This study aims to derive a qualitative model for energy requirements of the wood chipping process. A relationship is shown between energy requirements and properties of biomass, which is a quite variable material.The relationship between comminution machinery and energy which is necessary for the process is highlighted. The derivation of the model is focused on chipping, but it is generally possible to make it available for both different types of biomass (f. ex. agricultural residues)and different types of comminution machinery (f. ex. hammermills) by using different material properties adjusted to the machinery mechanics. The properties which are used in the derivation are meant to be easy to measure. Furthermore, the model is meant to be used as a base for a quantitative model that, thanks to measurements taken from real comminution machinery and thanks to using wood with known properties, could answer two important questions: - Would hypothetical changes in the desired size of output material increase the total system efficiency, taking into consideration the lowest efficiency of the combustion process (e.g., higher amounts of unburned fuel)? - Considering the energy used for the process, how can comminution as an operation in the biofuel supply chain be optimised? Answers for the above questions could highlight new possibilities in terms of further energy savings and a maximising of the energy efficiency of the bioenergy sector. Furthermore, the results could motivate optimized choices of comminution machinery for the biofuel supply chain as well as for other applications. Another important feature of this study is its unique holistic point of view that takes into consideration aspects from the fields of mechanics, material sciences and natural sciences to deliver the full picture to the reader.

Textprobe: Kapitel 1.3, Structure of biomass - wood as an example: Biomass has a composite structure. It consists of fibres which are made of cellulose and matrix that binds fibres together. Matrix consists of hemicelluloses and in case of ligno-cellulosic (woody) biomass also lignin. Biomass is highly anisotropic material which means that it has different properties, strongly depending on coordinates - namely fibre (cell wall) direction. The most important thing about wood that should be understood is a basic fact that it has evolved for millions of years to serve three main functions in a plant as an organism (U.S. Forest Products Labolatory, 2010): - conduction of water and nutrients from the roots to the leaves. - mechanical support of the plant body. - storage of bio-chemicals. ‘There is no property of wood, no matter physical, mechanical, chemical, biological or technological - that is not fundamentally derived from the fact that wood is formed to meet the needs of the living tree. By understanding the function of wood in the living tree, we can better understand the strengths and limitation it presents.’ (U.S. Forest Products Labolatory, 2010). In most of the cases wood is used as a material in terms of trees, when stumps and leaves are usually not utilised. In Bioenergy segment this statement is also true and in case of herbaceous biomass stalk is the main part being used (straw) and although it looks little bit different it’s designed by nature to meet the similar needs. Properties concerning comminution of woody biomass are to some extend true also for other types of biomass as well as other fibrous materials which are mostly of biomass origin. Trunk of the tree (stem) is composed of various materials present in the concentric bands (U.S. Forest Products Labolatory, 2010): - Outer bark (Fig. 1.2 - ob) provides mechanical protection of the softer inner bark and also helps to limit evaporative water loss. - Inner bark (Fig. 1.2 - ib) it’s the tissue through which sugars (food) produced by photosynthesis are translocated from the leaves to the roots or growing parts of the tree. Minerals and nutrients are also transported from the roots to the green parts. - Vascular cambium (Fig. 1.2 - vc) is the layer between bark and the wood that produces both of these tissues each year. - Sapwood the active tissue which is responsible not only conduction of sap and water but also for storage and synthesis of photosynthate like starch and lipids. - Heartwood is a darker-coloured wood in the middle of most trees. It’s not conductive and functions as a long term storage of biochemicals (extractives). Extractives are formed by parenchyma cells at the heartwood-sapwood boundary and then exuded through pits into adjacent cells (U.S. Forest Products Labolatory, 2010). - Pitch (Fig. 1.6 - p) is located at the very centre of the trunk and is the remnant of early growth of the trunk before it was formed.Figure 1.2 - Macroscopic view of a transverse section of a trunk (U.S. Forest Products Labolatory, 2010).

• Biomass

• Bioenergy

• Comminution

• Chipping

• Wood