Summary

Grinding is very important among the methods of processing biomass before it is finally used. The reduction of the particle size is a mechanical process, and usually consists of cutting or triturating. The processes are conducted in machines with rotary working parts. It is assumed that the grinding process results in particle size reduction down to 80 mm. The material obtained with grinding has higher density, while process parameters selected properly allow for obtaining particles of defined size. During the process the specific surface of the raw material increases. These parameters significantly influence the process of biomass compaction and finally obtaining good quality pellets or briquettes. Biomass processing, e.g. by grinding, decreases the cellulose crystallinity, which is required in bioconversion processes. Furthermore, in the biomass hydrolysis reactions appropriate size of the input material is necessary to reduce heat and mass movement resistance.

Depending on the character and the properties of the biomass material, the grinding process will be different, and it will require the use of suitable devices. For the grinding of wood and wood waste biomass which have anisotropic properties – in addition are non-homogeneous materials – mills with spinning elements and appropriate separation sieves are used. The spinning elements may be beaters or multi-lobe rotors. Today there is a wide offer in the market, which makes it possible to select an optimum grinding system for each biomass type. Switch grass (straw, reed) requires grinding before further processing. Due to its character, switchgrass biomass requires the use of grinding devices that perform the size reduction process mainly by cutting.

The selection of the grinding machine is performed individually, taking into account to the following factors: degree of pressing, amount and type of contaminations, as well as the purpose of the grinding product. The most frequently used is the straw grinding product having 6-10 mm long fibres. Straw biomass having such a fibre size is obtained in slow-rotation cutting mills. The technology description shows the basic parameters characterising the biomass grinding process. Examples of machines used for the grinding of wood biomass, wood waste and straw are also presented.

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