Summary

During the processing and transport of biomass we may encounter some technical and economic problems, which are mainly related to the large volume of biomass, causing high transport costs and increased requirements for storage space. Also, high water content causes biological degradation as well as the freezing of fuel, which brings some obstacles in the transport system. In addition, differences in water content cause problems in finding the optimal operation and management of energy facilities. All these problems can be partially minimized by the densification of the material, which provides more uniform fuel properties.

The main advantages of fuel densification compared with not densified fuels are as follows:

• Increased bulk density (from 80-150 kg/m³ for straw and 200 kgm³ for sawdust up to 600-700 kg/m³ after the densification, although it is possible to achieve even higher values). This reduces transport costs, reduces the need for very large storage spaces, and simplifies the handling of the fuel.
• Increased energy density and more homogeneous composition provide better options for burning, and therefore a more efficient combustion process.

The main disadvantage of densification is the relatively high cost of input energy, which is needed for the production of pellets and briquettes. This will also increase the price of the output product. The output product is in the form of briquettes or pellets. Calorific value, water content and chemical composition are approximately same for both, but the density and strength is usually higher for pellets. The main difference is in size. Pellets are 4-5 times longer than their diameter (ranging between about 6-12 mm), while briquettes have a diameter of 80-90 mm, or dimensions 150 x 70 x 60 mm in the case of prisms. Briquettes and pellets provide a simple use in the fully automatic operation from home use to large scale applications.

Briquettes are primarily used instead of firewood for manually charged domestic stoves. Pellets can be used in automatically charged stoves and boilers due to their good flowability, uniform water content, grain size and chemical composition. Consequently, pellet stoves and boilers for domestic applications offer a comfortable operation similar to oil-fired systems. In Sweden pellets are primarily used to substitute coal in large-scale power plants. For this purpose, sawdust is only pelletized in order to reduce transport and storage cost, and the pellets are milled before combustion.

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