Coal is categorised according to the hazards associated with it. Whenever coal is shipped from any place, the history of the previous shipments must be known, so as to be aware of the hazards of the particular type of coal.
Coal may have any or all of the following hazards:
Coal is very liable to spontaneus heating. Freshly mixed coal absorbs oxygen, forming peroxides which break up into carbon monoxide and carbon dioxide. This is an exothermic reaction and the heat produced causes further oxidation and more heat.
Co (carbon monoxide) has a large flammable range (12% to 75%) by volume and besides is also highly toxic. If this heat is not dissipated then spontaneous combustion can occur.
Oxidation depends on the surface area available for absorbtion of oxygen, hence breakage of coal into smaller pieces while it is being loaded is to be prevented.
If conciderable breakage occurs the smaller pieces shall accumulate in the centre of the hold while the larger pieces shall roll to the sides, this action aggravates the situation as the large pieces of coal give way for the air to flow to the smaller pieces where spontaneous combustion is most likely to occur.
Emissions of methane:
Coal emits methane immediately after loading and when newly worked or freshly broken. Methane is a flammable gas and when mixed with air forms an explosive mixture. It is lighter than air and therefore accumulates in the upper regions of the hold or other spaces. This gas can find itself into tanks cofferdams etc and pose a flammablity hazard so these compartments have to be well ventillated at all times and tested before man entry. Recommendations for tank entry procedures and check lists shall be provided in the bulk carrier code appenidx f.
‘pond coal’ is the term given to coal left over from earlier mining operations which has been dumped into freshwater ponds and later reclaimed for shipment. It has a high moisture and sulphur content. This type of coal releases high temperatures from self heating and the sulphur content reacts with water to give off sulphuric acid resulting in corrosion of the ships hull. The ship should have instruments for measuring the p.h. Content of the bilge waters and this must be done regularly.
It is a process where the moisture in the cargo migrates to the surface due to compaction and vibration resulting in the development of a flow state. This is particular in the case of coal slurry, coal duff and mud coal. The surface of the cargo behaves like a liquid and a transverse shift developes, if this occurs in reduced ships stability the condition is extremely dangerous.