Decarbonization Potential of Alternative Fuels in Container Shipping: A Case Study of the EVER ALOT Vessel

Abstract

Environmental emissions from the maritime sector, including CO₂, NO_x, and SO_x, contribute significantly to global air pollution and climate change. The International Maritime Organization (IMO) has set a target to reduce greenhouse gas emissions from international shipping to reach zero GHG by 2050 compared to 2008 levels. To meet these goals, the IMO strongly encourages the transition to alternative fuels, such as hydrogen, ammonia, and biofuels, as part of a broader decarbonization strategy. This study presents a comparative analysis of converting conventional diesel engines to dual-fuel systems utilizing alternative fuels such as methanol or natural gas. The methodology of this research is based on theoretical calculations to estimate various types of emissions produced by conventional marine fuels. These results are then compared with the emissions generated when using methanol and natural gas in dual-fuel engines. The analysis is conducted using the EVER ALOT container ship as a case study. The evaluation focuses on both environmental and economic aspects of engines operating in natural gas–diesel and methanol–diesel dual-fuel modes. The results show that using 89% natural gas in a dual fuel engine reduces nitrogen oxides (NO_x), sulfur oxides (SO_x), carbon dioxide (CO₂), particulate matter (PM), and carbon monoxide (CO) pollutions by 77.69%, 89.00%, 18.17%, 89.00%, and 30.51%, respectively, while the emissions percentage will be 77.78%, 91.00%, 54.67%, 91.00%, and 55.90%, in order, when using methanol as a dual fuel with percentage 91.00% Methanol. This study is significant as it highlights the potential of natural gas and methanol as viable alternative fuels for reducing harmful emissions in the maritime sector. The shift toward these cleaner fuels could play a crucial role in supporting the maritime industry’s transition to low-emission operations, aligning with global environmental regulations and sustainability goals.

1. Introduction

The urgent need to address global environmental changes necessitates transforming our energy production and consumption practices [1,2]. The shipping industry, a critical component of global trade, significantly contributes to greenhouse gas emissions [1,3,4,5,6]. This study focuses on evaluating the feasibility of transitioning from conventional marine fuels to alternative options such as LNG and methanol, with particular emphasis on their environmental impact within the maritime sector. LNG, methanol, and biofuels each offer different trade-offs in emission reduction, cost, and scalability [7]. A well-to-wake analysis of marine fuels reveals considerable variation in emissions depending on the fuel type and production pathway [8]. Given that maritime trade accounts for more than four-fifths of worldwide trade activity, the maritime sector’s emissions significantly affect global greenhouse gas levels. Vessels release large quantities of CO₂, NO_x, SO_x, PM, and CO [9]. Addressing these emissions is crucial for environmental sustainability and human health. According to the latest statistics from the International Maritime Organization (IMO), ships produce about 1.1 Gt CO₂ annually, which accounts for three percent of worldwide GHG emissions [10,11]. Global shipping NO_x and SO_x emissions account for about thirteen and twelve percent, in that order, of total NO_x and SO_x emissions from anthropogenic sources. In addition to 1.4 million tonnes of PM and 936 kilotonnes of carbon dioxide, global shipping contributes significantly to these emissions. Figure 1 shows that around 54,743 merchant ships are involved in the international movement of cargo and passengers. These vessels contribute 55 percent of overall carbon dioxide output [12].

Among these ship types, container ships emit the highest percentage of CO₂ emissions. This can be attributed to their ability to unload and offload quickly, which increases their underway time and consequently their contribution to emissions. This trend is well-documented in several studies on containerization [13]. In the Fourth IMO GHG Study 2020, the operational hours of different ship types were analyzed. The study found that container ships spend a significant amount of time underway compared to other ship types. Specifically, container ships are underway for approximately 70% of their operational time, while bulk carriers and tankers are underway for about 50–60% of their operational time. Further data from the ICCT report [12] supports this finding, indicating that container ships, on average, operate for more hours per year compared to bulk carriers, tankers, and other ship types. This increased operational time directly contributes to higher CO₂ emissions, as the ships are burning fuel and emitting CO₂ for longer periods. Additionally, container ships generally operate at higher Froude numbers compared to crude oil tankers. The Froude number, a dimensionless parameter that compares the ship’s speed to the wave speed, is higher for container ships due to their higher operational speeds. Container ships typically operate at a Froude number of around 0.2–0.3, while crude oil tankers usually operate at a lower Froude number of about 0.1–0.15. This higher Froude number indicates that container ships experience more significant wave resistance, which can lead to higher fuel consumption and increased CO₂ emissions. By comparing the operational hours and understanding the high percentage of time container ships spend underway, it becomes evident that their operational profile significantly contributes to their higher CO₂ emissions. This highlights the need for targeted measures to reduce emissions from container ships, such as improving fuel efficiency and adopting alternative fuels. The study uses the EVER ALOT, the world’s largest container ship, as a critical case study. Currently powered by an 11X92-B engine manufactured by WinGD (WinGD Ltd., Winterthur, Switzerland), the ship is proposed to be converted to a dual-fuel variant (11X92DF). The vessel’s movement efficiency is evaluated using the IMOs indicators. There is a lack of detailed, ship-specific comparative analysis of emissions from conventional marine fuels versus alternative fuels (such as LNG and methanol) under realistic operational conditions. Few studies use real vessel data (like the EVER ALOT container ship) to quantify emissions and assess the feasibility of converting to dual-fuel engines using theoretical models in the context of IMO decarbonization goals. This study deals with the following question: How do emissions from conventional marine fuels compare to those from methanol and LNG when used in dual-fuel engines on a large container ship such as the EVER ALOT, and what is the environmental benefit of such a fuel transition under IMO regulatory frameworks? This research offers a novel contribution to the decarbonization literature by presenting a detailed, ship-specific case study on the EVER ALOT, the world’s largest container vessel, to evaluate the feasibility of converting its main propulsion system from conventional diesel to dual-fuel configurations using liquefied natural gas (LNG) or methanol. Unlike previous studies that focus on single fuels or generic vessel categories, this work conducts a direct, side-by-side comparison of LNG–diesel and methanol–diesel dual-fuel modes under identical operational conditions. The analysis integrates environmental impact assessment, Energy Efficiency Design Index (EEDI) and Energy Efficiency Operational Indicator (EEOI) evaluations, and International Maritime Organization (IMO) regulatory compliance for both current and future decarbonization targets. The study also outlines a practical conversion pathway for modern ultra-large container ships, providing actionable technical and regulatory insights for shipowners and policymakers aiming to meet the IMO 2050 zero-GHG strategy.

