How are wood pellets produced?

Wood pellets are made either from the bits of wood left over from other uses of wood (e.g. the sawdust from timber mills), or from wood that is grown and harvested for the purpose of energy-production (e.g. from forests or "short-rotation coppicing").

The wood needs to be dried to a very low moisture-content (< 12%) before it can be pelletised. Smaller chunks of wood are easier to dry, so the first step, if the wood is not already in small pieces (e.g. sawdust) is to chip the wood into small chunks. The chunks or sawdust are then put through a dryer.

The dried wood is ground to produce very fine particles. These particles are squeezed through a die - typically a drum with lots of holes and a press rotating inside, so that the wood particles fed into the drum are squeezed through the holes at high pressure. The squeezing also raises the temperature of the wood as it passes through the holes. The combination of temperature and pressure causes the lignin in the wood to melt and bind the particles together, creating solid cylinders of wood as they emerge from the holes in the die. The cylinders either break off once a certain length has emerged from the die, or they are cut automatically if they reach the maximum allowable length (five times the width) before they break off.

The pellets are cooled as they are moved away from the die, and screened to remove the smaller particles that were not successfully bound into the pellets. Once the pellets are cool and clean enough, they are put into store, ready either to be bagged or loaded loose onto lorries for delivery in bulk.