Decarbonation Effects of Mainstream Dual-Fuel Power Schemes Focus on IMO Mandatory Regulation and LCA Method

Abstract

Recently, the IMO has completed the guidelines on the life cycle greenhouse gas intensity of marine fuels to accelerate the application of alternative fuels. Low-carbon fuels may persist for decades and have become a key transitional phase in replacing marine fuels. A more comprehensive methodology for evaluating the carbon emission levels of marine fuels was explored, and the carbon emissions and environmental impacts of a 150,000-ton shuttle tanker under 19 dual-fuel power scenarios were evaluated using the Energy Efficiency Design Index (EEDI) and life cycle assessment (LCA) method. The results show that liquefied natural gas (LNG) has a higher carbon control potential level compared to liquefied petroleum gas (LPG) and methanol (MeOH), while LPG is superior to MeOH based on EEDI evaluation. LCA analysis results show that MeOH (biomass) has the best carbon control potential considering the carbon emissions of the well-to-tank phase of the fuel, followed by LNG, LPG, MeOH (natural gas, NG), and MeOH (coal). However, MeOH (NG) and MeOH (coal) had greater negative environmental impacts. This study provides method support and a direction toward improvement for revising related technical specifications and regulations for dual-fuel vessel performance evaluation, considering the limitations of various maritime regulations.

1. Introduction

1.1. Background

The shipping industry is an important part of the international trade and logistics system, as well as a significant contributor to greenhouse gas (GHG) emissions and marine ecological damage [1,2]. To address the associated environmental issues, the International Maritime Organization (IMO) has successively introduced the Energy Efficiency Design Index (EEDI), Ship Energy Efficiency Management Plans (SEEMPs), and tier series standards to limit ship emissions during operation. According to Chapter 4 of the Annex VI amendment to the International Convention for the Prevention of Pollution from Ships (MARPOL), all newly built ships with a gross tonnage of ≥400 have been required to comply with the EEDI since 2013 [3,4,5]. Meanwhile, calculating and reporting the Energy Efficiency Existing Ship Index (EEXI) and Carbon Intensity Indicator (CII) for existing ships have been mandatory since 2023 [6]. Subsequently, the 2023 IMO Strategy on the Reduction of GHG Emissions from Ships was adopted, based on which the global shipping industry strives to achieve net-zero emissions by 2050 [7].

1.2. Literature Review

1.2.1. Evaluation of Decarbonization Effect Based on Different Marine Fuels and Traditional Technology Applications

The shipping industry’s most important conventional propulsion systems are marine engines fueled with marine diesel oil (MDO) and heavy fuel oil (HFO). Due to their high carbon contents, the combustion of these fuels produces large amounts of GHGs and contributes to the acceleration of global warming [8]. Researchers have conducted a series of studies to effectively respond to IMO regulatory standards to reduce carbon emissions during ship operations and improve energy efficiency. More specifically, Adland et al. [9] used the performance parameters and weather data from 2012 to 2016 to study the effects of periodic hull cleaning on tanker energy efficiency. Cleaned hull and propeller surfaces were found to reduce the daily fuel consumption by decreasing the drag affecting the hull. Meanwhile, Norlund and Gribkovskaia [10] used a supply ship as the research object. By adding a suitable speed optimization strategy to the ship schedule and utilizing the waiting time to reduce the speed, they achieved energy savings and emission reduction. Jia [11] systematically analyzed the structural optimization design process of 4500 deadweight tonnage bulk carriers and the impact of structural lightweighting on ship energy efficiency. The results showed that by decreasing the structural weight, the ship’s EEDI value could be moderately lowered and carbon emissions can be reduced; additionally, the load capacity can be increased or the main engine power can be reduced. Key optimization technologies include ship shape optimization, propeller modification, and the development and application of new energy-saving equipment can also further achieve the effect of carbon control and reduction in carbon emissions [12,13,14,15].

However, based on recent research trends, reducing carbon emissions based on existing optimization techniques and the single use of MDO/HFO remains challenging. Moreover, the mandatory requirements for new shipbuilding are difficult to meet, particularly in EEDI Phase III. Therefore, existing marine fuels are gradually shifting from single MDO/HFO to dual fuel, making studying new energy applications to replace traditional fuels an important research topic.

Alternative fuel selection prioritizes low-carbon options such as biofuels, methanol (CH₃OH, MeOH), liquefied natural gas (LNG), and liquefied petroleum gas (LPG), etc., and zero-carbon alternatives such as hydrogen and ammonia [16]. The literature shows that compared to conventional fuels, LPG, LNG, and MeOH can significantly reduce SOx and NOx emissions, reduce CO₂ and produce minimal particulates [17,18,19,20,21,22]. Specifically, LNG achieves 90–99% SOx reduction, 20–30% NOx abatement, and approximately 25% carbon reduction potential; LPG shows even more complete SOx elimination (90–100%) with 10–15% NOx reduction and 13–18% lower carbon emissions; while MeOH provides comparable SOx control (90–97%) but superior NOx reduction (30–50%), albeit with a more modest 5% decarbonization potential. Most notably, all three fuel alternatives achieve over 90% reduction in particulate matter emissions. Jafarzadeh et al. [23] conducted a study on the application of LNG to fishing vessels. They found that, as a clean and efficient energy source, LNG can be effective in improving the environmental footprint to some extent. Moroianu and Postolache [24] analyzed the performance of marine engines using MDO, HFO, and MeOH, identifying MeOH as an attractive alternative fuel given its reduced pollutant emissions and cost-effectiveness. Carbon emissions can also be reduced by using similar alternative fuels such as hydrogen, ammonia, and biofuel [25,26,27,28,29,30,31,32]. While considering factors such as energy availability, technological readiness, economic feasibility, environmental performance, fuel energy density, and regulatory frameworks, alternative fuels for ships are dominated by low-carbon fuels, with LNG, LPG, and MeOH being the primary type for ships, compared to zero-carbon fuels like hydrogen and ammonia. LPG, LNG, and MeOH are currently the main choice for newly built ships; even considering the future evolution of the shipping industry from low- to zero-carbon technologies, a long transition period remains for low-carbon fuels.

