Section 8.2: Coal (Self-heating, Methane Emission, Dust)

Coal is another cornerstone commodity in global bulk trade, fueling power generation and industrial processes worldwide. It is transported in vast quantities by vessels of all sizes, from Handysize to Capesize. However, coal is far from being an inert cargo; it is a chemically active substance that presents several significant hazards, primarily the risks of self-heating and spontaneous combustion, emission of flammable methane gas, and the generation of copious amounts of dust. Different types of coal exhibit these hazards to varying degrees, making it essential for the Master and crew to understand the specific characteristics of the coal being carried, as declared by the shipper and detailed in the IMSBC Code.

1. Types of Coal and Their General Characteristics:

Coal is a combustible; sedimentary rock formed from ancient plant matter. Its properties vary widely depending on its geological origin and rank (degree of coalification). Common types include:

Anthracite: The highest rank of coal, hard, black, and lustrous. Generally has a lower volatile content and is less prone to self-heating and methane emission than lower-rank coals.

Bituminous Coal: A broad category, widely used for steam generation (thermal coal) and coking (metallurgical coal). Its properties vary significantly. Some bituminous coals are highly prone to self-heating and methane emission.

Sub-bituminous Coal: Lower rank than bituminous, with higher moisture content and often a greater propensity for self-heating.

Lignite (Brown Coal): The lowest rank of coal, with high moisture content and high volatile matter. Very prone to self-heating and disintegration. Rarely transported long distances by sea due to its instability and low calorific value.

Coal Slurry / Coal Fines: Finely crushed coal, sometimes mixed with water for pipeline transport. If shipped in bulk with high moisture content, it can be a Group A cargo (risk of liquefaction) and may also retain Group B hazards.

Petroleum Coke (Petcoke): A carbonaceous solid derived from oil refinery coker units. While not strictly “coal,” it’s often handled and discussed alongside it. It has its own IMSBC Code schedule and can be prone to self-heating and dust. Different types exist (e.g., green, calcined).

2. IMSBC Code Classification and Key Considerations for COAL:

BCSN: COAL

Group: Typically classified as Group B (cargoes which possess a chemical hazard) due to its propensity for self-heating and methane emission. Some specific types of fine coal or coal slurry may also be classified as Group A and B if they are liable to liquefy and also possess chemical hazards.

Hazards (as listed in the IMSBC Code schedule for COAL):

May self-heat and then spontaneously combust.

May deplete oxygen in the cargo space.

May emit methane, which is flammable.

Some coals may be corrosive if wet.

Key Information from Shipper: The shipper’s declaration for coal is critical and must include:

Characteristics of the coal (e.g., rank, particle size).

Information on its liability to self-heat and emit methane.

Any specific precautions required.

Often, a weathering certificate may be required for certain types of coal, indicating it has been stockpiled for a period to reduce its initial reactivity.

The IMSBC Code schedule for coal provides detailed test procedures for determining its self-heating properties and methane emission potential, and shippers may need to provide results of such tests.

3. Key Hazards and Precautions Associated with Coal:

A. Self-Heating and Spontaneous Combustion:

Mechanism: Coal reacts with oxygen in the air (oxidation). This is an exothermic process. If the heat generated is not dissipated, the temperature of the coal rises. As the temperature increases, the rate of oxidation also increases, leading to a thermal runaway that can eventually result in ignition. Sulphur content (pyrites) can also contribute to self-heating.

Factors Influencing Self-Heating:

Rank of Coal: Lower-rank coals (sub-bituminous, lignite) and some bituminous coals are more prone than anthracite.

Particle Size: Finer coal has a larger surface area exposed to oxygen, increasing reactivity. Coal dust is particularly hazardous.

Moisture Content: While very dry coal can oxidize, a certain amount of moisture can accelerate self-heating in some types by promoting pyritic oxidation or by affecting heat dissipation. Excessively wet coal can also be problematic.

Temperature at Loading: Loading hot coal (e.g., directly from a mine or from sun-baked stockpiles) significantly increases the risk. The IMSBC Code often specifies maximum loading temperatures (e.g., not exceeding 55°C).

Presence of Pyrites (Iron Sulphide): Oxidation of pyrites is highly exothermic.

Previous History: Coal that has been freshly mined, crushed, or exposed to rain may be more reactive.

