Breakage Strength of Wood Sawdust Pellets: Measurements and Modelling

Józef Horabik,Marek Molenda,Agnieszka Adamczuk,Grzegorz Józefaciuk,Maciej Bańda,Rafał Kobyłka,Joanna Wiącek,Piotr Parafiniuk,Cezary Polakowski,Mateusz Stasiak

doi:10.3390/ma14123273

Józef Horabik, Marek Molenda + Show 8 more

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https://doi.org/10.3390/ma14123273

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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.

Highlights

IntroductionWaste biomaterials, including wood processing residues, are important renewable energy resources [1,2]
The order of magnitude of the bond elasticity modulus (107 –108 Pa) and the corresponding range of the normal stiffness coefficient of the bond (1010 –1011 N m−3 ) applied in our study were similar to the values of the bond elasticity and stiffness applied in the Discrete element method (DEM) modelling of loading of pinewood chip briquettes by Xia et al [38], the durability of wood pellets found by Mahajan et al [39], and the breakage of biomass pellets studied by Gilvari et al [40]
The main novelty of the present study was the successful simulation of the diametral compression test of wood pellets and reproduction of the profiles of ductile crack formation using the DEM with the implemented bonded particle model (BPM)

Summary

Introduction

Waste biomaterials, including wood processing residues, are important renewable energy resources [1,2] Since, in their native loose forms, such materials have low bulk density and their storage and transport are difficult and costly [3], densification is applied to reduce these disadvantages [4]. Binding forces are higher when a contact zone between particles covers a larger area; that is, the forces increase with an increase in compaction pressure and a decrease in particle size [8]. The moisture content (MC) markedly affects the binding forces It has both antagonistic (with water molecules replacing wood polymer bonds) and protagonistic (decreasing the melting temperature of lignin) actions on pellet durability [2]; an optimum MC should be maintained during the pelletisation process [9]. Pressure (contact between particles), temperature (plastic deformation of lignin), particle size, and optimum

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