2. IMO Regulations

The IMO has been busy throughout the process of improving maritime energy competency and implementing measures to reduce ship emissions [14]. According to the International Convention for the Prevention of Pollution from Ships (MARPOL) Annex VI, the IMO has approved several rules to restrict emission increases [15]. Regarding NO_x pollution, the IMO passed the technical NO_x Code, which came into force on October 10, 2008, and applies to ship diesel engines with an output power of more than 130 kW [16]. SO_x and PM emissions are proportional to the sulphur content of the fuel and are harmful to human health, causing respiratory and pulmonary diseases. In 2005, the IMO adopted regulation 14 of MARPOL Annex VI to limit the sulphur content of fuel used on board ships. In 2020, a new requirement on the sulphur content of diesel fuel used in ships went into effect, known as “IMO 2020” to enhance the quality of the air, and save the environment [17]. Regarding CO₂ emissions, the International Maritime Organization (IMO) has implemented two methods to evaluate whether ships meet global standards. One of the key approaches to improving environmental performance in the maritime sector involves the use of the Energy Efficiency Design Index (EEDI) and the Energy Efficiency Operational Indicator (EEOI). The EEDI measures the energy efficiency of a newly built ship by calculating the amount of CO₂ emitted per transport work (g CO₂/ton-mile), thus promoting the design of more fuel-efficient vessels. In contrast, the EEOI evaluates a ship’s actual operational performance by assessing CO₂ emissions per unit of cargo and distance traveled during voyages. Both indicators play a vital role in monitoring and reducing greenhouse gas emissions, supporting compliance with IMO environmental regulations and driving the shift toward sustainable maritime transport [18]. As shown in

Table 1. Several techniques to cut marine emissions.

Emissions	Type of Operation
SOx emissions	Switching to fuels with low sulfur content (e.g., marine gas oil or compliant heavy fuel oil) And scrubber systems
NOx emissions	Pre-combustion	Water injection into the air and adjusting combustion parameters to reduce NOx formation.
	Combustion	(EGR) Redirecting a portion of exhaust gases back into the engine to lower combustion temperature and NOx formation.
	After treatment	A post-combustion treatment that uses urea to convert NOx into nitrogen and water.
CO2 emissions	Concerning Design	Enhancing hull design, propulsion systems, and voyage planning to reduce fuel consumption
	Engine modifications	Waste heat recovery, engine modifications, and auxiliary system modification such as improving pump efficiency
	Operational measures	Implementing slow steaming, optimized routing, and energy-saving practices.
	New technologies	Carbon Capture Storage (CCS), Replacing conventional marine fuels with LNG, methanol, biofuels, or hydrogen.

, there are several methods that can be implemented to reduce marine emissions to meet EEDI requirements.

Among technical and operational standards, replacing traditional fossil fuels with alternative fuels with lower environmental impact appears to be the most useful way to achieve shipping sector decarbonization. In addition, most alternative fuels have limited levels of nitrogen and sulphur, which is critical for ships operating in Emission Control Areas (ECAs), where emissions requirements are extremely more stringent than international ones [19].

