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Treesearch Insight 2023

4. End-of-life thermally modified wood as a feedstock for pelleting

Paula Israelsson 1, Clemens Knill 2, Michael Finell 1, Gunnar Kalén 1, David A. Agar 1*

1 Swedish University of Agricultural Sciences, Department of Forest Biomaterials and Technology, SE-901 83 Umeå, Sweden

2 Swero GmbH & CO. KG, Wangen im Allgäu, Germany

*Corresponding author: Tel.: +46 72 450 6968. E-mail: 

Wood is a natural carbon sink and EU policy supports its cascaded use to extend its lifetime in the bioeconomy. Thermally modified wood (TMW) building products (e.g. flooring, deck and façade planking etc.) are increasingly utilised in modern construction and have extended lifetimes due to their enhanced durability and resistance to weathering and degradation. TMW is manufactured using only heat and steam (i.e. no chemicals) and this simplifies its recycling and reuse potential for many applications.

In this study, sixteen-year-old end-of-life thermally modified pine planking was recycled, pre-processed and reused as a feedstock for pelleted in a pilot-scale ring-die press. Quality analysis of produced pellets showed that they had high durability (97-98%), high bulk density (685-715 kg m-3), low production fines (0.7-1.0%), low moisture content (4.8-7.6%) and low ash content (0.13-0.24%). Based on higher heating value determination the pellets exhibited a high volumetric energy density (13.4-14.0 GJ m-1) indicating that the sequestered carbon in the material was well conserved. These values demonstrate that, whether as a material carrier for subsequent biorefining processes or for thermal conversion, pelleted end-of-life TMW is a high-quality biomaterial. The results are evidence that the cascading principle for TMW has potential in an EU bioeconomy and offers a strategy for further extending the lifetime of wood and sequestering biogenic carbon.