Knowledge April 8, 2017

Handling of Wood Pellets

The energy density of a fuel is prime consideration in reducing shipping costs. This is why most of the biomass exported from Canada to Europe is in the form of densified wood pellets.
Wood pellets tend to break up every time they are handled; they break up at every transfer point and even from rubbing together when being transported. When they break up, a very fine, talc-like dust is produced, which is difficult to contain and presents a significant fire and explosion risk. The more times pellets are handled the more they break up and the greater the fibre loss.
Current practices have pellets being handled many times between the manufacturing plant and the transport ship. In addition, most wood pellets are shipped in bulk form in 30 – 40,000 tonne ships. In order to avoid demurrage costs, these ships must be loaded and unloaded as quickly as possible; this requires that high capacity, material handling facilities be constructed at the loading and unloading ports. Care must be taken in the design of these systems to minimize pellet degradation.

Conveyor Systems

All conveying systems that encourage impacts or rubbing, wedging or grinding actions should be avoided; including chain conveyors and screw conveyor feeder / extractors. The least damaging method of handling wood pellets is carrying them on belt conveyors; however, care must be taken to properly design the conveyor transfers to minimize the impact points. Additionally, care must be taken to protect the wood pellets from the weather.
Conveyors should be equipped with heat, smoke and flame sensors, sprinklers and fire suppression equipment.

Storage and Reclaiming

Wood pellets must be stored under cover and kept dry to minimize deterioration due to weather. Their good flowability allows the use of storage silos, but care must be taken in the design of silo loader and discharger. Pellets falling from a great height will be broken; therefore a spiral loading chute or a cascade loading chute is preferred.
The action of mechanical dischargers underneath the load of a high silo can cause significant damage to pellets. However, pellets flow well, therefore silos can be designed without a mechanical discharger; but care must be taken with the design of the silo bottom.
A good silo discharging system would utilize a long belt feeder passing underneath the centerline of a line of silos. Each silo has a bottom that is sloped towards the center of the silo; multiple gates above the belt feeder can be sequentially opened to allow the pellets to flow through.
Silos and hoppers should be equipped with deluge systems and rupture vents and dust suppression should be considered.

Land Transportation

For long overland distances, train transport is usually more economical than that by truck. However, standard gondola cars that require emptying on a roll-over dumper should be avoided due to the pellet damage associated with the high material impacts. Bottom-discharge gondola cars are suitable.

Screening / Metal Removal

Many importers insist on the product being screened to remove the fines immediately prior to ship loading.

Screens that impart vibratory or wedging and grinding actions should be avoided, as they will contribute to product degradation. Wood pellets are best screened on flat, gyratory screens.

Drum magnets and self-cleaning belt magnets are suitable for removing tramp ferrous metal from wood pellets. The best location for tramp metal removal is over a belt conveyor head pulley where the material is suspended in air.

Dust Collection / Fire Protection

When handled, wood pellets produce a very fine dust. The dust is highly flammable and explosive, so all enclosures should be well ventilated and the ventilation air processed through a dust collector. Well designed and maintained dust collection systems utilizing baghouses are essential. High efficiency cyclones have difficulty separating the fine dust particles from the air stream and should be utilized only as a pre-separator, prior to a baghouse. Dust collection systems should be equipped with spark detection systems, deluge systems and abort gates. However, it is far better to design the handling systems to minimize the amount of dust generated.

Ship Unloading

Pneumatic ship unloaders are particularly hard on wood pellets and should be avoided. Clamshell bucket unloaders or screw unloaders also cause pellet damage.
Wood pellets flow well and are best transported in self-unloading ships which utilize special-built holds with live-bottom belt feeders that discharge into a sandwich-belt type of ship unloader that elevates the pellets up and out of the ship.