Wood pellets are made from sawdust which is a by-product of wood processing. Sawdust is compressed through dies by high pressure rollers to produce very dense pellets of either 6 or 8 mm diameter.
The quality of the wood pellet is of paramount importance. A pellet should have an ash melting point of greater than 1300 °C, be 100% wood with no bark, no recycled element or other outside contaminants. They should have a moisture content of between 6% and 8%. If these standards are not met the boiler efficiency will be compromised. A good pellet will be a uniform (yellow) colour throughout and will have an energy content similar to that of high quality coal.
Heating with pellets is very suitable for the domestic market because they are clean, easy to handle and store, easy to ignite and they flow like liquid, making most boilers as fully automatic as gas and oil burners, but more efficient.
A European standard (CEN/TC 335) is being developed for wood pellet fuel.
Pellet delivery
Pellets can be delivered in two forms of packaging: bagged or bulk. The bagged pellets usually come in 15 Kg bags. This is a more expensive way of buying wood pellets but, generally, less space is required for their storage.
Buying in bulk is the cheaper way of receiving pellets but a purpose made hopper is required to receive the pellets. Also, in this situation there needs to be a feed system for delivering the pellets to the boiler. Usually the pellets are either fed to the boiler by an auger or they are blown using an air driven system.
Storage Conditions
Wood pellets need to be stored in a dry environment. The absorption of moisture into the pellet will reduce the calorific value and, therefore, the effectiveness of the pellet at producing heat. Therefore, the silo should be in a covered area which is well ventilated.
