Biofuel development from flax straw: Torrefaction, steam explosion, and pelletization of treated biomass

This study explores the effects of steam explosion and torrefaction on the chemical and fuel characteristics of flax straw. Flax straw was torrefied at temperatures of 250°C, 275°C, and 300°C, which significantly increased its fixed carbon content and higher heating value, enhancing its energy density as a biofuel. While torrefaction had a pronounced effect on the fuel properties, steam explosion resulted in minimal changes to the biomass characteristics, in comparison. The study also examines the pelletization of torrefied flax straw and evaluates the impact of adding a lignin binder on the quality of bio-pellets. The results indicate that as torrefaction temperature increased from 250 to 300°C, relaxed density of pellets decreased from 894 to 814 kg/m³, and mechanical durability dropped from 79.6 % to 74.2 %. This decline in pellet properties is attributed to the development of pores and a reduction in particle density during thermal treatment. The inclusion of 10 wt% lignin as a binder significantly enhanced pellet durability across all torrefaction temperatures, facilitating fiber inter-diffusion and bond formation. Pellets produced from torrefied flax straw exhibited lower moisture uptake compared with those made from raw flax straw, as reported in literature, because torrefaction eliminated oxygen-containing functional groups, thereby improving their hydrophobicity. However, lignin-modified pellets exhibited a higher moisture uptake (23.3–26.3 wt%) compared with binder-free pellets (13.8–14.8 wt%), due to lignin’s hydrophilic properties relative to torrefied flax straw. This study highlights the potential of torrefied flax straw as a sustainable biofuel feedstock and demonstrates that lignin addition can improve pellet durability.