Torrefaction of agricultural crop residues in a continuous screw-auger-type reactor: evaluation of solid-fuel characteristics and economic feasibility

Abstract Torrefaction is mild thermochemical pretreatment technology that enhances biomass by changing its physico-chemical characteristics to produce a high-quality solid biofuel. The present study evaluated the performance of a continuous screw-auger-type torrefaction system for upgrading agricultural residues rice husk (RH) and wheat straw (WS) into energy-dense, hydrophobic biomass suitable for cofiring applications. Torrefaction experiments were conducted at three temperatures of 250°C, 275°C, and 300°C with a total residence time of 12 minutes. The results revealed that the mass yield decreased with temperature, from 85.60% to 75.00% and from 83.20% to 70.80% for RH and WS, respectively. Higher heating values increased significantly with torrefaction severity, reaching 19.26 MJ/kg for RH and 20.25 MJ/kg for WS when torrefied at 300°C, representing a 22%–25% improvement over raw biomass. Moisture uptake dropped substantially from 38.6% to 1.9% and from 41.5% to 2.1% for RH and WS, respectively, indicating the enhanced hydrophobicity. Combustion indices have also showed desirable trends; the fuel ratio increased to 0.54 and 0.57, and the volatile ignitability exceeded 15 MJ/kg for both biomass samples torrefied at 300°C, indicating improved ignition characteristics suitable for cofiring. Techno–economic analysis revealed a net profit of 138 520 Indian rupees/year, with a payback period of 5 years and a benefit–cost ratio of 1.27, supporting the economic feasibility of the system. Overall, the study confirms that torrefied biomass presents improved fuel characteristics and combustion behaviour, making it a viable solution for clean-energy transition and sustainable waste management.