The birth of the wood pellet can be traced back to 1973 when, in Idaho (USA), an engineer created a new kind of wooden fuel. In the beginning, it was created for an industrial use, but rapidly its use spread to the domestic boilers market. By the term pellet we mean a densified bio-combustible, normally cylindrical-shaped (usually with a variable diameter between 5 and 8 mm and length of 10-20 mm), that is obtained by pressuring the pulverized biomass with or without the help of pressing bidding. At present, the pellet is exclusively obtained from the wooden biomass, but it is necessary to underline the existence of studies that aim to develop the pellet spinneret from herbaceous cultures or from a mixture of these cultures with wooden biomasses. However, the different quality of the starting biomass, especially in terms of calorific power and cinder content, leads to unavoidably dif­ferent final qualities of the pellet, for different purposes. To avoid these variations, we should make use of the pellet spinneret production technology for the two different biomass typologies [2, 20].

Through a frame, the pellet making process can be divided into the following phases: drying, cutting, pelletization, cooling, separation, storage/bagging. In some cases (if the typology of the biomass requires), before the drying phase there are also the phases of roughing, grinding and deferrization. If the working material is rough-shaped (logs or slashes), grinding should be done first, using rotor knives the biomass is reduced to flakes.

Before entering the grinding mill, the raw material is subjected to the magnetic action, which separates from the raw material iron elements (ironing) whose pres­ence could damage the extruder. The most well-known technologies of pelletiza­tion do not allow the raw material pressing, if this has a high humidity. After the primary crushing, it is necessary for the material to be dried (usually through a rotary dryer). In this way, the biomass reaches the appropriate humidity grade and allows the lignin that is present in the raw material to be the binding material. At the end of the drying phase, the material reaches a maximum humidity of 10% which allows proceeding with the following treatment steps.

In the second step of grinding, or before the grinding step (if the starting mate­rial in the production cycle already has small dimensions: shavings, sawdust, wooden or herbaceous chips, etc.), the material is crushed (usually through a ham­mer mill) to reduce and level the width down to 3 mm. Such dimensions allow to obtain a standard characteristic for the product. After the material crushing, it passes to the conditioning step, where it is prepared to enter the pelletization spin­neret. This phase can also include the embedding of bidding or additive agents. Usually, this conditioning operation is realized using dry water vapour to frizz the wooden fibres and to induce a partial gelling of the biomass.

The pelletization that results from compression comprises perforated, cylindrical and flat forms (matrix) through whose holes the conditioned bio­mass is pushed at high pressure (up to 200 atm) using roll systems. The pellet is formed due to the transformations that happen during the passage of the fibre through the extrusion holes, when the temperature is up to 90°C. At these temperatures, there is a fluidization of the lignin that comes out from the cellular structure; this allows the fibres to stick to each other. Appropriate blades cut, at the desired length, the compressed material and the surface bakelized that overflows from the matrix holes. The extruded and cut material then passes to the following cooling step (realized through ventilation plants), where the product undergoes hardening. Subsequently, in the separation sec­tion, the whole pellet is deleted and it is re-entered in the extrusion system. At the end of the cycle, the pellet is stored in silos or bagged (storage/bagging steps) [2]. The mechanical energy spent for the pellet production is equal to the 2% of the final product’s energy content. As a consequence, pellets are considerably better than the fossil energy sources, for which 10-12% of their energy content is used for refining [1]. The cost of pellet production varies between 0.05 and 0.16 €/kg based on the applied technology and on the biomass used. The wholesale price is equal to 0.11-0.21 €/kg, whereas the detail price is 0.21-0.30 €/kg [20]. The national pellet production, which was estimated at around 240,000 tons in the 2004/2005 season, shows a strong growing trend [22].

The pellet changing allows obtaining a product characterized by high energy density. It is also easily transportable: in terms of motion, in fact, it acts simi­lar to fluids. This allows a high grade of automation of the tools and the com­bustion plants. This pellet property is due to the particular from, dimension and homogeneity of its elements that can be sent to the combustion oven through simple mechanical instruments (transport foils, scrolls or suction systems), with important advantages such as automatic control, dosing and continuous feed [1, 2].

Compared to the non-densified biomasses (sawdust, chips and slashes, etc.),

pellets show some advantages that make it more valuable in the market [2]:

1. High apparent density (bulk density, bio-fuel volume for unit mass): vari­able between 650 and 780 kg/m3. The pellet density is seven times higher than the density of sawdust and chips and this optimizes its transport and storage.

2. Low humidity content: low hydric content improves the combustion yield and contributes to reduced transportation costs. Furthermore, during the storage phase, the combustible does not show risk of undergoing fermentative phenomena.

3. High calorific power for unit weight: it depends on the composition and the structure of the biomass applied for pellet making. The wood pellet has a LCP of about 4,000 kcal/kg (high energetic weight among the bio-fuels).

4. Homogeneity of the material has both physical and qualitative characteris­tics points of view: The first, together with the small dimensions of the pellet, allows to easily move the product through scrolls, transport foils or wheel suction systems and the possibility to use them in automotive boilers, whereas the second allows a better regulation of the combustion and a better control of the emissions.