Among potential options, green ammonia has emerged as a promising zero-carbon fuel. Green ammonia offers significant advantages, such as carbon-free combustion and compatibility with pre-existing infrastructure.
Green Ammonia as a Pathway to Maritime Decarbonization
Raj Shah, Malvika Rao, Gavin Thomas | Koehler Instrument Company
Introduction
The maritime shipping sector facilitates approximately 80% of global trade and is responsible for nearly 3% of global carbon dioxide emissions. Historically, this sector depends on heavy fuel oil (HFO) and marine gas oil (MGO), fossil fuels with high carbon and sulfur content that contribute significantly to climate change. However, given recent regulatory pressures to decarbonize, such as the International Maritime Organization’s (IMO) 2020 sulfur cap and long-term targets for emission reductions by 2030 and 2050, the maritime sector demonstrates a growing and increasingly urgent interest in alternative marine fuels. Among potential options, green ammonia has emerged as a promising zero-carbon fuel. Green ammonia offers significant advantages, such as carbon-free combustion and compatibility with pre-existing infrastructure. However, its potential role in decarbonization may be hindered by substantial economic, safety, and regulatory challenges.
Problem Identification
This IMO has set a goal of net-zero GHG emissions by 2050, with an intermediate goal to be reached by 2030. Maritime transport currently accounts for around 940 million tons of CO2 emissions annually, making the sector an important hurdle to tackle to meet IMO goals [1]. The maritime sector currently has a dependency on fossil fuels with high carbon and sulfur content, although small steps have been taken to begin the transition to a more environmentally friendly future. HFO is exceptionally energy dense and cost-effective but also produces significant carbon dioxide (CO2), sulfur oxides (SOx), and fine particulate emissions, making it a major contributor to climate change and adverse health impacts [2]. While liquefied natural gas (LNG) and liquefied petroleum gas (LPG) has been widely adopted as a transitional fuel due to reduced sulfur and particulate emissions, it remains fossil-based and only offers modest 6-10% GHG reductions [3].
Hydrogen-based fuels, which include green hydrogen, e-methanol, e-methane, and e-ammonia, offer deeper decarbonization potential. However, they face significant barriers to implementation, specifically cost, energy efficiency, infrastructure and technical readiness, and safety [4]. Subsequently, the issue of an environmentally friendly and easily implementable maritime fuel faces significant challenges related to identifying a fuel pathway that achieves meaningful emission reductions while remaining technically, economically, and operationally viable.
The Promise of Green Ammonia
Green ammonia is synthesized by reacting nitrogen with green hydrogen produced through renewable-powered electrolysis [4]. Water is split into hydrogen and oxygen using renewable electricity, and the hydrogen is reacted with nitrogen from air via the Haber-Bosch process, shown in Figure 1, resulting in ammonia with near-zero carbon dioxide emissions [5].
Figure 1: Haber-Bosch Ammonia synthesis process. Sourced from [4].
This production pathway fully disassociates ammonia synthesis from fossil fuel feedstocks, eliminates carbon emissions at the point of combustion, and leverages an already well-established global infrastructure for ammonia storage and transport. As a result, ammonia offers a scalable and practical pathway for decarbonization of maritime shipping.
Unlike carbon-based fuels, ammonia does not emit CO2 during combustion and is sulfur-free, subsequently eliminating SOx emissions [1]. Crucially, ammonia benefits from decades of industrial experience in global production, storage, and transport due to its widespread usage in the fertilizer industry [4]. Millions of tons are already transported annually by ship, rail, and pipeline, providing a foundation upon which marine bunkering systems can be developed. This existing industrial infrastructure lowers the adoption risk of green ammonia, as it allows the maritime sector to repurpose preexisting infrastructure rather than develop an entirely new supply chain.
Recent technological and operational advancements further strengthen ammonia’s promise as a marine fuel. Industry engine manufacturers such as Everllence and Wärtsilä have already made progress towards fuel-flexible and ammonia-capable propulsion systems. Wärtsilä Gas Solutions has unveiled a novel Ammonia Fuel Supply System for ships, designed to enable the safe storage, handling, and controlled delivery of ammonia onboard while mitigating the risks associated with toxicity and leakage through dedicated safety and fuel conditioning systems [6, 7]. The Everllence B&W ME-LGIA, a two-stroke dual-fuel engine specifically engineered to operate on ammonia while retaining pilot fuel capability for ignition stability, represents a major milestone in ammonia propulsion development, offering a scalable solution for both new builds and retrofits and positioning ammonia as a viable zero-carbon option for large ocean-going vessels [8].
