Handling bulk coal - hazards involved



Vessels shipping coal should at all times carry on board instruments for measuring methane, oxygen and carbon monoxide gas concentrations, so that the atmosphere within the cargo space can be monitored. The instrument should be regularly serviced and calibrated so that it can provide the crewmembers with reliable data about the atmosphere within the cargo space. Care needs to be exercised in interpreting methane measurements carried out in the low oxygen concentrations often found in unventilated cargo holds.

The catalytic sensors normally used to detect methane rely on the presence of sufficient oxygen for accurate measurement. This phenomenon does not affect the measurement of carbon monoxide or measurement of methane by infrared sensor. However, additional guidance should be sought from the manufacturer of the instrument.
An instrument required for measuring methane, oxygen and carbon monoxide concentrations should be fitted with an aspirator, flexible connection and a length of tubing, thus enabling a representative sample to be obtained from within the square of the hatch.



Stainless steel tubing approximately 0.5m in length and 6mm nominal internal diameter with an integral stainless steel threaded collar is often preferred. The collar is necessary to provide an adequate seal at the sampling point.

A suitable filter should be used to protect the instrument against the ingress of moisture as recommended by the manufacturer. The presence of even a small amount of moisture would compromise the accuracy of the measurement

Bulk Coal Discharging
Fig: Bulk Coal Discharging


Australia is the world's largest coal exporter with an estimated 240 million tonnes exported every year. Other significant coal exporters include the USA, Canada and South Africa.

Major coal importing countries include Japan, Korea, the UK, Germany, India and Italy to generate electricity.

There are four categories of coal:

Coal Slurry
This is a mixture of solid coal containing water and is obtained as a by-product during mining. The coal particle size is usually less than 1 mm. It is regarded as a cargo that may liquefy due to its high moisture content.

Coal Duff
This is a mixture of coal and water with the largest coal particles around 7 mm. Less liable to liquefaction than coal slurry, but does require its moisture content to be monitored.

Coke
Solid residues obtained by distillation of petroleum products, or half burnt coal with less gas content but with a tendency to absorb moisture at up to 20% by weight.

Small Coal
Contains particles of coal less than 7 mm in size; small coal is likely to develop a flow state due to its high moisture content.

According to IMO classification, coal is considered MHB. Three characteristics of coal need to be considered:

Methane Emission
Most coals emit methane, which is highly explosive if a gas concentration of 516% is present in atmosphere. A naked flame or spark is sufficient to ignite it. Methane is lighter than air and so it flows towards the top empty part of the cargo compartment and may even travel to adjacent compartments, including a hatch top. Coals that emit methane should be monitored carefully and, if the methane level becomes unacceptable, surface ventilation should be carried out as recommended by the IMSBC Code.

Spontaneous Combustion
Some coals are liable to spontaneous combustion due to the presence of moisture that causes exothermic oxidation (a reaction that results in production of heat) of coal at ambient temperature. If this heat is not dissipated, the temperature rises and the coal may ignite.

Corrosion
Some types of coal react with water to produce acids that can cause excessive corrosion of the ship's structure, known as `cargo corrosion'. As a result of the chemical reaction during the process of forming acid and then corrosion, colourless and odourless gases such as hydrogen are produced.

Precautions for Carriage of Coal

General precautions for the carriage of coal include:
  1. The Master should be informed about the cargo in the `Cargo Information Form' ( ie. Shipper's Declaration/Cargo Information ) and the material safety data sheet (MSDS) prior to loading.


  2. The precautions that need to be taken depend upon the information provided, eg if the cargo is liable to emit methane, then the Master should refer to the IMDG Code to obtain the loading, stowage and carriage information.


  3. It is, therefore, imperative that the cargo information be supplied to the Master prior to commencing loading, or they may not allow the operation.


  4. Before loading cargo:
    • i) Bilge wells should be cleaned, residual cargoes removed, suction tested and covered with taped down double wrapped burlap.
    • ii) Hatch top wheels and associated equipment should be greased to ensure that no sparks are caused during opening and closing.
    • iii) Electrical cables, cargo hold lights and any other electrical instruments within cargo holds should be checked for insulation damage to ensure that they are safe for use in an atmosphere containing explosive gases. On bulk carriers, as no lighting is needed, the fuses should be pulled to isolate electricity.
    • iv) Ships that carry coal are required to carry instruments to measure:
      a) Methane, oxygen and carbon monoxide levels in cargo holds
      b) cargo hold temperature (between 0-100C)
      c) the pH of bilge water.

      The installation of these instruments or the taking of measurements, should be possible without entering the cargo space. Arrangements should also be made to calibrate and test the instruments.


  5. Extra monitoring should be implemented when coal is loaded in holds adjacent to hot areas, such as heated fuel DB tanks and engine room bulkheads.


  6. The ideal place to monitor the temperature of the load is the centre of the stow. If the temperature is measured from the side, top or corners of the hold it must be remembered that the temperature at the centre of the cargo will be several degrees higher.


  7. A no smoking policy should be fully implemented on the ship and hot work should not be allowed, particularly in the vicinity of cargo compartments.


  8. Except where the shipper provides specific instructions to ventilate the cargo, compartments containing coal should only be ventilated for the first 24 hours after departure from the loading port. There should be regular monitoring of atmosphere to check the concentration of pH of bilge water. Ventilation should only be continued if the amount of methane rises above the acceptable level. Coal should also be ventilated prior to discharge owing to the danger of the build-up of gases that could be ignited by a spark from the opening of the hatches.


  9. The gases may escape the cargo compartment to adjacent stores, mast houses, etc. These spaces should also be monitored on a regular basis.


  10. A higher pH value reading indicates the likelihood of increased corrosion. In such cases the bilges should be kept dry by pumping out any accumulated water. However, records should be kept in the deck logbook for the quantity of bilge water discharged to justify any claims of cargo shortage.


  11. If any suspected problem is observed during the passage the shipper should be contacted, not only to update their information but also to seek any clarification.

Special Precautions for Coals Emitting Methane

If the shipper has declared that the cargo is liable to emit methane or the methane concentration in the cargo compartment is above 20% of the Lower Explosive Limit (LEL), the following additional precautions should be taken:
  • Surface ventilation should be maintained
  • hatch covers should not be opened until the accumulated gases have been removed. Hatch covers should then be opened carefully ensuring no sparks are initiated
  • all enclosed spaces such as store rooms, tunnels and passageways, etc, should be thoroughly monitored for methane
  • enclosed hatch covers should be ventilated.

Special Precautions for Self-Heating Coals

If the shipper has declared that the cargo is liable to self-heat, then the following additional precautions should be taken:
  • If required, the Master should seek additional guidance from the shipper on the adequacy of the existing precautions
  • if analysis of the atmosphere of the cargo compartment indicates an increased carbon monoxide concentration, then


  • i) the hatch covers should be closed after loading and additional seals applied

    ii) surface ventilation should be kept to minimum

    iii) carbon monoxide concentration should be regularly measured and recorded

    iv) if the hold temperature exceeds 55C or the carbon monoxide concentration rises steadily, expert advice should be sought.


Read more on
  1. Special precaution & IMSBC code guideline for handling bulk coal


  2. Various grade coal -Anthracite coal & Bituminous coal Handling guides


  3. Coal loading and related safety considerations





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