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 slow 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 non-combustible containers.
Charcoal
Charcoal is a very porous substance and when freshly prepared can absorb nearly 10 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 a 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.
Coal
Coal, similar to charcoal, will oxidise on its surface and generate heat. The 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.
* 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.
2. 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
* 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.
3. Chemical reactions
Some chemicals react with one another and will heat spontaneously. The 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
* 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.
* 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.
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