1.2.2. Evaluation of Decarbonization Effects Under Mainstream Methods

Figure 1 shows that consistent with the application of technologies to reduce carbon emissions, quantitative evaluation methods have been applied to assess the effects of carbon emissions on the shipping sector. Based on IMO regulations, the EEDI represents the carbon dioxide emissions per ton of cargo transported by a maritime vessel over one nautical mile. Initially, it was proposed as a predictor of the energy efficiency of newly built ships [33]. A lower EEDI indicates higher energy efficiency. However, the EEDI only utilizes a single value, while the effects of multiple conditional factors (including changes in sailing speed and improvements in the design of key ship equipment) are not considered. This index can only be used for the operational phase of a large ship, not for quantitatively evaluating the degree of carbon emissions during the design, construction, or dismantling phases of a ship’s life cycle.

The principles of the newly introduced EEXI and CII are consistent with the EEDI. Other indicators and methodologies must be considered to assess ships’ carbon emissions more comprehensively and promote sustainable development throughout the ship cycle. Life cycle assessment (LCA) has recently been gradually applied to the marine sector [16,31,34,35]. Specifically, the IMO has developed 2024 guidelines on the life-cycle GHG intensity of marine fuels (2024 LCA Guidelines), focusing on the life-cycle GHG/carbon intensity of various types of low- and zero-carbon fuels [36]. IMO is increasingly emphasizing the importance of LCA in quantifying carbon emissions. The overall framework and content of LCA is based on ISO 14040 [37] and ISO 14044 [38]. Fang [39] used the LCA method to establish a spatiotemporal network model of the carbon footprint of a ship’s life cycle and analyzed the movement paths and spatial trajectories of the bulk carrier “HUA YUN 5” during its life cycle. The analysis confirms that the carbon footprint of this type of ship is most significant during the shipping phase. Moreover, Han et al. [40] sought to further determine the impact of the structural design margins of fiber-reinforced polymer (FRP) fishing vessels on air pollution. To this end, LCA was used to conduct an environmental impact assessment of FRP materials. The results show that the increase in raw construction materials due to the hull structure design allowance negatively impacted atmospheric pollution; hence, the design rules must be further revised and improved. Chatzinkolaou and Ventikos [41] proposed a new framework for assessing gas emissions from ocean-going vessels in conjunction with the LCA and conducted a case study with Panamax tankers. During the operational phase of a ship, gas emissions from machinery and equipment accounted for a large proportion. The LCA can also be an important basis for comparing and selecting ship power and propulsion systems [42,43,44]. Currently, LCA applications in the marine sector primarily target the entire life cycle of small vessels such as tugboats, yachts, fishing boats, and ferries; the carbon emissions per gross ton of small ships are generally much higher than those of large commercial ships, likely exerting a greater environmental impact [45,46,47].

Compared with the EEDI, LCA can be used for the operational phase of a ship as well as for a full life cycle assessment. Therefore, LCA has been proposed as a statutory measure to decrease carbon emissions in the shipping industry [48,49,50,51]. Accurately assessing the environmental impacts of ships throughout their life cycles, including energy consumption, carbon emissions, and water pollution, is of great significance to determining the environmental impacts of ships. Moreover, LCA results are relatively conservative compared to experimental results, such as main engine emission tests. This is primarily due to current mainstream databases, such as Ecoinvent 3, defining marine fuel as a production, with the production stage critical in life cycle inventory data [34,35,46].

1.3. Research Gap and Objectives

As mentioned previously, although the EEDI can effectively evaluate the energy efficiency and the carbon emission level of newly built ships, it only applies to the operational phase. Furthermore, the issue of methane slips and their impact on carbon emissions cannot be further considered. While the LCA can quantitatively evaluate ships’ carbon emissions and environmental impact performance from a complete life-cycle perspective, certain limitations remain. In particular, even the more comprehensive LCA databases are unable to accurately consider the carbon emissions of marine fuels during the combustion phase.

To address the research gap, a carbon emission level assessment method for marine fuels based on the EEDI and LCA is proposed. Taking a typical ship as the research object, different groups of dual-fuel power design schemes were developed from the perspective of marine fuels, particularly low-carbon fuels. Subsequently, EEDI and LCA evaluations were conducted, and a solution was proposed to address the lack of data in the LCA database. Meanwhile, the limitations of the EEDI in quantifying marine fuel carbon emissions during the application phase and the key role of LCA in the sustainable development of the shipping industry are discussed. The study provides a comprehensive quantitative framework for ship carbon emission assessment, and the findings can provide guidance for selecting dual-fuel solution for ships and promote the implementation and formulation of IMO regulations.

This paper is structured as follows. Section 2 examines the results of prior research and methodology, including target vessel type selection, dual-fuel power scheme development, and EEDI application. Section 3 analyzes the results of the LCA simulation, encompassing the entire process, from the definition and scope of the objectives to the interpretation of the results. Additionally, the results of prior EEDI research are discussed. Section 4 provides a multifaceted analysis and discusses the EEDI and LCA results. Finally, the main conclusions are summarized, while the limitations and future perspectives of the study are further discussed in Section 5.

