Decarbonization in Shipping—The Hopes and Doubts on the Way to Hydrogen Use

Abstract

This article presents the initial processes of changing ship fuels aimed at reducing emissions of carbon dioxide and other greenhouse gases. A significant reduction in GHG emissions is only possible by using carbon-free fuels. The process of reducing CO₂ emissions was forced by legal regulations introduced in recent years by the International Maritime Organization and the Parliament of the European Union. The year 2050 was set as the target year for achieving the intended goals, but intermediate goals should be achieved already in 2030 and 2040. This article attempts to analyze the ongoing changes in the fuel market in maritime transport on the way to achieving the threshold of climate neutrality with this form of transport. A number of hopes related to this were indicated but also so were obstacles that may slow down this process. In 2023, there was an increased interest among shipowners in adapting ship engines to burn more ecological ship fuels. However, it is far from our expectations. Meeting the gradually increasing emission limits through imposed regulations was possible in the years 2020–2023 by using dual-fuel engines in which gaseous fuels, mainly LNG and LPG, were used for long periods of operation. The next step is the use of biofuels or synthetic fuels, which, however, will not meet the requirements after 2030. Interest is moving towards the use of ammonia and, ultimately, after 2040, hydrogen. The aim of this article is to analyze the ongoing processes and assess the directions of changes that justify the sense of the actions taken.

1. Introduction

The increase in Earth’s temperature threatens the development of civilization. Human activity has contributed to the increase in the concentration of greenhouse gases in the Earth’s atmosphere over the last two hundred years, which is especially visible in the 21st century. Despite the Earth Summit in Kyoto in 1997 and the signing of the protocol, greenhouse gas emissions, especially carbon dioxide, are increasing year by year. The actions taken are insufficient, and the temperature increase threshold of 1.5 °C compared to 1990 may be exceeded. Analyzing the share of maritime transport in global carbon dioxide emissions into the atmosphere over the last 20 years, it ranged from 2.5 to 3.3%, depending on the growth rate of cargo by sea. It is the cheapest form of transport, consuming the least amount of energy in terms of transport effect. In international shipping, the International Maritime Organization and, in shipping within the European Union, the Parliament of the European Union have taken actions aimed at reducing GHG emissions from fossil fuels, mainly fuels derived from crude oil processing, in favor of biofuels or synthetic fuels, and as a result, achieving the intended ecological effects [1,2,3,4].

The transition to alternative fuels was accelerated in 2020 when the International Maritime Organization (IMO) reduced the fuel sulfur limit in international shipping from 3.5% (m/m) to 0.5% (m/m).

The last step was taken in July 2023, when the IMO presented a revised greenhouse gas emission reduction strategy with numerical targets [2,3]. The 2050 goals have milestones in 2030 and 2040. The life cycle of greenhouse gases emitted into the atmosphere associated with ship fuels is taken into account to achieve the following targets:

• To achieve net-zero GHG emissions by or around 2050;
• As a minimum 5% to 10% uptake of zero GHG emission fuels, etc., by 2030;
• At a checkpoint of 40% reduction in CO₂ emissions (per transport work) by 2030 (compared to 2008) and the indicative checkpoints: 20% to 30% reduction in GHG emissions by 2030 (compared to 2008) and 70% to 80% reduction in GHG emissions by 2040 (compared to 2008).

The main goal is to achieve climate neutrality from international shipping by or around 2050.

The new rules entered into force on 5 June 2023 in the EU ports. Since 1 January 2024, the EU’s Emissions Trading System has been extended to cover CO₂ emissions from all large ships (of 5000 gross tonnage and above) entering EU ports, regardless of the flag they fly (Figure 1).

When ships sail between European Union ports or at berth in port under the jurisdiction of an EU Member State, CO₂ emissions are counted as 100%, while if only one of the ports (exit or entry to a port) is located within the European Union, the emissions are counted at 50%.

An entity must buy or receive allowances to cover future emissions. The current price for a CO₂ certificate is approximately EUR 70 per ton of CO₂ emissions. If the fuel index for heavy fuel is 3.122, this means an additional cost of approximately EUR 220 per ton of fuel used.

The price of marine fuel is around USD 450–600 per metric ton. Purchasing carbon dioxide emission allowances will increase the cost of fuel by approximately 50%. DNV’s Director of Environment for Maritime Eirik Nyhus also has a similar opinion. The EU ETS will lead to additional costs for the industry of roughly up to EUR 10 billion a year once fully implemented in 2026, due to the need to acquire carbon credits corresponding to GHG emissions. This will effectively increase fuel-related costs by about 50% [6].

Greenhouse gas emission limits (mainly carbon dioxide) in maritime transport were set by the IMO in 2015–2020, with specific limits at transitional stages, but generally, by the end of 2022, there have been minor changes in the use of new, more ecological marine fuels. An estimated 99.5% of the fuels used came from crude oil processing, with only 0.51% coming from other fuels such as LNG, LPG, ammonia, and hydrogen [4,7].

Important changes began to occur in 2023, when interest in bio-methanol, ammonia, and synthetic liquefied natural gas increased significantly.

Meeting the requirements imposed by the IMO will be necessary by 2050. To achieve this goal, intermediate steps must be completed in earlier years.

Table 1. Annual greenhouse gas emissions from maritime transport in 2008 and 2021 and maximum emissions levels that will allow international shipping to reach the checkpoints [8] [unit: million tons CO_2eq].

