Review of the state-of-the-art of alternative marine fuels: A viable approach to zero-carbon shipping

The shipping industry, responsible for transporting 90% of global goods, is a major source of pollution and greenhouse gas (GHGs) emissions. In response to the increasingly stricter global and regional emission control regulations, the maritime industry has adopted various operational and technical measures to improve vessel energy efficiency so as to reduce emissions. However, these measures might not be able to effectively address the core issue of emissions, which arises from a heavy reliance on carbon-intensive energy sources. To reduce the emissions from the whole shipping industry more fundamentally, this review evaluates the viability of five alternative marine fuels — liquefied natural gas (LNG), methanol, ammonia, biofuel, and hydrogen — as potential solutions for maritime decarbonization. This review adopts the systematic search flow (SSF) approach, using iterative search refinement and thematic analysis for a structured synthesis of maritime alternative fuel literature. It first introduces each type of alternative fuel with an emphasis on production methods and sources, which are distinctively categorized by “color.” Following this, a comprehensive comparison of the fuels is presented, focusing on technical feasibility, economic viability, emission reduction capabilities, availability, and safety considerations. The practical application of these fuels is further explored through an analysis of their adoption in operational fleets and new orders, as well as the readiness of port infrastructure to support these changes. This review also examines the role of alternative fuels in the development of green shipping corridors, underscored by an analysis of green shipping finance initiatives. The findings provide valuable insights into the viability of these fuels, supporting the International Maritime Organization (IMO)’s 2050 decarbonization goals and paving the way towards zero emissions in global shipping.