The shipping industry faces significant environmental challenges due to marine engine emissions, including NOx, SOx, PM, and CO2. Existing after-treatment technologies, while helpful, are insufficient to meet the International Maritime Organization's (IMO) ambitious 2050 goal of a 50% reduction in carbon emissions and a target of zero emissions by the end of the 21st century. Focusing solely on energy efficiency improvements is insufficient to meet these targets. Alternative fuels, including liquefied natural gas, liquefied petroleum gas, methanol, biodiesel, hydrogen, and ammonia, are being investigated. In the short term, liquefied natural gas, liquefied petroleum gas, and methanol are considered more mature options, whereas biodiesel, hydrogen, and ammonia are seen as long-term solutions. Green fuels, produced from renewable sources and resulting in low or zero emissions, offer a crucial pathway toward decarbonization. Ammonia, hydrogen, and methanol are being actively investigated as green fuels. This paper addresses the gap in research concerning the pathway from renewable energy sources to the production of these green fuels for the shipping industry.

The paper reviews existing literature on decarbonization efforts in the shipping industry, including the IMO's strategies and national-level regulations. It summarizes studies on alternative fuels and their potential for ships, highlighting the advantages and challenges associated with each option. The paper also extensively reviews the production methods of renewable energy sources such as hydropower, wind power, solar power and bioenergy. It examines the role and importance of green power in carbon emission reduction by focusing on the production technologies and their potential challenges. The challenges posed by the intermittent nature of renewable sources such as wind and solar are discussed. The literature also reviews different carbon capture technologies (pre-combustion, post-combustion, and O2 fuel combustion) and their applications in reducing CO2 emissions from ships. A comprehensive review of existing studies of green hydrogen production (electrolysis, biomass), green ammonia production (Haber-Bosch process, electrochemical methods, photocatalysis, biocatalysis), and green methanol production (biomass and CO2 conversion) are presented to contextualize the work.

The paper uses a qualitative review methodology, systematically examining and synthesizing existing research on green fuel production for shipping. The authors review and analyze existing literature on various aspects of green fuel production, including renewable energy generation, carbon capture, and the specific production methods for green hydrogen, ammonia, and methanol. This involves examining different production methods for each fuel type, comparing their advantages and disadvantages concerning the shipping industry, and discussing the challenges associated with their implementation. The analysis focuses on the production pathways, energy efficiencies, environmental impacts, and economic considerations for each fuel. The paper also explores the application of these fuels in marine engines, discussing potential challenges such as onboard storage and infrastructure requirements. Quantitative data points such as energy densities and greenhouse gas emissions are incorporated to support the qualitative analysis. The paper then synthesizes this information to suggest a pathway for decarbonizing shipping.

The study finds that green power production from renewable sources (hydropower, wind, solar, and bioenergy) is crucial for generating green hydrogen, ammonia, and methanol. Electrolysis of water, particularly seawater electrolysis, is a promising method for green hydrogen production. The Haber-Bosch process, when powered by renewable hydrogen, is a viable method for green ammonia synthesis, although alternative methods like electrochemical nitrogen reduction and photocatalysis are being explored. Green methanol can be produced from biomass or via CO2 hydrogenation. For marine applications, methanol is considered the most promising near-term solution due to its higher energy density and ease of storage and handling. However, methanol is carbon-containing, so combining it with carbon capture is essential for near-zero carbon emissions. Ammonia is also promising but faces challenges related to its toxicity and combustion properties. Hydrogen has the potential for long-term use but requires significant advancements in onboard storage and supporting infrastructure. The paper highlights that cost remains a major obstacle to wider adoption of green fuels. Improvements in production technology and renewable energy sources are needed to increase efficiency and reduce costs. Onboard carbon capture can serve as a short-term solution to bridge the gap until green fuel technologies become fully mature.

The findings of this study address the research question by outlining a viable pathway for decarbonizing the shipping industry through the use of green fuels. The significance of the results lies in the comprehensive review of various green fuel production methods and their respective challenges and opportunities in the context of the shipping sector. The results highlight the need for technological advancements in renewable energy generation, fuel production, and onboard storage to make green fuels economically competitive with fossil fuels. This paper's relevance to the field lies in its contribution to a crucial discussion on how to achieve carbon neutrality in the shipping industry, providing valuable insights into the production, storage, and utilization of alternative fuels. The paper's comprehensive review of diverse green fuel production routes offers valuable guidance for policymakers, researchers, and industry stakeholders involved in the maritime decarbonization effort.

The paper concludes that the future of green fuels in shipping is promising, with green hydrogen, ammonia, and methanol potentially replacing fossil fuels. However, significant advancements in renewable energy technologies, fuel production efficiency, and cost reduction are necessary. The development of supporting infrastructure for green fuel storage and distribution is equally important. Future research should focus on improving the efficiency and cost-effectiveness of green fuel production, developing advanced storage technologies, and exploring new and innovative methods for carbon capture and utilization.

The paper primarily focuses on the technological aspects of green fuel production and utilization, with less emphasis on the broader economic and political factors influencing adoption. The analysis of life-cycle assessments for various fuels is limited in scope. The review of existing literature may not encompass every relevant publication, and future studies may benefit from more comprehensive data collection and analysis. Further, the evaluation of some technological options is based on projected future developments.