Integrated torrefaction-anaerobic digestion of bamboo waste for enhanced energy recovery: process optimization,...

This study evaluates an integrated valorization pathway for bamboo waste that combines torrefaction and anaerobic digestion (AD) to recover energy from both solid and liquid process streams within a circular economy framework. Specifically, a dual-stream valorization approach is developed to maximize energy recovery from both bio-coal and torrefaction condensate. Bamboo's high lignin content (24-27 wt%), low ash content (< 3 wt%), and favorable volatile composition enable enhanced thermochemical upgrading and subsequent biochemical energy recovery. Torrefaction at 290 °C for 60 min increased the higher heating value (HHV) of bio-coal from 17.6 [Formula: see text] 0.4 MJ/kg to 25.4 [Formula: see text] 1.5 MJ/kg, accompanied by ~ 66% reduction in volatile matter and significant lignin enrichment, resulting in improved fuel quality. The aqueous condensate, typically underutilized, containing biodegradable organic acids with low inhibitor concentrations achieved a biomethane potential of 493 [Formula: see text] 1.7 mL-CH₄/g-VS during AD, indicating its suitability as a secondary energy source under the tested conditions. The integrated process delivers a net energy recovery of approximately 21 GJ/ton, which is higher than rice husk and rice straw evaluated under identical optimized operating conditions, highlighting feedstock-dependent performance differences under similar conditions. A techno-economic assessment of a 50,000 t/y integrated torrefaction-AD pilot plant facility in India indicated economic feasibility with an internal rate of return (IRR) of 15.8% and a payback period of 6.5 years. Notably, the economic analysis is directly based on experimentally derived mass and energy balances, enhancing its practical relevance. By enabling near-complete utilization of bamboo residues, the proposed pathway supports the development of decentralized biorefineries, sustainable resource use, circular economy principles, and UN Sustainable Development Goals (SDGs) 7, 12, and 13.