Which is best 'cleaner' fuel for shipping?

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

The study, conducted by researchers at Liverpool John Moores University’s Global Centre for Maritime Innovation, led by Professor Eddie Blanco-Davis and Dr Sean Loughney, employed a hybrid multi-criteria decision analysis combining the analytical hierarchy process and TOPSIS methodology to evaluate three fuels—hydrogen, ammonia, and methanol—across technical, environmental, economic, and social criteria.

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

Environmental performance emerged as the most important factor, accounting for 42.8% of the total criteria weight. 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.

Hydrogen? Methanol?

Deep-sea vessels account for approximately 80% of global maritime CO2 emissions, according to the research. Liquid hydrogen, ammonia and methanol are considered the most applicable fuels for deep-sea vessels.

Advantages of ammonia include the fact it has a three-times higher energy density than compressed hydrogen and contains 50% more hydrogen than liquid hydrogen, it 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.

Hydrogen, ranked second, is “the cleanest marine fuel” in terms of combustion emissions, producing only water when burned. 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 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.

Methanol from biomass

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.

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.

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 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.

Professor Blanco-Davis said: “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.”

Other authors were Dr Sean Loughney (LJMU) and graduate engineer Rushdie Rasheed from local company Brookes Bell.