Page 86 - Dust Explosions in the Process Industries

Dust Explosions: An Overview 59

e Adjustment of powder and dust properties to acceptable levels by cooling, di-ying,and
the like, whenever required.
e Ensuring that heated solid bodies (e.g., a steel bolt heated and loosened by repeated
impact) do not become embedded in the powder or dust mass.
e Continuous monitoring of temperature in the powder mass at several points by ther-
mometer chains.
@ Monitoring possible development of gaseous decomposition and oxidation products
for early detection of self-heating.
@ Rolling of bulk material from one silo to another, whenever the onset of self-heating
is detected or as a routine after certain periods of storage, depending on the dust type.
@ Inerting of bulk material in silo by suitable inert gas, such as nitrogen.
Thermometer chains in large silos can be unreliable because self-heating and smol-
dering may occur outside the limited regions covered by the thermometers.
Inerting by adding nitrogen or other inert gas may offer an effective solution to the
self-heatingproblem. However, it introduces a risk of personnel being suffocated when
entering areas that have been made inert. In the case of nitrogen inerting, the negative
effects of lack of oxygen in the breathing atmospherebecome significant in humans when
the oxygen content drops to 15 vol% (air 21 ~01%).
If inerting is adopted, it is important to take into account that the maximum permis-
sible oxygen concentration for ensuring inert conditions in the dust deposit may be con-
siderably lower than the maximum concentration for preventing explosions in clouds of
the same dust. Walther (1989) conducted a comparative study with three different dusts,
using a 20 liter closed spherical bomb for the dust cloud experiments and a Grewer fur-
nace (see Chapter 7) for the experiments with dust deposits. In the case of the dust
clouds, oxidizability was quantifiedin terms of the maximum explosionpressure at con-
stant volume, whereas for the dust deposits, it was expressed in terms of the maximum
temperature difference between the test sample and a reference sample of inert dust
exposed to the same heating procedure. The results are shown in Figure 1.67.In the case
of the pea flour, it is seen that self-heatingtook place in the dust deposit down to 5 vol%
oxygen or even less, whereas propagation of flames in dust clouds was practically impos-
sible below 15 vol% oxygen.Also, for the coals, there were appreciabledifferences.
Extinction of smoldering combustion inside large dust deposits, such as in silos, is a
dual problem. The first part is to stop the exothermic reaction. The second, and perhaps
more difficult part, is to cool down the dust mass. In general, the use of water should be
avoided in large volumes. Limited amounts of water may enhance the self-heatingprocess
rather than quench it. Excessive quantities may increase the stress exerted by the powder
or dust mass on the walls of the structure in which it is contained, and failuremay result.
Generally, addition of water to a powder mass, up to the point of saturation, reduces the
flowability of the powder and makes discharge more difficult (see Chapter 3).
Particular care must be taken in the case of metal dust fires, where the use of water
should be definitely excluded.Possible developmentof toxic combustion products must
be taken into account.
The use of inert gases such as nitrogen and carbon dioxidehas proven successful both
for quenching the oxidation reaction and the subsequent cooling of smolderhg com-
bustion in silos. However, large quantities of inert gas are required, on the order of 10
tonnes or more, for a fair size silo. In the case of fine-grained products such as wheat