Additionally, in March 2025, NYK Line completed a three-month voyage of the Sakigake, a commercial vessel retrofitted from an LNG-powered tugboat that ran on ammonia fuel [9]. The Sakigake is the world’s first ammonia-fueled commercial vessel and was retrofitted by replacing all LNG-fueled equipment with ammonia-compatible machinery, such as a four-stroke engine that underwent extensive combustion testing to ensure stable operation with high ammonia content and near-zero carbon dioxide emissions [10]. The Sakigake’s trip achieved a 95% reduction in GHG emissions and provided a model for the modification of existing vessels to transition to ammonia fuel usage. A particular emphasis was placed on safety due to
ammonia’s heightened toxicity and corrosiveness relative to LNG [10]. Risk assessments informed extensive design modifications, including leak detection, remote monitoring systems, and new operational manuals and maintenance protocols. The Sakigake represents the possibility of achieving decarbonization via retrofitting of existing ships.
The feasibility of green ammonia as a marine fuel hinges on coordinated progress across technology, infrastructure, policy, and markets. Global coordination through the IMO, harmonized safety standards, carbon pricing mechanisms, and targeted policy incentives will be critical to overcoming first-mover risks. Current trends, such as ammonia-ready ship designs, pilot bunkering projects in major ports, and increasing regulatory clarity, suggest that ammonia could play a dominant role in shipping.
Challenges
Despite its advantages, green ammonia faces substantial technical, economic, and safety challenges. Foremost among these is cost. Green ammonia is currently estimated to be 3.5 to 4.5 times more expensive than HFO, driven largely by the high energy intensity of electrolysis and the Haber–Bosch synthesis process [9]. Producing one ton of e-ammonia requires approximately 10.3 MWh of electricity, implying a massive expansion of global renewable energy capacity to support large-scale adoption [11]. Furthermore, as seen in Figure 2, e-ammonia has a low volumetric energy density compared to conventional marine fuels such as MGO and HFO. This would necessitate a larger on-board storage volume (as denoted by the size of the bubbles) for e-ammonia to deliver an equivalent amount of energy as conventional fuels [11]. This does present a challenge of vessel safety and stability, but it could be mitigated through cargo design optimization and frequent refueling [11].[GT1]
Figure 2: Comparative energy density, carbon intensity, and storage volume requirements of e-ammonia and other marine fuels, sourced from [11].
Life-cycle assessments suggest that while ammonia-based systems, particularly SOFCs, offer high climate mitigation potential and cost-effectiveness relative to other zero-carbon options, all decarbonization pathways currently impose costs 2.5 to 4 times higher than MGO and increase electricity demand [2].
From a technical standpoint, ammonia exhibits poor combustion characteristics, including high ignition temperature, low flame speed, and narrow flammability limits [12]. As a result, most ammonia engines require pilot fuels, and incomplete combustion can lead to nitrogen oxide and nitrous oxide emissions. Mitigation technologies such as advanced injection strategies, exhaust gas recirculation, and selective catalytic reduction systems are therefore essential to control emissions and ensure stable operation, though they increase system complexity and cost [13]. Dual-fuel engines represent the most realistic transitional solution, enabling gradual adoption while mitigating combustion challenges. However, despite fuel cells consistently outperforming internal combustion engines in environmental performance, ammonia
ICEs are likely to dominate near-term deployment due to technological maturity [2].
Safety concerns further complicate adoption. Ammonia is toxic and corrosive, necessitating robust leak detection, ventilation, crew training, and harmonized safety regulations [9]. Recent projects demonstrate that these risks can be effectively managed through layered engineering and operational controls. Proposed solutions include double-walled fuel piping, continuous leak detection, enhanced ventilation, segregated fuel spaces, and automated shutdown systems, combined with rigorous crew training and updated operational procedures [13]. While substantial regulatory knowledge exists, gaps remain in fuel-specific maritime standards and quantitative risk assessments.
