FPASA BULLETIN GF 07
SPONTANEOUS HEATING
Spontaneous heating can lead to spontaneous combustion in many materials. For this
reason the phenomenon should be well understood if effective preventive action is to be
taken.
The process of increase in temperature of a material without drawing heat from its
surroundings is known as spontaneous heating and can lead to spontaneous combustion.
Whether oxidation is the cause of shelf heating or not, actual ignition will depend on the
rate of heat generation, the air supply, and the insulation properties of the immediate
surroundings.
Some materials, especially organic materials based on carbon, can react with oxygen at
room temperature. If this occurs within a material which is a good thermal insulator, the
heat generated cannot escape and, the temperature therefore rises, increasing the reaction
rate progressively. This could lead to the ignition temperature being reached.
Most organic substances prone to oxidation will evolve heat, but the reaction is usually sow
and the heat released is transferred to the atmosphere almost as rapidly as it is generated,
thereby preventing combustion.
Circumstances under which spontaneous heating occurs falls under three main
headings.
1.
Slow oxidation in air
Substances which are prone to oxidation without external heat being applied
include liquids, particularly oils, and solids such as oil seeds, coal and some metals.
Oils used in industry may be divided into mineral, animal and vegetable, and of
these mineral oil is not liable to spontaneous heating. Most animal and vegetable
oils will absorb oxygen from the air. The more they absorb the more hazardous
they are. Some vegetable oils are known as “drying oils” implying that they absorb
oxygen.
Some of these oils are used in paints and varnishes and, when oxidising, cause
either a skin to form over the liquid or the formation of a solid residue. When these
coatings are thinly spread over a large surface, as in painting a door, the heat
generated by oxidation is dissipated as quickly as it is generated.
However, if the same quantity of oil-based coating was contained in a mass of
combustible material such as cotton waste, the heat would be contained and the
temperatures of the oily mass would rise. This process takes place without any
external heat sources whatsoever. Polish mops left in cupboards have been known
to ignite for similar reasons.
Oils prone to spontaneous heating should be kept in cool environments as heat
from steam pipes, ovens and dryers will accelerate their self heating characteristics.
Safeguards

Stocks of oil liable to spontaneous heating should not be stored near
carbonaceous
material.

Oils should not be stored close to heat sources.

Combustible waste contaminated by oils should be disposed of in noncombustible containers.
Oil liable to spontaneous heating
Name
Bone oil
Cod Liver oil
Cotton seed oil
Lard oil (commercial)
Linseed oil (raw)
Mustard oil (wild)
Paint
Heating tendency
Moderate
High
High
High
Very high
High
Varying
Name
Palm oil
Perilla oil
Pine oil
Rape seed oil
Sperm oil
Tallow
Whale oil
Heating tendency
Moderate
Very high
Moderate
High
Moderate
Moderate
High
Charcoal
Charcoal is a very porous substance and when freshly prepared can absorb nearly ten
times its own volume of air. This exposes a large surface area to oxygen and heat is
generated which may be sufficient to cause slow oxidation.
When the charcoal is several days old the risk of heating is lessened because the pores
become filled with moisture and air can no longer be absorbed.
Safeguards

Store in weather-tight, well-ventilated building with automatic sprinkler protection or,
in bulk bins of non-combustible construction protected from the elements.
Ventilation must be provided from above only because air flowing up through the
material can cause self-heating.
Explosion venting should be provided where finely divided charcoal is likely to be
present.
Do not store wet and dry charcoal together.
Charcoal silos or storage bins should be monitored by a temperature alarm.
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Coal
Coal, similar to charcoal, will oxidise on its surface and generate heat. He softer and more
finely divided the coal is, the more the absorption of oxygen. Coal dust and pulverised coal
are more hazardous because of the huge surface areas subjected to oxidation.
Safeguards


Do not store coal in contact with hot areas such as boiler walls, flues and steam pipes.
Prevent air movement through coal stacks, eg: do not stack piles over manhole covers
of pipe trenches or gutters as this may allow air into the pile.
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2.

