Page 421 - Dust Explosions in the Process Industries

388 Dust Explosions in the Process Industries

the upper stable flame propagation branch. On cooling, that is, increasing U, n, or both
in equation (5.lo), the rate of reaction is reduced. However, the reaction continues right
down to (5) in Figure 5.2, from which the system temperature drops to a stable condi-
tion in the extinguished regime.
The scheme illustrated in Figures 5.1 and 5.2 is quite general and applicable to dif-
ferent types of systems. More extensive treatments of the general ignitiodcombustion-
stability theory were given, for example,by Gray and Lee (1967), Gray and Sherrington
(1977), and Bowes (1981). The classical basis for this type of analysis was established
by Semenov(1959) and Frank-Kamenetzkii (1969). The book by Bowes (1984) provides
a unique, comprehensiveoverview of the field of self-heating and ignition, not the least
in solid materials, including dust layers and heaps.
Although the basic considerationsimplied in Figures 5.1 and 5.2 to some extent pro-
vide a satisfactory general definition of ignition, the precise theoretical definition has
remained a topic of scientific discussion. One example is the dialoguebetween Lermant
and Yip (1984, 1986) and Essenhigh (1986).

5.2
SELF-HEATINGAND SELF-IGNITION
IN POWDER DEPOSITS

5.2.1
OVERVIEWS

Bowes (1984) reported the state of the art of experimentalevidence and theory up to the
beginning of the 1980s.Considerableinformationwas available,and theory for predicting
self-heatingproperties of powders and dustsunder various conditions of storage had been
developed.
There were nevertheless some gaps in the quantitativeknowledge, one of which is bio-
logical heating. In vegetable and animal materials such as feedstuffs and natural fibers,
self-heating may be initiated by biological activity, in particular if the volume of mate-
rial is large, its moisture content high, and the period of storage long. However, because
the microorganisms responsible for the biological activity cannot surviveat temperatures
above about 75"C,biological heating terminates at this temperature level. Further heat-
ing to ignition, therefore, must be due to nonbiologicalexothermic oxidation, for which
theory exists. It is possible, however, that the long-term biological activity in a real
industrial situation may generate chemically different starting conditions for further
self-heatingthan the conditions establishedin laboratory test samples heated artificially
to 75°C by heat from the outside. Further research seems required in this area.
Starting with the extensive account by Bowes (1984), Beever (1988) highlighted the
theoretical developments that she considered most useful for assessing the self-heating
and ignition hazards in industrial situations. In spite of many simplifying assumptions,
the models available appeared to agree well with experimental evidence. However,
extrapolating over orders of magnitude, from laboratory scale data to industrial scale,
was not recommended. Biologicalactivity was not involved in the self-heatingprocesses
considered. See also Sections 9.2.3.3 and 9.3.5.2 in Chapter 9 for further references.