2. Examination of Prior Research and Methodology

2.1. Target Ship Selection

Figure 2 shows the research object of this study, which was a 150,000-ton shuttle tanker built by COSCO Shipping Heavy Industry (Zhoushan) Co., Ltd., China. As a representative vessel from the company’s core product line, this shuttle tanker series specializes in crude oil transportation from offshore fields to onshore terminals. The vessel class, certified by Det Norske Veritas (DNV) with unrestricted navigation certification, currently comprises six operational units in this series. The main engine model (MAN B&W 6G70ME-C10.5) uses MDO as the primary fuel. The current EEDI value of the N786 ship is 2.774, which is ~28.84% lower than the baseline value of 3.574. Despite meeting the second phase of the EEDI through technologies such as hull form optimization, main engine power reduction, speed optimization, energy-saving devices, and hull structure weight reduction, the N786 ship did not meet the third phase. Consequently, the N786 ship was selected as the research object for conducting comprehensive analysis of viable decarbonization solutions.

Table 1. Main characteristics of the shuttle tanker.

Parameter	Value	Parameter	Value
Length (m)	278.90	75% MCR fuel consumption of main engine (g/kWh)	159.16
Distance between perpendiculars (m)	265.00	Number of generators and MCR (kW)	4000 for 5 units and 1600 for 1 unit
Molded breadth (m)	47.00	50% MCR fuel consumption of generators (g/kWh)	198.79
Molded depth (m)	24.90	Correction factor for reinforcement of ship voluntary structure	1.0105
Summer draft (m)	17.30	CSR correction factor	1.0150
Empty ship weight (t)	29,067	Design speed (kn)	14.5
Deadweight tonnage (t)	154,955	Baseline EEDI	3.574
Number of main engines and MCR (kW)	14,675 for 1 unit	Reached EEDI	2.774

MCR, maximum continuous rating; CSR, common structural rules.

lists the main dimensions, engine, speed, EEDI values, and other characteristics.

2.2. Effect Analysis of Dual-Fuel Selection Scheme Based on EEDI

A dual-fuel power design solution is an excellent choice to decrease carbon emissions. Hence, an examination of the results of prior research must be prioritized. In recent years, COSCO Shipping Heavy Industry (Zhoushan) Co., Ltd. has successively provided design schemes for dual-fuel-powered vessels for this series of target vessels and LPG, LNG, and MeOH. All three design schemes have obtained Approval in Principle (AiP) certification from DNV, and research continues. EEDI evaluation was used to analyze the effect of improvements in various power schemes.

The study ship was designed for a cruising range of 18,000 nm and a speed of 14.5 kn. Assuming that the range of low-carbon fuels is adjustable and controllable, different ranges were determined. A total of 19 groups of low-carbon fuel range design schemes were developed, and the difference in the low-carbon fuel range of each group of schemes was set to 1000 nm. The traditional MDO power scheme was used for a low-carbon fuel range of 0 nm.

Table 2. Design solutions for main engine selection.

Fuel	Main Engine Model	SMCR/NCR	Specific FOC	SPOC/SGC
LPG	MAN 6G70ME-C10.5 LGIP-HPSCR	15,128 kW × 74 rpm/13,615 kW × 74 rpm	157.3 g/kWh	6.34/140.1 g/kWh
LNG	MAN 6G70ME-C10.5-GI	14,675 kW × 74 rpm/13,208 kW × 74 rpm	161.9 g/kWh	2.93/130.7 g/kWh
MeOH	MAN 6G60ME-C10.5-LGIM-SCRBP	14,675 kW × 94.5 rpm/13,208 kW × 94.5 rpm	168.8 g/kWh	8.90/343.0 g/kWh

SMCR/NCR, specified maximum continuous rating/normal continuous rating; FOC, fuel oil consumption; SPOC/SGC, specific pilot oil consumption/gas consumption.

lists the specifications of the main dual-fuel engines selected for the different design options, based on AiP certified designs. Based on the data in Table 2, the fuel oil consumption, pilot oil requirement, and gas consumption for different dual-fuel power schemes were subsequently calculated. The SPOC is determined by SPOC/SGC item in Table 2. For example, in LPG applications, the consumption of 140.1 g of SGC requires 6.34 g of SPOC.

According to the guidelines for the calculation of EEDI for new ships [52], calculations for 19 groups of schemes were conducted. As shown in Equation (1), the EEDI is used as a design index to assess the energy efficiency of a ship, where the numerator represents the CO₂ emissions of the ship, and the denominator represents the ship’s total transportation capacity at the design speed.

$$\mathrm{EEDI}={\displaystyle \frac{{\mathrm{CO}}_2 \mathrm{emission} \mathrm{from} \mathrm{ship}}{\mathrm{Completed} \mathrm{work}}}={\displaystyle \frac{{\mathrm{CO}}_{2 \mathrm{Main} \mathrm{engine}}{+\mathrm{CO}}_{2 \mathrm{Auxiliary} \mathrm{engine}}- \mathrm{Reductions} \mathrm{by} \mathrm{different} \mathrm{energy} \mathrm{efficiency} \mathrm{techniques} }{Capacity\times V_{ref}}},$$

(1)

where Capacity is the deadweight tonnage (in tons, t) and V_ref is the ship speed (knots, kn).

The EEDI calculation for the dual-fuel power schemes is outlined in Figure 3, and the specific calculations are shown in

Table 3. EEDI values of 19 groups of dual-fuel power schemes.

