Characterisation of torrefied agricultural residue pellets with bran and peat binders

Pellets were produced from wheat straw, barley straw, and oat husks using two practically selected binder formulations: 85% biomass with 15% peat and 90% biomass with 10% barley bran. Preliminary trials identified these proportions as the highest biomass contents yielding intact pellets. The pellets were torrefied under nitrogen at 220, 270, and 320 °C for 1 h. Carbon content, ash content, solid yield, higher heating value, hygroscopicity, mechanical durability, compressive strength, and bulk density were evaluated. Increasing temperature was accompanied by higher carbon and ash contents and higher heating value, but lower solid yield, bulk density, and hygroscopicity, together with reduced mechanical durability and compressive strength in several formulations. At 320 °C, the higher heating value reached 24.8 MJ·kg⁻¹ and mass loss reached 43.5%, whereas treatment at 220 °C retained more than 90% of the initial mass and generally preserved mechanical properties. Van Krevelen analysis showed shifts towards lower H/C and O/C ratios for all formulations. Pareto analysis identified several favourable trade-offs between volumetric energy density and mechanical durability without indicating a universally optimal condition. Three-way ANOVA confirmed significant overall contributions of biomass type and torrefaction temperature to all eight properties, while the contribution of binder formulation was property-dependent. These results demonstrate the formulation-dependent trade-offs associated with the torrefaction of agricultural-residue pellets.