Integrated torrefaction-anaerobic digestion of bamboo waste for enhanced energy recovery: process optimization, product characterization, and techno-economic evaluation

Abstract

Abstract 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 $$\:\pm\:$$ 0.4 MJ/kg to 25.4 $$\:\pm\:$$ 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 $$\:\pm\:$$ 1.7 mL-CH 4 /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.