Low-Carbon Fuel Range (nm)	Dual Fuel	Primary Fuel or Not (Y/N)	EEDI Value	Low-Carbon Fuel Range (nm)	Dual Fuel	Primary Fuel or Not (Y/N)	EEDI Value
0	LPG–MDO	N	2.774	10,000	LPG–MDO	N	2.666
	LNG–MDO				LNG–MDO		2.418
	MeOH–MDO				MeOH–MDO		2.724
1000	LPG–MDO	N	2.838	11,000	LPG–MDO	N	2.647
	LNG–MDO		2.738		LNG–MDO	Y	2.032
	MeOH–MDO		2.769		MeOH–MDO	Y	2.694
2000	LPG–MDO	N	2.819	12,000	LPG–MDO	Y	2.463
	LNG–MDO		2.702		LNG–MDO		2.030
	MeOH–MDO		2.764		MeOH–MDO		2.691
3000	LPG–MDO	N	2.800	13,000	LPG–MDO	Y	2.461
	LNG–MDO		2.667		LNG–MDO		2.028
	MeOH–MDO		2.759		MeOH–MDO		2.688
4000	LPG–MDO	N	2.780	14,000	LPG–MDO	Y	2.458
	LNG–MDO		2.631		LNG–MDO		2.026
	MeOH–MDO		2.754		MeOH–MDO		2.686
5000	LPG–MDO	N	2.761	15,000	LPG–MDO	Y	2.456
	LNG–MDO		2.595		LNG–MDO		2.024
	MeOH–MDO		2.749		MeOH–MDO		2.683
6000	LPG–MDO	N	2.742	16,000	LPG–MDO	Y	2.453
	LNG–MDO		2.560		LNG–MDO		2.022
	MeOH–MDO		2.744		MeOH–MDO		2.681
7000	LPG–MDO	N	2.723	17,000	LPG–MDO	Y	2.451
	LNG–MDO		2.524		LNG–MDO		2.020
	MeOH–MDO		2.739		MeOH–MDO		2.678
8000	LPG–MDO	N	2.704	18,000	LPG–MDO	Y	2.449
	LNG–MDO		2.489		LNG–MDO		2.018
	MeOH–MDO		2.734		MeOH–MDO		2.675
9000	LPG–MDO	N	2.685
	LNG–MDO		2.453
	MeOH–MDO		2.729

. Table 3 shows a significant decrease in the LNG-MDO fuel type as the low-carbon fuel range passes from 10,000 nm to 11,000 nm. Meanwhile, the same phenomenon of MeOH-MDO and LPG-MDO occurred when the low-carbon fuel range passed from 10,000 nm to 11,000 nm and 11,000 nm to 12,000 nm, respectively. The main reason is that when the low-carbon fuel range exceeds the abovementioned ranges, the low-carbon fuels are considered primary fuels, due to the f_DFgas is greater than 0.5 in these scenarios according to Figure 3. Therefore, the calculation of the EEDI only considers the fuel consumption of both SPOC and low-carbon fuels, no longer considering the impact of conventional fuel oil on carbon emissions.

To further compare and analyze the optimization differences in different dual-fuel power schemes, the enhancement in the EEDI reduction factor is considered. The EEDI reduction factor is defined as the percentage change in the EEDI value achieved by the designed ship relative to the baseline EEDI value of the ship, as shown in Equation (2). Furthermore, the improvement of EEDI reduction factor is defined as the difference in the change in the reduction coefficient before and after optimization.

$$\mathrm{EEDI} \mathrm{reduction} \mathrm{factor}={\displaystyle \frac{\mathrm{Baseline} \mathrm{EEDI}-{\mathrm{EEDI}}_{attained}}{\mathrm{Baseline} \mathrm{EEDI}}},$$

(2)

Figure 4 compares the improvement effects of the EEDI reduction factor when the low-carbon fuel was used as non-primary and primary fuel. The low-carbon fuel range of 10,000 nm and 18,000 nm were selected for the comparison. This selection was based on the solution with the most substantial EEDI reduction effect. Utilizing low-carbon fuels, whether non-primary or primary, directly decreases the ship’s carbon emissions. When using low-carbon fuels as non-primary fuels, the larger the range proportion of low-carbon fuels, the lower EEDI value. The LNG–MDO combination improved the EEDI reduction factor by 9.97% compared with the use of the MDO solution, outperforming the LPG–MDO and MeOH–MDO solutions by 3 and 7 times, respectively. When low-carbon fuels were used as primary fuels, the LNG–MDO solution improved the EEDI reduction factor by 20.80% compared with the use of the MDO solution, significantly outperforming the other two solutions. In addition, among low-carbon alternative fuels, whether used as non-primary or primary fuels, LNG has the highest EEDI reduction factor, followed by LPG and MeOH, respectively.

2.3. Research Approach

In Section 2.2, different dual-fuel power options based on the EEDI were analyzed and discussed. The EEDI represents the CO₂ emissions per ton of transported cargo, which reflects the level of carbon emissions during ship operations. However, the results of prior studies showed that the EEDI evaluation technique does not consider the carbon emissions generated by the fuel during the production or transportation phases nor the issue of methane escape in the LNG power scenario. Meanwhile, when using low-carbon fuels as the primary fuel, there are some limitations and shortcomings in EEDI evaluation, especially the impact of the range of low-carbon fuels on carbon emissions reduction which cannot be quantitatively evaluated. Therefore, in this study, the N786 150,000-ton shuttle tanker was used as the research object and LCA was adopted to further study the three types of dual-fuel power scenarios in Section 2.2. The results were compared and analyzed using the EEDI assessment method.

