2. Low-Carbon Fuels
Low-carbon fuels refer to cleaner fossil fuels with a lower carbon content than conventional marine fuels
[11][10]. Natural gas is a low-carbon fuel with no sulphur and nitrogen atoms compared to conventional marine fuels. Due to that, it can easily be used for operation in Emission Control Areas (ECAs)
[12,13][11][12]. For transportation purposes, it can be used in compressed form, i.e., Compressed Natural Gas (CNG), or in liquefied form, i.e., Liquefied Natural Gas (LNG)
[14][13]. Natural gas is liquefied by cooling to −163 °C to make handling easier, occupying 600 times less volume than in its gaseous state
[15][14]. Nowadays, most LNG-powered ships are powered by dual-fuel engines, which ensure a smooth transition from fuel to fuel without affecting performance and efficiency
[16][15]. However, current investment costs, undeveloped infrastructure, and safety issues are major limitations for its use as an alternative fuel
[17,18,19][16][17][18].
Liquefied Petroleum Gas (LPG) is also considered an alternative to conventional marine fuels due to its high energy density and clean burning properties
[20][19]. According to Yeo et al.
[21][20], LPG is suitable for small to medium-sized domestic ships, such as fishing vessels. Moreover, as onboard LPG energy systems are compatible with ammonia-fuelled systems with only minor modifications, LPG can serve as a transitional fuel for zero-emission shipping with ammonia
[22][21].
Another low-carbon fuel already used in the shipping sector is methanol. Due to its liquid state, methanol can be used in existing diesel infrastructure with minor modifications
[23][22]. Many studies investigated methanol as a marine fuel and concluded that its use reduces harmful emissions
[24,25,26,27][23][24][25][26]. Its major drawback is energy density, which is more than 50% lower than the energy density of conventional fuels
[28][27]. However, methanol is still a suitable alternative fuel for the shipping sector, and nowadays, it is being used onboard ferries, cruisers, tankers, etc.
[29][28].
Dimethyl-ether (DME), a clean-burning liquid fuel of high density, is produced through methanol dehydration. Since its physical properties are similar to LPG, DME can be used in LPG infrastructure and dual-fuel engines intended for LPG
[30,31][29][30]. When combusted, it results in low CO
2 and NO
X emissions, while SO
X and PM are not emitted
[32][31].
3. Carbon-Neutral Fuels
Carbon-neutral (or climate-neutral) fuels refer to biofuels due to the general opinion that CO
2 emissions released during biofuel combustion will be absorbed by new biomass further used for biofuel production. In this manner, combustion-related CO
2 emissions are not considered in the environmental footprint of a biofuel
[33][32]. The first generation of biofuels refers to biofuels produced from edible biomass (e.g., corn, rapeseed, soybean, sugar cane, etc.), while the second generation represents biofuels derived from inedible biomass (e.g., poplar, switchgrass, corn stover, organic waste, etc.). The third and fourth generations of biofuels refer to fuel produced from microalgae and genetically modified microalgae
[34][33].
Gilbert et al.
[35][34] showed that using biofuels as marine fuels reduces GHGs by 57–59% compared to conventional marine fuels. However, their wider use onboard ships faces limitations such as availability, high cost, and sustainability of fuels
[36][35]. Like its fossil counterpart (LNG), Liquefied Biogas (LBG) has been identified as a potential alternative fuel for the shipping sector. The transition from using LNG as ship fuel to LBG does not require additional equipment or cost. Since combustion-based CO
2 emissions are not considered, LBG is more environmentally friendly than LNG
[37][36]. The most common biofuel that is being investigated as a marine fuel is biodiesel, which is mainly produced from edible biomass by the transesterification process
[34][33]. Its use onboard has been investigated in many studies
[38[37][38],
39], but it is not a pure fuel. It is limited to blends with diesel (usually 80–95% of diesel and 5–20% of biodiesel) due to poor cold flow properties, which can result in damaging power systems, and limited storage stability
[40,41,42][39][40][41].
