Integrated biofuel production from rice husk and rice straw via torrefaction and anaerobic digestion: Optimization and economic viability
Himanshu Kachroo, Tharaka Rama Krishna C. Doddapaneni, Vishal Verma, Priyanka Kaushal, Rohan Jain
Abstract
• ANN based approach successfully modeled the torrefaction process with R 2 > 0.99. • Rice husk-derived bio-coal has HHV of 22 MJ/kg comparable to lignite. • Torrefaction condensate from rice husk produced 508 ml/g VS of methane yield. • Better economic feasibility via lower production costs (US $3 M/y) in India. • Favorable IRR (11.54 %) and high revenue (US $3.6 M/y) via integrated approach. Advancing global energy demands and the need for sustainable alternatives to fossil fuels highlight the importance of bioenergy. This study investigates the torrefaction of rice husk and rice straw, significant agricultural residues in India, and converting them into bio-coal and valuable torrefaction condensate. Utilizing an artificial neural network (ANN) model, the study optimized torrefaction parameters at 290 °C for 60 min with a heating rate of 10 °C/min. This optimization yielded 28.44 ± 0.85 wt% of torrefaction condensate for rice husk and 25.71 ± 0.92 wt% for rice straw, with bio-coal higher heating values (HHV) of 22.27 ± 0.03 MJ/kg and 18.15 ± 0.05 MJ/kg, respectively. Temperature was identified as the significant factor, while the heating rate had the most negligible impact on product yield and quality. Detailed characterization revealed the potential of rice husk/straw-derived bio-coal as a substitute for medium-rank coal and highlighted the value-added-rich nature of condensate. This study is the first to report the anaerobic digestion of rice husk/straw-derived torrefaction condensates, achieving bio-methane yields of 471–508 ml/g-VS under mesophilic conditions. Economic analysis confirmed the enhanced feasibility of integrating torrefaction with anaerobic digestion, achieving superior economic performance through reduced production costs (US$ 2.96 million/year) in the Indian context. The integrated system exhibited a favorable internal rate of return (11.54 %) and substantial revenue generation (US$ 3.6 million/year), establishing a sustainable and economically viable framework for biomass valorization and renewable energy production.