Ventilation: Improper ventilation can exacerbate self-heating by supplying oxygen to a developing hot spot without adequately removing heat.

Monitoring During Voyage:

Temperature Monitoring: Regular temperature readings of the cargo in each hold are essential. This can be done using sounding pipes fitted with thermocouples, portable temperature probes, or fixed temperature sensors. Readings should be taken at various depths and locations within the stow.

Gas Monitoring: Monitoring for carbon monoxide (CO) is a key indicator of self-heating. CO is produced at an early stage of oxidation, often before significant temperature rises or smoke is detected. Increasing CO levels are a serious warning sign.

Ventilation Strategy (Critical and Complex):

The IMSBC Code schedule for COAL provides specific guidance. Generally:

Surface ventilation is usually recommended to remove methane and potentially some heat from the ullage space.

Through-stow ventilation (forcing air through the body of the coal) is generally NOT recommended for coal liable to self-heat, as it can supply oxygen to incipient fires and accelerate combustion.

If self-heating is detected (rising temperatures, increasing CO), ventilation should usually be stopped, and holds sealed as much as possible to restrict oxygen supply, while continuing to monitor temperatures and gases. The decision to alter ventilation strategy in such cases must be made carefully, considering all factors and IMSBC Code guidance.

Emergency Response: If fire occurs, the primary method is often to seal the hold and use the ship’s fixed CO2 system (if fitted for cargo holds and suitable for coal fires – CO2 may react with burning carbon at very high temperatures to produce CO, but it’s still a primary extinguishing agent listed). Boundary cooling with water may also be employed.

B. Methane (CH₄) Emission and Flammable Atmosphere:

Mechanism: Methane is a natural component of coal seams and is released when coal is mined, crushed, and exposed. The rate of emission varies with coal type, particle size, temperature, and time.

Hazard: Methane is colorless, odorless (in its pure form), and highly flammable. If it accumulates in a cargo hold or adjacent spaces and mixes with air in concentrations between its Lower Flammable Limit (LFL, approx. 5% by volume) and Upper Flammable Limit (UFL, approx. 15% by volume), it can be ignited by a spark or flame, leading to an explosion.

Monitoring:

Regularly monitor the atmosphere in cargo holds (ullage space) and adjacent enclosed spaces (e.g., forecastle store, mast houses if vents terminate there) for methane using a calibrated combustible gas indicator (methanometer).

Measurements should be taken frequently, especially in the initial period after loading when emission rates are often highest.

Ventilation:

Mandatory: Cargo holds containing coal must be ventilated (usually surface ventilation) to prevent methane accumulation.

Ventilation systems must be designed to prevent methane from entering accommodation or machinery spaces.

All ventilation outlets from coal holds must be fitted with flame screens of appropriate mesh size, which must be kept clean and intact.

Ignition Control:

Strictly prohibit smoking, naked lights, and any non-intrinsically safe electrical equipment in or near cargo holds, ventilation openings, or areas where methane might accumulate.

Use only certified safe (explosion-proof) equipment if entry into holds is necessary (though entry should be avoided unless absolutely essential and after thorough ventilation and testing).

C. Oxygen Depletion:

Mechanism: The oxidation process that leads to self-heating also consumes oxygen from the hold atmosphere. Methane emission can also displace oxygen.

Hazard: An oxygen-deficient atmosphere poses a serious asphyxiation risk to anyone entering the hold or an adjacent poorly ventilated space.

Precautions: Strict adherence to enclosed space entry procedures is mandatory: thorough ventilation and atmosphere testing for oxygen (and flammable/toxic gases) before entry. Never assume a coal hold is safe to enter without testing.

D. Dust:

Hazard: Coal dust can be a nuisance, an explosion hazard (if a fine dust cloud is ignited), and a health hazard (inhalation can lead to respiratory problems like Coal Workers’ Pneumoconiosis – “black lung disease”).

Precautions:

Minimize dust generation during loading and discharge (e.g., by adjusting loading spout height, using dust suppression sprays at the terminal).

Crew and stevedores should wear appropriate respiratory protection (dust masks/respirators) and eye protection.

Keep non-working hatches and accommodation/engine room openings closed and sealed as much as possible.

