Chemical Engineering June 2017 - 65

Solids Processing
Guarding Against Fires and Explosions
Caused by Self-Heating
The two experimental methods presented here can be used to quantify
powder self-heating hazards in layer and bulk conditions in industrial processes
Guibing Zhao
Chilworth Technology, a DEKRA
Insight company
B
ulk storage of heated powders
and powder accumulations
on hot surfaces or in
heated environments can
potentially lead to self-heating and
spontaneous ignition. Examples of
locations within processing companies
where self-heating of powder
accumulations may be expected
include uninsulated steam pipes,
inappropriately rated or selected
electric motors and lighting fixtures,
mechanical mills and inside surfaces
of various dryers. Self-heating is a
complicated phenomenon consisting
of both an exothermic chemical
reaction and a heat-transfer process,
which can cause material smoldering
and even lead to fires and dustcloud
explosions.
The parameters that affect the
actual self-heating for a given material,
include bulk size and shape,
temperature, time and oxygen/air
accessibility. Appropriate tests have
been developed to predict selfheating
hazard under plant-scale
conditions. This article introduces
the " layer powder " screening and
the advanced " basket " tests (Figure
1) that may be used to deduce the
reaction kinetics and therefore to
extrapolate the laboratory-scale test
data to the plant-scale so that the
data can be used to establish safe
operating limits for operating temperature,
critical bulk size, and residence/exposure
time.
The dangers of self-heating
There is potential for a thermal runaway
and spontaneous combustion
if a material can self-heat due
to decomposition or exothermic reaction
with air. This article focuses
FIGURE 1. The powder layer test (insert) and the oven-bake test
(main photo) can be used to establish safe operating temperatures
on self-heating of solid materials.
However, it should be noted that
self-heating may also occur in gas
or liquid phases.
Self-heating is a complicated phenomenon
related to exothermic reaction
kinetics of the material, heat
transfer (sometimes including mass
transfer) in both body and external
surface of the bulk material, induction
time, geometry and size of bulk
material, and ambient temperature.
Self-heating of solid materials may
lead to smoldering, flaming or dust
explosions, particularly when the
smoldering material is disturbed and
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exposed to air. Self-heating reactions
may also compromise product
quality or produce flammable gases,
which may lead to gas explosions in
process vessels. Self-heating incidents
in processing equipment, such
as silos, dryers, dust collectors and
flexible intermediate-bulk containers
(FIBCs; super sacks), have caused
fires and explosions in industry.
Typical materials that may experience
self-heating include sawdust,
coal, sewage sludge, grain and organic
salts, especially those containing
unsaturated bonds or peroxides,
for instance.
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Chemical Engineering June 2017

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