Page 295 - Dust Explosions in the Process Industries

264 Dust Explosions in the Process Industries

and a gas temperature of 1170 K, devolatilization and combustion of volatiles is com-
pleted within about 0.5 s, whereas the burning-off time of the char increases markedly
with decreasing content of volatiles.
Levendis,Flagan, and Gavals (1989) studied mechanisms and rates of oxidationof char
particles in the size range from a few pm to several tens of pm. The specific surface area
of the char particles varied with the origin of the char (polymers with pore-forming
additives). When heated in an inert atmosphere,the char particles maintained their amor-
phous nature up to 1600K. However, when oxidized at 1600K, the carbon matrix under-
went partial graphitization.
Vareide and Sonju (1987) developed approximate computer models for predicting
burn-off of char particles. Two alternative assumptions concerning the particle size
and density were adopted, constant density/decreasing diameter and constant
diameteddecreasing density. The total burn-off time decreased with initial particle
diameter.In the shrinking particle model, the total bum-off time at 15vol% O2and 1500K
was about 1 s for a 100 ,um particle and 0.1 s for a 10 pm particle. The corresponding
burn-off times predicted by the constant particle-diameter model were about 0.3 s and
0.04 s.
Essenhigh, Misra, and Shaw (1989) provided a comprehensive survey of the status on
coal particle ignition in the light of the historical development over the previous two
decades. The possibility of extending the single-particleresults to dust clouds was exam-
ined. Theories are available,but experimental verification is incomplete.The boundary
between conditions that produce heterogeneous ignition and those producing homoge-
neous ignition is not fully identified.

4.1.5
WOOD

Malte and Dorri (1981) developed a complete theory for the life of a single wood parti-
cle, of diameter from 100 pm upward, in a wood waste furnace of the grate type. The
particle was followed from the moment of injection via drying and pyrolysis to completion
of combustion.A main objective was to study the extent to which small particles were
entrained by the upward airflow before combustion was completed.
Equation (3.16) in Chapter 3 was used to calculate the gravitational terminal settling
velocity v,of the particle. The drag coefficient C, was determinedexperimentallyfor var-
ious particle sizes and shapes. One problem is that v, depends on particle drying and
devolatilization,because these processes reduce the particle density.
The homogeneous particle temperature was calculated by integrating the following
equations, (4.11)-(4.15). The drying process was described by

e dT dM

dt
dt
m,, = (C+ MC,)--hv- (4.11)
(4.12)