Technical, environmental and economic analysis of utilizing hydrogen-rich fuel in decarbonized container ships

Abstract

This paper analyzes the substitution of conventional fuels with hydrogen-rich fuel derived from ammonia for two different types of container ships, focusing on technical, environmental, and economic perspectives. Four operation modes are investigated including marine diesel oil (MDO), dual-fuel (50 : 50 and 25 : 75 percentages of MDO : H₂-rich fuel) and pure H₂-rich fuel. The environmental impact of using H₂-rich fuel is assessed based on the tank-to-wake and well-to-wake CO₂-equivalent emissions, considering different ammonia production pathways. The results reveal that all the alternative modes exhibit decreased tank-to-wake emissions compared to MDO. The minimum reduction percentage is related to the 50 : 50 mode at about 44%, and an average well-to-wake reduction of 3.5 and 6.3 g per t NM is achievable by using blue and green ammonia, respectively. Moreover, to avoid any increase in the total costs of alternative modes compared to the reference mode, the future ammonia fuel price should be less than 384 $ per t. The research demonstrates that H₂-rich fuel is a viable alternative fuel for container ships, providing notable environmental benefits. While initial costs are higher, long-term economic advantages can be achieved through carbon pricing.

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1. Introduction

The transport sector emits approximately 25% of global CO₂ emissions and the marine industry is responsible for 11% of these emissions, totaling about 900 million tons of CO₂ per year.¹ Therefore, the International Maritime Organization (IMO) has set a minimum reduction target of 40% for carbon emissions from maritime transport by 2030 compared to 2008 levels.² Fossil fuels such as Heavy fuel oil (HFO) and Marine Diesel Oil (MDO) are the dominant fuel in international shipping, producing a range of other harmful substances including sulfur dioxide, carbon monoxide, particulate matter, and heavy metals. Therefore, in order to support Green-House-Gas (GHG) emission reduction targets in the long term, carbon-free and sulfur-free fuels especially hydrogen (H₂) and ammonia (NH₃) will play the most important roles as alternative fuels for ships.³

Hydrogen presents some advantages such as a high gravimetric heating value (120 MJ kg⁻¹) and the potential for production from renewable sources. However, using hydrogen fuel presents significant challenges such as its very low density and low volumetric heating value, which are 10% and 24% of diesel, respectively.⁴ It is flammable over a wide range of concentrations in air (4–75 vol%) and even small hydrogen leaks have the potential to ignite. Hydrogen is commonly stored in two ways: as a highly compressed gas at pressures up to 700 bar and a temperature of 25 °C, or as a liquid, which requires extremely low temperatures of −253 °C at 1 bar. These storage methods are both energy-intensive and costly, posing a major obstacle to the viability of hydrogen-based applications.⁵

To overcome such challenges, ammonia has been recognized as one of the most promising carbon-free hydrogen providers as it has the highest gravimetric hydrogen density of 17.8 wt%, and an appropriate lower heating value (LHV) of 18.8 MJ kg⁻¹, meeting the criteria set by the US Department of Energy.⁶ Ammonia can be liquefied under mild conditions and stored in a simple, inexpensive pressure vessel since its vapor pressure at room temperature is 9.2 bar.⁷ Moreover, considering the potential for ammonia production from renewable resources, its global production capacity is projected to increase from approximately 236 million tons in 2021 to nearly 290 million tons by 2030.⁸

Despite the numerous advantages provided by ammonia as an energy carrier, there are certain limitations, particularly related to net ammonia combustion. For example, ammonia's laminar flame speed is 15 cm s⁻¹, which is significantly lower than hydrogen's, at approximately 269 cm s⁻¹.⁹ Moreover, its combustion generates three specific emissions raising concerns regarding safety, health, and climate: NH₃ slip, NO_X, and N₂O.¹⁰ Therefore, one of the best practical ways to enhance the chemical reaction rate of ammonia combustion and reduce such emissions is to use hydrogen in the ammonia-bound form and, immediately before use, decompose ammonia for producing hydrogen-rich fuel (HRF). This approach simplifies fuel storage and minimizes the need for extensive modifications to ammonia engines.^11,12Table 1 summarizes the properties of conventional marine fuels compared to ammonia and hydrogen.