In the marine sector, with a particular focus on marine fuel modules, LCA is used to assess the GHG emissions of the entire fuel chain involved in ship operations (“well-to-wake”, as shown in Figure 5). From the initial production phase of the fuel, through extraction and cultivation, to processing and refining, transportation, storage, and ultimately use on board. Depending on the state of fuel bunkering, the whole process is mainly divided into the “well-to-tank” part and “tank-to-wake” part [36]. The environmental impacts of the well-to-tank fuel phase are influenced by several conditions such as mining, refining, and purification. The internationally dominant LCA-related Ecoinvent 3 database, which provides pre-combustion environmental impact emission factors, such as the mining of fuel products, was used to evaluate the environmental impacts in the well-to-tank phase. However, the database is not suitable for the combustion phase of the fuel [34,35,46], whereas the Intergovernmental Panel on Climate Change (IPCC) database provides detailed emission factors for the combustion phase (“tank-to-wake”) of the fuel. The database was created and maintained by hundreds of experts and is supported by multiple corporate and non-profit organizations worldwide. The high credibility of the data provides an excellent solution for obtaining emission factors during the fuel combustion phase. Note that emission factors for the fuel combustion phase can also be obtained from actual main engine emissions tests or other databases. However, the Ecoinvent 3 database is also a comprehensive integration database of multiple data sources and methods, rather than being based entirely on actual testing of experimental methods. To ensure consistency in research methodology more effectively, the authoritative IPCC database was selected for the fuel combustion phase. Therefore, these two databases were used for LCA in this study and the IPCC method was used for environmental impact assessment.

Figure 6 shows the evaluation method used in this study for carbon emission levels of marine fuel. This method mainly covers the following three parts:

• Research preparation: Complete the data statistics of the target ship and develop corresponding dual-fuel power schemes based on the selection of the main engine.
• Environmental impact assessment: Conduct environmental impact assessments for different scenarios based on the Ecoinvent 3 database and IPCC database.
• Comparative analysis based on the EEDI and LCA: Considering the different focuses of the EEDI and LCA methods, the impact of different schemes on the results of both methods and the carbon emissions profile during the well-to-wake phase are considered.

From Figure 6, the method mainly combines the EEDI and LCA methods, aiming to overcome some limitations of the two individual methods. Meanwhile, a 150,000-ton shuttle tanker was selected as the study object and three groups of dual-fuel power design schemes (LNG–MDO, LPG–MDO, and MeOH–MDO) were developed to compare the effects of different dual-fuel power design options on the carbon control potential and environmental performance of ships.

3. LCA Simulation Result Analysis

3.1. Methodological Framework

The methodological framework of LCA was used in this study to evaluate and compare the environmental impact performance of three types of dual-fuel power schemes, that is, LNG–MDO, LPG–MDO, and MeOH–MDO, with that of conventional marine fuel (MDO), focusing on changes in the Global Warming Potential (GWP). The entire evaluation process strictly follows the standard procedures of LCA, including the definition of goal and scope, analysis of life cycle inventory, assessment of environmental impact, and interpretation of result. Furthermore, the obtained LCA results are compared with the prior EEDI results to explore the carbon control potentials of different dual-fuel power schemes. This will be more useful to fully reveal the differences in the actual effectiveness and potential of different dual-fuel power schemes in controlling carbon emissions.

3.2. Goal and Scope Definition

In the LCA simulations of this study, the main goal was to prioritize the comparative analysis of the environmental impact of three types of dual-fuel power scenarios, that is, LNG–MDO, LPG–MDO, and MeOH–MDO, for a total of 19 groups of low-carbon fuel range design scenarios used to replace the conventional marine fuel MDO over the full cycle range of marine fuels (“well-to-wake”). Both “well-to-tank” and “tank-to-wake” phases were included. The functional unit of the system is defined as the entire process from production to application of marine fuel, covering the processing and production of crude oil to the energy conversion of the combustion process in the operational phase. However, the fuel transportation process was given less consideration, as its environmental impact in the well-to-tank phase is relatively insignificant compared to raw material extraction and production processes. Although fuel transportation of great significance to LNG supply chains, relevant studies have shown that the transportation phase accounts for only 2% of total well-to-wake emissions for LNG [16,53]. The analysis primarily focused on the tank-to-wake phase to compare the results with those of a prior EEDI study. The overall system boundary framework applied to the LCA simulation is shown in Figure 7.

3.3. Life Cycle Inventory

The analysis of life cycle inventories is a key component that focuses on accurately quantifying all resource inputs and environmental outputs associated with the specific functional unit. In this study, a comprehensive and systematic comparison and analysis of the environmental impacts of marine fuels in all phases was achieved by examining in detail the data from the acquisition of raw materials up to the final application. Corresponding to the content of the 2024 LCA Guidelines developed by the IMO [36], both the “well-to-tank” and “tank-to-wake” phases of marine fuels were considered. The well-to-tank phase and fuel consumption derived from the 19 sets of dual-fuel power scenarios were used as inputs for the inventories. As previously mentioned, these inventories used the Ecoinvent 3 database provided by the LCA software SimaPro 9.6, which is one of the most comprehensive databases available. The tank-to-wake phase inventories utilized the IPCC results database [54,55,56,57]. This is because the emission factors for fuels in the application phase (i.e., fuel combustion phase) are not available in the current phase of Ecoinvent 3.10 database. The IPCC database, on the other hand, is regarded to be the authoritative repository of records on global climate change and includes a wealth of data on the rate of climate change, climate models, carbon emissions, and effects of climate change.

Table 4. Fuel properties provided by Ecoinvent 3.10 implemented in SimaPro 9.6 and IPCC emission factor database.