Regularly clean accumulated coal dust from decks and structures to reduce secondary dust clouds and fire risk.

Ensure electrical equipment in dusty areas is suitably protected (explosion-proof or dust-tight).

E. Corrosivity:

Some coals, particularly those with high sulphur content, can become acidic when wet, leading to corrosion of steel hold structures.

Keeping holds dry and maintaining good paint coatings helps mitigate this.

F. Cargo Shifting:

While not as prone to liquefaction as Group A fines, coal can still shift if not properly trimmed, especially finer grades or if the angle of repose is low.

Trimming reasonably level as per IMSBC Code Section 5 is necessary to minimize void spaces and prevent shifting.

4. Pre-Loading Preparations:

Hold Cleanliness: Remove all residues of previous cargo. Holds should be clean and dry. Any oily residues must be thoroughly removed as they can promote self-heating of coal.

Bilge Wells: Clean, strum boxes in place, suctions tested. Protect bilge wells from ingress of fine coal.

Ventilation Systems: Ensure all ventilators and flame screens are clean, intact, and operational.

Gas Monitoring Equipment: Calibrate and test methane detectors, CO detectors, oxygen meters, and any temperature monitoring equipment.

Shipper’s Declaration and Certificates: Obtain and scrutinize the shipper’s declaration, including details on self-heating properties, methane emission, and any weathering certificates.

IMSBC Code Schedule for COAL: Review it thoroughly with all officers.

Fire-Fighting Equipment: Ensure all FFA is ready, especially the fixed CO2 system for cargo holds (if fitted and intended for coal) and SCBAs.

5. Loading Operations:

Temperature Checks: If possible and safe, check the temperature of the coal being loaded. Refuse cargo that is excessively hot (e.g., >55°C, or as specified in the Code/by shipper).

Dust Control: Implement measures as discussed above.

Trimming: Ensure cargo is trimmed reasonably level to minimize shifting and aid even gas distribution/ventilation.

No Smoking/Naked Lights: Strictly enforce.

Monitoring: Begin gas and temperature monitoring as soon as practicable after loading starts and continue regularly.

Weather: Avoid loading coal during heavy rain if possible, as excess moisture can exacerbate some problems.

6. Precautions During Voyage:

Regular Gas Monitoring: Monitor methane (CH₄), carbon monoxide (CO), and oxygen (O₂) levels in each hold ullage space daily, or more frequently if readings are concerning or if advised by the IMSBC Code schedule for the specific coal. Record all readings.

Regular Temperature Monitoring: Monitor cargo temperatures at various points and depths. Record all readings. Be alert for any steady rise in temperature or any localized hot spots.

Ventilation: Implement the ventilation strategy as per the IMSBC Code schedule (usually surface ventilation). Adjust or stop ventilation if signs of self-heating develop, based on careful assessment and Code guidance.

Seal Holds (if self-heating detected): If significant self-heating occurs, the primary action is usually to seal the hold as effectively as possible to cut off oxygen supply.

Avoid Entry: Do not allow personnel to enter cargo holds unless absolutely essential, and only after rigorous enclosed space entry procedures.

7. Discharge Operations:

Dust Control: A major concern.

Monitor for Hot Spots/Fire: As cargo is discharged, watch for any smoldering areas or signs of fire that may have developed within the stow. Have fire hoses ready.

Gas Levels: Be aware that methane may still be present.

8. Master’s Key Responsibilities Specific to Coal:

Vigilance on Self-Heating and Methane: These are the primary safety concerns. Ensure robust monitoring programs are in place.

Correct Ventilation Strategy: This is critical and can be complex. The Master must ensure the strategy aligns with IMSBC Code guidance for the specific coal and the observed conditions.

Strict Enforcement of Safety Rules: No smoking, proper use of PPE, adherence to enclosed space entry procedures.

Readiness for Emergency: Ensure the crew is well-drilled in responding to a coal fire or a high methane situation.

Understanding Shipper’s Information: Pay close attention to any specific properties or precautions declared by the shipper, especially regarding the coal’s propensity for self-heating or gas emission.

The carriage of coal demands a proactive and knowledgeable approach to safety. By understanding its various hazards and diligently applying the precautions outlined in the IMSBC Code and industry best practices, Masters can significantly mitigate the risks associated with this vital but potentially dangerous commodity.