Emerging Technologies in Sustainable Marine Fuels: A Comprehensive Review

1. Introduction

The maritime industry stands at a critical juncture in its efforts to reduce greenhouse gas emissions and mitigate its environmental impact. In recent years, there has been a surge in technological advancements in the development and implementation of sustainable marine fuels. These innovations span a wide range of approaches, from harnessing biomass and algae for biofuel production to exploring the potential of hydrogen and ammonia as carbon-free alternatives. As the global shipping sector continues to develop in response to environmental pressures and regulatory demands, these emerging technologies and fuel alternatives are poised to shape the future of maritime propulsion and contribute significantly toward maritime decarbonization efforts.

However, significant challenges remain in scaling up production, developing comprehensive infrastructure, and improving economic feasibility. The global shipping industry contributes approximately 3% of global greenhouse gas emissions, with projections indicating a significant increase without intervention3. Reducing greenhouse gas emissions in the international shipping industry could cost hundreds of thousands of dollars over the coming decades, potentially impacting profitability and freight rates while necessitating massive investments in new technologies and infrastructure. In response, the International Maritime Organization has set ambitious targets to reduce the carbon intensity of international shipping by at least 40% by 2030 and 70% by 2050, compared to 2008 levels4. These goals have catalyzed intense research and development in sustainable marine fuels.
Figure 1 depicts the different types of fuels in the sustainable marine fuel market. As depicted Ammonia, Hydrogen, and Biofuels constitute approximately 50 percent of the current sustainable marine fuel market. Note that while methanol currently comprises 30 percent, its use is limited due to its low energy density combined with a low specific gravity; these factors do not align with the weight and space requirements of modern ships. This paper aims to provide a comprehensive overview of recent advancements in Biofuels, Hydrogen, and Ammonia specifically for maritime applications. As the global shipping sector continues to evolve in response to environmental pressures and regulatory demands, these emerging technologies and fuel alternatives are poised to shape the future of maritime propulsion and contribute significantly toward maritime decarbonization efforts.

2.1 Lignocellulosic Biomass Conversion

Recent years have seen significant progress in converting lignocellulosic biomass -- plant biomass composed of cellulose, hemicellulose, and lignin -- into marine-grade biofuels.
As Figure 2 depicts, converting lignocellulosic biomass into biofuels involves harvesting plants that have stored solar energy through photosynthesis, preparing the feedstock, and then subjecting the cellulose, hemicellulose, and lignin components to pretreatment and enzymatic saccharification to break them down into simpler sugars, which are then fermented and purified to produce biofuels.
One such study conducted by Johnson et al. in 2022 demonstrated a novel catalytic pyrolysis process using corn stover, producing fuel with properties comparable to conventional marine gas oil while reducing lifecycle greenhouse gas emissions by up to 80%2. The novel catalytic pyrolysis process optimized the conversion of corn stover, a common agricultural waste product, into a high-quality biofuel. The researchers employed a specialized catalyst to break down the complex carbohydrates in the corn stover at high temperatures, a process known as pyrolysis. This catalyst was designed to selectively promote the formation of hydrocarbon compounds similar to those found in conventional marine gas oil. The process involved several key steps: pre-treatment of the corn stover to remove impurities, followed by rapid heating in the presence of the catalyst under controlled oxygen-free conditions. The resulting bio-oil was then further refined and upgraded to meet the specifications required for marine engines. The researchers’ innovation in catalyst design and process optimization allowed them to achieve a product with energy density and flow properties closely matching those of conventional marine fuels, while significantly reducing the carbon footprint of the fuel production process. This breakthrough addresses the previous challenges in biofuel energy density and compatibility with existing engine systems. By demonstrating the ability to produce biofuels with properties comparable to conventional marine gas oil while substantially reducing lifecycle greenhouse gas emissions, this research addresses critical challenges in biofuel energy density and compatibility with existing engine systems, paving the way for more widespread adoption of biofuels in the maritime industry.

