions, such as in the defi-
nition of the concept of minimum explosibledust concentration.Under gravity conditions,
the limiting dust concentration for flame propagation depends on whether the propaga-
tion occurs upward, downward, or horizontally (see Section 4.2.6.2).
Wagner et al. (1987) studied the ignition and combustion of single coke and coal par-
ticles of diameters 63-125 pm in a vertical reactor containing hot oxidizing gas, through
which he particles settledfor predeterminedperiods (distances)before being captured and
cooled rapidly. The initial volatile content for the materials investigated varied from 4.5%
to 37%. The experimental data were compared with predictions by a numerical computer
model, based on the earlierwork by Field (1969) and Smith (1971).The model also treated
the devolatilization process, by considering it as a single-stage reaction of activation
energy 228.5 kJhnole. The combustion was considered controlled partly chemically and
partly by diffusionprocesses.Both convectiveand radiative heat transfer were considered.
Figure 4.6 shows a set of experimentalresults for particles burning in airat atmospheric
pressure and the corresponding predictions by the computer model. For all three coals

100

80

-
E 60
U
U
4
W
z
z
5 40
m
Figure 4.6 Burning-offof 63-125 p coalparticles
of various volatile contents as functions of residence
20
time in hot gas (1 170 K) in a vertical reactor:
0 = 37.1% volatiles
= 20.7% volatiles
0 x = 7.3 % volatiles
0 0.5 1.0 1.5 - = computer model predictions
RESIDENCE TIME [SI (From Wagner et al., 1987).