GHG Emissions	2008 (Base Year)	2021 (Latest)	2030 (20% Reduction from 2008)	2040 (70% Reduction from 2008)
Life cycle GHG emissions (WtW)	731	798	585	219
GHG emissions (WtT)	110	122	88	33
GHG emissions (TtW)	621	676	497	186

shows the schedule to achieve the indicative checkpoints. The year 2008 is the base year for the set target of reducing equivalent carbon dioxide emissions from maritime transport. Due to the fact that reducing emissions of other harmful substances into the atmosphere (nitrogen oxides, particulate matter, black carbon, and hydrocarbon slips) is more difficult to achieve, carbon dioxide emissions must be reduced to a greater extent. In 2040, the level of carbon dioxide equivalent emissions is to reach a minimum of −70%, which means that only carbon dioxide emissions must be reduced by approximately 90%.

Table 1 shows annual greenhouse gas emissions from maritime transport in 2008 and 2021 and maximum emissions levels that will allow international shipping to reach the checkpoints, taking into account two components well-to-tank (WtT) and tank-to-wake (TtW) and their sum well-to-wake (WtW).

2. Equivalent Energy for Different Marine Fuels

The world’s annual demand for petroleum-based marine fuels is 300–330 million tons. As the weight of cargo by sea increases by approximately 3%, the demand for energy contained in the fuel also increases by 3% [4,9].

Assuming that the engine efficiency does not depend on the type of fuel burned, but only on the amount of energy generated in the combustion process, it is possible to estimate the quantitative demand for potential ship fuels with different calorific values. Other potential marine fuels have different lower calorific values. Assuming the same amount of thermal energy in the combustion process, the demand for these fuels will vary. The higher the calorific value, the lower the demand, and vice versa (

Table 2. Amount of marine fuel in energy and volume equivalent [own elaboration].

Type of Marine Fuel	Equivalent Energy 1 [million ton]	Equivalent Volume [million m3]	Energetic Volume Density [MJ/m3]	Global Production (Data from 2021 to 2023) [million ton]
Petroleum-based marine fuels	330	370	38.6	>330
Methanol	696.5	879.4	15.76	111
Ammonia	737	1080	12.82	150 2
Methyl ester	370	425.3	32.6	74.8
S-LNG/bio-methane/LNG trade	285	433.8	31.9	no data/ 30/401.5
Liquefied hydrogen	115.5	1635	8.5	94

¹ taking the following into account: petroleum marine fuels 42 MJ/kg; bio-methanol 19.9 MJ/kg; ammonia 18.8 MJ/kg; methyl ester (biodiesel) 37.5 MJ/kg; S-LNG 48.6 MJ/kg; hydrogen 120 MJ/kg [4,10]. ² global production of renewable ammonia was zero in 2022.

).

When burning marine fuels from crude oil processing in 2022, carbon dioxide emissions would amount to approximately 1.03 Tg (1.03 billion tons). Due to the associated emissions of other greenhouse gases such as soot, nitrogen oxides, and sulfur from marine diesel engines, CO₂ equivalent emissions (CO_2eq) are sometimes used. The difference between CO₂ and CO_2eq is rather small, about 7–9% larger for CO_2eq, and is quite stable. This means that CO_2eq emissions amounted to 1.09 Tg.

Uniquely, fossil LNG offers significant GHG emissions reductions when used as a marine fuel compared with VLSFO—up to 23% on a full life cycle (well-to-wake) basis (without taking into account slip). By contrast, the use of fossil methanol (14% more), ammonia (47% more), and (liquid) hydrogen (64% more) produces emissions much higher than those associated with VLSFO due to the large amounts of electricity (largely derived from the combustion of fossil fuels) needed for their production (e.g., hydrolysis) and long-term storage [11].

The lower calorific value and lower density of alternative fuel require a much larger volume of fuel tanks. In the case of liquefied fuels (NH₃, LNG, and H₂), thermal insulation of the tanks is required, which additionally increases the required space for reserve fuel. The necessary increase in the required volume of fuel reserve tanks compared to currently traditional marine fossil fuels is up to approximately 2 to 7, respectively. For ships intended to transport cargo, this represents a significant reduction in available cargo space at the expense of increased space for fuel reserve tanks.

3. Expected Effects of Decarbonization of Marine Fuels

In March 2023, the European Parliament and the Council of Europe reached an agreement on the use of cleaner fuels, calling for a reduction in ship emissions by 2% by 2025 and 80% by 2050. The European Commission has introduced FuelEU regulations, effective starting from 1st January 2025, which assume a reduction in CO₂ equivalent emissions from shipping every 5 years, to the level of 18.23 g CO_2eq/MJ in 2050 (80% less compared to 2020) counting total well-to-wake emissions.

Under the initial agreement, container ships and passenger ships (using the most quantity of fuel and, at the same time, emitting the most GHGs into the atmosphere) starting from 2030 will be obliged to use a land-based power source to meet all energy needs when mooring at the quays of main EU ports (there are such intentions, as long as there are such possibilities in the port, because the demand for electricity reaches 2–5 MW per ship). Starting from 2035, it will also apply to other EU ports if these ports have shore power. Certain exceptions will apply, such as stays in a port of less than two hours, use of own zero-emission technology, or port calls due to unforeseen circumstances or emergencies [4].