4. Zero-Carbon Fuels
Zero-carbon fuels are fuels whose use does not result in CO
2 emissions. These fuels represent promising measures for ship decarbonisation and reaching the IMO’s 2050 goal
[42][41].
The electrification of ships represents a game changer for the decarbonisation of the shipping industry. There are three types of electrified ships, i.e., plug-in hybrid ships, hybrid ships, and all-electric ships. Both plug-in hybrid ships and hybrid ships include diesel engines and batteries, while all-electric ships refer to the sole use of batteries for ship power
[43][42]. The main drawbacks of full electrification are limitations regarding battery capacity, degradation and weight, investment costs, charging infrastructure at the docks, and sailing distance
[44,45,46][43][44][45]. Different types of batteries are available for maritime purposes. Perčić et al.
[47][46] investigated three batteries (lithium-ion (Li-ion), nickel-metal hydride, and lead batteries) for use in ferries. Li-ion batteries were highlighted as the most environmentally friendly and cost-effective option. With further development of battery technology, i.e., metal–air batteries
[48][47], the full electrification of ships that operate in the open sea could be feasible.
Hydrogen use onboard ships also achieves zero-emission shipping. Based on its cleanliness, i.e., the sources used for its production, hydrogen can be classified by different colours (grey, brown, blue, yellow, pink, green, etc.). However, hydrogen is still primarily produced from natural gas by steam reforming (known as grey hydrogen)
[49][48]. Due to its low volumetric energy density, hydrogen is difficult to store. Often stored in its liquid form, hydrogen evaporates due to heat leakage into the cryogenic tank, known as boil-off gas, which represents a drawback of liquid hydrogen storage
[50][49]. Due to the fast kinetics of electrochemical reactions and its only by-product being water, hydrogen represents the most appropriate fuel for fuel cells. There are different types of fuel cells that are classified based on their operating temperature: low-temperature fuel cells (~80 °C), intermediate-temperature fuel cells (~200 °C), and high-temperature fuel cells (650–1000 °C)
[51][50]. The application of fuel cells onboard usually refers to satisfying auxiliary power needs
[52,53][51][52]. However, their use for propulsion is entering a new phase, starting with the first ferry fully powered by fuel cells fuelled with liquid hydrogen which has been in operation in Norway since March 2023
[54][53].
Hydrogen can also be used in an Internal Combustion Engine (ICE), which is less expensive to produce, has a longer lifetime, and does not require fuel purification before use (which is required for low-temperature fuel cells)
[55][54]. However, its use in ICEs encounters several challenges, e.g., potentially high combustion temperatures, which lead to high NO
X emissions
[56][55].
Ammonia is a hydrogen-rich fuel whose storage onboard ships is easier than that of hydrogen. It is the second most produced chemical in the world, used mainly as a fertiliser. Its use on board (in ICEs or fuel cells) does not result in CO
2 and SO
X emissions, while NO
X emissions can be eliminated with the proper catalyst. Its main drawbacks are toxicity (for humans and marine life) and corrosiveness, low energy density, and infrastructure, which should be expanded to cover the maritime sector
[42][41].
5. Electro-Fuels
Electro-fuels are synthetic fuels produced with electricity by combining hydrogen and carbon atoms, either from CO
2 captured from industrial processes through carbon capture and utilisation or direct intake from the atmosphere, known as direct air capture. They can be divided into non-carbon-based e-fuels, like hydrogen and ammonia (belonging to zero-carbon fuels), and carbon-based e-fuels, such as e-methanol, e-methane, etc. (belonging to carbon-neutral fuels)
[42,57][41][56]. Generally, e-fuels are more expensive than their fossil counterparts, and due to that, subsidies are necessary for their production and use, as well as funding future pilot projects regarding e-fuels.
6. Comparison of Fuels
Some properties of conventional and alternative marine fuels are presented in
Table 1.
Table 1. Comparison of different marine fuels [58,59,60,61]. Comparison of different marine fuels [57][58][59][60].
