200756 - Wood pellets - fire hazards/CO2 emissions
| Wood pellets - fire hazards/CO2 emissions |
MARS Report 200756 |
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There have been several recent incidents of stevedores being overcome by fumes associated with the carriage of wood pellets. This cargo mainly originates in North America and Scandinavia.
The pellets are produced by drying sawdust and wood shavings and then milling them into particles of up to approximately 2mm particle size. The particles are then compressed to approximately a third of the original volume into pellets which are typically 10 - 20mm long and 3 - 12mm in diameter. The compression leads to an increase in temperature and stored bulk piles of wood pellets can self-heat in parts with high moisture contents and it is reported that this process can lead to the spontaneous combustion of the material after a period of time.
Due to transport movements and physical handling, some breakage of the pellets occurs and this means that the material loaded aboard a ship consists of pellets, pieces of broken pellets and wood dust. Further, the wood pellets are readily combustible and can be ignited by a range of ignition sources, while the dust associated with the pellets, when dispersed in the air and ignited, can give rise to a dust explosion.
In addition to the combustion hazards, wood pellets also undergo oxidation to produce carbon monoxide and carbon dioxide. In a closed space such as an unventilated ship's hold, this can lead to a dangerous reduction in the oxygen concentration in the hold as well as the development of a dangerous concentration of carbon monoxide which is toxic (and flammable). In a recent case a carbon monoxide concentration of approximately 1 per cent was measured in a sealed cargo hold of a ship containing wood pellets some 18 days after the cargo was loaded. The oxygen concentration at this time was less than 1 per cent.
The commodity has previously sometimes been classed as 'wood pulp pellets', which is entered in Appendix B of the solid bulk cargoes (BC) Code. However wood pulp is not normally formed into pellets and the wood pellets are not pulp. Furthermore, the entry for wood pulp pellets, while referring to oxygen depletion and the generation of carbon dioxide, does not refer to the formation of carbon monoxide. As a result, in the 2005 Edition of the BC Code, a new entry for wood pellets refers specifically to the hazard associated with the generation of carbon monoxide. Accordingly, ships' officers must ensure that both stevedores and ships' crews and others who may need to enter a cargo hold which has, or had recently contained wood pellets, are made fully aware of the dangers and that all appropriate precautions are taken.