Liquified Natural Gas (LNG)

LNG is considered a mature alternative fuel option. However, there are some technology choices that need to be made depending on specific vessel design and operational requirements.

Benefits of LNG as marine fuel

• LNG-fuelled vessels can reduce a vessel’s EEDI rating by 20 percent and the corresponding Carbon Intensity Indicator rating by approximately the same amount. 
• At the same time, improved vessel design and engine technologies can reduce GHGs by up to 25 per cent. And with more bunkering options now available and others planned, LNG represents a good, medium term option to reach compliance. 
• An LNG fuelled vessel can use lower/zero carbon fuel options like SNG, or biogas with minimal conversion – offering a potential path to decarbonized operations.

Technical considerations

• Because LNG has a lower volumetric energy density than fuel oil, onboard gas storage requires larger tanks than conventional fuel oil storage to provide the same operational range. 
• And due to the low temperature of LNG, the tank insulation and required gas handling systems additional space and equipment is required. 
• Depending on their preferences and priorities, owners can choose between two main types of engine technologies with different characteristics that are now available to the market. 

Liquefied Petroleum Gas (LPG) 

LPG, which is any mixture of propane and butane in liquid form, is a widely available fuel that has recently been introduced as a marine fuel. 

Benefits of LPG as marine fuel

• Liquified Petroleum Gas combustion results in CO2 emissions that are approximately 15 per cent lower than those of Fuel Oil FO. 
• When accounting for the complete life cycle, including fuel production, the CO2 savings amount to roughly 17 per cent. 
• The cost of installing LPG systems on board a vessel is roughly half that of an LNG system. This is because there is no need for special materials for handling cryogenic temperatures. 

Technical considerations

• Currently only two-stroke diesel engines are commercially available for using LPG as a ship fuel. 
• Four-stroke engines have also been developed but so far are only used for power generation on shore, not for marine applications.
• LPG fuelled vessels often install shaft generators to take advantage of LPG for auxiliary engines. 
• LPG can be stored under pressure or refrigerated, but bunkering options may not always be available in the temperature and pressure range a ship can handle. Therefore, pressurized tanks are typically selected. 
• The bunkering source and the ship must carry the necessary equipment and installations for safe bunkering. 
• LPG tanks can also be suitable for ammonia, so long as their pressure rating is appropriate. Engine technology is also quite similar, making LPG designs the easiest to retrofit to utilise ammonia as fuel at a later stage.

Methanol

Methanol is the simplest alcohol with the lowest carbon content and highest hydrogen content of any liquid fuel. Methanol can be made available through existing infrastructure in more than 100 ports globally. 

Benefits of methanol as ship fuel

• Methanol combustion in an internal combustion engine reduces CO2 emissions by approximately 10 per cent compared to FO. 
• When considering the complete life cycle, including the production of the fuel from natural gas, the total CO2 emissions are equivalent to or slightly higher (ca. 5%) than the corresponding emissions of petroleum-based fuels. 
• In the future, “green methanol” is likely to become available, with the potential for significantly lower GHG emissions.

Technical considerations

• There are two main options for using methanol as fuel in conventional ship engines: A two-stroke diesel-cycle engine or in a four-stroke, lean-burn Otto-cycle engine. 
• Methanol is a liquid fuel and can be stored in standard fuel tanks, but modifications are required to accommodate its low-flashpoint properties to comply with the IMO’s IGF Code.

Biofuels

Biofuel is a collective term for a range of energy carriers produced by converting primary biomass or biomass residues into liquid or gaseous fuels. While the number of vessels running on biofuels today is relatively small, DNV has identified sustainable biofuels as one of few options available for deep-sea shipping, especially for existing vessels, to achieve the IMO targets.

Benefits of using biofuels

• Biofuels from advanced processes derived from sustainable feedstocks can achieve substantial GHG reductions while minimizing other effects. 
• Biofuels can be blended with conventional fuels or used as drop-in fuels as substitutes for conventional fossil fuels. 
• A drop-in fuel can directly be used in existing installations without major technical modifications, making  them very attractive for existing tonnage.  

Technical considerations

Because biofuels are derived from organic materials, fuel quality can be compromised by microbial growth and oxygen degradation. Also, some liquid biofuels can have poor flow properties in low temperatures. Biofuels are also solvents, so during conversions, operators must flush the fuel system when switching from diesel to biofuel to avoid deposits or clogged filters. Managing these challenges is important to ensure trouble-free operations. 

Batteries

Batteries (and hybrid power plants) represent a transformation in the way energy is used and distributed on board vessels. While not yet a viable replacement to FO for deep sea transportation, batteries are increasingly being used for ferries and short sea shipping.  

Benefits

• In addition to being emissions free, electric power systems using batteries are more controllable and easier to optimize in terms of performance, safety and efficiency. 
• As ship power systems become increasingly electrified and battery technology improves and becomes more affordable, new opportunities emerge. 

Technical considerations

• Batteries produced by different manufacturers use different chemistries, resulting in significant differences in performance. Depending on the vendor, even batteries with the same nameplate chemistry can have very different properties. 
• Developments in battery technologies are expected to be the result of incremental improvements in terms of cost and performance. 
• New battery technologies, which would represent a disruptive change for deep sea shipping, may be as much as ten years away.