Table 1 Properties of ammonia compared to other fuels^13–16

Parameter	Ammonia (liquid)	Hydrogen (liquid)	MDO	HFO
Molecular formula	NH3	H2	C12·H23	C20·H30
Density (kg m^−3)	603	71	825	968
LHV(MJ kg^−1)	18.8	120	42.5	40.2
Volumetric energy density (MJ L^−1)	11.3	8.5	36	33
Stoichiometric air/fuel ratio (kg kg^−1)	6.14	34.3	14.4	14
Flammability limit (vol%)	16–25	4–75	0.6–5.5	0.7–5
Laminar flame speed (cm s^−1)	15	269	87	84
Auto-ignition temperature (°C)	650	585	230	250
Boiling point (°C)	−33.3	−252.9	282–338	175
Flash point (°C)	−64.2	−259.2	73.8	61

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Some studies have examined the role of HRF, and the integration of ammonia decomposition with internal combustion engines (ICE), with a primary focus on single-cylinder engines. Zhao et al.¹⁷ showed that when the energy fraction of hydrogen was less than 17% of the total energy needed for a 4-stroke single-cylinder engine, the HRF could hardly burn. In our previous paper,¹⁸ we demonstrated that increasing the hydrogen volume percentage in the NH₃/H₂ mixture from 0% to 50% raised the maximum adiabatic flame temperature from 2079 K to 2216 K. Moreover, Wang et al.¹² experimentally investigated ammonia decomposition to produce HRF for a single-cylinder dual-fuel engine operating at a constant load. Their findings indicate that when the HRF produced by waste heart recovery supplied 5% of the primary diesel fuel, carbon emissions were effectively reduced, while the NO₂/NO ratio increased. In another study, to compare with the ammonia operation mode, the effect of co-fueling a single-cylinder dual-fuel SI engine with HRF was investigated by Ryu et al.¹⁹ They revealed that operation with HRF resulted in about 19% higher power output and 80% reduction in the unburned NH₃ emissions.

Most previous studies in the field of ammonia maritime applications have focused on the ammonia-based engines or fuel cells for different case studies. Ejder et al.²⁰ conducted a techno-economic analysis for a bulk carrier ship, revealing that the ICE–ammonia system offered a 6% advantage over conventional systems in terms of maintenance costs, although it incurred a higher initial cost. In another study, Ejder et al.²¹ compared techniques such as the use of ammonia, LNG, low-sulfur fuels, and selective catalytic reduction (SCR) for container ships. They showed that dual-fuel engines, particularly for long-lasting and newly built ships, offer the most effective environmental benefits, while scrubbers combined with SCR are cost-effective for currently operating ships. Wu et al.²² environmentally and economically investigated ships using ammonia and hydrogen fuels arriving at Guangzhou Port, demonstrating that hydrogen led to higher overall expenses compared to ammonia. In another similar study, Karvounis et al.²³ investigated the economic and environmental sustainability of the diesel fuel substitution with hydrogen or ammonia in marine engines. They concluded that the primary challenges for hydrogen are its high price and low volumetric energy, while for ammonia the main challenge is its low heating value. Moreover, Wu et al.²⁴ conducted a viability assessment of implementing an ammonia SOFC system on an ocean-going vessel highlighting that challenges like volumetric power density, lifespan, and NOx emission need to be addressed for the widespread adoption of ammonia fuel cell technology.

Depending on the method of ammonia production, some studies assessed whether ammonia could serve as an eco-friendly fuel for ships from a life cycle perspective. Ahmed et al.²⁵ investigated the environmental impacts of a green ammonia-powered oil tanker compared to HFO and LNG. Considering the entire life cycle from manufacturing to operation, they revealed that using green ammonia led to about 90% reduction in CO₂ emissions compared to HFO. In another environmental study on bulk carriers, Chalaris et al.²⁶ conducted a parametric trend life cycle assessment considering different ammonia production methods and their findings indicated that the highest carbon emissions stemmed from brown ammonia produced via underground coal gasification, while the lowest emissions were associated with green ammonia produced through the Copper–Chlorine cycle.

Furthermore, a multi-criteria analysis by Kanchiralla et al.²⁷ demonstrated that the GHG reduction potential of alternative fuels varies by ship type and the ammonia usage in internal combustion engines (ICEs) was found to be more cost-effective than fuel cells, primarily due to its lower capital expenditure (CAPEX). In a similar study, Pericic et al.²⁸ conducted a techno-economic analysis on different propulsion methods and fuels across several vessels, concluding that among zero-carbon fuels, ammonia offers better economic performance. Kim et al.²⁹ analyzed four ammonia-based propulsion systems (ICEs and fuel cells) with a focus on technical and economic aspects and concluded that the ammonia-based ship would require 1.4–1.6 times more weight than conventional ones, and it costs 3.5–5.2 times more from a total lifecycle perspective.

Considering the abovementioned studies in maritime applications focusing on ammonia and the importance of using HRF in ICEs, this study aims to make an addition to the literature by conducting a comprehensive energy, environmental, and economic analysis on the use of HRF for container ships by considering various ammonia production pathways. While previous studies did not consider the effect of N₂O of ammonia-based engines on GHG emissions, in this article, tank-to-wake CO₂ equivalent (TTW) and well-to-wake (WTW) emissions are analyzed. Another important aspect of this paper is the consideration of an appropriate NH₃/H₂ ratio for the HRF, aiming to achieve a LHV comparable to that of diesel fuel. Therefore, two different types of container ships (small and large ones) are studied by focusing on four scenarios including MDO, dual-fuel modes (50 : 50 and 25 : 75 percentage of MDO : HRF) and pure HRF ICEs. Also, as holistic approaches to evaluate emission reduction strategies, based on the findings related to small and large container ships, an average overall potential for emission reduction in these types of vessels is estimated by utilizing criteria such as the Energy Efficiency Design Index (EEDI) and Marginal abatement cost (MAC). Furthermore, considering an average international carbon price, sensitivity analyses are performed based on various ammonia production methods and the optimum combination of grey, blue, and green ammonia is proposed to achieve both environmental and economic benefits.

