Ammonia has been identified as the most feasible alternative fuel for ocean-going deep-sea vessels seeking to meet the International Maritime Organization’s 2050 greenhouse gas emission reduction targets, according to research published in the MDPI journal Sustainability on 12 February.

The study, authored by Rushdie Rasheed, Managing Marine Engineer at Brookes Bell, in collaboration with Sean Loughney and Eduardo Blanco-Davis from Liverpool Logistic, Offshore and Marine (LOOM) Research Institute, Liverpool John Moores University (LJMU), employed a hybrid multi-criteria decision analysis combining the analytical hierarchy process and TOPSIS methodology to evaluate three alternative fuels — hydrogen, ammonia, and methanol — across technical, environmental, economic, and social criteria.

The analysis drew on survey responses from 57 maritime experts and secondary data from existing literature. Ammonia achieved a closeness degree value of 0.6241 in the preference ranking, followed by hydrogen at 0.5840 and methanol at 0.4160.

Environmental performance emerged as the most important factor in fuel selection, accounting for 42.8% of the total criteria weight, according to the expert survey. Technical attributes received one-third of the environmental weighting, while economic and social attributes accounted for 17.6% and 11.3%, respectively.

Within environmental criteria, greenhouse gas reduction carried the highest sub-criteria weight at 13.4%, followed by safety concerns. The combined weights of safety and environmental sub-criteria amounted to 55% of total local weights.

Deep-sea vessels account for approximately 80% of global maritime CO2 emissions, according to the research. The study focused on liquid hydrogen, ammonia, and methanol as the most applicable fuels for deep-sea vessels, noting that these options are more suitable than alternatives for vessels requiring wide availability and international regulatory compliance.

The survey participants represented diverse professional roles, with marine engineers comprising 71.9% of respondents. In terms of experience, 61.4% of participants had over 15 years in the industry. Educational backgrounds varied, with 42% holding STCW professional qualifications, 18% holding master’s degrees, and smaller proportions holding diplomas, bachelor’s degrees, and PhDs.

The research identified several characteristics of ammonia relevant to its ranking. The fuel has a three-times higher energy density than compressed hydrogen and contains 50% more hydrogen than liquid hydrogen. Ammonia can be stored at −33°C and atmospheric pressure in bulk quantities, or as pressurised liquid at ambient temperature.

However, the study noted ammonia’s drawbacks, including low flame speed, high auto-ignition temperature, high heat vaporization, and toxicity. The fuel requires pilot fuel use and selective catalytic reduction systems to mitigate nitrogen oxide generation. Ammonia can be corrosive and toxic in concentrated form, causing irritation to skin, eyes, and respiratory systems, with a recommended safe maximum threshold limit value of 50 ppm over eight hours.

Hydrogen, ranked second, is described in the research as the cleanest marine fuel in terms of combustion emissions, producing only water when burned in oxygen. The fuel has the highest energy content per unit mass but low volumetric energy density, requiring larger storage volumes than traditional marine fuels.

Current hydrogen production predominantly comes from fossil fuels, with only a small percentage generated through renewable electrolysis. The European Union aims to install 6 GW of renewable energy electrolyser capacity by 2024 to produce one million tonnes of green hydrogen annually, with plans to increase capacity to 40 GW by 2030 for 10 million tonnes of production. Australia anticipates exporting one million tonnes of hydrogen by 2030.

Hydrogen storage presents challenges, requiring either high-pressure tanks for gas form—needing four times the storage space of conventional fuels—or cryogenic tanks capable of withstanding temperatures of −253°C for liquid form. These constraints currently limit hydrogen to short-sea voyages.

Methanol, ranked third, emerged as a transitional option due to its cost, capability to integrate with existing technology, and current availability. The fuel is biodegradable and readily dissolves in water. Methanol can be used in internal combustion engines or as a fuel source for fuel cells, with combustion emitting water and CO2 as by-products, no sulphur oxides, and negligible nitrogen oxides.

The research noted methanol’s lower energy density compared to traditional marine fuels, requiring approximately twice the volume of marine diesel oil to produce equivalent energy. Safety risks include flames propagating at low temperatures with no smoke and invisible flames in daylight. Existing bunkering infrastructure would require minor modifications to accommodate methanol due to its low flashpoint.

Large-scale methanol production currently relies on fossil feedstocks such as natural gas and coal, though the fuel can be produced from renewable feedstocks or as an electro-fuel. Southeast Asia has abundant biomass feedstock available for methanol production.

The study conducted sensitivity analysis by altering the geometric mean of pairwise comparison survey results by −15% to +15% in 5% increments. The ranking order of ammonia, hydrogen, and methanol did not change despite variations in relative weights of all criteria and sub-criteria.

Within technical criteria, safety aspects carried the highest weight at 42.3%, followed by energy efficiency at 22.2%, technology readiness level at 21.8%, and energy density at 13.2%. For economic criteria, fuel cost and operational expenditures each received 30% weighting, capital expenditures 22.9%, and crewing and training costs 17.2%.

Social acceptance received 37.6% weighting within social criteria, followed by socio-economic development and compliance with regulations. The consistency ratio for social sub-criteria showed greater variation than other categories, potentially reflecting difficulty in assessing social criteria without objective data.

The research was supported by the UK National Clean Maritime Research Hub under the HYDRO-Port project, examining safety management and risk assessment of liquid hydrogen bunkering and storage in ports. The study received approval from Liverpool John Moores University’s Faculty Research Ethics Committee in June 2022.

The authors noted limitations in survey representation, with 71.9% of participants being marine engineers and limited participation from shipowners, charter parties, fuel suppliers, and engine manufacturers. Nine of 57 survey responses were discarded due to inconsistency, representing 15.7% of total responses.

The research suggests that IMO and national regulators should prioritize ammonia and hydrogen in long-term decarbonization frameworks, with targeted investments in fuel production, bunkering infrastructure, and safety regulations, while supporting methanol development as a transitional fuel.