2.2 Algae-based Biofuels

Algae has emerged as a promising feedstock for marine biofuels due to their high oil content and rapid growth rates. Zhang et al. (2023) reported a 30% increase in lipid productivity when cultivating microalgae using wastewater from marine aquaculture15. The study explored the potential of microalgae as a feedstock for marine biofuels, focusing on an integrated approach that combined microalgae cultivation with wastewater treatment from marine aquaculture. This innovative method addresses two significant challenges simultaneously: the need for sustainable marine fuels and the management of aquaculture wastewater. By creating a more efficient and sustainable system for biofuel production, this study suggests a viable path forward in developing sustainable marine fuels, potentially contributing to efforts to reduce the environmental impact of the shipping industry. The integrated approach of cultivating microalgae using wastewater from marine aquaculture not only enhances biofuel production efficiency but also contributes to water treatment, demonstrating the potential for synergistic solutions in sustainable maritime operations and underscoring the promising role of algae-based biofuels in the future of maritime energy.

2.3 Enhanced Fuel Properties

Recent advancements in biofuel technology have made significant strides in addressing two critical challenges for maritime applications: energy density and cold flow properties. These improvements are bringing biofuels closer to becoming viable alternatives to conventional marine fuels, potentially reducing the maritime industry’s environmental impact. One such study conducted by Liu et al. in 2022 focused on addressing the challenge of lower energy density in biofuels, a key barrier to their widespread adoption in the maritime industry8. The researchers employed hydrothermal liquefaction of mixed algal biomass to produce a high-energy-density bio-crude. Hydrothermal liquefaction is a process that converts wet biomass into crude-like oil using high pressure and temperature, effectively mimicking the natural geological processes that create fossil fuels but in a much shorter timeframe. Their innovative process resulted in a biofuel with an impressive energy density of 40 MJ/kg, which closely approaches the energy density of conventional marine diesel fuel (42-44 MJ/kg). Notably, this bio-crude maintained significantly lower sulfur content and greenhouse gas emissions compared to conventional fuels, demonstrating its potential as a more environmentally friendly alternative for maritime applications.
Another study by Rodriguez et al. in 2023 tackled another critical challenge in biofuel adoption: poor cold flow properties. Their research focused on developing a novel winterization process for biodiesel produced from waste cooking oil. Through their innovative approach, the researchers successfully improved the cold filter plugging point of the biodiesel from -5°C to -20°C11. This significant enhancement in cold flow properties makes biodiesel more suitable for use in colder climates without the need for additives or blending with conventional diesel, addressing a major limitation in the use of biofuels in diverse environmental conditions.
By improving these critical characteristics, these research teams have brought biofuels closer to being viable alternatives to conventional marine fuels. These innovations contribute to the broader goal of reducing the maritime industry’s reliance on fossil fuels and decreasing its environmental impact, aligning with global efforts towards sustainable and cleaner shipping practices.

3. Hydrogen: Production and Application Innovations

Hydrogen fuel has emerged as a promising alternative to traditional fossil fuels, offering a clean energy source that produces only water as a byproduct when used. As the maritime industry seeks to reduce its carbon footprint, hydrogen fuel cells are gaining attention for their potential to power ships with zero emissions. However, the sustainability of hydrogen fuel depends greatly on its production methods, leading researchers to focus on renewable hydrogen production techniques. Most hydrogen is produced from methane, which has a substantial carbon footprint. Recent advancements in renewable hydrogen production have made significant strides towards making this fuel sustainable for marine applications.