The primary effect of reducing CO₂ emissions will depend on the amount of biodiesel and bio-methanol used. The transition to renewable fuels is necessary in the transitional process of the decarbonization of shipping [12]. This process will be supported by increasing the consumption of liquefied ammonia. It is possible to increase ammonia production by 20–30% per year. However, it should be noted that in 2022, ammonia production from renewable sources was close to zero. Storing liquefied ammonia in its production processes and using excess electricity (which cannot be used directly) from renewable sources can contribute to the stated goal of decarbonizing shipping [13,14].

Achieving the decarbonization goal by 2050, i.e., switching to a significant share of hydrogen as a primary energy source in shipping, will depend on the method of hydrogen production (its color), i.e., whether energy from “dirty” sources was not used in the process of its production.

Table 3. Possible decrease in CO₂ emissions using different marine fuels [own elaboration].

Type of Marine Fuel	Fuel Factor [kg CO2/kg of Fuel]	Decrease in CO2 Emissions 1 [%]
Petroleum-based marine fuels	3.12–3.23	0
Methanol	2.9	10
Renewable bio-methanol	0	100
Ammonia with 15% MDO	0.46	85
Methyl ester/B100	3/0	5/100
Biodiesel/FAME/HVO/SVO	0–2	40–100
Fossil LNG/methane	2.75	23
Renewable bio-methane	0	100
S-LNG	0–5	−55–100
Renewable hydrogen	0	100

¹ HFO-MDO CO₂ emission = 100%.

shows the possible decrease in CO₂ emissions from different marine fuels.

The technology of fuel production is important, especially the carbon footprint left in this process. If the primary source is fossil fuels, even if they are processed, they will be transitional fuels. In the case of the production of synthetic fuels or biofuels, recognizing them as more or less ecological fuels depends on the amount of electricity used, and especially from what primary energy sources it was produced [4].

Achieving the target of reducing greenhouse gas emissions from maritime transport will be possible if emission reductions are properly distributed according to the capacity and type of ship. For example, container ships with large carrying capacity, equipped with main engines with the highest power of 50–80 MW, must reduce emissions to a greater extent than the presented total target, because for small-capacity ships (a capacity below 400), fishing vessels, and specialized vessels, it will be difficult to achieve proportional indicators or it will be outside of the regulations. For example, the most carbon-polluting cargo shippers in 2022 were the following: MSC—10.5 million t CO₂, CMA CGM—5.4 million t CO₂, and Maersk—5.3 million t CO₂. Whereas the most polluting cruise companies in 2022 were the following: Carnival—2.5 million CO₂ t, MSC Cruises—1.4 million CO₂ t, and Royal Caribbean International—0.8 million t CO₂ [15].

Levels of ambition directing the 2023 IMO GHG Strategy are as follows [3]:

• Carbon intensity of the ship to decline through further improvement of the energy efficiency for new ships (these methods have been used for 50 years and their possibilities have essentially been exhausted);
• Carbon intensity for international shipping to decline (by using fuels considered more ecological, e.g., biofuels);
• Other targets to be achieved by 2050 were given earlier.

A significant barrier to the transition to alternative and renewable fuels is the distribution system and availability of the bunker network, or essentially its scarcity 13,14]. As a result, depending on the availability of fuel, ships may be bunkered locally in port shipping, coastal shipping, or within the ship’s autonomy radius and the port (harbor roadstead)—the bunkering site. The first important step towards the decarbonization of shipping was the decision of shipowners of large container ships to switch to dual-fuel engines, with the option of long-term operation on liquefied natural gas (LNG). Currently, it is a fuel of fossil origin, but with limited gas leaks, CO₂ emissions are reduced by up to 23%. Technologies for producing synthetic LNG are energy-intensive and, as a result, do not still solve the problem of renewable energy. Actions have been taken to power engines with bio-methanol. This is a much better solution, allowing us to meet the requirements related to emission limits and, at the same time, being future-proof, enabling ships to be operated for many years until 2050. A similar effect can be achieved by switching to fueling the engines with ammonia. However, there remains the problem of the technology for obtaining ammonia, the production of which should use electricity generated from renewable sources [14]. One of the seriously considered proposals is the construction of ammonia storage facilities, produced at a time when there is a problem of surpluses in electricity production. Another considered option is the construction of hydrogen storage facilities near large wind or photovoltaic farms. An important problem is to obtain a high energy recovery coefficient as the ratio of the energy obtained in end devices to the amount of primary energy used in the production of a given storage medium. In this way, it will be possible to enter the era of renewable fuels, including hydrogen.

Due to the dominant share of fuel costs in ship operation, the shipowner’s decisions regarding adapting the ship’s power plant to burn a different type of fuel must be balanced, and the assessment of the financial consequences will be a decisive factor when analyzing the selection of the optimal type of fuel.

4. Possible Additional Financial Costs and Environmental Losses

IMO regulations on international shipping regarding carbon dioxide emission limits from maritime transport impose an obligation on shipowners to meet them in order to continue operating their ships (there is a risk of their ships being scrapped). A direct reduction in GHG emissions by reducing fuel consumption as a result of increasing the overall efficiency of engines and optimizing the selection of the number of operating engines and their loads does not allow for meeting these requirements. Opportunities to increase overall propulsion efficiency have already been exploited. The subsequent improvement may be very small, perhaps a few percent. Reducing hull resistance can have a similar effect. Optimizing the cruise route means savings of 1–3%. It is no longer possible to achieve a reduction in fuel consumption by another 10–30% this way. The only option is to change the type of fuel burned to one currently considered more ecological (biofuels), especially those without carbon in the molecule (ammonia and hydrogen) [16,17]. Changing the type of marine fuel requires adapting the fuel system and engine. The use of fuel with similar density and calorific value and similar combustion process parameters requires slight changes in the fuel system (e.g., for synthetic hydrocarbon fuels). Unfortunately, the trace production of synthetic fuels, considered ecological, creates a significant barrier due to lack of availability and high prices. Bio-methanol may be a transition fuel until around 2040. The role of green ammonia will significantly increase if investments in ammonia synthesis are completed, increasing its production by 2030 at least tenfold, while reducing the price several times. Moreover, it is estimated that the technological processes of producing these fuels generate a significant carbon trace. Despite fluctuations in the price of marine fossil fuels (Figure 2), they remain competitive with the alternative fuels considered. Even after the introduction of emission fees in European Union ports (EU ETS), which will increase fuel costs by approximately 50%, this situation will not change significantly.

