Techno-economic performance optimization and energy integration analysis of ammonia synthesis system with different electrolyzers

Converting renewable electricity into carbon-free ammonia fuel will not only solve the problem of renewable energy utilization but also contribute to reducing the traditional chemical industry's dependence on fossil energy and achieving a low-carbon transformation in chemical production. This study employs a multi-objective optimization algorithm to optimize the waste heat recovery unit of the ammonia synthesis system based on alkaline water electrolyzer, proton exchange membrane electrolyzer cell, and solid oxide electrolyzer cell, designs the system heat exchange network, and explores the effect of improving the system efficiency by using the system's low-temperature waste heat to produce domestic hot water. The results show that improved system energy efficiency comes at the expense of increased ammonia production costs due to increased investment in auxiliary equipment, and the energy efficiency of the ammonia synthesis system based on solid oxide electrolyzer cell is the highest, ranging from 72.1 % to 73.3 %. The payback time of the ammonia synthesis system based on alkaline water electrolyzer is the shortest, ranging from 7.7 to 8.1 years, due to the minimal investment in alkaline water electrolyzer. The exergy analysis results show that the maximum efficiency point further improves the system energy efficiency by optimizing the utilization of the system's medium and high-temperature waste heat. In addition, at the maximum efficiency point, if the low-temperature waste heat of the system is used to produce domestic hot water, the system's exergy loss will be further reduced, resulting in an increase in system exergy efficiency of about 0.3–0.5 %, and a shortening of the investment payback period by 0.2–0.6 years. Economic analysis results show that when the electricity price is greater than 56.8 $/MWh, the reduced electricity cost when the ammonia synthesis system adopts the solid oxide electrolyzer cell to produce hydrogen can offset the electrolyzer's high investment cost, making it more economical than the proton exchange membrane electrolyzer cell. The above conclusions provide an important reference for the integration design and technology selection of ammonia synthesis processes in regions with different resource conditions and technological maturity.