Alternative Fuels for Marine Propulsion: A Technical Analysis

1. Liquefied natural gas (LNG)

Features and benefits

Liquefied natural gas (LNG) is natural gas that has been cooled to a liquid state, to a temperature of about -162°C, for storage and transport. LNG consists primarily of methane (CH4) and is considered a cleaner alternative to conventional marine fuels such as heavy fuel oil (HFO) and marine diesel oil (MDO).

• Lower emissions:LNG combustion results in significantly lower emissions of sulfur oxides (SOx), nitrogen oxides (NOx) and particulate matter (PM). It also reduces carbon dioxide (CO2) emissions by about 20-25% compared to traditional fuels.
• Energy density:LNG has a higher energy density than compressed natural gas (CNG), allowing more energy to be stored in a smaller volume, crucial for long journeys.

Challenges

• Cryogenic storage:LNG must be stored in cryogenic tanks, which require specialized insulation and handling systems to maintain the low temperatures needed to keep the gas in a liquid state.
• Methane slip:One of the technical challenges associated with LNG is methane leakage, which is the release of unburned methane, a potent greenhouse gas, into the atmosphere during combustion or refueling.
• Infrastructure:LNG deployment is limited by the availability of bunkering infrastructure. Although LNG bunkering facilities are expanding, they are still not as common as conventional fuel storage facilities.

2. Biofuels

Features and benefits

Biofuels are produced from organic materials such as vegetable oils, animal fats and waste biomass. They can be used as a direct replacement or blended with conventional marine fuels.

• Carbon neutrality:Biofuels are considered carbon neutral because the CO2 emitted during combustion is compensated by the CO2 absorbed by plants during their growth cycle.
• Compatibility:Biofuels can be used in existing marine engines with minimal modifications, making them an attractive option for retrofitting existing vessels.

Challenges

• Availability of raw materialsBiofuel production depends on the availability of feedstocks, which may be limited by competition with food production and other land uses.
• Energy density:Biofuels tend to have a lower energy density than traditional marine fuels, requiring larger volumes to achieve the same range.
• Life cycle emissions:While biofuels are considered carbon neutral, their overall environmental impact depends on the entire life cycle, including growing, processing and transporting the feedstock.

3. Hydrogen

Features and benefits

Hydrogen is a zero-emission fuel that can be used in fuel cells or internal combustion engines. When used in fuel cells, hydrogen is combined with oxygen to produce electricity, with the only byproduct being water vapor.

• Zero emissions:The combustion of hydrogen produces no CO2, SOx or PM emissions, making it an ideal fuel for reducing the environmental impact of maritime transport.
• Energy efficiencyHydrogen fuel cells are extremely efficient, converting chemical energy directly into electrical energy with an efficiency exceeding 60%.

Challenges

• Storage and distribution:Hydrogen has a very low volumetric energy density, therefore high-pressure tanks or cryogenic storage are necessary to obtain adequate energy density for marine applications.
• Production:Most hydrogen is currently produced from natural gas via steam methane reforming (SMR), which produces CO2 emissions. Green hydrogen, produced via electrolysis using renewable energy, is more sustainable but currently more expensive.
• InfrastructureAs with LNG, the lack of widespread hydrogen bunkering infrastructure is a serious obstacle to its widespread deployment.

4. Ammonia

Features and benefits

Ammonia (NH3) is a carbon-free fuel that can be synthesized from nitrogen and hydrogen. It can be used in internal combustion engines or fuel cells.

• Zero carbon dioxide emissions: Burning ammonia produces nitrogen and water vapor, without emitting CO2. This makes it an attractive option for achieving future decarbonization goals.
• Storage and handling:Ammonia is easier to store and transport than hydrogen because it can be stored as a liquid at relatively low pressure and temperature.

Challenges

• Toxicity: Ammonia is highly toxic and poses serious health and safety hazards if not handled properly. This requires specialized handling procedures and safety protocols.
• Combustion characteristics:Ammonia has a lower energy density and different combustion properties than conventional fuels, requiring engine modification and optimization to operate efficiently.
• NOx emissions: While burning ammonia does not produce CO2, it can generate nitrogen oxides (NOx), which are harmful pollutants. Effective NOx control measures are necessary to mitigate this problem.

5. Methanol

Features and benefits

Methanol (CH3OH) is a liquid alcohol that can be produced from natural gas, coal, biomass or captured CO2 and renewable electricity.

• Easy to store:Methanol is a liquid at ambient temperature and pressure, making it easy to store, handle and transport using existing infrastructure.
• Lower emissions: Methanol combustion produces lower SOx, NOx and PM emissions compared to conventional marine fuels. It also offers the potential for carbon neutral operation if produced from renewable sources.

Challenges

• Energy density:Methanol has a lower energy density than traditional marine fuels, meaning ships need to carry larger volumes of it to achieve the same range.
• Corrosive:Methanol is corrosive to some metals and requires the use of appropriate materials in storage tanks, fuel lines and engines.

The adoption of alternative fuels for marine propulsion is essential for the marine industry to meet future environmental regulations and reduce its carbon footprint. Each alternative fuel offers unique benefits and challenges, and the choice of fuel will depend on a variety of factors, including vessel type, operating conditions and availability of bunkering infrastructure.

As the industry continues to innovate and adapt, a mix of alternative fuels could prove to be the optimal solution for different segments of the marine sector. The transition to these fuels will require significant investment in new technologies, infrastructure and safety protocols. However, the potential benefits in terms of environmental sustainability and compliance with global emissions standards make this a key area of ​​focus for the future of marine propulsion.