2. Case studies and the description of scenarios

Within the global fleet, container ships accounted for the largest share (23%) of CO₂ emissions with an average monthly CO₂ emission of about 17 million tonnes, while this amount for other types of ships such as oil tankers was 9 million tonnes per month between 2011 and 2021.³⁰ Therefore, in order to analyse the effect of using HRF which is produced by ammonia decomposition, the technical data and real fuel consumption of two different typical container ships (small and large ones) are utilised (Table 2).

Table 2 Main specifications of the typical container ships^24,31

Type of ship	Large container	Small container
Engine capacity (kW)	56 380	13 500
Fuel tank (m^3)	15 142	1800
DWT	189 766	30 000
TEU	20 182	2500
HFO consumption (kg per year)	27 923 000	13 507 000
IMO average distance (NM)	99 770	75 380

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Considering the IMO's ambitious targets for reduction in GHG emissions, for the dual fuel modes, it is assumed that 50% or 75% of the input energy to the engine is supplied by HRF and the rest is supplied by diesel so that eqn (1) shows the HRF energy fraction definition (EF_HRF) and Table 3 presents different scenarios in this paper.

Table 3 Different scenarios based on EF_HRF for both considered ships

No. of scenarios	EFHRF
SC1	0
SC2	50
SC3	75
SC4	100

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According to data obtained from the EnginLink database, the majority of marine engines with power higher than 1 MW include diesel engines.³² The efficiency of such engines depends on the type of engine, with two-stroke engines achieving a brake thermal efficiency even higher than 50%, while four-stroke units typically range between 40 and 50%.³³ Therefore, typical engine efficiencies of 50% and 42% are considered for 2-stroke (large container) and 4-stroke (small container) engines, respectively.³⁴ For these types of big power output engines, significant potential for waste heat recovery can be anticipated, considering heat sources such as exhaust gas, charge air coolers, scavenge air coolers, jacket water and lubrication oil along with exhaust gas recirculation (EGR).³³ Moreover, according to our previous paper,¹⁸ in order to achieve a HRF which has a LHV close to that of diesel (40 MJ kg⁻¹), a conversion rate of about 60% is needed in an ammonia decomposition reactor and the H₂ energy fraction should be about 63%. The energy needed for ammonia decomposition is considered to be supplied by waste heat recovery from the engine.^35,36

3. Methodology for energy–environment–economy analyses

The evaluation includes the analysis of technical performance, GHG emissions, and cost factors. A script in the MATLAB environment is implemented to evaluate different criteria for various fuel scenarios. The analysis addresses different aspects such as annual fuel consumption, lifecycle emissions, and cumulative costs, providing a comprehensive evaluation of the HRF potential in maritime applications.

3.1. Technical criteria

Fuels such as liquified natural gas (LNG), H₂, and NH₃ in their cryogenic liquefied forms continuously lose some mass due to their low boiling points and the temperature difference with the ambient environment, resulting in boil-off gas (BOG).³⁷ One way to avoid BOG generation is by using pressurized tanks to prevent leakage. However, storing large quantities of such liquefied fuels without BOG ventilation requires the installation of multiple pressure cylinders on the ship. This method may conflict with the ship design due to limitations such as space wastage, increased tank weight, and greater operational and instrumentation complexity. Therefore, employing non-pressurized tanks could be an alternative approach for storing large quantities of these energy carriers.³⁸ Even if ammonia is stored at atmospheric pressure, it benefits from a lower boil-off rate, approximately 0.025 vol% per day, compared to hydrogen and LNG which have rates of about 0.52 and 0.12 vol% per day respectively.^38,39 According to eqn (2), BOG is affected by factors such as the overall heat transfer coefficient for the tank (U_ov), the temperature difference between the tank and outside air (ΔT), and the surface area of the tank (A_s).⁴⁰where m_st is the total amount of the stored fuel and Δh_vap is the heat of vaporization.

