Over the last few decades, extensive use of fossil fuels has resulted in increased Greenhouse Gases (GHGs) in the atmosphere. These emissions refer to the emissions of carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and fluorinated gases at low concentrations. They form a thin layer in the atmosphere that prevents solar irradiation from being reflected from the Earth’s surface to space, and as a result, it causes the greenhouse effect, which leads to global warming [1]. The Glasgow Climate Pact of 2021 is the most recent climate agreement, which reaffirmed the Paris Agreement’s ambitions for limiting the global average temperature increase to 1.5 °C above pre-industrial levels ^[2][3][2,3]. To combat the global warming problem, some urgent measures need to be taken, including a sharp GHG reduction in all sectors of the economy, including the shipping sector.

According to the Fourth GHG Study of the International Maritime Organisation (IMO), the share of shipping GHG emissions in global anthropogenic GHG emissions in 2018 was 2.89%, an increase of 9.6% compared to 2012. Without any emission reduction measures, the study predicts a great increase in emissions by the end of 2050 [4]. Due to that, the IMO set the Initial Strategy for GHG reduction, which is in line with the Paris Agreement temperature goals. The strategy has three levels of ambitions: reduction in carbon intensity (CO₂ emissions per transport work) through the implementation of further phases of the Energy Efficiency Desing Index (EEDI) for new ships; reduction in carbon intensity by at least 40% by 2030 and 70% by 2050; and reduction in total annual GHGs by at least 50%, compared to 2008 levels [5]. In order to achieve IMO decarbonisation goals, the strategy indicated measures with the following timelines: short-term (2018–2023), mid-term (2023–2030), and long-term (2030–) measures. Short-term measures represent the start of reducing shipping emissions with national plans, a tighter EEDI, the Ship Energy Efficiency Management Plan (SEEMP), speed reduction, etc. [6]. As a mid-term measure, the IMO has adopted new ship energy efficiency regulations for existing ships, i.e., the Energy Efficiency Existing Ship Index (EEXI) and Carbon Intensity Indicator (CII), which entered into force on the 1st of January 2023. The EEXI is a technical measure of energy efficiency related to the design of a ship, while the CII represents the operational measure of energy efficiency embedded into the SEEMP, and it measures CO₂ emissions per transport work for cargo, cruise, and ro-ro passenger ships over 5000 gross tonnage (GT). Like its predecessor, the EEDI, the EEXI should be applied for all ships above 400 GT, and their calculated, i.e., attained EEXI needs to be less than or equal to the required EEXI ^[7][8][7,8].

The mid-term measure whose implementation represents an incentive towards zero-carbon technologies is the inclusion of the shipping sector in the Emission Trading System (ETS). As of 2024, commercial cargo and passenger ships of above 5000 GT operating in the European Union will be required to purchase carbon allowances for each ton of released CO₂ emission [9].

2. Low-Carbon Fuels

Low-carbon fuels refer to cleaner fossil fuels with a lower carbon content than conventional marine fuels ^[10][11]. 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) ^[11][12][12,13]. 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) ^[13][14]. Natural gas is liquefied by cooling to −163 °C to make handling easier, occupying 600 times less volume than in its gaseous state ^[14][15]. 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 ^[15][16]. However, current investment costs, undeveloped infrastructure, and safety issues are major limitations for its use as an alternative fuel ^[16][17][18][17,18,19]. Liquefied Petroleum Gas (LPG) is also considered an alternative to conventional marine fuels due to its high energy density and clean burning properties ^[19][20]. According to Yeo et al. ^[20][21], 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 ^[21][22]. 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 ^[22][23]. Many studies investigated methanol as a marine fuel and concluded that its use reduces harmful emissions ^{[23][24][25][26]}[24,25,26,27]. Its major drawback is energy density, which is more than 50% lower than the energy density of conventional fuels ^[27][28]. However, methanol is still a suitable alternative fuel for the shipping sector, and nowadays, it is being used onboard ferries, cruisers, tankers, etc. ^[28][29]. 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 ^[29][30][30,31]. When combusted, it results in low CO₂ and NO_X emissions, while SO_X and PM are not emitted ^[31][32].

3. Carbon-Neutral Fuels

Carbon-neutral (or climate-neutral) fuels refer to biofuels due to the general opinion that CO₂ emissions released during biofuel combustion will be absorbed by new biomass further used for biofuel production. In this manner, combustion-related CO₂ emissions are not considered in the environmental footprint of a biofuel ^[32][33]. 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 ^[33][34]. Gilbert et al. ^[34][35] 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 ^[35][36]. 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 CO₂ emissions are not considered, LBG is more environmentally friendly than LNG ^[36][37]. 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 ^[33][34]. Its use onboard has been investigated in many studies ^[37][38][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 ^[39][40][41][40,41,42].

4. Zero-Carbon Fuels

Zero-carbon fuels are fuels whose use does not result in CO₂ emissions. These fuels represent promising measures for ship decarbonisation and reaching the IMO’s 2050 goal ^[41][42]. 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 ^[42][43]. The main drawbacks of full electrification are limitations regarding battery capacity, degradation and weight, investment costs, charging infrastructure at the docks, and sailing distance ^[43][44][45][44,45,46]. Different types of batteries are available for maritime purposes. Perčić et al. ^[46][47] 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 ^[47][48], 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) ^[48][49]. 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 ^[49][50]. 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) ^[50][51]. The application of fuel cells onboard usually refers to satisfying auxiliary power needs ^[51][52][52,53]. 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 ^[53][54]. 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) ^[54][55]. However, its use in ICEs encounters several challenges, e.g., potentially high combustion temperatures, which lead to high NO_X emissions ^[55][56]. 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 CO₂ 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 ^[41][42].

5. Electro-Fuels

Electro-fuels are synthetic fuels produced with electricity by combining hydrogen and carbon atoms, either from CO₂ 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) ^[41][56][42,57]. 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. 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. ^[61][62] 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. ^[21][22] 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 ^[62][63], 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 ^[63][64] 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.