In the wood pelletization process, raw wood is compacted into a homogeneous product with higher energy density and lower moisture content and made into uniformly sized cylindrical shapes, facilitating transportation, handling, and usage.
Pellets can be produced from round wood but have mostly been made from cheaper waste residues of other wood-processing activities, primarily sawdust and shavings from sawmills and furniture factories. If made from roundwood, the full range of steps involving debarking, chipping, drying, and hammermilling must be done. Residues require less preparation because they are already much reduced in size, are mostly bark free, and are drier. Either way, the moisture content is a critical variable and must be confined within a range of about 12% to 17% (wet basis).
Otherwise, if too dry, the heat build-up induced by friction in the pelletizer burns the surfaces, but if too wet, the trapped steam pressure weakens internal bonds and reduces the mechanical properties, increasing breakage and dust during subsequent handling.
Once dried to specifications, particles are sorted by size and overly large pieces are hammermilled to gain further size reduction. Steam conditioning may be used to soften the lignin that binds the cellulose together to facilitate pellet formation during extrusion and shape consolidation thereafter. Finally, binding agents may be added to minimize breakage during transport, though for most uses that is not necessary because lignin acts as a binder. Some additives may also be applied to improve chemical characteristics, such as kaolin or calcium oxide to limit slagging.
Following these preparations, particles are extruded through dies and the emerging ribbons are cut to desired lengths. The hot pellets are cooled in a counter-flow cooler to allow the lignin to reset and form a hardened, compact unit. Finally, the finished product is bagged or shipped in bulk to market.
A variant of pelletization is the use of heat-treated wood called torrefied wood. Torrefaction is a somewhat slow (30 to 90 min) thermo-chemical treatment of biomass at a mild temperature range of between 200 and 300 °C (392 and 572 °F) in the absence of oxygen (Bergman and Kiel 2005). Torrefaction changes the properties of biomass: hemicellulose largely volatilizes and the remaining mass becomes hydrophobic, an important improvement from the viewpoint of transportation. Loss of hemicellulose also reduces the wood’s fibrous nature, improving its ability to be ground. The process volatilizes the organics in wood, losing some energy but increasing energy density of the remaining mass. However, most of the lignin is conserved, meaning that pelletization can proceed without the need for binder additives. The first torrefaction plant, a $12 million, 150,000-tonne facility in Georgia, is being built and is expected to begin production in late 2009. In a study investigating costs of furnish to gasification plants, torrefied pellets were deemed the most economical.