Innovative Technologies and Low-Impact Alternative Fuels for Maritime Sector Decarbonisation: Applicability, Integration, Techno-Economic Assessment, and Operational Simulations

In this study, ammonia is extensively analysed as an alternative zero-carbon fuel for marine applications from a techno-economic perspective. Within the framework of the green energy transition, integrating low- or zero-carbon alternative fuels into the maritime energy mix is a key strategy to comply with the long-term measures imposed by the International Maritime Organization (IMO) and governmental regulations. Ammonia has been selected for investigation due to its zero-carbon content, favourable volumetric energy density, and lower technical challenges for onboard storage compared to cryogenic fuels such as liquefied natural gas (LNG) and liquid hydrogen. To evaluate its feasibility in maritime applications, a tailored multi-criteria approach has been developed and applied to assess the suitability of different alternative fuels across various vessel types and sizes. The results indicate that ammonia is a promising solution, particularly for ships with lower range requirements. However, the strong dependence on fuel costs has been highlighted, making the approach highly sensitive to future market scenarios. In this regard, a comprehensive analysis of the ammonia market outlook has been conducted from different key perspectives: future blue and green ammonia production capacity, projected fuel demand, and ammonia-fuelled vessel order books, as well as the availability of port storage and bunkering infrastructure. To further detail ammonia's implementation onboard, a multi-criteria approach alone is insufficient, as it provides only a general feasibility assessment. Given the highlighted toxicity concerns, a mathematical model has been developed to simulate the thermodynamic behaviour of the ammonia handling system’s main component, represented by the storage. Following an extensive literature review on applicable modelling approaches, the Saturated Homogeneous Model (SHM) was selected for its balance between generalization potential and detailed thermodynamic representation. A flexible model was then built to simulate multiple operational conditions, including mass flow extraction for engine feeding, with the aim of analysing saturation pressure and temperature trends. Two case studies were conducted: a 20,000 m³ Type-A tank serving a large containership and a 1,250 m³ Type-C tank for a cruise ship, both analysed under sealed and mass-flow conditions. The findings highlight scenarios where a boil-off gas (BOG) management system is necessary to control tank pressure. For the smaller tank and at high mass flow rates, a depressurization effect was observed due to evaporative cooling. This analysis underscores the high sensitivity of smaller tanks to both self-pressurization and depressurization. However, no major critical issues were identified regarding ammonia storage. The primary challenge associated with ammonia, its toxicity, has been further investigated through a regulatory review and subsequent risk assessment. In this context, a Formal Safety Assessment (FSA), as proposed by the IMO, was conducted for an LNG-fuelled cruise ship. This vessel type was chosen for the risk analysis due to its stringent safety requirements. Furthermore, the selection of an LNG-fuelled ship as the baseline is justified by the strong alignment between the LNG and ammonia requirements, making it a more feasible solution from a cost perspective. The results highlight potential hazards related to ammonia release in both enclosed spaces and sensitive outdoor areas, necessitating the development of mitigation measures to reduce associated risks.