Figure 3 shows the costs of generating 1 kWh of electricity in SOFC fuel cells for various types of fuel [18]. The list does not include the costs of fuel storage. For liquefied hydrogen, these costs are significant, approximately 15–100 times higher than for other fuels, and at the same time, they generate higher costs (20–50 times higher) than subsequent energy conversion [18]. Similar costs will occur using modern marine diesel engines.

For comparison,

Table 4. Direct cost in USD of MWh output from marine diesel engines powered by various fuels at prices from 2022 to 2023 (on-board system CAPEX not included) [own elaboration].

Type of Marine Fuel	Cost of 1 MWh in USD
Marine diesel oil (MDO)	80–100
Fossil liquefied natural gas (LNG)	60–150
Used cooking oil methyl ester (UCOME)	120–215
Bio-methanol	160–320
Bio-LNG (from biomass)	170–275
Bio-LNG (price forecast in 2035)	130–220
Liquid hydrogen (L-H2)	285–375
E-ammonia *	220–325
E-methanol *	240–320
E-LNG *	270–360

* electro-fuels.

shows the direct costs of generating 1 MWh of energy from marine diesel engines powered by various fuels.

Fossil fuels are by far the cheapest source of energy in maritime transport. Comparable costs of obtaining energy may only apply to fossil liquefied natural gas. This is related to the energy policy of many countries, which want to keep the prices of crude oil and fuels obtained from it low. This allows us to reduce the production costs of, among others, consumer goods, increasing demand and boosting the state’s economy. The introduction of emission fees (essentially the purchase of greenhouse gas emission permits) in maritime transport equalizes the costs of obtaining energy and favors the energy transformation. The introduction of Carbon Intensity Indicator (CII) requirements [19,20,21,22,23], due to the annual requirement to report CO₂ emission levels, will accelerate this process because shipowners will face a dilemma: make the necessary adaptations of the fuel system and engines themselves to ecological fuels or replace the ship with a new generation [19].

The characteristics of emissions of harmful substances into the atmosphere from the combustion of various ship fuels are presented in

Table 5. Air pollutants and GHGs from marine fuel combustion [own elaboration].

Pollutant/Marine Fuel	HFO, MDO, MGO	LNG, Methane	Ammonia	Hydrogen
CO2 1	large quantities	large quantities 25% less	without a trace	without a trace
CmHn/VOC	trace amounts	trace amounts/slip	without a trace	without a trace
CO	trace amounts	trace amounts	without a trace	without a trace
NOx	trace/needs SCR on EC area	Otto engines meet EC area without SCR	present/needs SCR on EC area	trace/probably needs SCR on EC area
SOx	trace amounts	without a trace	without a trace	without a trace
PM	trace amounts	trace amounts	trace amounts	without a trace
Ammonia (catalyst)	low	without a trace	unknown	unknown
CH4 slip	not present	trace amounts	without a trace	without a trace
PAHs	trace amounts	trace amounts	without a trace	without a trace

¹ Fuel coefficients are presented in Table 3.

.

Taking into account the impact of fuel combustion products on the natural environment is important for assessing their suitability as marine fuels [24,25,26]. Efforts are being made to estimate product life cycle costs (LCCs) for marine fuels. An example of the actions taken is presented in

Table 6. Life cycle equivalent (WtW) carbon dioxide (CO_2EQ) emissions and SO_x, NO_x, and PM emissions in kg per kWh using different alternative marine fuels (on base [27]).

Type of Marine Fuel	Pollutant
	CO2EQ	SOx	NOx	PM
MDO, SCR, 2-stroke	0.664	0.247 × 10−3	1.24 × 10−3	0.330 × 10−3
MDO, SCR, 4-stroke	0.808	0.301 × 10−3	1.51 × 10−3	0.402 × 10−3
Green NH3, 5%MDO, SCR, 2-stroke	0.114	12.3 × 10−6	1.02 × 10−3	32.2 × 10−6
Blue NH3, 5%MDO, SCR, 2-stroke	0.418	12.3 × 10−6	1.45 × 10−3	37.1 × 10−6
Green H2, 1.5%MDO, 4-stroke	0.0503	4.67 × 10−6	2.11 × 10−3	14.7 × 10−6
Blue H2, 1.5%MDO, 4-stroke	0.397	4.67 × 10−6	2.60 × 10−3	20.2 × 10−6

.

Switching to fuels such as ammonia and hydrogen brings a number of additional costs, but at the same time, a number of benefits compared to marine diesel oil, which are presented in [22]. The estimated environmental impacts associated with fuel switching in the WtW process are greater when using ammonia or hydrogen compared to MDO (by approximately 50%). Only for blue hydrogen during combustion in four-stroke engines is the estimated environmental impact factor (WtW) lower by about 30% [27].

