Circular bioeconomy through a closed-loop anaerobic digestion-pyrolysis biorefinery: techno-economic, environmental...

Renewable gaseous fuels are increasingly promoted for cross-sector decarbonization. While anaerobic digestion (AD) is a commercialized technology for biogas or renewable natural gas (RNG) production, its wide deployment is constrained by limited conversion efficiency. This study proposed a novel integrated thermo-biological biorefinery that couples pyrolysis with AD to enhance fuel production from limited resources. This closed-loop AD-Pyrolysis system improves methane yield by incorporating solid digestate-derived biochar and co-treating aqueous pyrolysis liquid, while also producing liquid biofuel. A techno-economic-environmental assessment indicates 83% greater greenhouse gas (GHG) reduction than stand-alone AD and maximum reductions of -1152 kgCO₂-eq/tDM through flexible credit allocation among co-products. However, the uncompetitive minimum selling price (MSP) of RNG ($49-59/GJ) compared to cheap fossil natural gas ($4/GJ) highlights the need for capital cost reduction, yield improvement, and policy support. AD-Pyrolysis RNG is economically attractive under volume-based or carbon intensity-based policies at current traded carbon price, offering valuable insights for decarbonization strategies and stakeholder decision-making.