Marine alternative fuels for the energy transition

The International Maritime Organization (IMO) launched an updated strategy in July 2023 [1] to encourage the reduction of greenhouse gas (GHG) emissions produced by ships. The strategy aims to achieve a reduction of 20% to 30% (compared to 2008 levels) in GHG emissions produced by ships by using technologies, fuels, and energy sources with zero or near-zero GHG emissions. The ultimate decarbonization target of this strategy is to reach net zero GHG emissions by 2050. While different technical and operational measures could be used to achieve these targets, the utilization of alternative fuels is currently viewed as the best option for achieving IMO reduction targets [2,3].?

There are a variety of alternative fuels which may be used for power generation onboard ships, such as Liquefied Natural Gas (LNG), Liquefied Petroleum Gas (LPG), methanol (MeOH), bio-fuels (such as bio-diesel), hydrogen (H2) and ammonia (NH3). From a statistical point of view, 98.2% of in-service fleets use conventional fuel while 1.8% use alternative fuels - distributed as shown in Figure 1 [4]. The contribution of alternative fuel fleets will be enhanced after the operation of on-order ships, however, and will become 26.2% (compared to 73.8% for conventional fuel). Moreover, Figure 2 [4] shows the distribution of alternative fuel fleets (in operation and on order) per ship type.

As depicted in Figure 3, different marine fuel pathways exist which may be shaped by the definition of primary energy source, processing method, distribution/ bunkering method, and the prime mover installed onboard. Primary energy sources may be classified into fossil fuels (crude oil – natural gas), biomass sources, and renewable energy sources (hydro, wind, solar). Each primary energy source may be used to produce marine fuels by using a specific processing method - for example, LNG may be produced from natural gas (via natural gas processing) or from electrolysis, after feeding it with electricity generated from renewable energy sources. The alternative fuels may then be stored and bunkered to vessels by using different storage options classified into ambient, cryogenic, and pressurized tanks. The selection of an appropriate storage option depends on the characteristics of the fuel itself. The last element in the marine fuel pathway is the definition of installed prime movers onboard, which may come in the form of either an internal combustion engine (ICE) or an advanced energy system such as fuel cells (FC).??

From a technical standpoint, the difference between alternative fuel options is based on their gravimetric and volumetric energy densities. This gives an indication of their applicability onboard ships and defines their potential application in terms of ship type. In order to assess the different alternative fuels and their applicability onboard ships, their mass and volume requirements should be compared with the current most common fuel (HFO or MDO), as shown in Figure 4.

?Although methanol can be stored as a liquid in ambient conditions onboard ships, its gravimetric energy density is around 19.9 MJ/kg (15.8 MJ/l), less than half compared to HFO/MDO. Therefore, the mass and volume of methanol required to deliver the same energy produced by HFO/MDO onboard ships for a specific trip is more than 2 times and 2.5 times, respectively. On the other hand, LNG has a higher gravimetric density (48.6 MJ/kg) and a lower volumetric density (20.8 MJ/l) than HFO. LNG is required to be stored at a cryogenic temperature (-161 ?C), so its handling system onboard ships must contain an evaporator skid which is permanently installed. For these reasons, LNG and HFO have a similar requirement in terms of mass, but its volume requirement is larger than HFO by 2 times.?

Due to its lower volumetric energy density (high volume requirements), there is a key drawback in storing Hydrogen onboard ships. That being said, it does hold a high gravimetric energy density (120 MJ/kg). As shown in Figure 4, the required hydrogen volume is about 3.6-4.5 times its HFO equivalent, based on the prime mover type. In addition, bio-diesel possesses a similar gravimetric and volumetric energy density (42.2 MJ/kg, 33 MJ/l) to HFO. It is therefore a potential alternative fuel which may be applied onboard ships as a short-medium term solution.?

From an economic standpoint, alternative fuel prices are not a straightforward indicator. This is because many factors may affect their comparison including energy content, market availability, production location, etc. DNV and Argus [4,5] published prices for grey alternative fuels in Europe from Jan 2022 till Jan 2024, as seen in Figure 5. These prices are expressed in terms of an energy equivalent based on marine gas oil (MGO), to allow a fair price comparison between different fuels. Recently, LNG had the lowest price (588 USD/t MGOe) followed by biofuel (1,100 USD/t MGOe), methanol (1,500 USD/t MGOe) and ammonia (1,667 USD/t MGOe). Moreover, the levelized cost of hydrogen produced by the methane reforming process (with the presence of Carbon Capture and Storage (CCS)) was 5.4 USD/kg-H2 in 2020, and is expected to be reduced to about 3.4 USD/kg-H2 by 2050. The cost of green hydrogen was about 6.2 USD/kg-H2 in 2020, produced by dedicated solar PV electrolysis. This is expected to be reduced to about 2.4 USD/kg-H2 by 2050 [6].

From an environmental standpoint, ship emissions may be classified into GHG emissions (carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O)) and non GHG emissions such as nitrogen oxide (NOx), sulfur dioxide (SOx), and particulate matter (PM). Figure 6 depicts the assessment of different emissions for marine alternative fuels relative to HFO levels, from its production to its utilization onboard ships: also known as well-to-wake (WTW) emissions.

Methanol produced from natural gas processing and utilized as a main fuel in ICE emits the highest GHG emissions compared to other fuels, followed by LNG. Installing a CCS system is recommended when using LNG and methanol onboard ships, to reduce carbon dioxide emissions and comply with IMO targets. Bio-diesel has lower GHG emissions overall, considering that biomass processing consumes CO2 during the bio-diesel pathway. The use of LPG as a marine fuel reduces GHG emissions by 17% compared to HFO, with a small contribution to NOx emissions when using a four-stroke Otto cycle engine. Moreover, hydrogen and ammonia produced from natural gas processing have lower GHG emissions than HFO. Since hydrogen or ammonia-fueled ICE is expected to have the same NOx emissions as HFO, their NOx emissions are based on an energy converter (this technology is still in the design phase). These NOx emissions are negligible, however, if a fuel cell system is used as an energy converter. On the other hand, bio-diesel tends to have the same NOx levels as HFO with low levels of SOx and PM emissions.

References

[1] IMO. 2023 IMO strategy on reduction of GHG emissions from ships. RESOLUTION MEPC377(80) 2023;July:1–18. https://www.imo.org/en/OurWork/Environment/Pages/2023-IMO-Strategy-on-Reduction-of-GHG-Emissions-from-Ships.aspx?

[2] Bouman EA, Lindstad E, Rialland AI, Str?mman AH. State-of-the-art technologies, measures, and potential for reducing GHG emissions from shipping – A review. Transp Res D Transp Environ 2017;52:408–21. https://doi.org/10.1016/j.trd.2017.03.022?

[3] Elkafas AG, Rivarolo M, Massardo AF. Assessment of alternative marine fuels from environmental, technical, and economic perspectives onboard ultra large container ship. Transactions of the Royal Institution of Naval Architects, International Journal of Maritime Engineering 2022;164:125–34. https://doi.org/10.5750/ijme.v164iA2.768?

[4] DNV. Alternative Fuels Insights (AFI) for the shipping industry. AFI platform. https://www.dnv.com/services/alternative-fuels-insights-afi--128171??

[5] Argus. Sample report: Argus marine fuels. 2023. https://www.argusmedia.com/en/oil-products/argus-marine-fuels

[6] ? ? ? ? DNV. Hydrogen forecast to 2050. Energy transition outlook 2022. https://www.dnv.com/focus-areas/hydrogen/forecast-to-2050.html?