Item		Unit	MDO	LNG	LPG	MeOH (Biomass)	MeOH (NG)	MeOH (Coal)
Low Calorific Value (LCV)		MJ/kg	42.7	48.0	46.3	19.9
Density		kg/m3	900	450	610	790
Emission Factors	CO2	g/kg	3212	2750	3000	1375
	CH4	g/kg	0.1	14.5	1	0.2
	N2O	g/kg	0.0798	0.0245	0.1770	0.01
	well-to-tank	t CO2-eq	0.84	1.07	1.06	0.68	0.88	4.46
	tank-to-wake	t CO2-eq	3.24	3.16	3.07		1.38	1.38

shows the key properties of the fuels used in this study. The preparation of MeOH was relatively diverse compared with that of the other two. Based on the database and research trends, the three main production and preparation processes of biomass, natural gas (NG), and coal were considered for methanol in this study. In addition,

Table 5. Consumption of alternative and traditional fuels for different design solutions.

Fuel Consumption (t)					Fuel Consumption (t)
No.	Low-Carbon Fuel		Diesel Fuel	Pilot Oil	No.	Low-Carbon Fuel		Diesel Fuel	Pilot Oil
1	LPG	0	2899.46	0	11	LPG	1452.52	1288.65	66.15
	LNG					LNG	1431.24		29.65
	MeOH					MeOH	3472.43		90.07
2	LPG	145.25	2738.38	6.61	12	LPG	1597.77	1127.57	72.76
	LNG	143.12		2.97		LNG	1574.36		32.62
	MeOH	347.24		9.01		MeOH	3819.67		99.08
3	LPG	290.50	2577.29	13.23	13	LPG	1743.02	966.49	79.38
	LNG	286.25		5.93		LNG	1717.49		35.58
	MeOH	694.49		18.01		MeOH	4166.91		108.09
4	LPG	435.76	2416.21	19.84	14	LPG	1888.28	805.40	85.99
	LNG	429.37		8.90		LNG	1860.61		38.55
	MeOH	1041.73		27.02		MeOH	4514.16		117.10
5	LPG	581.01	2255.13	26.46	15	LPG	2033.53	644.32	92.60
	LNG	572.50		11.86		LNG	2003.73		41.52
	MeOH	1388.97		36.03		MeOH	4861.40		126.10
6	LPG	726.26	2094.05	33.07	16	LPG	2178.78	483.24	99.22
	LNG	715.62		14.83		LNG	2146.86		44.48
	MeOH	1736.21		45.04		MeOH	5208.64		135.11
7	LPG	871.51	1932.97	39.69	17	LPG	2324.03	322.16	105.83
	LNG	858.74		17.79		LNG	2289.98		47.45
	MeOH	2083.46		54.04		MeOH	5555.89		144.12
8	LPG	1016.76	1771.89	46.30	18	LPG	2469.28	161.08	112.45
	LNG	1001.87		20.76		LNG	2433.10		50.41
	MeOH	2430.70		63.05		MeOH	5903.13		153.13
9	LPG	1162.02	1610.81	52.92	19	LPG	2614.54	0	119.06
	LNG	1144.99		23.72		LNG	2576.23		53.38
	MeOH	2777.94		72.06		MeOH	6250.37		162.13
10	LPG	1307.27	1449.73	59.53
	LNG	1288.11		26.69
	MeOH	3125.19		81.07

lists the total consumption of low-carbon and conventional fuels for the 19 groups of dual-fuel power schemes. Based on the data provided in Table 4 and Table 5, the GWP values were calculated using the rule of “activity data times by carbon emission factors”.

3.4. Evaluation of Impact Assessment and Result Analysis

Converting the environmental loads of marine fuels in both the well-to-tank and tank-to-wake phases into specific quantitative indicators is essential for comprehensively and accurately assessing the actual impact of these phases on the environment. In other words, quantifying the environmental loads during the different phases is a key component in the scientific assessment of the environmental effects. The IPCC 2021 method on climate change, that is, GWP100 was adopted. The GWP100 is a measure of the intensity of the greenhouse effect of a GHG relative to carbon dioxide over a 100-year timeframe and is one of the more authoritative assessment techniques available. Based on the CO₂ equivalent values, the GHG emissions were converted to GWP values using the latest GWP coefficients listed in

Table 6. Global Warming Potential (GWP) values of major greenhouse gases.

Industrial Designation or Common Name	GWP Values for 100-Year Time Horizon: Fifth Assessment Report (AR5)
Carbon dioxide (CO2)	1
Methane (CH4)	28
Nitrous oxide (N2O)	265
CO2-eq	${\mathrm{gCO}}_2{\text{-}\mathrm{eq}}_{\left(100\mathrm{years}\right)}=1\times {\mathrm{gCO}}_2+28\times {\mathrm{gCH}}_4+265\times {\mathrm{gN}}_2\mathrm{O}$

[56]. Simultaneously, the numerical conversion was completed to obtain the carbon emissions of the ship during the operation phase based on the EEDI values of the 19 groups of dual-fuel power options listed in Table 3.