So far, the assessment has been carried out by analyzing the environmental effects from the moment the fuel is in the ship’s tank until the moment of gas emissions into the atmosphere after the combustion process (tank-to-wake—TtW). Currently, environmental impacts are analyzed from the point of extraction, processing, or production of fuel to the point at which the fuel reaches the ship’s fuel tank (well-to-tank—WtT). The results of an environmental impact analysis that are the sum of both processes are considered to be more valuable (well-to-wake—WtW). In this case, alternative fuels, especially black hydrogen, achieve much higher indicators, which is contrary to the assessment of their environmental friendliness. On the other hand, the “well-to-wake” (WtW) parameter of methanol is close to MDO, which means that bio-methanol will be preferred as a transition fuel [4].

5. The Influence of Ship Type and Size on the Choice of Transition Fuel or Hydrogen

The lack of suggestions and recommendations as to what type of marine fuel will be preferred in the coming years meant that shipowners did not decide to switch to low-emission fuels in 2023. The number and type of ships dominating a given shipowner’s fleet had the greatest impact on decisions regarding the fuel used on a ship, followed by its deadweight capacity, type of cargo transported, shipping line, etc. One of the options that has been under consideration for over 20 years is natural gas. For example,

Table 7. Number of LNG-powered ships by type in operation or on order [4,25].

Type of Vessel	Number of Vessels in Operation	Number of Vessels on Order
Car/passenger vessels	43	7
Container ships	43	181
Crude oil tanker	49	40
Offshore supply vessels	36	1
Oil/chemical tankers	44	44
Tugs	22	16
Other	124	213
Total	361	502

shows the number of ships in operation and on order, powered by liquefied natural gas.

For about 40 years, methane evaporating from the cargo space (boil-off gas, BOG) has been used as a fuel because the energy costs for re-liquefying it are too high. In the 1990s, BOG was burned in boilers, producing steam to power steam turbines. After dual-fuel engines entered the shipping market (after 2010), due to their greater overall efficiency, they dominated the LNG transport market. Currently, ship engines are being built mainly adapted to burn LNG, not only for LNG carriers, but during engine start-up, stoppage, and in emergency situations, typical liquid ship fuels are switched to. Only about 50 gas carriers carrying LPG as cargo use the LPG combustion option in dual-fuel engines. In the case of LNG carriers, whose fleet is much larger, this solution is used on 361 ships. The use of methane evaporation from the cargo space (BOG) requires moving the gas outside the tank and its further processing, e.g., use as fuel in the ship’s energetic systems (the simplest solution is combustion in steam boilers). Currently, due to the amount of BOG, dual-fuel engines are used, and the excess amount of gas not used as fuel is condensed again, even though it is uneconomical, but these are environmental protection requirements (methane must not be released into the atmosphere under normal ship operating conditions).

Due to the large share of container ships in the total GHG emissions from maritime transport, there has been interest in the use of dual-fuel ship engines that can burn natural gas for long periods of operation. Despite many benefits for the natural environment, the problem of the source of this gas remains. There should be a shift from the use of brown natural gas to synthetic gas, whose carbon trace in new technologies can be smaller and smaller using electricity from renewable sources.

Interest in methanol as a marine fuel, especially bio-methanol, has increased significantly, especially among container shipowners. The fleet of ships using methanol in 2022 was 25 ships, of which methanol was used on 22 chemical tankers, 1 ro-pax ship, and 1 tug. Of the 59 ships ordered, 47 are container ships, 4 offshore ships, 2 bulk carriers, and 1 cruise ship [25]. The ongoing changes would most affect ships that consume the largest amounts of fuel, hence the search for alternative fuels (bio-methanol will be recognized as an ecological fuel, thanks to which emission requirements will be met) [4].

However, there is a bottleneck related to the volume of current methanol production. The demand for methanol is not very high because it is produced approximately 75 million tons per year. For maritime transport, using it as a marine fuel, world production would have to increase more than 10 times. Natural gas is mainly used as a raw material for the production of methanol [9,28]. In 2023, only about 220,000 tons of bio-methanol were produced. The equivalent amount of energy, with marine fuel consumed in the amount of approximately 300 million tons, will be 630 million tons of methanol. In order to switch to methanol as a marine fuel, its production would have to be increased, which requires many investments in this field, but on the other hand, it would be possible within 10–20 years. If the fleet of ships using methanol as a fuel increases, its scarcity (unavailability) may constitute a significant barrier to its popularization. Moreover, the “well-to-wake” (WtW) parameter of methanol is similar to that of diesel fuel (MDO), which means that mainly bio-methanol should be used.

Successful tests of ammonia-fueled marine engines were carried out in 2020–2022. With the current design of the engine combustion chamber (too low compression ratio), it is required to use 5–15% of the pilot dose of diesel oil (MDO) to initiate and accelerate the combustion process. This gives hope for significant changes in the use of ecological fuel in maritime transport. The combustion of ammonia makes it possible to meet carbon dioxide emission limits by 2050, despite the additional combustion of light fuel. If a decision is made to switch to ammonia as a new marine fuel, many additional problems remain to be solved (storage, distribution network, and bunkering procedures), although this has been possible in the port of Singapore since 2023. However, the main problem will remain the most ecologically possible production of ammonia at reasonable production costs [28,29]. A good sign is the investments made in Australia and Southeast Asia in the production of green ammonia (from electricity from renewable sources) and blue ammonia production (USA and Canada), with some announcing intentions to provide ammonia as an energy source for the maritime sector. Renewable ammonia production is expected to reach 20 million tons in 2030 and as much as 540–1140 million tons in 2050 [30]. The demand for ammonia only in maritime transport, taking into account the equivalent amount of energy, in 2023 would be approximately 600–700 million tons.