Besides the qualitative indicators shown in
Table 1, environmental and economic analyses are crucial for the decision-making process, i.e., choosing the appropriate alternative fuel for a particular ship that operates in a specific area. Perčić et al.
[62][61] performed a Life-Cycle Assessment (LCA) and Life-Cycle Cost Assessment (LCCA) of different marine fuels and indicated that among the considered alternatives, fully electrified ships are the most environmentally friendly and cost-efficient alternative to diesel power systems installed on ro-ro passenger ships.
Recent studies on alternative fuels in the marine sector are presented in
Table 2. Ha et al.
[22][21] performed an LCA of Heavy Fuel Oil (HFO), LNG, LPG, and methanol as marine fuels onboard a Korean bulk carrier. The study indicated that LPG has the lowest GHG emissions, but the country of import significantly affects overall emissions. Similar research was conducted by Spoof-Tuomi and Niemi
[63][62], who investigated an LCA comparison of Marine Diesel Oil (MDO), LNG, and LBG onboard ro-ro passenger ships. The results showed that the most environmentally friendly option is LBG, whose implementation in the shipping sector would be difficult to achieve without any subsidies. Jeong and Yun
[64][63] explored the cost-effectiveness of Low-Sulphur Fuel Oil (LSFO), LNG, and ammonia onboard container ships. Along with capital, investment, and operational costs, carbon cost was also included in the analysis. The study revealed the introduction of carbon allowances into the shipping sector would not be sufficient to replace conventional fuel with ammonia. However, such a tax policy would increase the chance of LNG being more profitable than LSFO.
Table 2.
Recent studies on alternative fuels in the shipping sector.
Year
|
Studies
|
Coverage
|
Scope
|
Fuels
|
Test Case
|
2023
|
Jeong and Yun [64][63]
|
LSFO; LNG; ammonia
|
container ship
|
Economic analysis
|
Kim et al. [20][19]
|
Diesel; gasoline; LPG; bio-LPG
|
small fishing vessel
|
LCA
|
Ha et al. [22][21]
|
HFO; LNG; LPG; methanol
|
bulk carrier
|
LCA
|
|
2022
|
Chen and Lam [65][64]
|
−136
|
|
Diesel; hydrogen
| 470
|
tugboat
|
LCA
|
Huang et al. [66][65]
|
MGO; LNG; methanol; ammonia
|
very large crude carrier
|
LCA
|
Lee et al. [67][66]
|
MGO; LNG; hydrogen
|
ferry
|
LCA
|
Solakivi et al. [11][10]
|
MDO; LSMGO; LNG; methanol; biodiesel; e-fuels (hydrogen, ammonia)
|
ro-ro ship
|
Economic analysis
|
Koričan et al. [68][67]
|
Diesel; electricity; methanol; LNG; ammonia; B20; hydrogen
|
fishing vessel (trawler)
|
LCA; LCCA
|
2021
|
Fan et al. [69][68]
|
Diesel; LNG; electricity
|
container ship; bulk carrier
|
LCA; LCCA
|
Perčić et al. [70][69]
|
Diesel; electricity; methanol; LNG; hydrogen; ammonia; B20
|
inland navigation ships (tanker; small passenger ship; dredger)
|
LCA; LCCA
|
Korberg et al. [36][35]
|
Biofuels, bio-e-fuels, and e-fuels (methanol; DME; diesel; liquefied methane gas; LBG; ammonia); hydrotreated vegetable oil; MGO; hydrogen
|
ro-ro passenger ship; general cargo ship, bulk carrier; container ship
|
Economic analysis
|
2020
|
Perčić et al. [62][61]
|
Diesel; electricity; methanol; DME; CNG; LNG; hydrogen; ammonia; B20
|
ferry
|
LCA; LCCA
|
Spoof-Tuomi and Niemi [63][62]
|
MDO; LNG; LBG
|
ferry
|
LCA
|
Hwang et al. [71][70]
|
MGO; LNG; hydrogen
|
ferry
|
LCA
|