Bioeconomy of an integrated anaerobic digestion ˗ hydrothermal carbonization of agricultural waste to energy

The integration of anaerobic digestion (AD) and hydrothermal carbonization (HTC) offers a promising approach for simultaneously recovering energy and valorizing waste within the circular bioeconomy. In this study, an integrated Aspen Plus AD-HTC system with a capacity of 1142 kg h −1 , operating at conditions of 230 °C, 60 min, and a 1:10 ratio, was explored for the valorization of crop residues into biogas and hydrochar. The process produced 48.6% hydrochar with a high heating value (HHV) of 25.4 MJ kg −1 and 0.68 m 3 CH 4 kg −1 VS compared to the HTC process only. The integrated AD-HTC process decreased net greenhouse gas emissions by 31.5% and increased total energy recovery by 37.8%. The economic evaluation revealed strong profitability, with a net present value (NPV) of USD 2.15 million, an internal rate of return (IRR) of 21.3%, and a payback period of 4.5 years. The study demonstrates the synergistic potential of integrating AD˗HTC for carbon sequestration from crop residues and sustainable energy production. The results provide quantitative evidence to support policy adoption of integrated thermochemical-biochemical systems as low-carbon bioenergy solutions in future agricultural waste management frameworks. • An Aspen Plus-based AD-HTC biorefinery model was developed for crop residue valorization. • The integrated system produces biogas and hydrochar at a 1142 kg h −1 feed rate. • Hydrochar yield reached 48.6 wt% (25.4 MJ kg −1 ), with 0.68 m 3 CH 4 kg −1 VS. • AD-HTC integration increased energy recovery by 37.8% and cut GHGs by 31.5%. • Techno-economic analysis shows strong feasibility (NPV USD 2.15 M, IRR 21.3%).