Ammonia production will compete with hydrogen production as an energy storage source from renewable sources. The low volumetric energy density of hydrogen is a particularly significant disadvantage when used as a marine fuel. Due to the need to store large amounts of fuel (ship autonomy), this reduces the cargo space on these ships. The volumetric energy density of hydrogen is low and amounts to 5.14 GJ/m³ when stored in a compressed state at a pressure of 80 MPa (in order to equal the energy density of diesel oil, it should be compressed to a pressure of approximately 420 MPa) and slightly higher at 8.55 GJ/m³ when stored in a liquid state in cryogenic conditions at −253 °C. Moreover, hydrogen mixed with air creates a very wide flammability (and explosive) range, i.e., 4.1–75% (compared to 5–15% for methane), and its greenhouse effect potential (GWP) within 100 years is estimated at 11. The basic technology for obtaining hydrogen seems to be water electrolysis. Due to the high energy expenditure associated with compressing hydrogen or its condensation in order to increase its volumetric calorific value and the large losses associated with its storage, an alternative may be the processing of hydrogen into ammonia, the cost of which is several times lower in storage. In 2022, the number of ships powered by hydrogen as a fuel was six ships, i.e., three ro-paxes, one tug, one ro-ro, and one cruise ship. For comparison, the number of ships in international shipping exceeds 100,000. An increase in the fleet of ships whose main source of energy is hydrogen-powered fuel cells is expected [25]. This development will currently concern relatively small vessels sailing over short distances. It is estimated that, by 2030, the number of ships that use hydrogen as marine fuel will be well below 100, and their share in tonnage will be small (less than 0.1%) [4,25].

6. Problems to Overcome (Doubts) on the Way to the Hydrogen Era

There are many obstacles to the popularization of hydrogen as a primary energy source, including marine fuel. Due to its high chemical reactivity, hydrogen on Earth occurs primarily in chemical compounds. The most popular is water. It is essentially non-existent in gaseous form in the atmosphere. However, it may accompany deposits of certain raw materials. For example, in France, large deposits of white hydrogen associated with water have been discovered in the Massif Central. Hydrogen can be produced for industrial purposes using various technologies [31,32]. Depending on the production technology, hydrogen colors are distinguished:

• Black or brown—using black coal or lignite in the hydrogen-making process;
• White—is naturally occurring (in underground deposits);
• Yellow—is made through electrolysis using solar power;
• Pink—is generated through electrolysis powered by nuclear energy;
• Green—is made by using clean electricity from surplus renewable energy sources;
• Blue—is produced mainly from natural gas (steam reforming process);
• Turquoise—is made using a process called methane pyrolysis;
• Gray—is created from natural gas (without the use of carbon capture and storage).

In the process of black, brown, gray, and blue hydrogen production, in addition to consuming energy in the technological process, greenhouse gases (carbon dioxide, carbon monoxide, and black carbon) are also produced, which means that these technologies are not neutral to the natural environment. Currently, hydrogen is produced in over 90% by these methods. Other technologies are more environmentally friendly provided that electricity from renewable sources is used, especially excess energy that cannot be used on an ongoing basis.

6.1. Physical Properties of Selected Potential Marine Fuels

The physical properties of a substance are the basis for analyzing the possibility of its production, storage, and use.

Table 8. Comparison of physical parameters of selected potential marine fuels [own elaboration].

Parameter/Type of Fuel	Ammonia	Methane	Hydrogen
Molecular mass [kg/kmol]	17.031	16.043	2.016
Boiling point [°C; K] at 1 atm; Tboiling	−33.34; 239.81	−161.5; 111.6	−252.8; 20.27
Tmelting Melting point [°C; K]	−77.7; 195.45	−182.5; 90.7	−259.2; 13.99
Critical temperature [°C; K]	132.4; 405.5	−82.59; 190.56	−243.8; 32.9
Medium specific heat [kJ/kgK] liquid, cp	4.45	35.8	14.29
Density at boiling point [kg/m3] liquid; gas	681.9; 0.86	422.8; 0.717	71; 0.09
R Heat of vaporization [kJ/kg]	1371.2	510	446.4
ΔT = Tamb − Tboiling [°C; K]	48.34	176.55	267.8
Δt = Tboiling − Tmelting [°C; K]	44.36	21	6.4
Q = R + cp × ΔT [kJ/kg]	1586.3	6830.49	4273.26
Physical state at a pressure of 1MPa and a temperature of +15 °C; 288.15 K Tamb	liquid	gas	gas

shows selected physical parameters for three potential marine fuels. Fundamental differences occurring in the boiling point (especially when there are differences in these temperatures in relation to the ambient temperature, e.g., 15°C) and density are taken into account. This is important for the required thermal insulation, in order to reduce the amount of heat entering the liquid and, as a result, the amount of liquid converted into gas and the increase in pressure in the tank. Long-term storage of substances with a low boiling point requires large financial outlays on the construction of a thermal barrier and a system for removing gas outside the tank, re-liquefying it, or directly using it. The best situation is when the daily fuel demand equals or exceeds the daily rate of evaporation of the liquid in the tank.