The carbon emission assessment results for the conventional MDO (zero low-carbon fuel range) and three dual-fuel power schemes (LNG–MDO, LPG–MDO, and MeOH–MDO) are shown in Figure 8, Figure 9 and Figure 10. The LCA results show that the GWP values of LNG–MDO, LPG–MDO, and MeOH (biomass)–MDO gradually decrease with increasing range, whereas the GWP values of MeOH (NG) and MeOH (coal) increase, which raises the load on the environment. The observed variations primarily result from the combined effects of the specific carbon emission factors associated with each low-carbon fuel and differentiated fuel/gas consumption patterns among the three dual-fuel power schemes. These mechanisms will be thoroughly examined in the Discussion section. In contrast, using CO₂ emissions as a benchmark, the EEDI shows a “broken” trend based on the low-carbon fuel range. The main reason is that when the low-carbon fuel is considered as a primary fuel, the calculation of the EEDI only considers the fuel consumption of both SPOC and low-carbon fuels no longer considering the impact of conventional fuel oil on carbon emissions. Generally, when low-carbon fuels are used as non-primary fuels (i.e., LNG, MeOH < 11,000 nm; LPG < 12,000 nm), the range and CO₂ emissions exhibit a positive correlation, with emissions decreasing with increasing range. When low-carbon fuels are used as primary fuels (i.e., LNG, MeOH ≥ 11,000 nm; LPG ≥ 12,000 nm), the EEDI cannot quantify the correlation between CO₂ emissions and range. Further comparative analysis revealed that, compared with the EEDI evaluation method, LCA quantitatively analyzes the changes in the GWP values of the well-to-tank and tank-to-wake phases simultaneously, which reflects the environmental impacts of GHGs. The EEDI can only quantitatively analyze CO₂ emissions during the tank-to-wake phase and does not include low-carbon fuels as the main fuel. In addition, the analysis of the GWP composition ratios of marine fuels under different substitution scenarios shows that CO₂ contributes the most, followed by CH₄ and, finally, N₂O (Figure 11). Other GHG contributions, which are very small and have little influence on GHG effects, were not considered here.

4. Discussion

In this section, the previous research results of EEDI and LCA simulations were combined to further analyze and discuss the inventory inputs, carbon emissions, and life-cycle impacts of the research process. To determine the current applicable dual-fuel power scheme, three aspects were considered: the effects of emission factors on GWP, the carbon control potential of different alternative schemes, and the prospect analysis of low-carbon alternative fuels.

4.1. Effects of Emission Factors on GWP

Emission factors directly affect the estimation of GHG emissions, which represents a significant influence on the results of GWP calculations. Specifically, a higher factor means that producing the same amount of energy will release more GHG emissions, resulting in a higher GWP. The focus was placed on the full well-to-wake cycle of marine fuels including both the well-to-tank and tank-to-wake phases. The emission factors of the different marine fuels throughout the well-to-wake cycle are shown in Figure 12. Overall, the effects of the emission factors on the GWP can be ranked from smallest to largest as follows: MeOH (biomass) < MeOH (NG) < MDO < LPG < LNG < MeOH (coal). The integrated well-to-tank process for marine fuel oil covers the fuel process of crude oil procurement, subsequent processing, and production. In this phase, the effects of the emission factors on the GWP can be ranked from smallest to largest as follows: MeOH (biomass) < MDO < MeOH (NG) < LPG < LNG < MeOH (coal). For the tank-to-wake phase, which considers the use of marine fuels (i.e., process of fuel combustion), the emission factors can be ranked from smallest to largest as follows: MeOH (biomass) < MeOH (NG) = MeOH (coal) < LPG < LNG < MDO. MeOH (biomass) is a type of “renewable carbon dioxide”. Although carbon dioxide is produced during subsequent methanol combustion, carbon emissions are captured in a loop; therefore, the loop is closed and carbon emissions are zero [58]. In addition, methane escape during this phase was considered. Data from the IPCC database were used. The methane escape ratios were generally similar to the results of actual dual-fuel mainframe emission tests [59]. The conventional methane slip ratio of 2% was prioritized. When the escape ratio was reduced, the LNG emission factor was further reduced. Based on analyzing the effect of the emission factors on the GWP, MeOH (biomass) is the most environmentally friendly alternative fuel for marine use. However, because of the effect of the low calorific value of the fuel (Table 4), the lower the calorific value of the fuel is, the less energy is supplied and more consumption is required for the same number of nautical miles of operation. Therefore, low-carbon fuels require further comparative analysis. The carbon emission factor in the EEDI calculation mainly considers the carbon emission during fuel combustion. However, through the analysis of this subsection, it can be seen that the impact of the extraction, refining, and production stages of fuel cannot be ignored, and it also shows the improvement direction of the carbon factor in carbon emission accounting of the existing EEDI and other relevant regulations so as to more comprehensively evaluate the carbon control potential of ships from the perspective of the whole life cycle of fuel.

4.2. Carbon Control Potential of Different Alternative Schemes

To further compare and analyze the carbon control potential of different low-carbon fuel alternatives, the results of the EEDI and LCA simulations of the conventional MDO power scheme were used as a baseline. Two typical working conditions of low-carbon fuels, that is, non-primary fuels (with a range of 10,000 nm) and primary fuels (with a range of 18,000 nm), were selected for further discussion, as shown in Figure 13 and Figure 14.

Figure 13 shows the carbon emission assessment results for low-carbon fuels as non-primary fuels under different alternative schemes. The results show that the application of LNG, LPG, and MeOH reduced the carbon emissions by ~12.85% (993.86 t CO₂), 3.89% (301.33 t CO₂), and 1.81% (139.79 t CO₂), respectively, compared to the EEDI results of the traditional MDO case. Based on the LCA simulation results, compared to the traditional MDO case, the application of LNG, LPG, and MeOH (biomass) reduced the GWP by ~3.36% (397.01 t CO₂-eq), 2.56% (303.31 t CO₂-eq), and 32.49% (3843.38 t CO₂-eq), respectively. In contrast, the application of MeOH (NG) and MeOH (coal) increased the GWP by ~13.89% (1643.06 t CO₂-eq) and 118.97% (14,074.35 t CO₂-eq), respectively.

