Thermal Pretreatment of Sugarcane Straw: Physicochemical Properties, Combustion Behaviour and Emission Prediction

Sugarcane straw is an agro-industrial residue with potential for use as a solid biofuel in thermochemical processes. However, its direct use presents limitations due to its high moisture content, low bulk density, fibrous structure, and high levels of alkalis, sulfur, and chlorine, which promote slagging, fouling, corrosion, and pollutant emissions. This study evaluates a combined pretreatment process—cutting, washing, and torrefaction—aimed at improving its physicochemical properties. The sugarcane straw was chopped (5–20 mm) and washed with distilled water under eight different conditions, varying temperature (20 °C and 80 °C), time (10 and 20 minutes), and stirring type, while maintaining a solid concentration of 3.33%. The optimal condition (80 °C, 20 minutes, continuous stirring) allowed for the highest removal of ash, alkalis, chlorine, and sulfur. The washed residue was then torrefied at 250 °C for 20 minutes in a controlled oxidizing atmosphere. The results show significant improvements: the higher heating value reached 22 MJ·kg^-1 (a 26.65% increase), ash content was reduced by 18.7%, and the resulting chlorine and sulfur contents were 0.01% and 0.12%, respectively. Additionally, 62.7% of Na_2O and 47.6% of K_2O were removed, reducing the risk of deposit formation. Energy density increased from 0.29 to 5.05 GJ·m^-3 bulk density increased thirteenfold, and both hydrophobicity and grindability were enhanced. Although total emissions increased slightly, more than 95% corresponded to CO₂ which is potentially recyclable via photosynthesis. The proposed treatment represents an effective strategy to enhance the energy value of this residue.