Page 457 - Dust Explosions in the Process Industries

424 Dust Explosions in the Process Industries

10 20 50 100 200 500 1000 BAM
AREA OF HOT SURFACE [rnm21

Figure 5.25 Influence of the area of hot surface on the minimum ignition temperature of clouds of
natural organic dust in air compared with results from BAM furnace tests (hot surface area is approx-
imately 2000 mm2) (From Pinkwasser, 1989).

5.5.2
THEORIES FOR PREDICTING THE MINIMUM IGNITION
TEMPERATURES OF DUST CLOUDS

In their theoretical investigation, Mitsui and Tanaka (1973) focused on the influence of
particle size on the minimum ignition temperature. They considered a spherical dust
cloud, inside which heat was generated by combustion and from which heat was lost
due to convection and radiation. They then assumed a combustionrate with an Arrhenius-
type exponential temperature dependence and proportional to the total particle surface
area in the spherical dust cloud. The critical ignition condition was specified as the rate
of heat generation due to chemical reaction being equal to the rate of heat loss. The result-
ing equation seemed to predict an influence of particle size in good agreement with
experimental results when using a tuning constant depending on the dust chemistry.
A similar study, focusing particularly on the geometry of the Godbert-Greenwaldfur-
nace (see Chapter 7), was undertaken by Takigawa and Yoshizaki (1982). They inves-
tigated natural organic dusts and found a reasonably good agreementbetween measured
and numerically predicted dependence of minimum ignition temperature on particle size.
The numerical model calculations further revealed that the residence time of the dust
particles in the furnace largely affects the ignition process. It was concluded that the
steady-state solution of the minimum ignition temperature is not applicable to the igni-
tion process in the Godbert-Greenwald furnace. Comparison of model predictions with
experimentaldata from other workers confirmed the validity of the predicted effect of the
residence time of the dust particles in the fmace on the minimum ignition temperature.
Nomura and Callott (1986) modified the Cassel-Liebman theory to make it account
for the influence of the residence time of the dust particles in the hot furnace. The theory