Conclusion
Green ammonia stands out as one of the most promising long-term solutions for decarbonizing maritime shipping. Its carbon-free combustion, sulfur-free composition, and compatibility with existing industrial infrastructure provide compelling advantages over other alternative fuels. However, high costs, energy intensity, combustion challenges, and safety risks present formidable barriers to widespread adoption. Compared with LNG, hydrogen, and e-fuels such as methanol, green ammonia offers superior scalability for long-distance shipping but requires sustained technological innovation and policy support. Ultimately, green ammonia’s success will depend on whether its environmental benefits can outweigh its economic and technical challenges. With continued advancements in engine technology, emissions control, renewable energy expansion, and global regulatory coordination, green ammonia has the potential to become a cornerstone of a fossil-free maritime future.
Dr. Raj Shah, is a Director at Koehler Instrument Company in New York, where he has worked for the last 25 plus years. He is an elected Fellow by his peers at ASTM, IChemE, ASTM,AOCS, CMI, STLE, AIC, NLGI, INSTMC, Institute of Physics, The Energy Institute and The Royal Society of Chemistry. An ASTM Eagle award recipient, Dr. Shah recently coedited the bestseller, “Fuels and Lubricants handbook”, details of which are available at ASTM’s Long-awaited Fuels and Lubricants Handbook https://bit.ly/3u2e6GY. He earned his doctorate in Chemical Engineering from The Pennsylvania State University and is a Fellow from The Chartered Management Institute, London. Dr. Shah is also a Chartered Scientist with the Science Council, a Chartered Petroleum Engineer with the Energy Institute and a Chartered Engineer with the Engineering council, UK. Dr. Shah was recently granted the honorific of “Eminent engineer” with Tau beta Pi, the largest engineering society in the USA. He is on the Advisory board of directors at Farmingdale university (Mechanical Technology), Auburn Univ (Tribology), SUNY, Farmingdale, (Engineering Management) and State university of NY, Stony Brook (Chemical engineering/ Material Science and engineering). An Adjunct Professor at the State University of New York, Stony Brook, in the Department of Material Science and Chemical Engineering, Raj also has over 700 publications and has been active in the energy industry for over 3 decades.
Ms. Malvika Rao is a Chemical and Molecular Engineering Undergraduate Student at Cornell University. She is also a part of a thriving internship program at Koehler Instrument company in Holtsville, NY underneath Dr. Raj Shah.
Mr. Gavin Thomas is part of a thriving internship program at Koehler Instrument Company in Holtsville, NY and is a recent graduate of the Chemical and Molecular Engineering program at Stony Brook University. He also works as a process engineer at Mill-Max in Oyster Bay, NY where he becomes hands-on with various production processes to ultimately improve safety, efficiency, and cost-effectiveness.
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Life-Cycle Assessment and Costing of Fuels and Propulsion Systems in Future Fossil-Free Shipping. Environmental Science and Technology, 56(17), 12517-12531. https://doi.org/10.1021/acs.est.2c03016
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(pp. 1–5). Oxford Institute for Energy Studies. http://www.jstor.org/stable/resrep64339.8
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https://www.sciencedirect.com/science/article/pii/S0308597X24004445
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https://www.wartsila.com/media/news/27-02-2024-wartsila-introduces-ammonia-fuel-sup ply-system-to-ease-shipping-s-transition-to-ammonia-fuel-3410284
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https://www.wartsila.com/marine/decarbonisation/future-fuels-development
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https://orcid.org/0000-0002-4614-7214
[GT1]Ammonia’s low volumetric energy density ( 2. MJ/L) relative to conventional marine fuels such as marine gas oil (MGO, 36.6 MJ/L) necessitates significantly larger storage volumes, approximately 4.1 times more than MGO, to deliver equivalent energy (see Table 3). As illustrated in Figure 7, the relative size of the bubbles represents the storage volume needed for each marine fuel type in comparison to conventional options. The figure also captures the carbon intensity (y-axis) and energy density (x-axis), which influences onboard storage requirements. While this raises concerns regarding vessel stability and cargo capacity, these challenges may be mitigated through design optimisation and more frequent refueling (Cames, Wissner und Sutter 2021, EMSA 2022)
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