What Are Wood Pellets?

Applications

Wood pellets supply heat to thermal power stations and industrial boilers, and are also used as the energy source for domestic and central heating systems.

Where They Come From

The raw material is naturally dried wood residue — sawdust, shavings and trees that have died naturally in the forest — with no chemical binders added.

How They Are Made

The cell walls of wood fibre consist mainly of lignin, cellulose and hemicellulose. Under high pressure the resulting heat ruptures those cell walls and releases the lignin, so no chemical binder is needed. Temperature and moisture content must be controlled throughout the process, with moisture held between 10% and 15%.

Production begins by passing the wood material through a hammer mill until it forms a uniform, dough-like mass. This is fed into a press and extruded through a die whose holes are sized to the required dimensions — typically 0.6 cm in diameter and 2–3 cm long. The finished pellets pass through a final screen to remove rejects before being trucked to the customer.

Quality Control

Most wood pellets on the market originate from sawmills, and the quality of the sawdust determines the calorific performance of the finished pellet — which makes quality control during processing essential. A number of European countries and international bodies have therefore established certification standards, among them Germany’s DIN 51731 and Austria’s ÖNORM M 7135. Since 2002 the European Biomass Association (AEBIOM) has consolidated these into EN plus.

Sources:
Weng, F.-Y. (2008). Wood Pellets: Star of the New Energy Era. Energy Monthly, March, pp. 23–26.
Weng, F.-Y. (2009). Wood Pellet Development in Europe and North America. Energy Monthly, August, pp. 28–31.
Wikipedia.