Valorization of Textile Cotton Waste and Textile Sludge into High-Quality Torrefied Biofuel Pellets: Fuel Characteristics and Optimization

This study investigates the conversion of textile wastewater sludge (TWS) and textile cotton waste (TCW) into solid biofuels through pelletization and torrefaction, addressing the growing need for sustainable waste management and alternative fuels in the textile sector. Blended feedstocks were conditioned to ~10% moisture, pelletized into 8 mm cylinders, and thermally upgraded at 200–240 °C for 30–90 min. Proximate and ultimate analyses, calorific value measurements, compressive strength testing, bulk and true density assessment, and TGA–DTG were used to evaluate fuel properties, while response surface methodology (RSM) optimized torrefaction parameters. The TCW-rich 20:80 (TWS:TCW) blend with 5% starch exhibited the most favorable overall performance, achieving a calorific value of 3377 kcal kg−1, ash of 10.3%, bulk density of 554 kg m−3, and maximum compressive strength of 14.9 N mm−2. Torrefaction at 200 °C for 60 min increased the GCV to 4083 kcal kg−1 with a high mass yield of 92%, indicating mild thermal decomposition and good energy retention. Further Torrefaction at 220–240 °C increased GCV to 4362–4565 kcal kg−1, accompanied by expected mass-yield reductions due to increased devolatilization. TGA–DTG confirmed faster and cleaner decomposition for TCW-rich pellets and higher residues for sludge-rich blends. RSM indicated temperature as the dominant factor governing mass and energy yields. These findings demonstrate that optimized co-pelletization and mild-to-moderate torrefaction can effectively transform textile residues into energy-dense, mechanically stable biofuels suitable for industrial heat applications.