Coal cargoes: Avoiding explosion and self-heating

Gitana Røyset, Gard Claims Executive, Arendal has written an article on the best ways to avoid self-heating and explosions on vessels carrying coal.

Royset noted that, despite its contribution to greenhouse gas emissions, global coal consumption climbed to an all-time high in 2022. It was on track for a record-breaking 2023 and 2024.

Coal is a fossil fuel with varying properties, depending on its source and handling before it is loaded aboard ship. Some coals can self-heat, and some emit methane – characteristics that can create fire and explosion risks to the crew, vessel and cargo.

The writer said that Gard had seen several cases involving problematic coal cargoes, particularly those originating in Indonesia. Royset said that her particular focus in this article was how to deal with a cargo that was both self-heating and emitting methane, and how to monitor for both conditions.  

The IMSBC Code requires that the shipper provides the Master with the characteristics of the coal, in writing, for all types of coals. As a minimum the information should include the coal’s moisture content, sulphur content, size and whether the cargo may be liable to emit methane or self-heat. In Gard’s experience it was common for coal cargo declarations to be inaccurate. For example, coal loaded in Indonesia / Borneo / Kalimantan often tended to self-heat, without being declared as such. Some charterparties or associated documents have required masters of vessels to treat such coal as liable to self-heat, regardless of the shipper’s declaration.

The IMSBC Code indicates that precautions for self-heating apply if the coal has been declared as liable to self-heat. However, Gard said that it was usually best to treat coal as if it was liable to self-heat and emit methane in the first instance, and use initial, frequent gas measurements to check the actual situation, and to act accordingly. “Given that the cargo declaration may be inaccurate, Gard’s advice has been to treat all coal as self-heating until it is shown that it is not.”

Some coal can emit methane (CH4), which will produce flammable mixtures with air / oxygen (O2) in hold ullages, thus presenting a risk of explosion. CH4 emission is usually dealt with by ventilating, thus keeping the level of CH4 well below the minimum that will support flaming combustion or explosion.

Many types of coal tend to self-heat, which can lead to toxic atmospheres, spontaneous combustion, and production of flammable gases. Self-heating is usually dealt with by excluding air / O2, by trimming stows flat and closing hatches and vents.

In some cases, although uncommon, coal can both self-heat and produce CH4 at the same time. This was more difficult to deal with because the two effects need opposite actions to bring them under control. Self-heating requires sealing to reduce the level of oxygen, while methane requires ventilation to decrease the concentration of methane.

The IMSBC Code does not give explicit instructions for this situation, but it is often (correctly) taken to indicate that ventilation should take priority, due to the acute nature of explosion risks.

In this situation expert advice is usually appropriate, and more detailed comments are below.

Because self-heating can produce flammable gases, and gas detectors are usually calibrated for CH4 and display results as CH4, self-heating is sometimes misinterpreted as CH4 emission. This can lead to holds being ventilated, which worsens the self-heating.

Coal that is liable to self-heat should not be loaded on board vessels if its temperature exceeds 55°C. This is because self-heating reaction rates increase exponentially as temperatures rise. As shippers’ declarations may not be reliable, proper temperature measurement before loading is very often appropriate, for example in Indonesia, even if the coal is not declared as liable to self-heat.

Temperature measurement of the coal to be loaded needs to be done at multiple points, to pick up hot spots. The temperature should preferably be measured below the surface, because if there is any self-heating the bulk will be hotter than the surface. If that cannot be done, then freshly exposed coal should be measured before it has been able to cool.

Once coal is on board, temperatures are more difficult to measure. Temperature sounding pipes within the holds are often used, but their readings are of limited value because bulk coal transmits heat poorly.

Therefore the focus should be on temperature measurement before loading.

The IMSBC Code indicates that coal stows should be trimmed ‘reasonably level’ to the hold boundaries. This is to minimise the exposed surface area, and to avoid cracks, hence minimising air entry and self-heating. It is good practice to take photos of the coal stows at the completion of loading to show the final trim, and to record the ullage size.

Holds and adjacent spaces must not be entered without proper precautions to ensure that the atmosphere is safe, because coal often removes O2 from air and produces toxic gases such as the odourless carbon monoxide (CO).

If there are delays of more than a few hours with no loading, consider closing holds and measuring gases in the meantime

Once holds are full, the IMSBC Code provides that, unless indicated otherwise, ventilate for the first 24 hours after departure from the load port, and measure gases once during this period, after closing vents for a suggested period of not less than 4 hours. With self-heating coal that might not be declared as such, venting may worsen self-heating. It is often best to measure gases early within the first 24-hour period, after closing vents for the suggested period for measurement, and to repeat gas measurements frequently thereafter until conditions are seen to be stable. This is to check early for excessive CH4 / flammable gas and CO.

If CH4 / flammable gas concentrations remain below 20 %LEL, then holds should remain closed and unventilated. This is to exclude air / O2 and hence minimise the potential for self-heating.

If CH4 / flammable gas is increasing and above 20 %LEL, ventilation needs to be considered as a priority, to avoid explosion risks, which are more acute than self-heating.

However, the %LEL action level for ventilating also depends on the ventilation history, the O2 and CO levels and other factors.

Ventilation can hide self-heating problems. This can suppress CO measurement results, the best indicator of self-heating.

Therefore, gas measurements need to be assessed carefully and ventilation needs to be controlled correctly, to avoid both explosion risks and self-heating.

If gas measurements indicate that CO is rising above 50 parts per million (ppm) in unventilated holds, that indicates that the coal has propensity to self-heat. Gard recommends seeking expert advice if the CO level is above 50 ppm, as is indicated in the IMSBC Code. In such cases gas measurements should be taken at least every 12 hours until the situation is stable.

For coal with any self-heating tendency, once the vessel arrives at the port of discharge it is recommended to keep all cargo holds and vents closed and to measure gases until the discharge operation is ready to start. This applies to all holds individually meaning those holds that are not actively being discharged should remain closed. Pre-discharge surveys should not cause holds to be opened, allowing air / O2 entry to the holds, unless discharge is to commence very shortly, or gas measurements have shown that there is no self-heating tendency.

If coal is showing signs of problematic self-heating, then it should be discharged all in one go, without delays. It is preferable to have a discharge plan to facilitate this, if it is possible within ship loading and stability limits.

If there are any significant delays during discharge in any holds, consider re-closing those holds and measuring gases.

In case of severe self-heating or fire in coal, the IMSBC Code mentions avoiding using water when at sea. Royset said that Gard’s experience was that spraying water could help to suppress heating and fire, usually at anchor or alongside. Care however must be taken to keep within loading and stability limits, for example by pumping out water to an appropriate tank. Fresh water is preferred because seawater is often detrimental to the end use of the coal.

Concluding remarks

Given that cargo declarations may be unreliable and faced with the prospect of catastrophic losses to people and property, Gard said that it was important that care be taken at each stage, from loading to discharge, to monitor loading temperatures and, critically, gas levels on board.

The writer noted that coal has other characteristics not addressed here. For example, some coal can also liquefy or produce acidic liquids which may corrode the vessel’s structure.