Alternative fuels for marine applications are generally categorized into two principal groups: liquid alternatives, such as methanol, ethanol, and biodiesel, and gaseous options, including natural gas, hydrogen, and propane. Among these, biodiesel stands out as a renewable fuel with notably lower emissions of particulate matter, unburned hydrocarbons, carbon monoxide, and sulfur dioxide, making it a cleaner substitute for conventional marine fuels [20]. Nevertheless, it does not perform well in cold conditions, and the rising need for its production has contributed to a food shortage, as it competes directly with food crops, particularly as demand grows. This competition can lead to higher food prices, food shortages, and food insecurity, particularly in regions where food production is already strained. Additionally, there is an increase in NO_x emissions associated with biodiesel production [20]. Research indicates that ethanol is a viable marine fuel option, producing only CO₂ and water when burned, although it struggles to ignite in cold weather and requires a lot of agricultural lands to produce them [13]. Methanol is another alternative fuel that has seen extensive research. Stena Germanica was the first ferry to run on methanol and marine gas oil in 2015 [21]. To investigate its suitability, IMO conducted a study that assessed the carbon footprint of methanol using Life Cycle Analysis (LCA). According to the results, methanol has lower life-cycle GHG emissions than conventional fuels. Although the application of methanol is feasible, there are issues associated with its use, including aldehyde emissions, cold starts, and cost effectiveness. Natural gas is used in transport in the form of liquefied natural gas (LNG) and compressed natural gas (CNG). Regarding the use of Compressed Natural Gas (CNG) as a marine fuel, it is important to note that some flag states, including the United States, do not currently allow for CNG storage of natural gas for propulsion. This limitation is due to safety and regulatory concerns surrounding the storage and handling of CNG on board vessels. LNG is the most used alternative fuel in shipping compared to CNG. Its use in ECAs is significant because of its low sulfur, carbon, and nitrogen content [22]. Many studies have shown that LNG is preferred for long-term use, cheaper for long-distance transport, environmentally friendly, and more powerful [10]. The use of LNG as a fuel is a feasible option for reducing the carbon impact. However, there are obstacles to using LNG such as high investment costs, an absence of LNG facilities in ports, and safety regulations. Another problem related to the combustion process is “methane slip”, in which unburned methane from the fuel is released with the exhaust gas. Nevertheless, this issue is virtually eliminated in advanced 2-stroke engines, advanced 2-stroke engines are known as “electronically controlled dual-fuel low-speed 2-stroke engines equipped with gas admission valves and pre-chamber combustion, such as WinGD X-DF series [23]. Many studies have been conducted to investigate the possibility of alternative fuels in the maritime sector. However, these studies mainly focused on the environmental effects as a marker of whether alternative fuel is acceptable for use in the maritime sector, without taking into consideration technical assessment, achieving IMO requirements, or economic impact such as cost effectiveness. The International Maritime Organization (IMO) regulations under MARPOL Annex VI have been pivotal in reducing emissions from ships and driving the maritime industry toward sustainability. These rules, including the IMO 2020 sulfur cap, Tier III NO_x limits, and Energy Efficiency Design Index (EEDI) requirements, have significantly lowered harmful emissions and promoted the adoption of cleaner technologies such as dual-fuel engines, scrubbers, and alternative fuels like LNG and methanol. However, compliance with these regulations poses challenges, including high retrofitting and fuel costs, limited global infrastructure for alternative fuel supply, and complex overlapping requirements with regional policies. Additionally, some measures have unintended consequences, such as methane slip from LNG and pollutant transfer from scrubber discharge. While IMO regulations provide a strong framework for environmental protection, they remain technology-neutral, leaving shipowners uncertain about the long-term viability of different compliance options. To enhance their effectiveness, these regulations should be supported by clear policy roadmaps, financial incentives, and global infrastructure development. Overall, the IMO’s efforts are commendable, but achieving the ambitious 2050 decarbonization targets will require more integrated approaches that address both technical and economic barriers. At the global level, the 2023 IMO GHG Strategy (adopted at MEPC 80) sets a sector-wide trajectory to reach net-zero GHG emissions by or around 2050, with indicative checkpoints to cut total annual GHG emissions by at least 20% (striving for 30%) by 2030 and at least 70% (striving for 80%) by 2040, relative to 2008. The Strategy also targets the uptake of zero/near-zero GHG fuels to ≥5% (strive 10%) by 2030 and advances a workplan toward a global marine fuel standard and an economic (pricing) measure.

Complementing this, the IMO’s short-term measures under MARPOL Annex VI are already in force: since 1 January 2023, ships have had to comply with the Energy Efficiency Existing Ship Index (EEXI) and are subject to the Carbon Intensity Indicator (CII) framework (annual reporting in 2023; first ratings in 2024). These measures link operational performance to decarbonization pathways and provide a baseline for evaluating alternative fuels on existing tonnage. In terms of air-pollutant controls, the Mediterranean Sea has been designated an SO_x Emission Control Area with a 0.10% sulfur cap effective from 1 May 2025, joining the Baltic, North Sea, North American and U.S. Caribbean ECAs; in parallel, NO_x Tier III standards apply in the Baltic and North Sea NECAs for ships constructed on/after 1 January 2021 when operating in those areas. For voyages touching EU ports, the EU “Fit for 55” package adds regional obligations. FuelEU Maritime (Regulation (EU) 2023/1805) imposes a well-to-wake GHG-intensity reduction on the energy used onboard—−2% in 2025 rising stepwise to −80% by 2050—and requires container and passenger ships to use on-shore power supply (OPS) or equivalent zero-emission technologies at berth from 1 January 2030 in AFIR-covered ports (expanding thereafter). In addition, the EU ETS now covers maritime emissions with phased surrendering: 40% of 2024 emissions in 2025, 70% of 2025 in 2026, and 100% from 2027. These measures directly affect fuel selection and onboard energy strategies for ships calling EU ports.

