Page 296 - Dust Explosions in the Process Industries

Propagation of Flames in Dust Clouds 265

where
Q is the rate of heat transfer to particle;
C is the specific heat of dry wood;
C, is the specific heat of liquid water;
mD, is the dry mass of wood particle;
T is the homogeneous particle temperature;
h, is the latent heat of vaporization, including differential heat of wetting;
M is the fractional moisture content: mass H,O/dry mass; parameter b (empirical corre-
lation) equals

(4.13)

The pyrolysis process, neglecting particle swelling, was described by
e

-= PC- dT -[C,(T -T,)-41- dP (4.14)
vP dt dt
(4.15)

where
p is the particle density at time t;
pFis the final particle density;
Vpis the particle volume;
C, is the specific heat of volatiles;
Tois the reference temperature;
q is the exothermic heat of pyrolysis at reference temperature;
k is the Arrhenius rate constant equal to A exp (-EIRT).
The value of k varies with temperature, activation energy, and the constant A. A and
E in turn vary with the details of the composition of the wood, the rate of heating, and
so forth. This aspect was investigated in some detail by Malte and Dorri (1981).
Acomputer model was used to simulate trajectories of wood particles of various sizes
and shapes, in the waste furnace. It could be shown that particles of diameters smaller
than 500 pm had a significant tendency to become entrained by the upward air in the fur-
nace and escape ignition and combustion at the hot grate in the furnace bottom.

4.2
UST FLAMES

4.2.1
LAMINAR FLAME PROPAGATION IN PREMIXED, QUIESCENT GASES

The basic concepts of flame propagation in dust clouds are adoptedfrom premixed gas prop-
agation theory. It is appropriate,therefore, to briefly introducesome central aspects of the latter.