3.2. Emission criteria

Among GHG emissions, CO₂ is the most important contributor to global warming. In 2018, the global CO₂ emissions from shipping amounted to 1.06 gigatons representing a share of 2.9% of global CO₂ emissions.⁴¹ Based on eqn (3), the yearly fuel consumption (m_F) for each scenario can be used to calculate their respective CO₂ emissions (E_CO2) using the CO₂ emission conversion factor (CF) for different fuels

E_CO2 = CF × m_F (3)

Other important gaseous contributors to GHG include nitrous oxide (N₂O) and methane (CH₄) emissions which have 100-year global warming potential values (GWP100) that are 265 and 28 times higher than that of CO₂, respectively.⁴¹ The other anthropogenic species contributing to global warming is black carbon (BC) with a GWP100 of 900. While CH₄ and N₂O are typically not classified as air pollutants, CH₄ contributes to the formation of tropospheric ozone in regions where background CH₄ dominates. N₂O emissions contribute to the depletion of stratospheric ozone and its role as an anthropogenic contributor is growing along with decreasing halocarbon emissions. N₂O is a potential by-product of both ammonia combustion and after-treatment technologies such as three-way catalyst or selective catalytic reactors. It is also part of the combustion emissions from conventional engines such as marine diesel or HFO engines with emission levels around 0.03 g kW⁻¹ h⁻¹. This index for CH₄ and BC can be considered as 0.01 g kW⁻¹ h⁻¹ and 0.005 (g kW⁻¹ h⁻¹) respectively.⁴² Therefore, in order to better understand the potential impact of N₂O, CH₄ and BC emissions on the total GHG emissions of the ship, TTW is calculated for different scenarios by considering eqn (4):

TTW = E_CO2 + 265 × E_N2O + 28 × E_CH4 + 900 × E_BC (4)

Regarding the application of ammonia fuel in the maritime industry, currently, N₂O emissions are not regulated by IMO. To provide a more accurate assessment of the effect of N₂O on GHG emissions of dual fuel and HRF modes, a literature review is conducted. Experimental studies by Yang et al.¹⁵ on a single-cylinder diesel engine at a constant speed of 1800 rpm demonstrated that when the hydrogen energy blending ratio reached only 10%, N₂O decreased by 50%. Further increases in the hydrogen ratio continued to reduce N₂O emissions, with reductions stabilizing at 97% when the hydrogen blending ratio (Rh) reached 30% or more. Hu et al.⁴³ experimentally investigated nitrogen-based emissions of ammonia/hydrogen blends in a SI engine and found that hydrogen addition significantly improved ammonia combustion stability. As the hydrogen energy ratio increased, N₂O emissions decreased, with a 38% reduction observed when the hydrogen energy ratio reached 20%. Also, Nadimi et al.⁴⁴ experimentally investigated the effect of various ammonia-diesel ratios on engine performance and they concluded that diesel must be replaced with more than 36% ammonia to decrease GHG emissions, with the average amount of N₂O being 0.3 g kW⁻¹ h⁻¹. Moreover, Schwarzkopf et al.⁴⁵ reported 0.78 g kW⁻¹ h⁻¹ N₂O emission for typical ammonia marine engines. In this study, the effect of N₂O is analyzed from two aspects: (A) the expected limit and (B) the average possible amount based on previous research. Therefore, the average N₂O emission factors can be considered as 0.15, 0.225 and 0.39 g kW⁻¹ h⁻¹ for the 50 : 50, 25 : 75 and HRF modes, respectively. It is worth mentioning that most of the previous studies have not considered the effect of N₂O on TTW emissions.^29,46

Considering the international environmental regulations, especially those set by the IMO, the EEDI is assessed to quantify CO₂ emissions per unit of transported distance based on eqn (5).

where DWT is the dead weight tonnage and L is the total yearly distance based on nautical miles (NM) of the ship.

3.3. Economic criteria

The economic analysis of using HRF-based engines is examined by considering three key components including capital expenditure costs (CAPEX), operating expenditure costs (OPEX), and environment expenditure costs (ENVEX). This analysis utilizes eqn (6) to determine the cumulative total annual cost (C_T).where i corresponds to each of the equipment. For example, for CAPEX, the main engine, fuel tanks and ammonia decomposition reactor are considered using their specific costs and installations expenses (eqn (7) and (8)).

CAPEX_en = P_en(C_en + C_in,en) (7)

CAPEX_tank = V_tankC_tank (8)

where P_en is the power of the engine in kW_,C_en is the engine specific power cost in $ per kW, C_in,en is the engine installation specific cost in $ per kW, V_tank is the volume needed for the fuel tank and C_tank is the tank's specific volume cost including installation ($ per kW). Table 3 shows the unit capital cost associated with different propulsion systems for small and large container ships.

All alternative fuels that may replace marine conventional fuels have lower volumetric densities.⁴⁷ The investment cost of fuel storage on board a ship includes the expenses related to the construction and installation of fuel tanks, pumps, valves, pipelines, fire suppression systems, ventilation, and spill containment. This value for MDO and NH₃ and liquid H₂ is 965, 600 and 2160 ($ per m³) respectively.^48,49 Additionally, the specific capital cost for the ammonia decomposition reactor, including installation expenses, is estimated to be $50/kW_en for the modes incorporating HRF.⁵⁰

OPEX refers to all operating and maintenance cost including fuel consumption costs as represented in eqn (9):

OPEX = MC + FC (9)

where MC and FC represent the costs of the ship's maintenance and fuel consumption. The annual maintenance cost of the engine is set at 2.5% and 4.5% of the total CAPEX per year for diesel-based and ammonia-based engines. For other equipment such as the decomposition reactor and fuel tanks, it is set at 1%.^50–52Table 4 displays the global average prices for different fuel types in 2023.

