By Neil Sanders
A small but significant number of marine cargo fire and explosion incidents are due to self-heating in some form. Selfheating can affect a surprising variety of cargoes including coal, iron in the form of direct reduced iron (DRI), metal turnings, charcoal, seed cake and biomass, in addition to the more obvious issues with solid and liquid chemicals. A new joint publication from The Swedish Club and Burgoynes, ‘Fire – the hidden causes’ is planned for later this year and will provide loss prevention advice on such incidents, in addition to examining those vessel fires caused by everyday sources such as cargo lights, fumigation, movement of cargo and of course smoking and hot work.
In this article we focus on self-heating, perhaps the most perplexing of all causes of fire. In general, self-heating occurs when an exothermic (heat-producing) chemical or biochemical reaction happens within a body of cargo. Loss of the heat produced by the reaction is restricted by the surrounding cargo, packaging, dunnage, containers etc. Due to restricted heat loss, the temperature within the cargo tends to increase. The rate of most chemical reactions increases with increasing temperature and it often roughly doubles for each 10°C rise in temperature. Therefore the heating and the temperature rise can worsen exponentially, which can ultimately lead to a fire in the cargo and surrounding materials.
Why do materials self-heat?
Two main types of reaction can lead to cargo self-heating:
- Oxidising reactions in which cargo reacts with oxygen in air and/or water. Common examples are coal, oil seed cake, DRI and metal turnings.
- Self-reaction or self-decomposition of cargo. Well-known examples are calcium hypochlorite and some fertilisers.
Loss prevention advice
There are a number of actions that can be taken which affect the severity of self-heating and whether it develops to a problematic level:
“Self-heating due to oxidation can only progress to a problematic level if enough oxygen can pass into the cargo to produce heat that cannot be dissipated.”
Effects of an incident involving reactive solids in a container ship hold
Tanker fire
Reduce the availability of oxygen: With many bulk cargoes, such as coal, the holds need to be kept closed and sealed. This limits the access of air (oxygen) to the cargo, thus restricting the oxidation reactions, so that temperatures do not rise to problematic levels.
Fire from self-heating of DRI
Reduce the availability of water: Some cargoes, such as DRI, need to be kept dry so that they cannot react with water. If water does enter holds containing DRI it can start a self-heating reaction which can then worsen, leading to oxidation reactions with air and then severe heating.
Unloading self-heating DRI
Prevent air diffusing into the cargo: Self-heating due to oxidation can only progress to a problematic level if enough oxygen can pass into the cargo to produce heat that cannot be dissipated. For example, some cargoes such as activated carbon may need to be carried in hermetically sealed bags to stop oxygen coming into contact with the cargo. Bulk coal cargoes should also be properly trimmed to give a compacted, flat surface that restricts entry of air into the stow.
Effects of a fire in a container involving rechargeable batteries
Monitor the size of the body of cargo: Heat is dissipated less effectively from a larger body, causing an increase of temperature within the cargo. This is why some cargoes have package size restrictions or maximum container load restrictions.
Self-heating of coal
‘Age’ the cargo by exposure to air: Some cargoes are particularly reactive and so need to be ‘aged’ by exposure to air, ensuring that the most easily oxidised parts have reacted before loading. This means that the rate of oxidation of the cargo is reduced and so self-heating is less likely. This method can be applied to DRI and also some charcoal cargoes.
Control the loading temperature of the cargo: Reactions occur faster at higher temperatures, so it makes sense to restrict the loading temperature of some cargoes so that oxidation rates are not too fast. Bulk coal, for example, shall not be loaded if it is above 55°C.
IMO requirements relating to self-heating
Both the International Maritime Solid Bulk Cargoes (IMSBC) Code and the International Maritime Dangerous Goods Code (IMDG) address factors affecting self-heating. For some cargoes there may be other relevant requirements or guidance. Some of the main IMO requirements are below – these vary depending on the type of cargo:
- Closing and sealing holds to exclude air and water.
- Flushing holds with inert gas to exclude air.
- Adhering to maximum cargo loading temperatures.
- Keeping such cargoes away from sources of heat, such as direct sunlight.
- Ensuring proper ageing before loading/stuffing.
- Following packaging/stuffing requirements, such as maximum package sizes.
“Some cargoes are particularly reactive and so need to be ‘aged’ by exposure to air, ensuring that the most easily-oxidised parts have reacted before loading.”
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Source: The Swedish Club
