Unsafe transfer of cargoes of wood pellets increases the possibility of fire

Photo by Steve Pomeroy, Empire Shipping Agency, Vancouver, B.C.
Photo by Steve Pomeroy, Empire Shipping Agency, Vancouver, B.C.

Peter Hazell, Assistant Vice President and Head of FDD Skuld, has looked at former coal fired power stations that have started to burn biomass as feedstock in order to enhance sustainability of fuel sources. This results in increased shipments of wood pellets intended to be burned.

Most of these shipments are completed without incident but there are significant hazards associated with the carriage of wood pellets that surveyors should be aware of.

Generally, to have safely transfer wood pellets the Club advises always to follow standard enclosed space entry procedures.

The wood pellets can include a binder additive but not all do so. Each of these types can self-heat when in bulk form.

As Mr Hazell noted, the IMSBC code has two plans for wood pellets. The only difference between them is that pellets with or without binders can be categorised as one that could involve flammable gases when wet. Yet, both are Group B commodities with the principle hazard when in bulk form being the oxygen depletion and carbon monoxide evolution.

In addition, the IMSBC Code advises that wood pellets can experience microbiological deterioration/fermentation when at a moisture content of over 15%. Therefore, cargoes of this kind are most often shipped at lower moisture levels, whereas the Code notes that gas concentrations resulting from fermentation tend not to reach flammable levels.

It is also known that wood pellets can result to fire by oxidation of the fibrous material. Although intact wood doesn’t heat that much by oxidation to cause any accidents, the wood pellets consist of fibrous material from the wood itself which has been broken up and reconstituted into pellets.

This exposes the fibres to the oxygen in the air adjacent to a pellet and the fibres can then slowly oxidise. This process depletes oxygen in a space containing freshly-loaded wood pellets. Fine material produced from the mechanical breakdown of pellets during handling can be expected to be more prone to oxidation.

The oxidation of wood pellets increases the heat. But if the heat is generated slowly the cargo temperature may rise a little and the situation will stabilise. Additionally, it is possible for wood pellet heating to become self-accelerating to the point of fire.

Concerning oxidation and the dangers of it, the most important factor to cause problems is the availability of oxygen.

Generally, it is recommended to transport wood pellets without applying any ventilation, meaning that any tendency towards self-heating and the consequences of that, including carbon monoxide evolution and oxygen depletion, is self-limiting. Britannia P&I recommends that cargo temperatures are taken if possible, using sounding pipes.

These unfortunately provide limited useful information on the temperatures elsewhere in a hold.

Transferring wood pellets without ventilation is also dangerous and might result to condensation wetting on the surfaces of the cargo stowages especially if the vessel is to be discharging in low ambient temperatures.

Under the possibility of smoke evolution, levels of carbon monoxide increasing above 100pm, or high temperatures experienced during carriage of wood pellets, all holds should be immediately sealed if not already and not opened until discharge is imminent.

It is however important to note that evolution of high levels of carbon monoxide is not necessarily an indication of a problem.

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