Continuous inclined-screw torrefaction with off-gas heat integration: pilot-scale residence-time impacts on severity and fuel quality

Abstract

Continuous torrefaction can upgrade biomass pellets, but pilot-scale evidence on how residence time governs thermal zoning, off-gas heat integration, and fuel upgrading in screw reactors remains limited; therefore, this study evaluates a pilot-scale inclined-screw torrefaction system to quantify these effects. Experiments were conducted at two feed rates (17.5 and 20 kg h⁻¹) by varying residence time through screw-speed control, covering 20-35 min in Condition 1 and 30-45 min in Condition 2. Temperature monitoring confirmed stable zones: 33-96 °C (drying) and 96-238 °C (torrefaction). Heat supply shifted from LPG (Liquefied Petroleum Gas) start-up to torrefaction off-gas combustion, demonstrating heat integration. At 40 min residence time, the off-gas contained CO<inf>2</inf> 48.8 vol%, CO 22.04 vol%, H<inf>2</inf> 12.7 vol%, and CH<inf>4</inf> 16.2 vol% (LHV 8.7 MJ kg⁻¹), indicating potential to reduce LPG use through co-firing. Increasing residence time increased the mass-fraction-weighted LHV up to 24.03 MJ kg⁻¹ but decreased mass yield (80.2-55.1% and 81.29-46.48%) and energy yield (93.17-81.58% and 92.78-68.94%) because devolatilization and deoxygenation intensified solid mass loss faster than the incremental LHV gain. Longer residence times produced predominantly black material (higher severity), and increasing the feed rate required ∼10 min longer residence time to reach comparable severity.