Wood pellets are renewable energy resources and are produced by pressing of sawmill residue and industrial waste wood. During transportation wood pellets are exposed to mechanical loading, storage and operations. Owing to various stress types, breakage and attrition of wood pellets take place resulting in an increase of fine fraction of the bulk. Investigation of the breakage and attrition mechanisms as well as parameters, which influence these mechanisms, can help to predict the fine fraction formation resulting from different conveying techniques or a change of process parameters, such as conveying velocity, mass flow or ambient conditions.
The aim of this research is to investigate the breakage and attrition behavior of wood pellets caused by transportation, processing, storage as well as handling, and to estimate the main factors, which influence the strength of wood pellets. Furthermore, a numerical model will be developed to describe the stress state, breakage behavior and agglomerate deformation under different types of mechanical loading conditions, e. g. compression, shearing and impact.
Different experimental techniques are used to obtain the agglomerate deformation, breakage and attrition behavior. By means of the single particle compression tester, sliding friction rig, the free fall apparatus and the pneumatic impact gun, the mechanical properties of single wood pellets or a bulk product are experimentally determined at static or dynamic loads. Based on the experimental results, the important material parameters such as modulus of elasticity, contact stiffness, strength, friction coefficients, coefficient of restitution, breakage energy and breakage function are obtained.
In the pneumatic impact gun a particle is accelerated by the air and collides with a target plate. The actual particle velocity is estimated from records captured by a high speed video camera. The particle size distribution is determined before and after the mechanical loading tests for characterization of fracture probability. In order to investigate the influence of pellet velocity and breakage/impact behavior of wood pellets different air velocities and impact plate materials are used.
Based on the experimental data, a numerical contact model for describing the breakage and deformation behavior of wood pellets will be established using the in-house-developed simulation framework MUSEN, which combines the Discrete Element Method (DEM) and Bonded Particle Model (BPM). Based on experimental and numerical results the breakage probability function and population balance model for pneumatic conveying of pellets will be developed.
Compression tests show that mechanical properties of pellets are strongly influenced by loading direction, pellet size and pellet internal structure. Pellets of smaller length feature less micro cracks resulting in a tighter pellet structure and higher mechanical properties. Furthermore, compression tests of pellets from different producers showed that elastic modulus is nearly independent of pellet internal structure and size.
Higher air speed increases pellet impact velocity and thus impact energy leading to higher breakage probability. Furthermore, the initial pellet length and pellet internal structure have a fundamental influence on the breakage behavior. Breakage probability increases with increasing pellet length.
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Mechanical Engineering, Department of Energy Plant Technology, Ruhr-University Bochum (V.Scherer)
The German Federation of Industrial Research Associations (AiF). Project number IGF 19116 N