Table 4 Unit costs for different marine engines

Engine type	Small engine ($ per kW)	Large engine ($ per kW)
MDO	350 (ref. 46)	505 (ref. 38)
MDO/HRF	500	720
HRF	600 (ref. 46)	790 (ref. 47)

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ENVEX represents the cost needed to address the degradation of the surrounding environment due to the release of emissions from the corresponding equipment. Therefore, an average international market-based carbon price is considered within the range of Pc = 40–110 $ per t_CO2 for the period between 2025 and 2040.⁵³

ENVEX = TTW_CO2P̄_c (10)

3.4. Some of the other assumptions

• The effect of BOG on the total cryogenic liquid ammonia needed on the ship is taken into account considering typical 300 sailing days per year.

• The CF for diesel and HFO engines is 3206 and 3114 (g_CO2 kg_fuel⁻¹).⁴²

• N₂O levels are expected to be at most around 0.06 g kW⁻¹ h⁻¹ by combining minimum safety and environmental requirements with realistic performance forecasts, based on ongoing development and optimization for engines and catalytic emission treatment technologies.⁴²

• The use of co-fuel engines can increase the capital cost of the system, but it is necessary for the proper operation of the engine. Therefore, a 20% increase in the capital cost related to dual-fuel modes compared to MDO is considered.⁵⁴

• The cost of engine installation is considered to be 10% of their capital costs.⁵⁵

• The expected lifetimes of the marine ICE and ammonia reactor are20 years and 7 years, respectively, so a replacement cost for the ammonia reactor is added to the CAPEX for the MDO/HRF and HRF modes.^56,57

4. Results and discussion

4.1. Fuel consumption and storage analysis

Considering total fuel consumption of the reference HFO marine engine, total energy required for the small and large ships is estimated to be 1.12 × 10⁹ and 0.54 × 10⁹ (MJ), respectively. Therefore, based on the LHV of fuels in different scenarios, the equivalent yearly fuel consumption can be determined. Fig. 1 (left) illustrates the annual fuel consumption for each scenario associated with the respective container ships. The results show that total fuel consumption in the alternative modes is higher than that of MDO due to the lower LHV of ammonia compared to diesel. Using dual fuel modes leads to less total fuel consumption in comparison to the HRF due to the higher LHV achieved. For instance, in the 50 : 50 mode, fuel consumption is 26% lower (14 500 tonnes for the large ship and 7011 tonnes for the small ship) than in the HRF ICE configuration. Moreover, Fig. 1 (right) demonstrates that the total specific fuel consumption (SFC) of the smaller ship is higher than that of the large one for all of the modes. It can also be concluded that using dual fuel modes of 50 : 50 and 25 : 75 leads to a 55% and 82% increase in SFC compared to MDO.

Fig. 1 Annual fuel consumption for each scenario (left) and the related specific fuel consumption (right).

Considering the fuel consumption, LHV and density of the fuels, Table 6 shows the results related to the effect of using HRF on the fuel tank volume of the ships compared to MDO. The results reveal that the storage volume needed for the alternative modes increases compared to the volume needed for MDO. For container ships, TEU can be considered as the cargo-carrying capacity. Therefore, considering the ratio of tank volume/TEU, the results indicate that using the alternative mode of 50 : 50 leads to a tank volume/TEU of 4.4% and 4.6%, while this value for MDO was estimated to be 2.0% and 2.2% for small and large containers, respectively. Moreover, the ratio of power to tank volume for all alternative modes and both ships is less than that for MDO due to the lower volumetric energy density of ammonia (11.3 MJ L⁻¹) compared to MDO (36 MJ L⁻¹).

Table 5 Fuel price values

Fuel price	Range ($ per t)	Assumed ($ per t)
HFO^58	460–620	540
MDO^58,59	550–720	635
NH3 grey^11	245–400	320
NH3 blue^10,60	275–425	360
NH3 green^60	560–800	670

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Table 6 Effect of using HRF on the fuel tank volume of the containers

Scenario	Tank volume/TEU (%)		Power/tank volume (kW m^−3)
	Small	Large	Small	Large
MDO	2.1	2.2	7.9	3.9
50 : 50	3.7	3.8	4.5	2.2
25 : 75	4.4	4.6	3.7	1.8
HRF	5.2	5.5	3.1	1.5

