Characterization of Torrefied Waste Hops for Bioenergy Applications: Moisture Sorption Kinetics and Thermal Analysis

Biomass is a promising renewable energy source; however, its direct use is limited by high hygroscopicity, low energy density, and challenging storage conditions. This study investigated the effect of thermal pretreatment temperature on the moisture sorption behavior and thermal stability of waste hops. Waste hops were torrefied at 250 °C, 350 °C, and 450 °C. Equilibrium moisture content (EMC) was measured at 30 °C under three relative humidity levels (50%, 70%, and 80%). The sorption data were fitted using four empirical models (Henderson, Chung-Pfost, Halsey, and Oswin), and model performance was evaluated using statistical parameters (R2, RMSE, RSS). Structural and compositional changes were analyzed by thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), and elemental and proximate analyses. For untreated hops, the Halsey and Oswin models described the data almost exactly (R2 ≥ 0.999), while the Halsey model was the most suitable for samples torrefied at 250 °C and 350 °C (R2 = 0.991 and 0.951, respectively). For the sample torrefied at 450 °C, none of the models provided an adequate fit. As the material became less hygroscopic, the agreement of all four models decreased (R2 of the best model from 1.000 for WH-raw to < 0.77 for WH-450). Because only three relative humidity levels were measured, the model ranking should be regarded as indicative. At 80% RH, torrefaction at 250 °C, 350 °C and 450 °C reduced the EMC by 30%, 50% and 64%, respectively, compared with that of the untreated material. With increasing torrefaction temperature, the onset decomposition temperature increased from 212.5 °C (WH-raw) to 412.3 °C (WH-450), and the final decomposition temperature from 512.9 °C to 581.4 °C. The improvement in hygroscopic and thermal stability with torrefaction temperature was not linear: the additional reduction in EMC became smaller at higher temperatures, and the higher heating value increased mainly between 250 and 350 °C, with only a marginal further gain at 450 °C. Total mass loss decreased from 77.11% to 39.77%, while the solid residue at 900 °C increased from 22.89% to 60.23%. FTIR spectra indicated degradation of hydrophilic functional groups. Thermal pretreatment improved the hydrophobicity and thermal stability of waste hops. The validated sorption models offer a useful tool for optimizing drying and storage conditions, enhancing the potential of waste hops as a sustainable bioenergy feedstock.