3. Methodology

This study lays out a comprehensive methodology for converting a ship’s propulsion system to utilize alternative fuels, aiming to clarify whether the study constitutes a rough order magnitude, feasibility study, or functional design. The evaluation of alternative fuel storage solutions is a critical component, involving detailed power estimates for the fuel storage systems, including the energy requirements for safety measures. Various storage arrangements are examined, integrating these systems into the ship’s existing design without compromising operational efficiency. The auxiliary systems needed to support fuel storage, such as cooling systems and safety mechanisms, are identified and specified. The study employs a rigorous environmental analysis methodology to evaluate the environmental impact of the conversion, defining specific environmental metrics such as CO2 and NO_x emissions and using advanced life cycle assessment tools to quantify these impacts. The methodology for estimating the Energy Efficiency Design Index (EEDI) involves a detailed calculation process, applying the EEDI formula with specific data inputs and explaining assumptions for transparency. The assessment of the Energy Efficiency Operational Indicator (EEOI) follows a methodical approach, collecting and analyzing operational data to calculate the EEOI and comparing it to pre-conversion values to evaluate improvements. This study adopts a ship-specific feasibility analysis to evaluate the conversion of the EVER ALOT container vessel’s low-speed WinGD 11X92-B diesel engine into dual-fuel configurations using either liquefied natural gas (LNG) or methanol. The approach integrates technical assessment, environmental impact estimation, and energy efficiency evaluation in line with International Maritime Organization (IMO) requirements. The technical assessment identifies two conversion pathways: (i) modification to a WinGD 11X92DF dual-fuel engine (LNG + marine diesel oil) with ISO VAC LNG tanks, and (ii) conversion to a MAN B&W ME-LGI dual-fuel engine (methanol + marine diesel oil) using adapted diesel storage facilities, both in compliance with the IMO IGF Code for low-flashpoint fuels. The environmental analysis quantifies CO₂, NO_x, SO_x, particulate matter (PM), carbon monoxide (CO), and hydrocarbons (HC) for cruising (85% load), maneuvering (20% load), and standby (5% load) conditions, using IMO-referenced emission factors and equations to determine total emissions per voyage and annually for each fuel blend (89% LNG + 11% MDO, 91% methanol + 9% MDO). Results are compared with IMO Tier III NO_x standards, IMO 2020 SO_x limits, and relevant ECA requirements. Energy performance is evaluated using the Energy Efficiency Design Index (EEDI) following MEPC.308(73) guidelines, calculating attained values for baseline and dual-fuel scenarios and benchmarking them against required EEDI targets for large container ships. Operational efficiency is assessed through the Energy Efficiency Operational Indicator (EEOI) based on the vessel’s 24,004 TEU capacity, 11,004 nautical mile route, and annual voyage frequency, applying IMO SEEMP methodology. Key assumptions include unchanged engine efficiency post-conversion, constant voyage frequency (eight round trips annually), and operational parameters derived from vessel documentation and peer-reviewed literature. The methodology outputs comparative emissions, EEDI, and EEOI values for each fuel scenario, assesses compliance with current and future IMO thresholds, and identifies the potential of LNG and methanol dual-fuel systems to meet 2030 and 2050 decarbonization objectives.

3.1. Alternative Fuel Storage on Board Ship

In general, NG could be stored on board vessels in either compressed (CNG) or liquefied form (LNG), but due to the larger quantity of NG used throughout this case study trip, it will not be workable to hold it in compressed form [24]. The only appropriate storage form here is a liquefied gas, which already has the advantage of being lighter and taking up less space than CNG. The specifications of LNG ISO tank containers that can be used for this purpose are shown in

Table 2. Specifications of the LNG containers.

Model	Pressure (bar)	Weight (kg)	Length (m)	Height (m)	Width (m)	Capacity (Gross)
ISO VAC 40-LNG	10	12,670	12.192	2.591	2.438	43,500 Ltr
ISO VAC 20-LNG	17	7875	6.058	2.591	2.438	20,000 Ltr

. Because of its liquid state, one of the major benefits of methanol fuel is that it is very comparable to marine diesel fuels. Because of its low flashpoint, the current diesel fuel facilities can be used for methanol fuel with minor modifications [25]. Regulations pertaining to the safety of fuel tanks, location, and venting have requirements for the storage of methanol or LNG. Fuel should not be kept in machinery or accommodation spaces, and a least horizontal distance of 760 mm shall exist between both the fuel tank side and the ship’s shell. Each tank must be able to support the propulsion system’s continuous rating and the generator plant’s typical operating load for at least eight hours while at sea. Fuel tanks must be equipped with a system for gas freeing and secure inert gas purging. When there is no full access from the open deck, fuel tanks must have enough ventilation entrances and outlets to ensure full gas-freeing, but no fewer than two inlets and two outlets per tank. To reduce any fire risks near the fuel tanks on the weather deck, special care must be taken. Depending on the circumstances, a fire safety assessment may be required to determine how to protect the low flashpoint liquid (LFL) fuel tanks from potential fires on board.

3.2. Environmental Analysis Methodology

First, the overall emissions produced by ships during a voyage per tonne can be calculated by using Equation (1). The emission value (EM) is influenced by the main engine’s output power ($P_w$) in kW, the running time in hours, the load factor of engine ($L_f$) and the fuel pollution factor $P_{f,i}$ in g/kWh; (i) is the type of emission, (f) is the type of fuel and (t) is the operating time.

$${EM}_{trip,i,f}=\sum \left[t\left(P_w·L_f·P_{f,i}\right)\right]$$

(1)

(1) Maritime emissions encompass a wide spectrum of pollutants, including sulfur oxides (SO_x), nitrogen oxides (NO_x), carbon dioxide (CO₂), particulate matter (PM), and carbon monoxide (CO). The specific emission rate, expressed in grams per kilowatt-hour (g/kWh), differs based on the pollutant type and fuel characteristics. In the case of CO₂, the principal factor influencing emission intensity is the carbon concentration inherent in the fuel, which varies significantly across different fuel types. Figure 2 illustrates the comparative carbon content associated with various fuels, such as Marine Diesel Oil (MDO), Natural Gas (NG), and Methanol (ME).

To mitigate nitrogen oxide emissions from ships, the International Maritime Organization (IMO) has established MARPOL Annex VI, Regulation 13, which specifically targets NO_x emissions. This regulation applies to marine engines with a power output exceeding 130 kW and is mandatory for vessels constructed on or after 1 January 2000. The control of NO_x emissions is categorized into three stages, Tier I, Tier II, and Tier III, which were introduced in 2000, 2011, and 2016, respectively, according to the engine’s rotational speed (n), as shown in Figure 3.