Do not vent coal stacks by means of pipes or flues as this usually supplies more
oxygen and assists with self-heating.
Coal can be stacked in two ways ie:
to allow maximum ventilation, eg: using large coals,
compacted to exclude air eg: stacked in layers and rolled flat by
mechanical means
Check temperatures at least once a week. The temperature should not exceed
71°C.
Low-grade coal should not be piled higher than 3 m and best-grade coal not higher
than 4,5 m unless piled by the roll-pack method. All yard piles should be at least 8
m from other combustibles and important buildings.
Action of micro-organisms
Vegetable fibres are liable to spontaneous heating if stacked in a damp condition.
Self-heating can also be produced by seeds ripening or germinating, but continued
respiration in vegetable cells, by bacterial or fermentation taking place, and even by
insects in grain or seeds.
The phenomenon of spontaneous heating being caused by micro-organisms occurs
during decomposition of material by organisms such as fungi and thermophillic
bacteria. Fungi will raise the temperature of hay, for example, to an optimum
temperature of about 38°C, whereas thermophillic bacteria has an optimum
temperature of about 60°C. Most micro-organisms will die at temperatures
exceeding 70°C, but sufficient charring would have been produced to allow the
chemical action of oxidation to be initiated.
Safeguards



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Stack hay in dry conditions if possible
Do not provide vertical air shafts to assist ventilation because if imperfect this
type of venting will only assist self-heating.
Contamination of vegetable fibres with materials liable to heat spontaneously
should be avoided and contaminated materials should not be allowed to
accumulate.
Stacks should have adequate breaks between them and they should not be
situated too close to important buildings.
Do not vent coal stacks by means of pipes or flues as this usually supplies more
oxygen and assists with self-heating.
Chemical reactions
Some chemicals react with one another and will heat spontaneously. He heat generated
may be very rapid and cause immediate combustion of the chemicals themselves eg:
sodium and potassium with water, calcium hypochlorite with brake fluid glycerine with
potassium permanganate, or the heating may be sufficient to cause ignition of other
combustibles eg: acids and wood and quicklime with water.
In the plastics industry, polymerisation of organic materials, if uncontrolled, can also lead to
overheating occurring eg: during the manufacture of plastic foams.
Safeguards

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Store chemicals in properly sealed containers
Keep chemicals cool and dry
Store reactive chemicals apart
Store oxidisable chemicals out of contact with the air
Combustible materials such as wood and paper should not be kept in chemical stores.
PRECAUTIONS IF SPONTANEOUS HEATING IS DETECTED
It is important to try to identify spontaneous heating during the early stages. This can be
done through constant observation of materials, the use of thermocouples or thermometers
to monitor temperatures at various levels, or even by using detectors to give an early
warning of smouldering.
Once spontaneous heating has been identified, the following precautions could be initiated.

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Reduce the volume of material by re-stacking or transferring to smaller containers
Control the ventilation by either opening the stacks or excluding the air altogether
Cool by means of water sprays
Suffocate the fire by using carbon dioxide. This presupposes that the material is in a
contained storage area such as a ship’s hold, a silo or a bin.
Materials liable to spontaneous heating
Material
Animal feeds
Liability to
spontaneous heating
Varying
Coal
Moderate
Fertilisers, artificial
Moderate
Foam
rubber,
consumer products
in
Moderate
Grain
Slight
Hay
High
Comments
Precautions
Chiefly made from grain,
oil, oil seeds and fibres –
the hazard depends on
the constituents. Adding
molasses to animal feeds
increases the hazard.
Hazard related to size of
coals, mode of stacking,
and nature of coal
Keep cool Avoid high
moisture contents
Hazard
depends
on
composition,
eg:
presence of nitrates and
free acid.
May continue to heat
spontaneously
after
being
subjected
to
heating eg: in a dryer
Autoignition
is
rare
except in milled grain
meal
Very likely to heat in hot
weather if damp. If no
heating has occurred
after 96 days, the stock
can
generally
be
considered safe
Avoid high temperatures.
Store in stacks preferably
not greater than 200 t.
Monitor temperatures
Avoid extremely high or
low moisture content
Heater articles should be
cooled
before
being
bundled or put away. Do
not allow foam rubber to
become
contaminated
with substances liable to
spontaneous heating eg:
oils.
Keep cool and dry
Keep cool and dry
Lime, unstacked
Moderate
Metal powder, eg: iron,
steel, brass aluminium
etc.
Oily cloths, rags etc.
Moderate
Waste and scrap of all
kinds
Varying
Heats in contact with
water
Moisture normally
accelerates oxidation
Varying
Hazard depends on
constituents. Oily or
charred waste is
particularly dangerous
Keep dry
Keep
in
containers
closed
Keep cool, dry and well
ventilated. Waste rage
etc should be treated as
for waste
Keep cool and in small
piles. Oily rags should
be placed in closed
containers until they can
be removed from the
building.
Avoid
contamination. All waste
should be disposed of at
the earliest opportunity
References:
Published by
Factory Mutual System, Loss Prevention Data
Fire Prevention No. 126, August 1978 FPA (UK)
Hazard No. 15, Royal Insurance
Fire Protection Association of
Southern Africa
(Incorporated Association not for Gain)
(Reg.No. 73/00022/08)
P O Box 15467
1472 Impala Park