Ammonia as fuel for marine dual-fuel technology: A comprehensive review

Abstract

Decarbonizing maritime transportation is a key objective for the future green shipping industry, with ammonia emerging as a promising candidate among hydrogen carriers and alternative fuels for CO₂-free energy systems. Its benefits include high volumetric hydrogen density, low storage pressure, high storage temperature than other alternative fuels, and long-term stability. This comprehensive review analyzes over 300 studies, researches and experiments from 2001 to 2024, evaluating the potential of ammonia supply systems for dual-fuel engines in both current and future maritime applications. The study explores the viability of using ammonia in dual-fuel marine diesel engines, combining it with diesel, hydrogen, biodiesel, LNG, LPG, or other fuels to achieve significant reductions in carbon emissions, supported by ammonia's low auto-ignition temperature. Maintaining the ammonia energy ratio below 60 % helps to lower the flame temperature, which in turn reduces NOx emissions. However, increasing the ammonia energy proportion also leads to higher N₂O emissions, which counterbalances the benefits of lower CO₂ emissions. Optimizing diesel injection timing helps to reduce N₂O and NH₃ emissions. Despite its potential, ammonia dual-fuel combustion still presents challenges, particularly concerning NOx emissions and high levels of unburned ammonia due to the fuel's nitrogen content. These issues necessitate the implementation of effective aftertreatment systems. Consequently, ammonia is currently regarded as a feasible option primarily for marine applications, power generation, and potentially cargo vessels, where space limitations are less restrictive. Furthermore, addressing critical factors such as safety, combustion efficiency, and technological advancements is essential before ammonia can be considered a complete replacement for conventional marine fuels.