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4.2. Environmental analysis

Fig. 2 (left) shows the TTW emission for each scenario including the error bars related to the minimum expected emission. The results indicate that CO₂ emissions decrease in all alternative methods due to the partial or complete diesel replacement with HRF. Moreover, the effect of N₂O on GHG emissions is significantly greater than BC and CH₄. Its contribution to the TTW for the mode 50 : 50 is 10.8% and 12% for small and large container ships. Even when considering N₂O emissions, all alternative scenarios can lead to a reduction in total GHG emissions compared to the diesel mode. The reduction percentage for the 50 : 50, 25 : 75 and HRF modes are about 44%, 64% and 82%, respectively. In the 50 : 50 mode, approximately 37 753 tonnes of GHG emissions can be reduced annually for the large container ship, while the small container ship can achieve a reduction of about 18 616 tonnes.

Fig. 2 TTW for each scenario (left) and the energy efficiency design index (right).

According to the IMO database,² the average distances travelled by large and small container ships are 99 770 and 75 380 NM. Therefore, Fig. 2 (right) demonstrates EEDI_TTW for the alternative scenarios compared to the reference mode. Due to the importance of N₂O emission, the adoption of such alternative methods is quite promising so that a minimum reduction of 8.23 and 1.99 (g_CO2 per t NM) for small and large containers can be achieved. For the HRF mode, these reduction amounts can increase to 15.6 and 3.72 (g_CO2 per t NM) for small and large containers, respectively.

Based on the IMO database,² global marine fuel consumption was estimated to be 213 million metric tonnes per year, contributing to approximately 1076 million tonnes CO₂ equivalent in 2019.⁶¹ In this study, we analyzed various categories of container ships based on the data from ref. 2. The weighted average of DWT·L is calculated to be 4.48 × 10⁹ (t NM). Therefore, the weighted average of the potential reduction in TTW based on the number of container ships is calculated to be 5.5 (g_CO2 per t NM). Assuming that 70% of container ships could be powered by the dual fuel mode of 50 : 50, regardless of the increase in the total number of ships, it can be predicted that an annual reduction of about 90 million tonnes in TTW emissions from container ships could be achieved, which will account for 38% of the total emission related to this sector.

Although the IMO announced a strategy for reducing TTW emissions, it is also crucial to evaluate the emissions associated with the production pathways of alternative fuels. Therefore, a WTW investigation is conducted which comprises the well-to-tank (WTT) phase including all stages of fuel production, processing, transportation, and distribution, as well as the TTW phase.⁶²Table 7 shows the average WTT emissions on a GWP100 basis for different methods of ammonia production including brown (from coal or lignite), grey (typically from natural gas), blue (including carbon capture) and green (from carbon-free sources and near-zero carbon electricity). Nowadays, around 72% of ammonia is produced through the steam reforming of natural gas.⁶⁹ For blue ammonia, a reduction in carbon intensity from 50% to 80% compared to grey ammonia is possible through the utilization of carbon capture units. Moreover, there are still some emissions associated with renewable-based ammonia production due to the potential use of fossil fuels for the manufacturing processes of photovoltaic panels (PV) or wind turbines, as well as emissions linked to the fuel distribution process.⁶⁴

Table 7 WTT GHG emissions of different fuels

Fuel	WTT (gCO2 MJ^–1)	WTT range (gCO2 MJ^–1)
Diesel	17 (ref. 62)	14–19 (ref. 63)
HFO	15 (ref. 62)	11–17 (ref. 63)
Brown ammonia	160 (ref. 64)	64–225 (ref. 65)
Grey ammonia	100 (ref. 64)	74–191 (ref. 66 and 67)
Blue ammonia	45 (ref. 67)	21–75 (ref. 68)
Green ammonia	8 (ref. 64)	3.5–25 (ref. 62)

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Fig. 3 (left) illustrates the relative WTW emissions for each scenario compared to the WTW of the diesel mode, which emits approximately 51 000 and 105 750 tonnes for small and large containers, respectively. The figure incorporates the average TTW emissions from the previous section, with error bars indicating the range of WTT emissions.

Fig. 3 Relative WTW-CO₂-eq emissions (left) and the effect of minimizing N₂O from the engine on WTW emissions compared to the average values (right).

The results demonstrate that using blue and green ammonia in the HRF-based scenarios leads to a decrease in WTW emissions. For blue ammonia, the 50 : 50, 25 : 75 and HRF modes lead to a reduction of 23%, 34% and 43%, respectively, compared to the reference mode. Therefore, integrating carbon capture, utilisation and storage systems with grey ammonia production is of vital importance considering the total GHG emission reduction. For all modes, the effect of TTW emission on the WTW is much more important when it comes to using blue and green ammonia. For the 50 : 50 mode, the proportion of the TTW emissions to WTW emissions for brown, grey, blue and green ammonia is about 34%, 44%, 60% and 78%, respectively. This contribution can be further reduced by considering the minimum expected N₂O emissions from the engine.

