In a study published in the journal Energy Conversion and Management, researchers Hilal Unyay, Andrii Kostyniuk, and their team evaluated the thermal upgrading of oxytree (Paulownia) biomass. Oxytree is a fast-growing energy crop favored in Europe for its rapid production and high-quality wood. However, necessary pruning during its growth generates massive amounts of waste biomass, often accounting for 70% of the tree’s total volume. The research compared dry torrefaction (DT) and wet torrefaction (WT) to determine which process best transforms these pruning residues into efficient, high-density solid biofuels. 

Torrefaction is a mild heat treatment used to improve the fuel traits of raw biomass by making it more carbon-rich and hydrophobic. Dry torrefaction is typically performed in an oxygen-free gas environment, while wet torrefaction occurs in pressurized water. The team found that while dry torrefaction increased the energy content of the oxytree by about 25%, wet torrefaction achieved much more dramatic results. Under the most intense conditions of 260°C, wet torrefaction boosted the higher heating value of the biomass from 16.3 MJ/kg to a substantial 27.9 MJ/kg—approaching the energy density of lignite coal. 

One of the most compelling findings was wet torrefaction’s superior ability to retain energy while removing unwanted oxygen and hydrogen. The pressurized water in the wet process acts as a catalyst, breaking down unstable plant components like hemicellulose more effectively than the dry method. This leads to a solid “hydrochar” that is not only richer in carbon (reaching 68.3% carbon content) but also more chemically stable for long-term storage and transportation. Furthermore, wet torrefaction preserved energy yields exceeding 74% at peak temperatures, clearly outperforming the dry samples under identical conditions. 

The researchers also analyzed how these different fuels actually burn. Using advanced thermal analysis, they determined that wet-torrefied oxytree is significantly more “combustion-ready.” It consistently ignited at lower temperatures and required less activation energy to sustain a flame compared to dry-torrefied material. In practical terms, this means hydrochar produced through wet methods acts like a highly reactive fuel that lights quickly and burns rapidly, which is ideal for stable flame propagation in industrial power plants. 

Beyond solid fuel, wet torrefaction offers additional benefits by producing a liquid phase rich in valuable organic compounds. The study identified high concentrations of acetic acid, furfural, and phenolic compounds in the aquatic byproduct, which can be harvested for use in chemical manufacturing. By adjusting the reaction time, producers can even control whether they prioritize these high-value chemicals or focus solely on solid fuel production. 

Ultimately, while dry torrefaction is simpler and better for maximizing total solid mass, wet torrefaction emerges as the superior choice for high-performance bioenergy. WT’s ability to handle high-moisture pruning waste without prior drying and its exceptional energy densification make it a promising strategy for decentralizing bioenergy systems. This research provides a clear roadmap for selecting the right thermal treatment to maximize the profit and environmental benefits of oxytree plantations. 

Source: Unyay, H., Kostyniuk, A., Szufa, S., Lewandowski, A., Likozar, B., & Wielgosinski, G. (2026). Dry vs wet torrefaction of oxytree biomass: A comparative study on fuel properties and energy yield. Energy Conversion and Management, 350, 120974.