Performance Evaluation of Green Ammonia Production for Decarbonizing Ship Fleet Operations

With the increasing need to decarbonize maritime transport, green ammonia (NH₃), produced from renewable energy sources, has emerged as a promising alternative marine fuel. In this study, the environmental performance of green ammonia production is evaluated for its application in fueling two container ships operating within Portuguese territory. The system is designed for a production capacity of 15,000 metric tons of ammonia per year, supported by a 10 MW synthesis plant and a 20 MW electrolysis unit to supply the required green hydrogen.The proposed system is compared with conventional ammonia production pathways, including grey ammonia produced via steam methane reforming (SMR), as well as grid-based ammonia scenarios reflecting the Portuguese electricity mix for 2025, 2030, and 2050. Results indicate that green ammonia production leads to emissions of 0.61 kg CO₂-eq per kg NH₃, achieving a 72% reduction compared to grey ammonia.Although ammonia production pathways exhibit higher upstream emissions than heavy fuel oil (HFO) production, the combustion phase of ammonia enables a reduction of up to 98% in emissions relative to HFO. When considering both production and utilization stages, ammonia-based pathways, particularly those relying on renewable electricity, result in lower overall emissions than HFO. Furthermore, the increasing share of renewable energy in future electricity mixes significantly enhances the environmental performance of ammonia production.