Low-temperature torrefaction improves net energy gain in anaerobic digestion of lignocellulosic biomass

High energy demand for size reduction limits the application of lignocellulosic biomass in anaerobic digestion. This study investigated the effects of torrefaction at 160-220 °C on wheat straw that was subsequently ground to < 1 mm and subjected to anaerobic digestion at 10% total solids. Methane yield per unit substrate showed a nonmonotonic trend and recovered at 180 °C (197.20 L/kg VS) to a level close to the control (202.29 L/kg VS). Energy balance analysis showed that torrefaction did not impose a net energy penalty but improved system net energy performance. After accounting for the drying energy for the 20% initial moisture feedstock and the condensate energy recovery, estimated net energy reached 2735.20 MJ/t at 180 °C, 25% higher than the conventional process, identifying 180 °C as the optimal torrefaction temperature window. Multiscale analysis showed that torrefaction increased straw brittleness through hornification, thereby reducing grinding energy consumption but also decreasing methane potential. At 180 °C, controlled hemicellulose degradation restored biodegradability, whereas temperatures above 200 °C caused substantial loss of biodegradable organic matter, leading to deterioration in anaerobic digestion performance.