Abstract
The aim of the study was to investigate the densification characteristics of raw, milled, and cut-milled pine and poplar shavings and determine the strength parameters of pellets, pastilles, and granules. In producing agglomerates from hard pine shavings compared to plastic poplar shavings, 19% more specific compaction work was required with over 2-times more specific work to push the agglomerate out of the die opening. Pine agglomerates exhibited lower linear expansion than poplar agglomerates, achieving a higher single density. Due to the elevated content of thermoplastic lignin in the wood (30.7 and 18.4%, respectively), pine agglomerates demonstrated superior radial compression strength parameters, including specific deformation energy, maximum tensile stresses at which agglomerates cracked, and the highest modulus of elasticity. Agglomerates made of cut-milled shavings had the highest single density, but their tensile strength was significantly lower than that of agglomerates made from raw shavings. The susceptibility to densification of the shavings during sequentially repeated densification of small doses during pellet production was the highest, resulting in pellets characterised by the smallest linear and radial expansion, as well as the highest single density of 1081 kg·m–3 and tensile strength among agglomerates. The smallest single density and strength were observed in granules produced with parameters recommended for particleboard production: a temperature of 170 °C and an agglomeration pressure of 12 MPa, compared to 93 °C and 70 MPa for pellets and pastilles, respectively. The higher temperature did not compensate for the much lower pressure. Shavings compaction parameters for granules are not recommended for particleboard production without a binder, typically urea–formaldehyde resin.
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Abbreviations
- AC :
-
Ash content in shavings (%)
- d 1, d 2 :
-
Agglomerate diameters in two perpendicular directions (mm)
- d a, d m :
-
The diameter of the agglomerate and the die opening, respectively (mm)
- E, E b, E p :
-
Modulus of elasticity; general, in bending and compression of the agglomerate, respectively (MPa)
- E j, E jb, E jp :
-
Specific energy; deformation, bending and compression until the agglomerate cracks, respectively (mJ·mm–2)
- E w :
-
Energy needed to evaporate water in standard conditions per 1% water from wet shavings (kJ∙kg–1)
- F, F m :
-
Compaction force; current and maximum, respectively (N)
- F b, F be, F bm :
-
Bending force; current, in the range of elastic load and maximum, respectively (N)
- FC :
-
Fixed carbon content (%)
- F p, F pe, F pm :
-
Compressive force; current, in the range of elastic load and maximum, respectively (N)
- h :
-
Agglomerate height in the die chamber after its compaction (mm)
- H:
-
Hydrogen content (%)
- HHV, LHV :
-
Higher heating value and lower heating value, respectively (MJ·kg–1)
- I :
-
Moment of inertia of the cross-sectional area of the pellet (mm4)
- I s :
-
Degree of shavings compaction (–)
- k s :
-
Coefficient of shavings susceptibility to compaction (J·m3·kg–2)
- l 1, l 2 :
-
Agglomerate length in two perpendicular directions (mm)
- l a, d a :
-
Length and diameter of the agglomerate after its expansion, respectively (mm)
- l b :
-
Distance between the pellet support points (mm)
- l c :
-
Die height (mm)
- L c :
-
Total compaction work (J)
- L s, L v :
-
Specific work; compacting and pushing the agglomerate out of the die opening, respectively (kJ·kg–1)
- m, m a, m d :
-
Mass; sample, agglomerate and single material dose, respectively (g)
- m A, m C, m M, m S :
-
Mass; crucible with shavings after heat test, empty crucible, with shavings before heat test and solids after shavings annealing, respectively (g)
- MC :
-
Material moisture content (% w.b.)
- m H2O :
-
Mass of water produced in the combustion process per unit of hydrogen (kg∙kg–1)
- p, p m, p u :
-
Pressure; current compaction, maximum compaction and maximum of pushing the agglomerate out of the die opening, respectively (MPa)
- p 1, p 2 :
-
Pressure; in the measuring and reference cells, respectively (MPa)
- R l, R d :
-
Agglomerate expansion rates relative to its length and diameter, respectively (%)
- S b, S p :
-
Cross section of the agglomerate during bending and compression, respectively (m2)
- s m, s u :
-
Piston displacement; to the maximum compaction pressure and to push the agglomerate out of the die opening, respectively (mm)
- t :
-
Die temperature (°C)
- V C, V A :
-
Volume; measuring and reference cells, respectively (mm3)
- VM :
-
Volatile matter content in the shavings (%)
- x :
-
Piston displacement during shavings compaction (mm)
- y :
-
Outer fibres distance from the pellet neutral axis (mm)
- ε :
-
Relative punch displacement in the range of elastic load (–)
- Δl :
-
Punch stroke until the agglomerate cracks under the load (mm)
- ρ a, ρ b, ρ s :
-
Density; single agglomerate, shavings volume and wood substance shavings (specific density), respectively (kg·m–3)
- σ, σ b, σp :
-
Maximum strength; general, bending and tensile, respectively (MPa)
- AF:
-
Agglomerate form; (pellet (P), granule (G), pastille (T))
- TL:
-
Type of load; (axial compression, AC, radial compression, RC, bending, B)
- TW:
-
Species of wood; (pine, PI, poplar, PO)
- WS:
-
Shavings condition; (raw, R, milled, M, cut-milled, CM)
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Acknowledgements
This research is part of a doctoral dissertation supported by the Ministry of Science and Higher Education in Poland. The authors thank their colleagues from the Department of Biosystems Engineering for technical assistance in the research.
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Conceptualization: PT and AL; methods: PT, JG, and AL; software: PT and AL; formal analysis: PT and AL; investigation: PT, AL, and JG; resources: PT and AL; data curation: PT and AL; writing—original draft preparation: PT; writing—review and editing: AL and JG; visualization: PT and AL; project administration: AL and JG; supervision: AL. All authors have read and agreed to the published version of the manuscript.
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Tryjarski, P., Lisowski, A. & Gawron, J. Pressure agglomeration of raw, milled and cut-milled pine and poplar shavings: assessment of the compaction process and agglomerate strength. Eur. J. Wood Prod. (2024). https://doi.org/10.1007/s00107-024-02046-6
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DOI: https://doi.org/10.1007/s00107-024-02046-6