The amount of heat flux entering a tank with a liquid at a lower temperature than the surroundings depends on the following:

$$Q=k\times S\times ∆T \left[\mathrm{k}\mathrm{W}\right]$$

(1)

$${\displaystyle \frac{Q}{V}}=k\times \left({\displaystyle \frac{S}{V}}\right)\times ∆T \left[{\displaystyle \frac{\mathrm{k}\mathrm{W}}{{\mathrm{m}}^3}}\right],$$

(2)

where

• $Q$—the amount of heat flux entering a tank [kW];
• $k$—heat transfer coefficient [${\displaystyle \frac{\mathrm{k}\mathrm{W}}{{\mathrm{K}\mathrm{m}}^2}}$];
• $S$—the external surface of the tank [m²];
• $V$—tank capacity (amount of liquid in the tank) [m³];
• $∆T$—temperature difference (ambient and liquid in the tank) [K].

Equation (2) shows that the daily relative evaporation rate can be reduced by building a tank with a larger volume, with the same heat transfer coefficient and temperature difference. By increasing the volume of the tank, its volume increases faster than the external surface. The greater the temperature difference, the greater the heat flow penetrating the tank. For liquefied hydrogen, this value is approximately six times higher than for ammonia.

6.2. Methods of Hydrogen Production

Hydrogen can be produced in many chemical and electrochemical processes [33,34]. Industrial-scale production requires technologies that are simple methods that use the least amount of energy possible. The steam reforming of natural gas is the most universal method. After removing contaminants from natural gas, it is exposed to steam at a temperature of 900 °C in the presence of a nickel-based catalyst (the method is known as Steam Methane Reformation—SMR). The overall reaction of the process is

CH₄ + 2H₂O→2CO₂ + 4H₂.

The main disadvantage of this process is the emission of approximately 9 kg CO_2eq per kg H₂. If a carbon capture system (CCS) with an efficiency of 80–90% is additionally used, carbon dioxide emissions drop to approximately 1 kg CO₂.

Partial oxidation and autothermal reforming (ATR) are other routes to produce hydrogen. This is a more efficient and compact reformer. Carbon monoxide and dioxide (after oxidation) generated in the production process must be captured by CCS and stored in a geological repository. The overall reaction of the process is

3CH₄ + O₂ + H₂O→3CO + 7H₂.

The production of greener hydrogen requires much higher investment costs than technologies based on its production from methane or by the gasification process of hard coal or lignite [33]. The share of green hydrogen in global production is approximately 4%. The basic technology for obtaining greener hydrogen is water electrolysis. It is necessary to develop and search for new zero-emission technologies.

6.3. Comparison of Storage Parameters for Ammonia, Methane, and Hydrogen

Hydrogen production and its current consumption at the point of production are very rare. As a result, there is a need to store hydrogen and transport it to its destination. This requires the construction of appropriate tanks for its storage and tank cars for transport or a network of pipelines for transmission. Due to the unfavorable physical parameters of hydrogen, it can be stored in two variants: liquefied or compressed.

Table 9. Comparison of energy parameters of selected potential marine fuels [own elaboration].

Type of Fuel/Energy Parameter	Lower Calorific Value [MJ/kg]	Volumetric Energy Density [GJ/m3]	Tank Volume Increase Factor for an Equivalent Amount of Energy 1 (for MGO f = 1]	Daily Evaporation [%]
Marine gas oil (MGO)	42.7	36.6	1	0
Liquid ammonia	18.6	12.7	3	0–0.1
Liquid methane	50	23.4	1.6	0.1–1
Liquid hydrogen	120	8.5	12	1–3
Compressed hydrogen (70 MPa)	120	7.5	14	0

¹ without taking into account the increase in volume due to thermal insulation.

compares the storage parameters of hydrogen, ammonia, and methane.

Fluid storage is possible in gaseous or liquid form. Due to significant differences (evaporation ratio for ammonia is 900, for methane 660, and for hydrogen 848) between the density of liquid and gas under normal conditions, the liquid state is preferred. This requires keeping the storage temperature below the critical temperature, and preferably at the boiling point. Due to the difficulties and costs of storing liquefied and compressed hydrogen, it can be processed into ammonia, the long-term storage of which is not a significant problem and does not cause high costs. This possibility should be considered, especially when marine engines adapted to burn ammonia are already available, and in some ports, it is possible to bunker it [27]. Ammonia may be a transition fuel until new competitive zero-emission hydrogen technologies are developed.

7. Analysis of the Ongoing Transformation Processes to Ecological Marine Fuels—Discussion

Including additional maritime transport charges (taxes) in the EU ETS may significantly increase costs. In 2022, the price of CO₂ emissions ranged from EUR 70 to 100 EUR/ton. With emissions of 3.12 tons of CO₂ per ton of HFO, this will increase the cost of fuel by approximately EUR 230–300. The cost of emissions will be an increase in fuel costs in ship operation by 30–50%, taking into account indirect costs which will be approximately 50%.

This is a proposal from the European Commission. The deadline for the implementation of the EU ETS depends on the results of negotiations between the Commission, the European Parliament, and the Council. It was planned to start billing in 2024, and these regulations will probably come into force starting from 2025 and may apply to large ships, over 5000 GT. Firstly, the amounts of fuel used in EU waters should be reported, and previously purchased CO₂ emission permits should be canceled. The obligation to pay emission fees applies to ships calling at EU ports. This may make maritime transport in EU ports less attractive (for economic reasons) and may increase the attractiveness of calling at ports outside the EU, with goods then delivered by land to EU countries. The introduction of additional fees is a big challenge for shipowners who will try to mitigate the effects of this process [4].

