Integrated co-digestion and dual fixed-bed Fe2O3/Ca(OH)2 biogas upgrading using fruit and vegetable waste and cattle manure

Abstract

Low-cost biogas upgrading remains a major barrier to decentralized biomethane deployment, particularly where conventional scrubbing or membrane systems are impractical. This study quantifies the performance envelope of a dual fixed-bed upgrading train operated in series (Fe₂O₃ for H₂S removal followed by solid Ca(OH)₂ for CO₂ capture) while treating real biogas generated on-site. Semi-continuous co-digestion of fruit and vegetable waste and cattle manure (30:70 on a VS basis) was used solely as a realistic and stable biogas supply to provide representative CH₄/CO₂/H₂S loads to the upgrading unit. Under favorable hydrodynamic conditions, biomethane-quality enrichment was achieved (CH₄ ≥ 99.4%) with near-complete CO₂ removal (∼99.7–99.95%) and H₂S reduced to low-ppm/sub-ppm levels (0.624–1.54 ppm; >99%). Performance losses at higher throughput indicated that CO₂ capture in the Ca(OH)₂ bed is the limiting step under demanding conditions. Post-use characterization (SEM, TGA/DTG, XRD) confirmed carbonation of Ca(OH)₂ to CaCO₃ and transformations consistent with sulfide capture in the iron bed, linking performance trends to sorbent evolution. System-level indicators yielded 66 m³ biogas·t⁻¹ and 44.22 m³ CH₄·t⁻¹ (≈1583 MJ t⁻¹, LHV basis) and a simplified benefit of ≈0.895 t CO_2eq t⁻¹ avoided within the defined boundary. The results demonstrate that biomethane-quality upgrading can be achieved under mild operating conditions using low-cost solid sorbents in a compact fixed-bed train.