3.1 Renewable Hydrogen Production

Renewable hydrogen addresses maritime carbon reduction needs while overcoming traditional production limitations. It offers zero-emission operation, energy storage, and versatility, complementing other sustainable fuels. Recent innovations in solar and wind-powered hydrogen production are improving efficiency and economic viability, paving the way for reduced fossil fuel dependence in the maritime industry. Current hydrogen production primarily relies on methane reforming, a process that is cost-effective but still emits greenhouse gases. This method contradicts the goal of reducing carbon emissions in the maritime industry. While hydrogen produced through electrolysis using renewable energy sources is cleaner, it remains more expensive. However, ongoing research and development efforts are focused on making renewable hydrogen production more economically viable. These initiatives aim to leverage advancements in solar, wind, and other renewable technologies to decrease the cost of green hydrogen, potentially making it competitive with methane-derived hydrogen in the future. Recent advancements, such as Nakamura’s highly efficient solar-powered hydrogen production and Siemens Gamesa and Ørsted’s offshore wind-to-hydrogen project, are making renewable hydrogen more efficient and economically viable. These innovations are crucial steps towards integrating hydrogen into the broader spectrum of sustainable marine fuels, providing the maritime industry with a viable path to reduce its dependence on fossil fuels and decrease its environmental impact.
A study by Nakamura et al. focused on developing a novel photocatalyst material to enhance the efficiency of solar-powered hydrogen production. The researchers engineered a multi-layered photocatalyst structure using a combination of rare-earth elements and transition metals. This composite material was designed to absorb a broader spectrum of sunlight and more efficiently convert photon energy into hydrogen production. The study employed advanced nano-fabrication techniques to create a precise arrangement of these materials, optimizing the interface between layers to minimize energy losses. They also incorporated a unique surface treatment to enhance the catalyst’s stability and longevity in aqueous environments. Through iterative testing and refinement, the study was able to achieve a remarkable 19% solar-to-hydrogen efficiency, significantly outperforming previous photocatalyst designs10. This breakthrough in photocatalyst technology could potentially transform the economic viability of large-scale hydrogen production for marine applications, making it a more attractive alternative fuel option for the shipping industry.
Complementing solar-based methods, the concept of offshore wind-to-hydrogen has gained considerable traction. In 2021, renewable energy giants Siemens Gamesa and Ørsted joined forces to launch a pioneering pilot project. This initiative aimed to demonstrate the feasibility of producing green hydrogen directly from offshore wind power at one of Ørsted’s Danish offshore wind farms12. The study explores a novel approach to renewable energy utilization, addressing challenges in energy storage and transportation while leveraging existing offshore infrastructure. This innovative concept could potentially revolutionize fuel production for the maritime sector, accelerating the adoption of hydrogen as a sustainable marine fuel and marking a significant step towards decarbonizing the shipping industry. By making large-scale hydrogen production for marine use more economically viable, it offers a unique solution to the challenges faced by the maritime sector in adopting cleaner energy sources.

3.2 Fuel Cell Technology

Advancements in fuel cell technology have made hydrogen a more viable option for marine propulsion. Kim et al.’s 2022 study, published in the Journal of Power Sources, marks a significant advancement in fuel cell technology for marine applications. The researchers successfully operated a 500-kW high-temperature proton exchange membrane (HT-PEM) fuel cell system, demonstrating its potential for maritime propulsion. Notably, the system used impure hydrogen produced from ammonia cracking, addressing logistical challenges associated with hydrogen storage and transport in marine environments. The fuel cell achieved an impressive 55% electrical efficiency, comparable to or surpassing advanced marine diesel engines while producing zero emissions7. This high efficiency, combined with the system’s ability to operate on cracked ammonia, showcases the potential of fuel cell technology to provide environmentally friendly, high-performance power solutions for the maritime industry. The study’s focus on a high-temperature PEM fuel cell likely contributed to its efficiency and tolerance for fuel impurities, further enhancing its suitability for marine applications. Typical marine diesel engines usually operate with thermal efficiencies in the range of 25-35%21. So, the 55% electrical efficiency achieved by the HT-PEM fuel cell system is significantly better than the most efficient conventional marine diesel engines. This is significant because fuel cells offer this high efficiency while producing zero emissions, unlike diesel engines which produce various pollutants.

4. Ammonia: A Promising Carbon-Free Fuel

In parallel with hydrogen, green ammonia has emerged as another promising carbon-free fuel for the maritime industry. In parallel with hydrogen, green ammonia has emerged as another promising carbon-free fuel for the maritime industry. Produced using renewable energy, water, and air, green ammonia offers a potentially sustainable alternative to conventional fossil fuels. Its high energy density and existing infrastructure for production, storage, and transport make it an attractive option for long-distance shipping. Recent advancements in production methods, including more efficient electrochemical processes and novel catalysts, are improving the economic viability of green ammonia. These developments aim to overcome the challenges of traditional ammonia production, which is energy-intensive and typically relies on fossil fuels. As the maritime sector seeks to reduce its carbon footprint, green ammonia is gaining attention for its potential to serve as both a direct fuel and a hydrogen carrier, offering flexibility in its application across various vessel types and sizes.

