Breakage Strength of Wood Sawdust Pellets: Measurements and Modelling

Abstract

Wood pellets are an important source of renewable energy. Their mechanical strength is a crucial property. In this study, the tensile strength of pellets made from oak, pine, and birch sawdust with moisture contents of 8% and 20% compacted at 60 and 120 MPa was determined in a diametral compression test. The highest tensile strength was noted for oak and the lowest for birch pellets. For all materials, the tensile strength was the highest for a moisture content of 8% and 120 MPa. All pellets exhibited a ductile breakage mode characterised by a smooth and round stress–deformation relationship without any sudden drops. Discrete element method (DEM) simulations were performed to check for the possibility of numerical reproduction of pelletisation of the sawdust and then of the pellet deformation in the diametral compression test. The pellet breakage process was successfully simulated using the DEM implemented with the bonded particle model. The simulations reproduced the results of laboratory testing well and provided deeper insight into particle–particle bonding mechanisms. Cracks were initiated close to the centre of the pellet and, as the deformation progressed, they further developed in the direction of loading.

DEM modeling of wood sawdust compaction and breakage strength of pellets determined in diametral compression test

“…The Poisson ratio ν = 0.35 and the coefficient of restitution e = 0.5 were in accordance with the typical values for wood materials (Kretschmann, 2010;Rackl et al, 2017). A yield strength of p y = 100 MPa and a bond radius of r b = 20 mm followed the findings of a study concerning the optimization of bond parameters for the compaction of wood sawdust samples performed by Horabik et al (2021). The material parameters of the mould and sawdust particles, and the interaction parameters used in DEM simulations are shown collectively in Table 2.…”

“…Pelletization increases the bulk density by up to 1 000-1 200 kg m −3 . This value is far above the bulk density of the intact wood block (400-700 kg m −3 ) and close to that of the intact wood solid-phase density including closed pores (1 050-1 400 kg m −3 ) (Shaw et al, 2009;Kretschmann, 2010;Horabik et al (2021).…”

“…The discrete element method (DEM) which was introduced by Cundall and Strack (1979) was extended with the parallel bonded particle model (BPM) of Potyondy and Cundall (2004) which appears to be very useful in providing a particle scale insight into the bonding mechanisms. DEM simulations reproduced the results of the experimental tests of the pressure compaction of biomass very well (Ilic et al, 2018), in terms of the mechanical behaviour of pinewood chips in cyclic loading (Xia et al, 2019), durability (Mahajan et al, 2017), and the breakage strength of wood (Horabik et al, 2021) and biomass pellets (Gilvari et al, 2020). DEM modelling of the mechanical behaviour of compacted biomass requires further study in order to verify its applicability to different types of biomass and experimental conditions.…”

“…Any change in δ n results in an increase in the tension force (δ n − δ n,0 < 0) or in the compression force (δ n − δ n,0 > 0) up to a limiting force (f c ), which leads to the breakage of the bond. Details concerning the applied mathematical formulas of BPM were presented in an earlier publication about DEM modelling of the breakage strength of wood sawdust pellets (Horabik et al, 2021).…”

Characterization of Bond Fracture in Discrete Groove Wear of Cageless Ball Bearings

“…The discrete element method was improved by Feng et al by establishing a fracture criterion for steel through the fracture criterion for bonds, effectively solving the problems of large deformation, plasticity and fracture of beam structures [ 19 ]. Horabik et al modeled agglomerate fragmentation using the discrete element method, replacing adhesion in relaxed particle contact with bonding, and providing greater insight into the mechanism of interparticle bonding [ 20 ]. Shilko et al used the discrete element method to describe the fracture process as a process from linked state to unlinked state, and on this basis, the fracture criterion of bonding bond was established [ 21 ].…”