Development of a Hybrid Anaerobic Digestion and Pyrolysis System for Integrated Bioenergy Production from Agricultural Waste with Carbon Capture Capabilities

Agricultural residues represent an underutilized lignocellulosic biomass resource capable of mitigating fossil fuel dependence through advanced bioenergy conversion pathways. In this study, an integrated hybrid system coupling mesophilic anaerobic digestion (AD) with secondary slow pyrolysis and an in-line biochar-based carbon capture stage was designed and experimentally validated using a co-digested maize stover and dairy manure feedstock. Anaerobic digestion achieved a specific methane yield of 284.6 ± 11.2 mL CH₄ g^-1 volatile solids (VS), leaving a recalcitrant lignocellulosic digestate that was mechanically dewatered, dried using recovered process heat, and subsequently pyrolyzed at temperatures ranging from 450 °C to 650 °C. Pyrolysis at 550 °C yielded 34.2 wt.% biochar with a specific surface area of 218.4 m² g^-1, alongside an energy-dense syngas (14.8 MJ Nm^-3) utilized for system parasitic thermal loads. The produced engineered biochar was modified via thermal-alkaline activation to serve as an in-situ CO₂ adsorbent for biogas upgrading, achieving an adsorption capacity of 2.14 mmol CO₂ g^-1 at 25 °C and 1 bar, successfully enriching raw biogas to 94.6% biomethane purity. Comprehensive thermodynamic modeling revealed that the hybrid configuration improved total exergy efficiency to 58.7%, representing a 22.4% increase over standalone anaerobic digestion. Life-cycle assessment metrics confirmed a net-negative carbon balance of -128.4 kg CO₂-equivalent per megawatt-hour of generated energy, demonstrating a scalable framework for bioenergy production coupled with durable carbon sequestration.