Characterization and quality analysis of wood pellets: effect of pelletization and torrefaction process variables on quality of pellets

A lab-scale single-channel press was employed for producing pellets and the effect of pressure, die temperature, moisture content (MC), particle size, and binding agent on pellet quality was investigated. Meanwhile, torrefaction subsequent to pelletization was performed and the quality of pellets was evaluated under different torrefaction conditions. From the analysis of variance (at p < 0.05), temperature (70–200 °C), pressure (70–160 MPa), and MC (5.5–28%) were found most significant factors for density, strength, and compression energy of pellets. Particle density and strength of raw wood pellets were significantly affected by temperature and pressure. The highest particle density and strength (1.307 g/cm³, 11.8 N/mm²) for particles of 0.25–0.5 mm and (1.300 g/cm³ and 10.7 N/mm²) for particles of 0.5–1.41 mm were found at a temperature of 200 °C (pressure ≥ 100 MPa and MC 5.5%), indicating that pelletization at a temperature of 200 °C was beneficial. The relaxed density of pellets remained the same as of particle initial density after storage time of 2 weeks. The expansion ratio of the pellet was found lower. Use of synthetic resin as a binder in the proportion of 1:9 was found optimum, the particle density increased from 1.19 to 1.24 g/cm³ (0.25–0.5 mm) and from 1.17 to 1.22 g/cm³ (0.5–1.41 mm) and energy consumption reduced by 9.39% (0.25–0.5 mm) and 8.7% (0.5–1.41 mm). Pellets made at a temperature of 200 °C were found water-resistive as compared to those made at other temperatures. The highest lower heating value, i.e., 26.76 MJ/kg of torrefied pellets (TOPs) was achieved at 300 °C and 120 min. Torrefaction process parameters adversely affected the particle density, volumetric energy density, strength, and durability of TOPs. The highest true density (1.85 g/cm³) and porosity (65 v %) for TOPs were achieved at 300 °C and 120 min, much higher than those of raw pellets. Moisture uptake of TOPs at 300 °C was 2.0–2.8 wt.%, showing strong water-resistant ability. From the results of FTIR, O–H bond was destroyed after torrefaction.