4.1 Green Ammonia Production

The study by Brown et al. (2023) presents a significant advancement in ammonia production compared to conventional methods2. While the traditional Haber-Bosch process relies on fossil fuels, operates at high temperatures and pressures, and has an efficiency of 45-55%, Brown’s team developed an electrochemical process using only air, water, and renewable electricity. This novel approach achieved 60% energy efficiency, marking a notable improvement19. The Haber-Bosch process combines nitrogen from the air with hydrogen, typically derived from natural gas, under high temperatures and pressures using an iron-based catalyst to produce ammonia. Unlike the centralized, large-scale facilities required for Haber-Bosch, this new method shows potential for decentralized, small-scale production, possibly even at ports. This could revolutionize the ammonia supply chain, particularly for the maritime industry, by enabling on-site production and reducing transportation costs and emissions. Renewable electricity and simpler raw materials make this process carbon-neutral, aligning with industry goals to reduce environmental impact. While promising, it’s worth noting that further research may be needed to scale this technology for widespread industrial use. Overall, this innovation represents a significant step towards more sustainable and efficient ammonia production, with promise for the maritime sector’s transition to cleaner fuels. Recent research has focused on developing novel catalyst materials to improve the efficiency of ammonia synthesis. For example, Guo et al. (2022) reported a ruthenium-based catalyst that significantly enhances ammonia production rates under milder conditions compared to traditional iron catalysts. This new catalyst demonstrates higher activity and selectivity, potentially reducing the energy requirements and improving the overall sustainability of the ammonia production process21.

Conclusion

The field of sustainable marine fuels has seen remarkable progress in recent years, with advancements in biofuels, hydrogen, and ammonia technologies offering unique pathways to maritime decarbonization. Improvements in efficiency, energy density, and engine compatibility are making alternative fuels increasingly competitive with conventional marine fuels. However, significant challenges remain, particularly in scaling up production, developing comprehensive bunkering infrastructure, and addressing the higher costs associated with many alternative fuels. Future research should address these barriers to facilitate the widespread adoption of sustainable marine fuels. As the maritime industry continues to evolve in response to environmental pressures, the technologies discussed in this review will likely play a crucial role in shaping a more sustainable future for global shipping.

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Authors

Dr. Raj Shah is a Director at Koehler Instrument Company in New York, where he has worked for the last 25 plus years. He is an elected Fellow by his peers at IChemE, AOCS, CMI, STLE, AIC, NLGI, INSTMC, Institute of Physics, The Energy Institute and The Royal Society of Chemistry. An ASTM Eagle award recipient, Dr. Shah recently coedited the bestseller, “Fuels and Lubricants handbook”, details of which are available at ASTM’s LongAwaited Fuels and Lubricants Handbook 2nd Edition Now Available https://bit.ly/3u2e6GY.
He earned his doctorate in Chemical Engineering from The Pennsylvania State University and is a Fellow from The Chartered Management Institute, London. Dr. Shah is also a Chartered Scientist with the Science Council, a Chartered Petroleum Engineer with the Energy Institute and a Chartered Engineer with the Engineering council, UK. Dr. Shah was recently granted the honourific of “Eminent engineer” with Tau beta Pi, the largest engineering society in the USA. He is on the Advisory board of directors at Farmingdale university (Mechanical Technology), Auburn Univ (Tribology), SUNY, Farmingdale, (Engineering Management) and State university of NY, Stony Brook ( Chemical engineering/ Material Science and engineering). An Adjunct Professor at the State University of New York, Stony Brook, in the Department of Material Science and Chemical engineering, Raj also has over 650 publications and has been active in the energy industry for over 3 decades. More information on Raj can be found at https://bit.ly/3QvfaLX
Contact: rshah@koehlerinstrument.com
Dr. Vikram Mittal is an Assistant Professor in Systems Engineering at the United States Military Academy at West Point, New York.  Before USMA, he was a senior mechanical engineer at Draper Laboratory in the Vehicles and Robotics Group.  He worked on several projects designing power systems for robotic platforms.  He earned his PhD in 2009 from MIT researching the relationship between fuel octane numbers and engine performance.  Additionally, he received an MS in Aerospace from Oxford and a BS in Aeronautics from Caltech.  He is a Reservist in the United States Army and a combat veteran.
Udithi Kothapalli is a chemical engineering student at Carnegie Mellon University, set to graduate in May 2025. She is also pursuing a minor in Biomedical Engineering. Udithi is actively involved in campus organizations, serving as the current president of the Indian Organization at her university. Additionally, she holds the position of industrial liaison for the American Institute of Chemical Engineers Chapter at Carnegie Mellon, demonstrating her commitment to both cultural and professional development within her field of study.