Fig. 3 (right) shows the effect of minimizing N₂O from the engine on WTW. The greatest reduction of 52.5% is observed in the green HRF mode, while the grey HRF mode shows a reduction of 9.8% for small container ships. The trend of the results for the large container ship is the same with a maximum difference of about 3%.

Considering the 50 : 50 mode and drawing inspiration from the definition of EEDI_TTW, based on eqn (11), an average WTW reduction of 3.5 and 6.3 (g per t NM) can be expected for using blue and green ammonia, respectively. Therefore, assuming that 70% of the container ships could be retrofitted with such a dual fuel mode, the annual reduction in WTW emissions for container ships could reach approximately 58 million tonnes and 104 million tonnes for blue and green ammonia, respectively.

4.3. Economic analysis

Fig. 4 illustrates the ratio of yearly fuel expenses for different scenarios compared to MDO. Based on the averages presented in Table 5, the annual fuel costs for large and small container ships under the MDO scenario are 16.7 and 8.1 M$, respectively. Error bars show the related expenses based on the averaged green ammonia price projections which will be in the range of 14–24 $ per GJ by 2040.¹⁰ The results show that using grey and blue ammonia in the 50 : 50 mode can lead to only approximately 3% and 10% higher yearly fuel costs compared to the reference mode. This figure for green ammonia is about 60%. Additionally, the sensitivity analysis reveals that a future reduction in the price of green ammonia could lead to a maximum increase of 20% compared to MDO, assuming that the price of MDO remains constant.

Fig. 4 Ratio of yearly fuel expenses to MDO.

Total expenses, including CAPEX, OPEX and ENVEX while considering the average carbon price during the lifetime of the system are shown in Fig. 5 for both large and small container ships. Moreover, error bars show the effect of the future price of green ammonia fuel. The results indicate that the fuel price is the most significant factor affecting total expenses, with green ammonia resulting in higher costs compared to other types of ammonia fuel.

Fig. 5 Total cumulative cost related to a small container ship (left) and a large container ship (right) including the predicted price of green ammonia.

Furthermore, the engine capital cost for the small and large ships constitutes about 70% and 64% of the total CAPEX. In the HRF-based scenarios, the costs of the ammonia reactor and fuel tanks account for a maximum of 2% and 6% of the total cumulative costs for small container and large container ships. For small engines using blue ammonia, the total cumulative costs for the 50 : 50, 25 : 75, and HRF scenarios are lower than the MDO costs of $309 million. In contrast, the cumulative costs for large ships exceed the MDO costs of $527 million. This difference arises because fuel costs constitute a larger proportion of the total cumulative costs for small ships, and the price of blue ammonia per unit mass is lower than that of MDO. For both types of ships, although the amount of ammonia is higher than MDO, due to the low cost of grey ammonia, the alternative scenarios lead to the lower total costs compared to the reference mode.

The marginal abatement cost (MAC) is another important criterion to assess the cost-effectiveness of different scenarios since it measures the costs of reducing one tonne of CO₂ equivalent emissions (eqn (12)).^70,71

where ΔCost and ΔCO₂ are the difference in total costs and the difference in CO₂ emissions between the alternative scenarios and the reference mode considering the lifetime. Since these two ships have independent characteristics and operation details, the weighted average MAC has been calculated by considering the number of container ships. Fig. 6 shows that even when using blue ammonia, it is possible to avoid an increase in the MAC compared to the carbon price. For green ammonia, the maximum MAC is related to the pure HRF mode and equals to about 240 $ per t_CO2 due to the higher price of the fuel and the higher initial cost of the engine.

Fig. 6 Average marginal abatement costs compared to the carbon price.

5. Appropriate combination of dual-fuel percentage and ammonia sources

By setting an expected target of reducing total TTW emissions by 50% compared to MDO, based on Fig. 2, it is concluded that the appropriate dual-fuel mode for MDO : HRF would be 44 : 56 and 42 : 58 for small and large container ships, respectively. Consequently, a representative mode of 40 : 60 is proposed.

Moreover, although the mentioned target focuses on TTW reduction, it is essential to identify the maximum allowable CO₂ emissions from the ammonia production pathways. This will ensure that a minimum reduction of 30% in WTW is achieved, provided that the dual-fuel configuration remains at a ratio of 40 : 60.

The results reveal that the maximum CO₂ emissions from different routes of ammonia production should be limited to 36 (g_CO2 MJ⁻¹) to achieve this goal, while the current average for blue ammonia is about 45 (g_CO2 MJ⁻¹). This underscores the importance of using green ammonia as well as combining grey or blue ammonia with green ammonia. Therefore, a blend of 0.31 grey/0.69 green ammonia or 0.78 blue/0.22 green ammonia results in a 30% reduction in WTW emissions and 50% TTW emission reduction.

