Maritime shipping is a critical component of global trade but remains heavily reliant on fossil fuels, contributing significantly to greenhouse gas emissions. As the industry seeks decarbonization pathways, hydrogen-derived fuels such as liquid hydrogen (LH2) and ammonia are emerging as viable alternatives for powering hydrogen carrier ships. These vessels, designed to transport hydrogen, can also utilize their cargo as fuel, creating a closed-loop system that reduces emissions and operational complexity. This article examines the engine technologies, bunkering logistics, and emissions reduction potential of using ammonia and LH2 for marine propulsion.
Engine Technologies for Hydrogen-Derived Fuels
The adoption of hydrogen-derived fuels in marine propulsion requires compatible engine technologies, primarily fuel cells and internal combustion engines (ICEs). Each has distinct advantages and challenges when using ammonia or LH2.
Fuel cells, particularly proton exchange membrane (PEM) and solid oxide fuel cells (SOFCs), are well-suited for LH2 due to their high efficiency and zero-emission operation. PEM fuel cells operate at low temperatures, making them compatible with LH2’s cryogenic storage requirements. SOFCs, while less common in marine applications, can also run on ammonia by cracking it into hydrogen and nitrogen internally, achieving efficiencies of up to 60%. However, fuel cells face scalability challenges for large vessels due to power density limitations and high costs.
Internal combustion engines modified for ammonia or LH2 offer a more immediate solution. Ammonia can be combusted directly in ICEs with minor adjustments to ignition systems and fuel injection. Trials by manufacturers like MAN Energy Solutions have demonstrated ammonia’s feasibility in two-stroke marine engines, though nitrogen oxide (NOx) emissions remain a concern. LH2-powered ICEs, while less developed, benefit from hydrogen’s high energy density and clean combustion, producing only water vapor. However, hydrogen’s low ignition energy and high flame speed require specialized engine designs to prevent knocking and ensure safety.
Bunkering Logistics for Ammonia and LH2
Bunkering infrastructure is a critical factor in the adoption of hydrogen-derived fuels. Ammonia has an advantage due to its established global supply chain, with over 180 ports already handling ammonia for fertilizer production. It can be stored at moderate pressures (-33°C) or ambient temperatures under pressure, simplifying bunkering operations. Existing liquid bulk terminals can be retrofitted for ammonia, reducing upfront costs.
LH2 bunkering is more complex due to its cryogenic storage requirements (-253°C). Insulated storage tanks and specialized transfer systems are necessary to prevent boil-off losses. Pilot projects, such as those in Japan and Norway, are testing LH2 bunkering at scale, but widespread deployment will require significant investment in cryogenic infrastructure. Safety protocols for LH2 handling, including leak detection and ventilation, are more stringent than for ammonia due to hydrogen’s flammability range.
Emissions Reduction Potential
The shift to hydrogen-derived fuels offers substantial emissions reductions compared to conventional marine fuels. Ammonia combustion produces zero carbon dioxide (CO2), though NOx emissions must be managed through selective catalytic reduction or engine tuning. Lifecycle analyses indicate that green ammonia, produced via renewable-powered electrolysis, can reduce well-to-wake CO2 emissions by over 90% compared to heavy fuel oil.
LH2’s emissions profile is even cleaner, with no CO2 or NOx when used in fuel cells. However, the energy-intensive liquefaction process affects its overall carbon footprint. If renewable energy powers liquefaction, LH2’s lifecycle emissions approach zero. Methane slip, a concern with liquefied natural gas (LNG), is irrelevant for LH2, eliminating a potent greenhouse gas source.
Operational Challenges and Future Outlook
Despite their promise, ammonia and LH2 face operational hurdles. Ammonia’s toxicity requires rigorous safety measures for crew and port personnel, while LH2’s low energy density per unit volume necessitates larger storage tanks, reducing cargo capacity. Both fuels currently suffer from higher costs than conventional options, though scaling production and infrastructure could narrow the gap.
The maritime industry’s regulatory landscape is evolving to support hydrogen-derived fuels. The International Maritime Organization’s (IMO) 2050 emissions targets are driving investment in zero-emission technologies, with ammonia and LH2 positioned as leading candidates. Classification societies like DNV have begun issuing guidelines for ammonia-fueled vessels, signaling growing acceptance.
In conclusion, hydrogen carrier ships powered by ammonia or LH2 represent a pragmatic step toward decarbonizing maritime transport. Fuel cells and ICEs offer viable propulsion options, while existing and emerging bunkering infrastructure can support their adoption. Emissions reductions are substantial, particularly when paired with green hydrogen production. Overcoming cost and technical barriers will require continued innovation and collaboration across the industry, but the foundation for a hydrogen-powered shipping future is being laid today.