The pellet production process consists of the following stages:
1st Stage - Receipt and Processing of Feedstock: Depending on the type and size of raw materials which are received at the factory, various processing machines are used, such as wood shredders, wood or bale chippers and grinders which reduce the feedstock size to chips of a mean diameter of 3 cm or even directly to wood sawdust. The raw material pre-processing stage is very often complemented by transporting and lifting devices in order to add automation.
2nd Stage - Drying: When the raw material acquires a suitable size, it is transferred to the biomass dryer which aims at the reduction of moisture content to a range of 10-15%, which is the required level necessary before the pelletisation process begins. The drying cost accounts for a significant percentage (usually between 28-35%) on the total production cost of a pellet factory, therefore it is necessary to use low-cost biomass residues which cannot be transformed to quality pellets as combustion materials in the dryer’s burner.
3th Stage - Grinding: Immediately after drying, the raw material wood chips are being grinded by means of one or more hammer mills and take the form of saw-dust or wood powder with a mean diameter of less than 5mm, while in parallel the raw material is being released from metal particles through the use of a metal separator.
4th Stage – Pelletisation: Before the startup of material compression for pelletising, it is necessary to add 1-2% of moisture at the form of steam (conditioning) and then to heat the material at a temperature of 70-80°C in order to release the natural lignin inherent in wood. Lignin is the bonding material which provides dimensional stability of wood pellet end products. As for agri-pellets (or agro-pellets), which do not contain lignin, other natural connecting additives are used, as for instance starch. During the compression of wood powder, the press is compressing the material through a perforated metal die of suitable hardness, through which the pellets are passing. There are two basic designs of pellet mill dies: The flat type die and the ring type die. The flat type die is being used at configurations of lower productivity (usually up to 1 ton per hour). According to this arrangement, a pair of rollers is revolving at very high speed at a few tenths of a millimeter above a circular flat die with holes of equivalent diameter to that of the final product. In this way, the material is being entrained and compressed through those holes. In the case of a ring type die, which is used at larger factories, there are small rollers running around the inside edge of the die and eject the compressed pellets through the holes which are present on the die’s edge. The diameter of the final products depends on the diameter of the die holes. Τhe most frequently used diameter of pellets for domestic heating stoves and burners is 6mm, whereas for industrial burners can be 8 to 10 mm.
5th Stage - Cooling: The hot and elastic pellets are exiting from the pellet mill and are transferred to a colling station in order to cool down to room temperature. Cooling procedure improves pellet durability and reduces dust formulation during transportation and storage of the end products. Further, the pellets are being screened through a suitable sieve and the surplus dust is fed back to the pelletising process.
6th Stage - Packaging and Storage: After the cooling and screening processes are complete, pellets are transferred by means of a bucket elevator to a raised end product buffer silo prior to getting weighed and packaged either into individual 15kg or 20kg bags for domestic heating customers or into jumbo bags of 500-1000 kg for industrial customers.