Tier III standards aim to achieve an 80% reduction in NO_x emissions compared to Tier I levels and are enforced within designated Emission Control Areas (ECAs). Additionally, under Regulation 14 of MARPOL Annex VI, the IMO introduced strict controls on sulfur content in marine fuels due to its significant contribution to SO_x emissions. In 2020, a global sulfur cap reduced the permissible sulfur content in fuel from 3.5% to 0.5% m/m, while stricter regional limits lowered it to 0.1% in ECAs as early as 2015, as depicted in Figure 4.

Now, for a dual-fuel engine which is operated between two fuels, the total emission factor for each fuel is calculated with the effect of fuel percentage, as shown in the equation below:

$$P_f=\left(Diesel\%\right)\ast P_D+\left(New fuel\%\right)\ast P_{gas}$$

(2)

where $P_f$ is the total emission factor for dual fuel engine, $P_D$ is the emission factor for pilot diesel fuel, and $P_{gas}$ is the emission factor for alternative fuel.

3.3. Energy Efficiency Design Index Estimation

IMO adopted the Energy Efficiency Design Index (EEDI) to evaluate how efficiently energy is consumed during ship operations and to show the environmental consequences of maritime transportation. EEDI is used to evaluate the energy performance of designated vessel types. MARPOL Annex VI addresses their concerns about ships that are 400 gross metric tons or more, including LNG carriers, container vessels, and oil tankers. The calculation procedure for EEDI is based on emissions, vessel velocity, and size. The Energy Efficiency Design Index (EEDI), established by the IMO, serves as a benchmark for assessing the energy efficiency of newly built vessels. According to [26,27], the EEDI is calculated using a standardized formula. IMO guidelines define two key values: the required EEDI, which reflects the expected efficiency target for each vessel type, and the regulatory EEDI limit, which varies based on the ship’s total deadweight and is enforced using Equation (3) (IMO, 2018).

$${EEDI}_{require}=\left(1-{\displaystyle \frac{X}{100}}\right)\ast {\displaystyle \frac{z}{{DWT}^y}}$$

(3)

where z and y are ship-specific parameters defined by the prediction curve fitting of a specified set of vessels with varying capacities, and for container ships, their values are 174.22 and 0.201, respectively. Based on the initial value, X% is the required EEDI reduction value every 5 years, which increases from 10% to 20%, and 30% in 2015, 2020, and 2025, respectively, and DWT is the Dead Weight Tonnage. Attained EEDI is the actual value that can be evaluated by using Equation (4). For IMO satisfaction, its value should be less than the required EEDI.

$${EEDI}_{attained}={\displaystyle \frac{\left(ME\left(s\right)+AE\left(s\right)-innovative technology\right) {CO}_2 emissions}{Transport work}}$$

(4)

The EEDI equation uses three terms to calculate CO₂ emissions per unit of transport work (g CO₂/ton·nm). To begin with, CO₂ emissions for main engines (ME CO₂ emissions) can be calculated by using Equation (5). $P_{ME}$ is the Power output from main engine. $C_{FME}$ is the fuel conversation factor from fuel consumption to ${CO}_2 emission$. ${SFC}_{ME}$ is the specific fuel consumption for main engines.

$$ME {CO}_2 emissions=\left\{{\sum }_{i=1}^{nME}{P}_{ME\left(i\right)}·C_{FME\left(i\right)}·{SFC}_{ME\left(i\right)}\right\}$$

(5)

Secondly, CO₂ emissions for auxiliary engines (AE-CO₂ emission) can be evaluated by using Equation (6) according to IMO for maximum continuous rating (MCR) above 10,000 kW. $P_{PTI\left(x\right)}$ is the shaft motor mechanical power divided by the generators’ weighted efficiency. $C_{FAE}$ and ${SFC}_{AE}$ are the fuel conversion factor and specific fuel consumption for auxiliary engines, respectively.

$$AE {CO}_2 emission=\left\{0.025\ast \left({\sum }_{i=1}^{nME}{MCR}_{ME}+{\displaystyle \frac{\sum _{i=1}^{nPTI}{P}_{PTI\left(x\right)}}{0.75}}\right)+250\right\}\ast C_{FAE}·{SFC}_{AE}$$

(6)

The third component of the emission reduction framework addresses the decrease in CO₂ emissions attributed to the integration of energy-saving innovations, such as auxiliary sail propulsion systems. These technologies contribute to lowering the effective engine power demand. In this context, $P_{eff\left(i\right)}$ represents the power reduction achieved through the i-th energy-efficient technology. $f_{eff\left(i\right)}$ denotes the efficiency factor associated with each technology.

$$Reduction {CO}_2 from innovative technology=\left\{{\sum }_{i=1}^{neff}{f}_{eff\left(i\right)}·P_{eff\left(i\right)}·C_{FME}·{SFC}_{ME}\right\}$$

(7)

The denominator in the CO₂ intensity calculation, commonly referred to as transport work, is derived using Equation (8) and includes several correction factors, which $f_i,f_1,f_w,{\mathrm{a}\mathrm{n}\mathrm{d} f}_c$ account for ship type-specific adjustments. $f_w$ is a non-dimensional correction factor that reflects environmental influences, such as weather conditions, and is determined using Equation (9). The parameter $V_{ref}$ stands for the ship’s reference speed. In passenger ships, the transport capacity is measured using gross tonnage (GT). For container ships, the capacity is assumed to be 70% of the deadweight tonnage (DWT). This 70% benchmark reflects industry-standard practices and operational realities, accounting for container size, stacking limitations, and loading efficiency.

