Techno-economic optimization of an integrated ammonia–methane synthesis system powered by LNG-assisted biogas oxy-fuel cycle and vanadium chloride hydrogen production

This study presents a novel multi-generation biogas-fueled power system integrating oxy-fuel combustion and thermochemical hydrogen production for simultaneous power generation, synthetic fuel production, and carbon management. The system combusts biogas with pure oxygen, produced via cryogenic air separation using LNG cold energy, in three sequential combustion chambers and turbines. High-temperature exhaust gases are directed to a vanadium chloride (VCl) thermochemical cycle for hydrogen production and an Organic Rankine Cycle (ORC) for additional power recovery. At the same time, radiative heat from the combustion chambers is converted to electricity through thermophotovoltaic (TPV) units. Separated hydrogen is divided between methanation and ammonia synthesis units, using CO2 from the final exhaust and nitrogen from air separation. Thermodynamic, techno-economic, and environmental analyses were conducted, followed by a multi-objective optimization using the Gray Wolf Optimizer. One scenario targeted maximum net power and exergetic efficiency with minimized product cost, another emphasized environmental impact reduction, and the third balanced subsystem exergetic efficiency with product cost. In the first case, the system achieved a net power output of 12,023.05 kW, an exergetic efficiency of 47.60 %, and a product cost of 19.21 $/GJ. The environmental-focused case reduced the environmental index to 0.4063 $/kWh with a product cost of 19.50 $/GJ, while the balanced case reached an exergetic efficiency of 45.23 %, a product cost of 19.12 $/GJ, and an environmental index of 0.4030 $/kWh. Increasing ammonia and methane prices to 1.6 $/kg and 0.6 $/kg shortened the payback period from 5.71 to 4.5 years. These results demonstrate the system's high efficiency, economic resilience, and renewable energy potential.