The aim of the introduced regulations is to reduce GHG emissions in EU maritime areas. The introduced fees will result in accelerated changes towards zero-emission fuels or fuels considered ecological, thanks to which CO₂ emissions from these biofuels will not be subject to fees. There are still too many unknowns at the moment. Shipowners are waiting for the final decisions of the EU administration. There is a risk that this may be a unilateral EU decision, without taking into account the opinions of shipowners, the International Maritime Organization, etc.

As of 2022, the number of ships in operation and ordered that do not use fossil fuels as marine fuel is presented in

Table 10. Number of ships in operation and on order using non-fossil fuels ¹ [4,25].

Number of Vessels	Hydrogen	Methanol	LPG	LNG
In operation	6	25	51	361
Ordered ships by 2026	19	59	78	502
Total	25	84	129	863

¹ Biofuels are FAME, HVO, SVO, DME, F-T diesel, liquefied biogas (LBG), upgraded pyrolysis oil, and more.

.

Competition in the shipping market meant that large players in this market (owners of container ships, tankers, and car carriers) had to be the first to decide to switch to alternative fuels. This resulted in the adaptation of engines to burn LNG (dual-fuel diesel engine, DFD), even though it basically comes from natural gas deposits.

Table 11. Number of vessels in service and on order using LNG as basic fuel [11].

Type of Vessel	Number of Vessels in Operation	Number of Vessels on Order
Cruisers	13	26
Ro-ro and ro-pax	32	16
Tanker	100	92
Container vessels	43	175
Car carriers	10	119
Bulk	18	50

shows the number of ships in operation and on order, which use LNG as the basic fuel. There is a significant increase in orders for these types of ships. Further developmental trends depend on the recognition by the IMO and the EU Parliament of the color assigned to LNG due to its source, origin, and production technology.

Marine fuels considered as transient are produced mainly using electricity, and their production will have a significant share of fossil fuels as a primary energy source. There is a need for far-reaching changes in production processes, especially the storage of excess electricity on land and at sea so that the production of new-generation marine fuels can be emission-free. Another significant problem in the transition to green (or temporarily to blue) fuels is their availability for the shipping market. Attention should be paid to the development and acceleration of processes taking place in the energy sector on land. The changes taking place on land will force the acceleration of processes taking place in maritime transport and vice versa; what is happening with sea transport has a strong connection with what is happening onshore. It should be expected that the processes of changing primary energy sources in the production of electricity and new-generation fuels will occur faster at sea than onshore.

The ongoing changes in the choice of marine fuels and forecasts for the future can be tracked in many studies and scientific publications [7]. The possibilities of using fuel cells, despite significant progress, are currently insufficient in terms of power for propulsion and the demand for electricity [35]. On the other hand, technologies for capturing carbon dioxide from exhaust gases from ship engines make it possible to temporarily meet the emission reduction requirements (Carbon Intensity Indicator—CII) [19,36], but ultimately, these actions are insufficient. With this method, it is impossible to meet the “Fit for 55” requirements.

8. Conclusions

Already after the first fuel crisis in 1973–1974, the share of fuel costs in ship operations became significant. This was one of the main reasons for moving away from turbo-steam propulsion in ships to diesel engines. To this day, the main barrier to the development of maritime transport is fuel costs. The actions taken are always aimed at minimizing the ship’s operating costs, especially fuel.

The requirements to reduce GHG emissions from maritime transport force the shift to transitional fuels and, ultimately, to zero-emission fuels. The current price of production, storage, transport, and distribution of biofuels, transition fuels, ammonia, and hydrogen is much higher than fossil fuels. A system of emission fees (EU ETS) (a form of tax) has been introduced in EU countries to accelerate the transformation of marine fuels, which generates additional costs when using fossil fuels.

A ban on subsidies (public money) for maritime transport by countries will slow down the processes of change. The flag state of the ship makes independent decisions on many matters that have not been determined by international regulations. One such situation is the recognition of a new type of fuel as ecological. At the same time, there may be pressure from the European Commission and the International Maritime Organization. Their assessment of the changes taking place may force a change in the decision of the flag state.

This article attempts to analyze the ongoing changes in the fuel market in maritime transport on the way to achieving the threshold of climate neutrality with this form of transport. A number of hopes related to this were indicated, but also so were obstacles that may slow down this process.

In 2024, interest in bio-methanol has increased significantly. The actions taken to increase its production may reach the threshold of approximately 30–60 million tons for maritime transport purposes in 2030. This is approximately 5–10% of the energy demand in this transport. Significant changes may concern the use of ammonia, especially in the green version. Investments made in Australia and Southeast Asia may achieve a production capacity of 60–100 million tons, which may constitute 10–15% of energy demand in maritime transport. This represents a significant breakthrough, but not yet a green revolution. A significant increase in the share of hydrogen as a marine fuel is unlikely to occur before 2040. As a result, the green revolution in maritime transport will be extended in time.

The process of transformation of marine fuels, still hardly noticeable in 2022, accelerated in 2023. Investments in the construction of low-emission fuel production infrastructure will result in a significant increase in the production of green hydrogen and, above all, green ammonia, a decrease in their prices, and an increase in interest in their use. Significant development is expected in the coming years, which will result in an increase in the share of renewable fuels in the marine fuel market. There is a need to provide practical solutions to remain competitive and compliant, and reduce risk in a rapidly changing world.