$$Transport work=f_i·f_1·f_w·f_c·V_{ref}·Capacity$$

(8)

$$f_w=0.0208\ast \ln \left(Capacity\right)+0.633$$

(9)

Moreover, Specific Fuel Consumption (SFC) and the fuel conversion factor (C_f) are dependent on the type of fuel used. In the case of dual-fuel engines, the product of SFC and C_f—which is used to compute fuel-related emissions—is evaluated based on Equation (10):

$${SFC}_{DF}\ast {CF}_{Df}={SFC}_{PF}\ast {CF}_{Pf}+{SFC}_{Gas}\ast {CF}_{Gas}$$

(10)

3.4. Energy Efficiency Operational Indicator (EEOI) Assessment

The Energy Efficiency Operational Indicator EEOI is a checking tool that was developed by The Ship Energy Efficiency Management Plan (SEEMP) to supervise ship and fleet efficiency overall performance. EEOI incorporates SEEMP recommendations to provide the most effective approaches for operating ships with optimized fuel efficiency for both existing and new vessels by estimating the amount of CO₂ emitted per ton of transport work. As described in Equation (11), i represents the voyage count, FC refers to the amount of fuel consumed, C_F is the conversion factor from fuel to CO₂ emission, m is the cargo weight on board, and D is the distance of a voyage in nautical miles.

$$EOI={\displaystyle \frac{{\sum }_{i}FC·C_f}{{\sum }_i{m}_{cargo}·D}}$$

(11)

4. Case Study

As a case study, the container ship EVER ALOT was chosen, for which it was proposed to convert its diesel engine into a dual-fuel engine running on alternative fuels. Therefore, it is crucial to study the environmental and economic consequences of using dual fuel. The study was based on a comparison of methanol and liquefied natural gas as alternative fuels. The container vessel EVER ALOT, which is owned by the Taiwanese ship operator Evergreen Marine and primarily serves trade routes connecting the Far East and Europe, was built in 2022 with a 24,000 TEU capacity and is sailing under the Panamanian flag.

Table 3. Main data of EVER ALOT ship.

Type	Container Ship
Ship’s Name	EVER ALOT
Year built	2022
IMO No.	9893955
Flag	Panama
Length over all (m)	400
Breadth over all (m)	61.5
Draught (Avg/Min/Max) (m)	13.2/3.5/25.5
Speed (kn)	20.1
Power (kW)	60,400 at 70 rpm
TEU	24,000
Deadweight (tons)	241,000
Gross Tonnage (GRT)	236,228
Main engine	CMD-WinGD 11X92-B
Generators (kW)	5 × 4300

depicts the ship’s critical data. The ship track should be selected to assess the environmental impact of ship operation. The ship’s liner service for this study is the route from Hamburg, Felixstowe, Rotterdam, Colombo, Tanjung Pelepas, Kaohsiung, and Qingdao via the Suez Canal and the total distance is 11,004 nautical miles. The ship is currently operated with a low-speed diesel engine (CMD-WinGD 11X92-B), at 70 rpm and 11 cylinders. The length and height of the engine are 21.215 m and 16.12 m; the bore is 920 mm, and the stroke is 3468 mm. The engine weight is 1960 MT. The engine uses marine diesel (0.1 S%) to move cargo between several ports with a maximum continuous rating of 66,440 kW. The connection between specific fuel consumption and maximum continuous rating (MCR) can be determined based on engine specifications. The specific fuel consumption (SFC) for a low-speed engine is 162.3 g/kWh. Regarding NO_x regulations, after January 2016, IMO implemented new limits of approximately 3.4 g/kWh for ships operating in existing NO_x Emission Control Areas (ECAs), or for vessels constructed or operating on or after the official recognition of a new ECA. In addition, IMO set a sulfur cap of 0.5% under the 2020 SO_x emission regulation.

Therefore, it is suggested to convert the main engine into an 11X92DF engine with the same speed and power, which will run on liquid natural gas as fuel. Since no major structural elements need to be changed, DF-capable engines can be converted into dual-fuel engines. Another proposal is to transfer the 11X92-B to a MAN B&W ME-LGI engine with the same speed and power that can run on methanol as a dual fuel. The ability of dual-fuel engines to maintain safe gas operation and perform accurate fuel oil and gas switching even in difficult weather conditions gives them an advantage. The combustion process for the ME-LGI engine occurs at 1300 °C to avoid methane slip and N₂O. The aim of the study is to evaluate the environmental and economic impacts of using LNG and methanol as a dual fuel instead of conventional fuel.

5. Results and Discussion

The study compared the use of methanol and NG as alternative fuels in dual fuel engines. Firstly, the emission rates were determined to evaluate the environmental benefits of using alternative fuels (Methanol and natural gas). The emission rates are compared based on IMO limitations. Secondly, the energy efficiency design index (EEDI) was calculated to evaluate CO₂ emissions and contrast the results with IMO standards across various stages. The energy efficiency was then assessed by analyzing how the conversion procedure influences the EEOI. Lastly, the effect of applying alternative fuels such as natural gas and methanol on economic efficiency is measured.

