Ammonia has emerged as a promising hydrogen carrier for maritime transport due to its high hydrogen density, ease of storage, and compatibility with existing infrastructure. Unlike pure hydrogen, which requires cryogenic temperatures or high-pressure systems for liquefaction, ammonia can be stored as a liquid at moderate pressures or at -33°C under ambient pressure. This makes it a practical medium for long-distance shipping of hydrogen, particularly for international energy trade.
One of the primary advantages of ammonia is its high volumetric hydrogen density. A single cubic meter of liquid ammonia contains approximately 121 kg of hydrogen, compared to 71 kg for liquid hydrogen at -253°C. This higher energy density translates to more efficient transport, reducing the number of shipments required for the same hydrogen delivery. Additionally, ammonia benefits from an established global supply chain, with over 180 ports equipped for ammonia handling, primarily for fertilizer production. This existing infrastructure can be repurposed for hydrogen logistics with minimal modifications.
However, ammonia’s toxicity poses significant safety challenges. Exposure to concentrations as low as 25 ppm can cause respiratory irritation, while higher concentrations are lethal. Leak detection and mitigation systems must be rigorously implemented to prevent accidental releases during loading, unloading, and transit. Furthermore, ammonia combustion produces nitrogen oxides (NOx), which contribute to air pollution. While catalytic converters can reduce NOx emissions, they add complexity to ammonia-based propulsion systems.
Another critical challenge is the energy-intensive process of cracking ammonia back into hydrogen. The reaction requires temperatures between 400°C and 600°C, along with catalysts such as ruthenium or nickel. Efficiency losses during cracking reduce the overall energy yield, with current systems achieving approximately 70-80% conversion efficiency. Research is ongoing to develop lower-temperature cracking methods and more durable catalysts to improve this process.
Several pilot projects demonstrate the feasibility of ammonia as a maritime fuel. In 2020, the Maritime Energy and Sustainable Development Centre of Excellence in Singapore launched a project to retrofit a vessel with an ammonia-powered engine. Similarly, Norway’s Eidesvik Offshore and Wärtsilä are collaborating on an ammonia-fueled supply ship expected to begin operations by 2024. These initiatives highlight the growing interest in ammonia as a zero-carbon fuel for shipping.
Conversion technologies for ammonia-to-hydrogen are also advancing. Membrane reactors, which integrate cracking and hydrogen separation, show promise for improving efficiency. These systems use palladium-based membranes to extract pure hydrogen while retaining unreacted ammonia for further processing. Another approach involves electrochemical cracking, which uses electricity to decompose ammonia at lower temperatures, though this method is still in early development.
Regulatory frameworks for ammonia transport are well-established due to its long history in agriculture. The International Maritime Organization (IMO) classifies ammonia as a hazardous material under the International Code for the Construction and Equipment of Ships Carrying Dangerous Chemicals in Bulk (IBC Code). Vessels must adhere to strict containment and ventilation standards to ensure safe handling. However, regulations specific to ammonia as a hydrogen carrier are still evolving. The IMO’s Greenhouse Gas Strategy encourages alternative fuels, but additional guidelines are needed to address ammonia’s unique risks in hydrogen logistics.
The economic viability of ammonia-based hydrogen transport depends on scaling up production and cracking technologies. Current estimates suggest that ammonia cracking adds $1-2 per kg to the cost of hydrogen, primarily due to energy inputs and catalyst expenses. However, as renewable energy costs decline and cracking technologies improve, these costs are expected to decrease.
In summary, ammonia offers a practical solution for hydrogen shipping, leveraging existing infrastructure and high energy density. Its toxicity and cracking inefficiencies remain hurdles, but ongoing projects and technological advancements are addressing these challenges. Regulatory support and further innovation will be crucial in establishing ammonia as a mainstream hydrogen carrier for global maritime transport.
Case studies illustrate the real-world potential of this approach. Japan’s Green Ammonia Consortium is exploring large-scale ammonia imports for hydrogen supply, with plans to utilize ammonia directly in power generation and ship propulsion. Similarly, Australia’s Hydrogen Energy Supply Chain project includes ammonia transport as a key component of its export strategy. These initiatives underscore the growing recognition of ammonia’s role in the hydrogen economy.
The future of ammonia as a hydrogen carrier will depend on continued collaboration between industry, regulators, and researchers. By addressing safety concerns, improving cracking efficiency, and expanding infrastructure, ammonia could become a cornerstone of international hydrogen trade. The maritime sector, with its need for high-density energy carriers, is poised to lead this transition, paving the way for broader adoption of hydrogen as a clean energy source.