Additionally, an economic analysis is conducted for the 40 : 60 mode aiming to equalize the total costs of the alternative scenarios with those of the reference MDO, while ensuring a 50% reduction in TTW emissions. The results, summarized in Table 8, illustrate that the ammonia fuel price should be less than 384 $ per t. This allows for the proposal of two combinations: 0.82 grey/0.18 green ammonia or 0.92 blue/0.08 green ammonia. The latter also achieves a 26.9% reduction in WTW.

Table 8 Appropriate dual-fuel modes and ammonia sources considering environmental and economic criteria

Optimization criteria	Environmental		Economic
Type of ammonia	31% grey	78% blue	82% grey	92% blue
	69% green	22% green	18% green	8% green
HRF energy fraction	0.6	0.6	0.6	0.6
CO2 reduction (%)	60	60	60	60
TTW CO2 reduction (%)	50	50	50	50
WTW CO2 reduction (%)	30	30	2.35	26.89
Fuel price ($ per t)	560	428	384	384
Fuel costs compared to MDO	1.51	1.25	1.16	1.16
Total costs compared to MDO	1.24	1.06	1.00	1.00

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6. Conclusions

This study evaluates the feasibility of using H₂-rich fuel derived from ammonia in container ships considering technical, environmental, and economic perspectives. Both tank-to-wake and well-to-wake CO₂ equivalent emissions are evaluated across different ammonia production pathways. To provide a robust assessment, two small and large container ships are analyzed under four fuel scenarios: marine diesel oil, dual-fuel (50 : 50 and 25 : 75 MDO ratios), and pure H₂-rich fuel. Moreover, using the energy efficiency design index and marginal abatement cost criteria, the overall emission reduction potential is estimated for these vessel types. Finally, sensitivity analyses incorporating average international carbon pricing enable the proposal of optimal combinations of grey, blue, and green ammonia to balance environmental and economic benefits.

The results indicate that utilizing H₂-rich fuel, especially in dual-fuel modes, enhances the practical applicability of ammonia by achieving a greater lower heating value. However, the 50 : 50 mode represents a loss of 2.4% in the cargo capacity compared to the MDO mode. The environmental analysis demonstrates that by replacing the conventional fuel with H₂-rich fuel, significant reductions in CO₂ emissions can be achieved. Moreover, even when considering N₂O emissions, all the alternative scenarios can lead to the reduction in tank-to-wake emissions compared to the reference mode. The reduction percentage for 50 : 50, 25 : 75 and pure H₂-rich fuel modes are about 44%, 64% and 82%. Additionally, the analysis of well-to-wake emissions highlights that the use of blue and green ammonia can further enhance the environmental benefits, with average energy efficiency design index reductions of 3.5 and 6.3 (g per t NM) for blue and green ammonia, respectively.

Economically, the study acknowledges the higher initial costs associated with transitioning to ammonia-based propulsion systems. However, it emphasizes that the long-term benefits especially by applying potential carbon pricing, can offset these initial investments. Moreover, blue ammonia can lead to an average marginal abatement cost of less than 75 $ per t_CO2. Finally, to prevent an increase in the overall costs of the alternative mode compared to the reference one, the price of ammonia fuel must remain below 384 $ per t. This would allow for the use of either a 0.82 grey/0.18 green ammonia or a 0.92 blue/0.08 green ammonia combination.

Overall, the study concludes that HRF derived from ammonia is a viable alternative fuel for container ships, offering significant environmental benefits and manageable economic implications, thus contributing positively to the maritime industry's sustainability goals. Further studies on advanced technologies for controlling and reducing N₂O emissions and other potential pollutants from ammonia-based engines, along with assessments of economic incentives like subsidies, tax breaks, and carbon credits, will be crucial in accelerating the adoption of ammonia-based fuels in the maritime sector.

Abbreviations

NH3	Ammonia
BOG	Boil-off gas
CAPEX	Capital expense
DWT	Deadweight tonnage
EEDI	Energy efficiency design index
ENVEX	Environment expenditure costs
GHG	Green-House-Gas
GWP	Global warming potentials
HFO	Heavy fuel oil
H2	Hydrogen
HRF	Hydrogen-rich fuel
ICE	Internal combustion engines
IMO	International Maritime Organization
LHV	Lower heating value
LNG	Liquified natural gas
MAC	Marginal abatement cost
MDO	Marine diesel oil
OPEX	Operating expenditure costs
TEU	Twenty-foot equivalent unit
TTW	Tank-to-wake
WTW	Well-to-wake

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

Payam Shafie: conceptualization, methodology, investigation, analysis, writing-original draft, Alain Dechamplain: conceptualization, supervision, analysis, review & editing, and Julien Lepine: conceptualization, supervision, analysis, review & editing.

Conflicts of interest

The authors declare that they have no conflicts of interest.

Acknowledgements

The project was financed by Chantier Davie Canada Inc, a premier shipbuilder in Canada, based on their work in the field of using alternative fuels for the marine industry.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4se01109k

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