Integration of amine-based CO2 capture with bio-methanol synthesis using waste methanol solvent: Toward energy-efficient and circular carbon solutions for biogas-fueled power plants

• Integrated CO₂ capture and bio-methanol synthesis from waste methanol. • DEA in methanol enables low-energy desorption and high CO₂ removal. • Waste methanol is a cost-effective and sustainable absorbent. • Optimized process reduces energy use while maintaining performance. • Stable CO₂ capture with real flue gas supports industrial feasibility. This study investigates the performance of monoethanolamine (MEA) and diethanolamine (DEA) in capturing carbon dioxide (CO₂) from the exhaust gas of biogas-fueled engine-generator systems, using water and methanol as solvents in a pilot-scale absorption–desorption process. DEA in methanol exhibited superior separation efficiency and lower energy consumption compared to aqueous systems. Notably, recovered waste methanol from previous methanol distillation showed comparable performance to commercial-grade methanol, demonstrating its potential as a sustainable and cost-effective solvent. The CO₂ concentration in the desorbed gas reached 98.00% for DEA in water, 97.75% in commercial methanol, and 95.27% in waste methanol. Total energy consumption ranged from 14.55 to 15.56 MJ/kg CO₂ captured. Process optimization operating absorption at 30 °C and desorption at 80 °C using a cooling tower in place of a chiller successfully reduced energy use to 11.46 MJ/kg CO₂. Real exhaust gas containing sulfur oxides (SOₓ), nitrogen oxides (NOₓ), and carbon monoxide (CO), at elevated temperatures post-combustion, was first cooled via a shell-and-tube heat exchanger and dehumidified using an adsorption dryer prior to CO₂ capture. A 24-hour continuous operation test confirmed the system's stability. These results support the implementation of methanol-based DEA absorption systems for efficient CO₂ capture, offering integration potential with biomethanol production and contributing to low-carbon energy systems and circular economy initiatives. The corresponding CO₂ removal efficiencies were 95.36% for commercial methanol and 88.90% for waste methanol, confirming the high effectiveness of DEA regeneration under mild-temperature conditions.