Coal is the second-highest-volume solid bulk cargo carried by sea, with global seaborne trade of approximately 1.2 to 1.3 billion tonnes per year. The cargo is regulated under the International Maritime Solid Bulk Cargoes Code (IMSBC) administered by the International Maritime Organization. The IMSBC schedule for coal is among the most complex in the Code because the cargo presents three distinct hazards: liquefaction risk for some coals when wet (Group A), self-heating that can lead to fire (Group B), and emission of flammable methane gas during the voyage (Group B). The 2009 IMSBC entry replaced the earlier Bulk Coal Code and consolidates handling, ventilation, monitoring, and emergency response requirements.
Group classification
Coal is classified into two groups under the IMSBC Code depending on its fines content and moisture characteristics:
- Group A and B (combined): Coal cargoes that contain a sufficient fines fraction to be susceptible to liquefaction in addition to the self-heating and gas hazards.
- Group B: Coal cargoes that are not susceptible to liquefaction but retain the chemical hazards of self-heating and methane release.
The shipper must declare the group classification on the cargo declaration. Coals that may liquefy require pre-loading TML/MC testing in addition to the gas and temperature monitoring required for all coals.
Self-heating hazard
All coals undergo low-temperature oxidation in air. The reaction releases heat. In well-ventilated, low-density storage the heat dissipates and the cargo remains stable. In compacted, sealed bulk-carrier holds the heat can accumulate, particularly in the upper layers near the trim where small air ingress is possible. Self-heating progresses through three stages:
- Slow temperature rise to approximately 50 to 60 degrees Celsius.
- Accelerated oxidation between 60 and 100 degrees Celsius with carbon monoxide release.
- Spontaneous combustion above approximately 100 to 150 degrees Celsius.
The IMSBC Code requires monitoring of cargo temperature throughout the voyage where practical. Carbon monoxide concentration in the cargo hold atmosphere is the most reliable indicator of incipient self-heating; CO concentrations above 50 parts per million in monitoring samples taken from sealed holds indicate active oxidation, and concentrations above 200 ppm or sharp upward trends warrant emergency response procedures.
Methane release hazard
Many coals, particularly higher-rank bituminous coals, retain methane absorbed during geological formation. The methane is released during loading and continues to evolve from the cargo throughout the voyage. The principal hazard is methane accumulation in the cargo hold headspace at concentrations within the flammable range (5 to 15 per cent in air). The IMSBC Code requires methane monitoring at hold-atmosphere sampling points throughout the voyage and prescribes ventilation procedures for methane-emitting coals.
Coals that emit methane are required to be ventilated using either continuous surface ventilation (where the headspace is positively flushed with outside air) or alternating ventilation regimes. Surface ventilation is the preferred method for methane control because it minimises the introduction of fresh oxygen to the cargo body, which would accelerate self-heating, while still venting methane from the headspace.
Gas-monitoring procedures
The shipper provides a cargo declaration that specifies whether the consignment is methane-emitting and self-heating, and the master implements monitoring accordingly. Standard monitoring practice includes:
- Carbon monoxide measurement in each hold at sealed sampling points, daily after departure.
- Methane and oxygen measurement in each hold daily, particularly during the first week of the voyage when methane evolution is highest.
- Cargo temperature measurement where temperature probes are installed (rare on older bulk carriers, becoming more common on newer designs).
The data is recorded in the cargo log and forwarded to the operator and shipper for trend analysis.
Liquefaction risk in fine coal
Fine coal cargoes, particularly coal slurry residues and some Indonesian and Chinese coal exports, contain a high fraction of particles below 1 millimetre and may exceed the transportable moisture limit. These cargoes are classified Group A and B and require pre-loading TML and moisture content certification using the same Flow Table or Proctor-Fagerberg methods used for iron ore fines. Liquefaction casualties involving fine coal cargoes have occurred and are a continuing focus area for class societies and flag states.
Loading and trimming
Coal is loaded by shore-based conveyor systems with shiploaders distributing the material across the hold. Larger fines are encouraged to settle at the bottom and centre of the hold; trimming with bulldozers may be needed to flatten the cargo surface and allow secure hatch closure. Holds should be inspected for cleanliness, hatch cover integrity, and bilge well clearance before loading. After loading, the holds are sealed (subject to ventilation requirements) and the cargo declaration is signed off.
Discharge and post-discharge cleaning
Coal discharges through grab-fitted shore cranes or via continuous-unloader systems. Coal residues are difficult to fully remove from cargo holds and require thorough washing and drying before the next cargo, particularly if the next cargo is grain or another sensitive cargo for which residual coal contamination is unacceptable. The cleaning procedure is set out in IMSBC Section 4 and class society guidance.
Notable casualties
Coal fires aboard bulk carriers have caused multiple total losses through the post-1990 period, generally associated with self-heating in incompletely vented holds or with insufficient pre-loading inspection of cargo condition. Shippers’ and shipowners’ procedures have progressively tightened in response, and most major coal exporters (Australia, Indonesia, Russia, the United States, and Colombia) now operate well-controlled loading terminals with comprehensive cargo certification.