Urban leaf waste is an underutilized biomass stream that can be upgraded into moisture-resistant solid biofuels. This study quantifies the hydrophobicity of torrefied urban leaf biomass using water droplet penetration time (WDPT) measured on standardized compressed pellets (Ø6 mm, 0.20 g; 15 μL droplet). Leaves from eight urban tree species, with Miscanthus grass and fir twigs/needles as reference feedstocks, were analyzed. Torrefaction was performed under stationary and vibration-assisted conditions at 240–280 °C for 2–10 min; process severity was expressed as TSF (T·t), and the vibration effect as ΔWDPT. Raw WDPT ranged from 5.4 s for Miscanthus to 105.6 s for oak. Temperature was the most consistent driver of hydrophobicity development, increasing WDPT to 10 2 –10 3 s and, in selected cases, above 10 3 s. Vibration effects were feedstock-dependent. At 280 °C and 4 min, vibration-assisted torrefaction increased WDPT from 496.3 to 958.0 s for beech, from 592.0 to 881.4 s for black locust, and from 113.0 to 211.8 s for magnolia. Particle-size fractionation significantly affected WDPT, while vibration increased energy densification by approximately 2–5% but reduced mass and energy yields. • WDPT of torrefied urban leaf pellets increased from 5.4 to 105.6 s to 10 2 –10 3 s. • Vibration effects were feedstock-dependent and non-universal across biomass types. • The strongest vibration effect increased beech WDPT from 1082 to 2430 s. • Pellet-based WDPT enabled cross-species comparison of torrefied leaf waste.
Sztybel et al. (Fri,) studied this question.