Wood pelleting

Fragola Spa and the
wood pelleting industry

Fragola offers its technology with highly efficient and sturdy machinery for wood pelleting, to guarantee efficiency and reliability over time.

SELECTION AND CLEANING: vibrating pre-cleaner, disintegrator separator, winnower

GRINDING AND REFINING: hammer mill

PRESSING: pellet mill

COOLING: static cooler, rotary cooler-dryer

SCREENING: vibrating screen, cryloc

The wood pelleting industry process

The production process can vary significantly depending on the starting material (logs, chips, sawdust,…), moisture (dry, wet) and depending on the type of initial storage and/or the lay-out required.

In general, the sections into which the pelleting plant can be divided are as follows:

  • Chipping and/or primary grinding
  • Selection and preparation of the material
  • Secondary grinding
  • Drying
  • Refining
  • Pressing
  • Cooling and sieving
  • Packaging and palletising

1. CHIPPING AND/OR PRIMARY GRINDING
The material must be virgin wood (logs and/or sawmill residues), never fermented material or wood affected by bacterial or fungal processes that deplete its resinous and lignocellulosic components.

At the beginning of the line, a debarker is required to remove bark from the logs. Since bark is rich in silicates, it causes significant wear on components such as knives, hammers, dies and rollers.

Silica contained in the bark is also undesirable in pellets because, besides providing no calorific value, it causes clinker formation in stove combustion chambers.

The chipping section essentially consists of a loading system for logs and sawmill residues which, via conveyors and/or vibrating feeders and after passing through a metal detector, feed a drum chipper equipped with knives.

Through the action of a rotor fitted with cutting blades, the machine slices the material to obtain coarse chips (G30–G40). The produced chips are usually stored in a moving floor, a horizontal silo with hydraulic rake extraction.

2. MATERIAL SELECTION AND PREPARATION

Material preparation essentially consists of screening the material to remove oversize particles and contaminants, which may include stones and ferrous or non-ferrous materials.

There are multiple ways to carry out this phase.
An ideal selection is obtained using roller screens which, in addition to rejecting oversize material, allow classification of the product; depending on the particle size, the material will undergo different treatments.

Another machine suitable exclusively for removing heavy materials from the process is the pneumatic separator, which allows the removal of stones and metal pieces that are not excessively small.
Clearly, a ferrous metal separator is indispensable, such as a magnetic belt or a magnetic drum separator.
The selection and preparation section is strictly linked to the type and origin of the wood being processed; therefore, the less contaminated the material, the fewer devices are required in the process.

3. SECONDARY GRINDING

The previously selected material is sent to a grinder whose purpose is to prepare the moist material for the subsequent drying phase.
The small particle size favors moisture migration from the inside toward the outside.

Grinding takes place by means of a rotor equipped with hammers; for the description of operation, see the refining section below, since the process is similar.

The result is homogeneous material of correct size for the dehydration phase.

4. DRYING

The drying phase has the function of extracting water from the raw material.

The moisture content of the material at the outlet will be approximately 13%; this moisture level allows the success of the subsequent processing stages and, due to the low moisture content that is further reduced in the following phases, allows storage and preservation of pellets without mold formation.

The dryer is a belt dryer equipped with heat exchangers.

5. REFINING

High peripheral speed swinging hammer mills are used for the impact disintegration of materials having different physical and mechanical characteristics.

The fragmentation process is carried out by exploiting mass forces; the material to be ground is discharged into the grinding chamber through a tangential inlet located at the top of the machine.
As soon as it enters the action field of the high-speed rotating beaters, the material receives high instantaneous acceleration and intensive fragmentation occurs.
The material is then thrown against the walls of the grinding chamber, obtaining further reduction by rebound; during its subsequent path over the screening surface, once the desired fineness is reached, it is discharged through the airflow that creates negative pressure in the grinding chamber; the remaining particles continue to undergo the constant action of the hammers until reaching the required size.

6. PELLETIZING

The extrusion section essentially consists of the pellet mill which, through mechanical action alone, transforms the product from sawdust into pellets.
Compaction is obtained through the compression of the rollers against the die, producing pellets at a temperature of approximately 80–90°C.
Before the pellet mill there is a fast mixer whose function is to homogenize the material and allow the addition of a small quantity of water.

The extruded pellet is cut by knives.

7. COOLING AND SCREENING

The counterflow cooler lowers the temperature of the pellet.

Indicatively, every reduction of 11°C corresponds to a 1% reduction in moisture.
Freshly extruded pellets must be cooled and dried to obtain a product suitable for storage and/or bagging.
When leaving the pellet mill, the pellet has a relatively fibrous structure that allows moisture to migrate from the inside to the outside due to capillary action; the cooler is designed to bring ambient air into close contact with the external surface of the pellet.

This air, which is not 100% saturated, removes moisture from the pellet surface.

Moisture is removed through an evaporation process, and this evaporation process causes the cooling of the pellet.

8. PACKAGING AND PALLETIZING

The pellets are conveyed to the weighing and bagging machine, which performs weighing and packaging into bags.

The finished product bags, through a palletizer, are arranged to form the desired pallet configuration.

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