Solid biofuel generation from Eucalyptus wood residues: An experimental comparison study of torrefaction, hydrothermal, and slow pyrolysis

Eucalyptus trees are extensively cultivated to meet paper pulp, timber, and fiber-demands, generating substantial amounts of wood residues (EWR), suitable for energy applications. This study evaluates the conversion of EWR via torrefaction (TOR), hydrothermal carbonization (HTC), and slow pyrolysis (SP) in lab-scale reactors, assessing temperature and time effects on fuel properties. The resulting solid fuel was characterized using proximate and elemental analyses as well as thermogravimetric analysis. Results indicate that product yields were significantly influenced by both temperature and residence time. The highest temperatures within the investigated operating ranges produced the most pronounced fuel upgrading, with volatile matter decreasing from 86.9% to 78.9%, 45.1%, and 14.6% for TOR, HTC, and SP, respectively, while their higher heating value increased to approximately 22, 26 and 33 MJ/kg. A central composite design experiment was implemented, with statistical analyses conducted on the resultant data, employing response surface methodology and analysis of variance. Under optimal condition of 194.32°C-14.59 min (TOR), 180.57°C-2.38 h (HTC), and 282.85°C-3.09 h (SP), energy yields of 97%, 84%, and 71% were obtained, respectively. The solid fuels showed combustion behavior comparable to coal and lignite. Emission estimations suggested lower environmental impact than fossil coal, reinforcing their potential as sustainable biofuels.