The production stages of the pellets

The production stages of the pellets

Wood pellets are produced from raw materials-by-products of the wood industry (e.g. from sawmills), from logging residues and from special sustainable forestry crops with a short rotation period. In addition, there is the possibility of producing agropellets which come from agricultural residues (e.g. straw) or from crop prunings. Agropellets are at a disadvantage compared to wood pellets mainly due to their significantly higher ash content.

There are 7 different stages in the production process of biomass pellets:

1) Storage of raw materials

2) Cleaning of raw materials from impurities Pretreatment

3) Biomass drying Biomass

4) Grinding-shredding of biomass

5) Pelletization

6) Cooling and Sieving of Pellets Production

7) Storing the pellets. Pellets

1) Storage of raw materials

A good raw material storage system is necessary to keep the biomass free from impurities and to protect it from rain, which can increase its moisture content to such an extent that it is unprofitable to dry it (and thus to use it in the production process). Automated feeding of raw material from storage to the production area (e.g. via conveyor belt or screw) is often preferred to reduce labor costs.

2) Purification of raw materials from impurities

This stage is essential especially in cases where recycled or raw wood (eg pallets) is used as raw material. So many units use mechanisms to separate timber from aggregates, such as stones, or magnetic separators to capture metal objects, such as nails and dowels. The presence of even the smallest amount of such impurities in the final product is unacceptable, while at the same time they can cause serious damage to the equipment of the production process, e.g. in the hammer mill and press.

3) Biomass drying

The majority of materials used for the production of pellets require drying in order to produce a product of satisfactory quality. Only a few dry-harvested materials, such as straw, can bypass this stage of the production process. Drying the biomass to a level between 10 and 15% is essential. The biomass dryers used are mostly drum dryers, although there are also several references to belt dryers. The fuel used to dry the biomass is either natural gas or part of the biomass itself, the second option being preferred for environmental, technical and economic reasons. It is noted that the specific stage is the most energy-intensive of the production process, while it contributes to the maximum extent to the operational costs of the unit. Consequently, the majority of research efforts and innovations in the production of pellets are located in the process of successfully drying the raw material.

4) Biomass Shredding-Grinding

Before introducing it into the pelletizing press, it is necessary that the biomass has acquired the appropriate homogeneity and granulometry. These characteristics are achieved by the use of various types of machines for cutting and grinding the raw material, such as hammer mills. The size of the chopped biomass to be fed into the press is not random: the particles must not be large enough to fit through the holes of the press die. On the other hand, however, if the material is very fine, there is a lower conversion efficiency of the biomass into pellets, as it is more difficult to aggregate the particles.

After the necessary properties have been obtained from the biomass in terms of its purity, its moisture and its dimensions, it is introduced into the basic equipment for forming and producing pellets: the pellet press.

5) Pelletization

The pellets are produced by pressing the granulated biomass in a special press through the holes of a stainless steel mold with which it is equipped and which gives the pellets their characteristic cylindrical shape. The capacity of each pellet press ranges from a few tens of kilograms to a few tens of tons per hour. Correspondingly, there is a large variation in the performance of the different types of press.

Applying high pressures to the incoming pelletized biomass forces the material into a movement through the holes of the circular die, with the diameter of the specific holes ultimately determining the diameter of the pellets produced. As the applied pressure increases, the friction between the biomass grains increases with a direct consequence of the increase in its temperature. The elevated temperature softens lignin (one of the three main groups of biomass compounds along with cellulose and hemicelluloses) which acts as an adhesive between the biomass particles. Setting the appropriate temperature to do this is achieved through proper mold design, which should create the required level of resistance to the flow of biomass particles to develop the desired pressures - and thus temperatures. If the mold holes are larger than they should be, the material easily escapes through them and sufficient pressure-temperatures are not developed to produce a solid product without chips. The existence, of course, of very small holes in the molds implies a sharp increase in temperature, the partial pyrolysis (incomplete combustion) of some particles and the production of a degraded quality of the final product.

The pelletizing stage is the only phase of the pellet production process in which additional compounds may be used. More specifically, to strengthen the cohesiveness of the pellets, some natural adhesive, such as starch, is sometimes added. The use or not of natural additives basically depends on the qualitative composition of the raw material, i.e. on the ratio of lignin to cellulose, hemicelluloses and ash of the material. The maximum allowed amount of additives in the final product is determined based on international quality standards.

With proper design of the press mold and careful control of the whole process, the pellets come out of the press with moisture less than 10%, density more than 600 kg/m³ and calorific value more than 4.7 kWh/kg.

6) Cooling and screening of the pellets

The pellets coming out of the press are too soft and hot (70-90 °C) to be stored and packaged as they are. Consequently, they are allowed to cool in air through a conveyor belt to stabilize the lignin as an adhesive for the cellulose fibers. After cooling and obtaining the desired hardness, the pellets are passed through industrial sieves from where they are separated from dust and debris. It is noted that the presence of debris in the final product may make it out of specification and make it difficult for it to be absorbed by the market. The separated chips are recycled to minimize biomass loss and increase overall process efficiency.

7) Storing the pellets.

As pellets are a high-quality fuel, its storage conditions are necessary to ensure the preservation of its quality. Storing it in standard bags of constant weight where they are protected from environmental impurities and moisture is a common practice, especially when the main consumers are small domestic users. In case of bulk storage, it is again advisable to place them either in a container or in a silo, likewise to protect them from moisture and impurities.

It is a logical conclusion that all the above individual stages for the production of the pellets must consume significant electrical energy. However, their environmental dimension cannot be dismissed: it is estimated that the energy requirements for the production of the pellets correspond to less than 22% of their energy content, leaving the pellets with a positive energy sign.

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