5.1. Environmental Assessment of Alternative Fuels

Based on experimental investigations, the present study supposes that the efficiency of the transformed dual-fuel engine with methanol or natural gas will be identical to the efficiency of the diesel engine, particularly in the case of high-performance engines [28]. The emission factors are the first step in the comparative study of methanol and NG as a fuel in dual-fuel engines. The pollution factors for NG and methanol were calculated according to Equations (1)–(4). The CO₂, NO_x, and CO pollution factors for NG fuel were 548.2 g/kWh, 2.16 g/kWh, and 0.92 g/kWh, while the emission factors for Methanol fuel 275 g/kWh, 2.47 g/kWh, 0.54 g/kWh and 0.9205 g/kWh, respectively. We are currently investigating the effect of dual-fuel injection on pollutant factors, calculated by Equation (5). According to Figure 5, using methanol as a dual fuel reduces CO₂ emissions by more than half compared to natural gas, while using a high proportion of methanol or NG reduces SO_x and PM emissions to negligible levels.

Methanol and natural gas have almost identical effects on NO_x emissions. However, the HC emission factor for methanol or natural gas dual fuel is slightly higher compared to marine diesel fuel. HC emissions for methanol and NG arise from incomplete combustion; methanol engines generally show higher aldehyde and HC emissions, especially during cold starts, according to Bilgili (2020) [13].

The next step is to compare NO_x and SO_x emission rates with IMO requirements to determine the optimal percentage of dual fuel relative to pilot oil. For NO_x pollution, the emission rate is calculated using Equation (1), with the power and loading factor for the engine based on the case study. Figure 6 depicts the effect of increasing the methanol or NG ratio on NO_x emissions and compares it to the IMO limit of 3.4 g/kWh for low-speed engines. The analysis shows that dual-fuel engines operating on pilot fuel at any proportion up to 11% for NG and 9% for methanol (ME) will meet the necessary IMO rates.

For SO_x emissions, IMO 2020 imposed new requirements that fuel oil used on board ships should not exceed 0.5% sulfur, which equals 0.09264 tons per hour. As shown in Figure 7, there was a similarity for using a dual fuel with NG or methanol that complied with IMO 2020 limitations for each dual fuel. As previously stated, (89% NG, 11% MDO) and (91% ME, 9% MDO) met IMO requirements for all types of emissions.

The total emission per trip and the annual emission for each emission factor must now be calculated.

Table 4. The emission factor during operation, maneuvering, and standby by using natural gas or methanol dual fuel.

	Type of Emission	Emission Factor (g/kwh)	Emission Rate During Operation (kg/h)	Emission Rate During Maneuvering (kg/h)	Emission Rate During Standby (kg/h)
89% (NG) dual fuel	NOx	3.7924	264.0079	62.11951	15.52988
	SOx	0.0396	2.756754	0.648648	0.162162
	CO2	563.6649	39,239.53	9232.831	2308.208
	PM	0.0209	1.454954	0.342342	0.085586
	CO	0.9728	67.72147	15.93446	3.983616
	HC	1.312	91.33488	21.49056	5.37264
91% Methanol (ME) dual fuel	NOx	3.7777	262.9846	61.87873	15.46968
	SOx	0.0324	2.255526	0.530712	0.132678
	CO2	312.2411	21,736.66	5114.509	1278.627
	PM	0.0171	1.190417	0.280098	0.070025
	CO	0.6174	42.9803	10.11301	2.528253
	HC	0.891655	62.07256	14.60531	3.651327

. shows she emission factor during operation, maneuvering, and standby by using natural gas or methanol dual fuel. To calculate the emission factor, a dual-fuel engine operating on 89% natural gas and 91% methanol was employed, as detailed in Table 4. The engine load factor (the percentage of rated power used) can be determined based on the engine’s average operating speed. For this, load factors are set at 85% for cruising, 20% for maneuvering, and 5% for standby operation. Differences in emissions can be observed during maneuvering and idle states. Methanol-fueled engines produce lower SO_x, CO₂, and particulate matter emissions than natural gas, though they result in a moderate increase in NO_x emissions. Each trip typically lasts around 30 days, depending on the ship’s route and velocity, and ships are estimated to operate around 8 voyages annually. By using Equation (1), the total and yearly emissions were calculated to evaluate the environmental benefits of utilizing a dual-fuel engine.

Table 5. Annual emission reductions for using dual fuel by methanol or natural gas.

Emission	Fuel Type	Emission t/Trip	Emission t/Year	Reduction t/Year	% of Reduction/Year
NOx	Diesel fuel	1102.7016	8821.613
	NG dual fuel	245.993	1967.946	6853.667	77.69%
	ME dual fuel	245.040	1960.318	6861.295	77.78%
SOx	Diesel fuel	23.351	186.811
	NG dual fuel	2.569	20.549	166.261	89.00%
	ME dual fuel	2.102	16.813	169.998	91.00%
CO2	Diesel fuel	44,678.226	357,425.805
	NG dual fuel	36,562.01101	292,496.088	64,929.717	18.17%
	ME dual fuel	20,253.457	162,027.652	195,398.153	54.67%
PM	Diesel fuel	12.324	98.594
	NG dual fuel	1.356	10.845	87.749	89.00%
	ME dual fuel	1.109	8.874	89.721	91.00%
CO	Diesel fuel	90.811	726.486
	NG dual fuel	63.100	504.804	221.682	30.51%
	ME dual fuel	40.048	320.380	406.106	55.90%
HC	Diesel fuel	38.919	311.351
	NG dual fuel	85.103	680.821	−369.470
	ME dual fuel	57.837	462.696	−151.345

shows the overall emission reductions achieved with dual-fuel technology. For instance, using 89% natural gas reduced CO₂ emissions to about 292,496.088 tons annually and decreased NO_x, CO₂, SO_x, PM, and CO by 77.69%, 18.17%, 89%, 89%, and 30.51%, respectively. Similarly, with 91% methanol, reductions were 77.78%, 54.67%, 91%, 91%, and 55.9%, in the same order. Nonetheless, hydrocarbon emissions increased with each dual-fuel configuration.

