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

www.nature.com/scientificreports OPEN Integrated torrefaction-anaerobic digestion of bamboo waste for enhanced energy recovery: process optimization, product characterization, and technoeconomic evaluation Himanshu Kachroo1,2, Tharaka Rama Krishna C. Doddapaneni3, Priyanka Kaushal4, Sabine Kutschke2 & Rohan Jain2 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 ± 0.4 MJ/kg to 25.4 ± 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 ± 1.7 mLCH4/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. Keywords Bamboo waste, Circular bioeconomy, Torrefaction, Anaerobic digestion, Bio-coal and biomethane, Techno-economic assessment Abbreviations AD Anaerobic digestion BMP Bio-methane potential COD Chemical oxygen demand GC Gas chromatography 1School of Interdisciplinary Research, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India. Institute Freiberg for Resource Technology, Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstraße 400, 01328 Dresden, Germany. 3Renewable and Sustainable Energy Research Center, Technology Innovation Institute, Masdar City, Abu Dhabi, United Arab Emirates. 4Clean Energy Laboratory, Centre for Rural Development and Technology, Indian Institute of Technology Delhi, Hauz-Khas, New Delhi 110016, India. email: ; 2Helmholtz Scientific Reports | (2026) 16:15878 | https://doi.org/10.1038/s41598-026-52760-9 1 HHV Higher heating value IRR Internal rate of return MS Mass spectrometry NPV Net present value SDG Sustainable development goals TCD Thermal conductivity detector TGA Thermogravimetric analysis Tof-SIMS Time of flight-secondary ion mass spectrometry VS Volatile solids Global energy demand is rising rapidly, driven by population growth, industrialization, and urbanization, while continued reliance on fossil fuels exacerbates greenhouse gas emissions. Coal and natural gas emit approximately 90–110 and 50–60 g CO2-eq/MJ, respectively, compared to 10–30 g CO2-eq/MJ for bioenergy pathways such as biogas and biodiesel1. Meeting the targets of the Paris Agreement requires the rapid deployment of low-carbon emission energy technologies2. The International Renewable Energy Agency estimates that bioenergy could supply up to 60% of global energy demand by 2050, avoiding up to 37 gigatons of CO2 emissions annually3. Currently, bioenergy accounts for nearly half of global renewable energy consumption, underscoring its pivotal role in the decarbonization transition. Bamboo biomass offers unique advantages in this context due to its high lignocellulosic content, rapid growth, low ash (~ 3 wt%), and wide availability in tropical and subtropical regions4. India has ~ 14 million hectares under bamboo cultivation, producing ~ 14.6 million tonnes annually, with over 50% concentrated in the northeastern states5. However, processing inefficiencies in industries such as furniture, flooring, handicrafts, and incense stick manufacturing can reach up to 80%, generating 2.1–4.9 million tonnes of bamboo waste annually6. Despite this abundance, bamboo waste remains underutilized for energy recovery in India, unlike rice husk and rice straw, which dominate biomass utilization in the Indo-Gangetic plains despite their high ash (~ 17–18 wt%), elevated silica levels, and associated slagging issues7,8. Bamboo’s low ash, higher energy density, and regional availability position it as a strategic feedstock for decentralized bioenergy production, particularly in bamboo-rich areas of Northeast India. Torrefaction is a mild thermochemical pretreatment conducted under inert conditions at 200–300 °C, which improves biomass fuel quality by reducing volatile matter, increasing fixed carbon, and enhancing hydrophobicity and grindability9,10. The resulting bio-coal can achieve energy densities comparable to lignite or sub-bituminous coal11–13, while requiring lower capital investment and operating temperatures than pyrolysis or gasification12. However, torrefaction also produces a condensable volatile fraction, referred to as the torrefaction condensate, which is often underutilized or discarded due to its high water content, acidity, and presence of corrosive compounds13,14. Notably, this condensate contains biodegradable organics (acetic, formic, propionic, and lactic acids) alongside low concentrations of inhibitory compounds. Despite this, most existing studies primarily focus on solid product enhancement, with comparatively limited attention given to valorization of liquid by-products generated during torrefaction. AD offers a promising pathway to convert these organic compounds into biomethane, enabling energy recovery from the liquid fraction. Previous studies have investigated AD of bamboo biomass15, as well as torrefaction condensates from feedstocks such as pine and rice husk, reporting methane yields of 470–510 mLCH4/g-VS (volatile solids)13,16. However, integrated approaches that combine torrefaction and AD for bamboo waste remain largely unexplored. In particular, studies addressing the simultaneous valorization of both solid and liquid streams within a unified framework are scarce, limiting the overall efficiency and circularity of biomass conversion systems. In our previous work16, the techno-econom (...truncated)