Adopting ammonia fuel for marine vessels and pushing toward wider use of liquid hydrogen fuel
By Toshiro Fujimori, Keiichi Okai and S.A. Sherif|December 2024
The Terrestrial Energy Systems Technical Committee works to advance the application of engineering sciences and systems engineering to the production, storage, distribution and conservation of energy for terrestrial uses.
Transitioning shipping to ammonia fuel is a high priority of the Japanese government’s “green growth strategy.” In August, the vessel Sakigake was delivered to the Port of Yokohama, making it the world’s first ammonia-fueled tugboat for commercial use, its developers said. Formerly powered by liquefied natural gas, Sakigake was remodeled by NYK Line and IHI Power Systems under Japan’s New Energy and Industrial Technology Development Organization. In offshore operation, each engine achieved a maximum 95% mixing ratio of fuel ammonia, and ammonia and nitrous oxide concentrations were sufficiently removed by an exhaust gas after-treatment system to ensure 90% greenhouse gas reductions. Ammonia fuel emits no carbon dioxide, leading to significant emissions reductions in power generation and transportation applications. For example, the International Maritime Organization adopted the 2023 IMO Strategy on Reduction of GHG Emissions from Ships, which aims to reduce carbon emissions in international shipping by 40% by 2030 relative to 2008 levels and reach zero emissions by or around 2050.
On the hydrogen fuel front, the U.S. Department of Energy in April announced the winners of phase two of its Hydrogen Shot Incubator Prize Competition, which funds “disruptive technologies that reduce the cost of producing clean hydrogen.” Electro-Active Technologies of Tennessee, Green Fortress Engineering of Indianapolis, NX Fuels of Michigan and PAX Scientific of California are each to receive $400,000 in lab vouchers and funds. This initiative calls for reducing the cost of clean hydrogen by 80% by 2030, which equates to $1 per kilogram. In May, DOE’s Hydrogen and Fuel Cell Technologies Office released its “Multi-Year Program Plan” for achieving that and related goals, including a target of $2/kilogram by 2026.
Once produced, hydrogen needs to be stored for use in different applications. In the transportation sector, for example, liquid hydrogen, or LH2, is convenient because it has a larger storage density relative to gaseous storage. Onboard storage in vehicles enables burning hydrogen in internal combustion engines or using it in a fuel cell. Liquefying hydrogen has historically consumed a significant part of the available combustion energy. For example, the minimum energy required for classical theoretical liquefaction cycles is greater than 10 kilowatt-hours per kilogram of LH2. While some expander cycles can in theory deliver liquefied hydrogen below 6 kWh/kg, some large plants in operation today still consume 13-14kWh/kg. The DOE has set a target of 6 kWh/kg.
Liquid storage is also typically associated with relatively high boil-off losses. To address that challenge, upgraded refrigeration/pre-cooling cycles such as the high-pressure Claude cycle and some mixed refrigerant cycles are used. Scaling up liquefaction plants to above 100 tons/day is key to higher efficiencies. This year’s advances in raising the liquefaction efficiency include the development of oil-free scroll compressors by “Beyond Scroll,” with compressors capable of compressing hydrogen from atmospheric pressure without contamination, thus reducing system components and maintenance costs. Beyond Scroll’s compact compressors operate at high speeds with only 120 kilowatt-electric of input power.
Other advances include the development of metal hydride hydrogen compressors, MHCs, for transportation. Greek startup CYRUS did this with thermal-powered MHCs absorbing hydrogen at low pressure and temperature with an external heat source. In the area of liquid storage, optimization of vessel shape and employing vacuum-jacketed insulation are being used. Research is currently addressing the whole LH2 production and supply chain, including using catalysts to speed-up ortho-para conversion during liquefaction, larger refrigeration units during transportation, baffles in trucks to reduce sloshing and finding innovative ways to reduce flashing during unloading of LH2. DOE’s increased emphasis on transitioning to a hydrogen economy prompted numerous researchers to work on all aspects of this transition. A review of these developments appeared in the Feb. 29 issue of the International Journal of Hydrogen Energy.