5.2. Energy Efficiency Assessment

In accordance with IMO regulations, evaluating the EEDI is considered the most efficient method to determine a ship’s energy performance. As mentioned, there are two types of EEDI values: the required value, which represents the maximum acceptable threshold defined by Equation (6). This value is determined using a vessel deadweight of 241,000 tons, and X denotes the percentage of reduction, which increased from 10% in 2015 to 30% in 2025, as shown in Figure 8.

The required Energy Efficiency Design Index (EEDI) is progressively tightened across three regulatory phases, decreasing from 14.43 gCO₂/ton·nm to 12.988, 11.545, and 10.102 gCO₂/ton·nm, respectively. In parallel, the attained EEDI is determined using Equation (7) and is evaluated against the applicable threshold—specifically, the Phase 2 limit under IMO regulations. According to the case study, the vessel is powered by a single main propulsion engine and is equipped with five auxiliary generators to meet electrical demands during both standby and active sailing modes. Diesel fuel is used for both the main and auxiliary engines. By assuming a design service speed of 22 knots as the reference speed V_ref and considering the vessel’s effective capacity as 70% of its deadweight tonnage (DWT), the calculated EEDI is approximately 13.0386 gCO₂/ton·nm. This value complies with the Phase 1 limit but exceeds the stricter Phase 2 requirement effective from 2020 to 2025, as illustrated in Figure 9.

To determine the energy efficiency that achieves the required EEDI, the reduction effects of using (89% NG, 11% MDO) and (91% methanol, 9% MDO) as dual-fuel engines were compared. For the natural gas dual-fuel engine, the EEDI has been reduced to 11.388 gCO₂/ton.nm to comply with phase 2 of the IMO’s regulations, which set specific targets for reducing CO₂ emissions by incremental phases. This reduction is primarily achieved due to the cleaner nature of LNG fuel, which produces less CO₂ compared to conventional marine fuels. Additionally, the use of more efficient engine technologies and other operational improvements contribute to this reduction. Similarly, the EEDI for the methanol dual-fuel engine has been reduced to 6.26226 gCO₂/ton.nm to meet the stricter future EEDI requirements set for 2030. These future requirements aim for even lower emissions, pushing the industry to adopt advanced technologies and cleaner fuels like methanol, which produce fewer CO₂ emissions compared to conventional marine fuels. These reductions demonstrate the potential of alternative fuels and advanced engineering solutions to meet and exceed regulatory requirements, contributing to the global effort to minimize the environmental impact of shipping.” requirements in 2030. The Energy Efficiency Operational Indicator (EEOI) represents an additional metric established by the International Maritime Organization (IMO) to evaluate a vessel’s operational energy performance and associated carbon emissions. As outlined in Equation (11), the EEOI is calculated using key operational parameters, including the total voyage distance, the number of transported TEU containers, and the fuel consumption per journey. For the EVER ALOT container ship, a total of 24,004 TEUs were carried over a distance of 11,004 nautical miles. During operation, the EEOI decreased from 0.00013 ton CO₂/TEU.nm when using MDO to 0.0001132 ton CO₂/TEU.nm and 6.22231 × 10⁻⁵ ton CO₂/TEU.nm when operating with 89% natural gas and 91% methanol, respectively. The calculations for maneuvering and standby were carried out based on 20% and 5% fuel consumption, in order.

6. Conclusions

The International Maritime Organization (IMO) emphasized various approaches to reducing ship exhaust emissions and enhancing marine energy efficiency through both operational and technical strategies. One significant long-term approach explored in this study for reducing emissions and enhancing energy performance is shifting from fossil fuel usage in traditional diesel engines to dual-fuel engines that run on alternative energy sources. Natural gas and methanol are gaining interest as substitutes for conventional marine fuels. This research conducted a comparison between methanol and natural gas usage in dual-fuel engines to determine the most environmentally efficient option. The analysis concluded the following:

• From an environmental perspective, the findings indicate that utilizing dual-fuel engines with a mix of 89% natural gas and 11% MDO results in reductions of NO_x, SO_x, CO₂, PM, and CO emissions by 77.69%, 89%, 18.17%, 89%, and 30.51%, respectively. In contrast, using 91% methanol as a dual fuel achieves emission reductions of 77.78%, 91%, 54.67%, 91%, and 55.9%, respectively. Furthermore, replacing conventional diesel engines with dual-fuel systems operating on natural gas or methanol will meet IMO 2016 and 2020 emission standards for NO_x and SO_x, assuming marine diesel oil content remains below 10%.
• From an energy efficiency perspective, based on the Energy Efficiency Design Index (EEDI) criteria, dual-fuel engines operating on either (89% NG and 11% MDO) or (91% ME and 9% MDO) are expected to comply with IMO targets. The EEDI reference value for the third phase was 10.1021 gCO₂/ton-nm, while the calculated EEDI for dual-fuel systems using natural gas and methanol was 11.388 gCO₂/ton-nm and 6.263 gCO₂/ton-nm, respectively. Moreover, both 89% natural gas and 91% methanol configurations fulfill current IMO EEDI limits and show potential to satisfy future standards, especially with ongoing progress in fuel efficiency and technology.