Similarly, Figure 14 shows the carbon emission assessment results for low-carbon fuels as primary fuel. The EEDI results shows that compared to the traditional MDO case, the application of LNG, LPG, and MeOH reduced the carbon emissions by ~27.24% (2107.84 t CO₂), 11.73% (907.38 t CO₂), and 3.56% (275.06 t CO₂), respectively. The LCA simulation results shows that compared to MDO case, the application of LNG, LPG, and MeOH (biomass) reduced the GWP by ~6.04% (714.56 t CO₂-eq), 4.62% (545.99 t CO₂-eq), and 58.48% (6918.05 t CO₂-eq), respectively. The application of MeOH (NG) and MeOH (coal) increased the GWP by ~25.00% (2957.53 t CO₂-eq) and 214.15% (25,333.87 t CO₂-eq), respectively. Regardless of the use of low-carbon fuel as non-primary or primary fuel, the application of LNG, LPG, and MeOH (biomass) has great potential for carbon control, reducing carbon emissions while minimizing the effects of GHGs on the environment. The results indicate that MeOH (biomass) is optimal, followed by LNG and LPG. In addition, MeOH (NG) and MeOH (coal) lead to a greater environmental load because of the production process.

4.3. Prospect Analysis of Low-Carbon Alternative Fuels

In Section 4.2, the carbon control potential of different fuel alternatives is discussed. Overall, based on the analysis of the evaluation results, MeOH (biomass) has a much better carbon control potential than LNG and LPG; however, many challenges remain. Fuel calorific value, fuel volume, and carbon emission factors are key in determining the total quantity of carbon emission. As mentioned before, MeOH has the lowest carbon emission factor during the operational phase among the evaluated alternatives, accounting for approximately about 43% and 44% of the LNG and LPG factors, respectively (excluding MeOH (biomass)). However, due to its smaller LCVs, MeOH may impact payload capacity. In contrast, LNG and LPG have higher LCVs, approximately 2.4 times that of MeOH, combined with their lower density and moderate emission factors, resulting in a smaller impact on payload. Therefore, under the same design conditions, MeOH consumption is higher, requiring more volume for fuel storage to meet the power demands of vessel operations. Additionally, the current state of raw materials poses certain challenges for MeOH production. Figure 15 shows the proportion of feedstock and resources used to generate global MeOH. Currently, the main sources of MeOH include natural gas, coal, and coke oven gas [58]. All of these raw materials need to go through complex processes and strict treatment steps in the production process to finally obtain qualified MeOH products. At the same time, insufficient supply of raw materials and complex production processes are also the main problems faced by current MeOH production. Therefore, measures are needed to improve the feasibility and economy of MeOH (biomass) production such as developing new biomass resources and optimizing pretreatment and conversion technologies [60,61,62].

The results of this study show that the carbon control potential of LNG and LPG is second only to that of MeOH (biomass). LNG is increasingly favored as a low-carbon fuel that is easy to obtain and cost competitive. Its popularity is due to a number of significant advantages [63,64]. For example, LNG has a very high thermal efficiency, which makes the energy conversion process more efficient and helps reduce energy consumption. Secondly, LNG produces far fewer NOx during combustion than conventional fuels, which has a significant effect on improving air quality and reducing environmental pollution. Furthermore, its lower risk of bursting brings higher safety to the transportation and usage process, reducing the occurrence of accidents. In addition, the high compression ratio of LNG means that more energy can be stored in the same amount of storage space, improving the economy of storage and transportation. Related studies suggest that more NG reserves may be discovered in the future, which will further contribute to lowering the cost of LNG. Note that potential emission taxes could increase the interest in LNG [23,65].

Although LPG is widely recognized globally as a viable alternative fuel option for all types of vessels operating on local voyages and international routes, its popularity in practical applications in the maritime sector has not yet met widespread expectations [66]. The main reasons can be summarized as follows [67]: when LPG is used, ships must be refueled within a few weeks, which is inconvenient for long sea voyages and operations; despite the fact that the global infrastructure for LPG exhibits a high degree of completeness and is in good condition, this guarantees the safe and efficient storage and transportation of LPG. However, it should not be overlooked that current production capacity and supply facilities are relatively limited. This limitation is not only reflected in the fact that the expansion of production capacity may not be able to keep up with the rapid growth of market demand, but also in the lack of flexibility and scalability of the existing supply facilities in responding to sudden demand or large-scale applications. In addition, compared with LNG, LPG faces more serious challenges in terms of safety. Due to the higher density of LPG compared to air, once a leak occurs, the gas often spreads along the ground and is difficult to detect in a timely manner, greatly increasing the potential risk of accidents. Meanwhile, considering the low flash point temperature of LPG, it is extremely easy for it to burn rapidly once it encounters an ignition source or high temperature, thus triggering a fire or even a more serious explosion.

The world’s famous shipping company Maersk previously led the charge in making methanol fuel an important development strategy, and more recently appears to be shifting toward LNG as a marine fuel. This may be due to the dwindling supply and high cost of green methanol compared with LNG based on energy density and extraction costs. Therefore, considering the carbon emission reduction effect based on the GWP of fuels from well-to-wake and cost issues, etc., LNG remains a good choice for the current low-carbon fuel transition phase. However, with the gradual maturity of biofuel preparation technology and the gradual expansion of biofuel production capacity, the optimal choice for the distant future is the promotion of green fuels, such as bio-methanol, which can help achieve net-zero emissions.

5. Conclusions

The shipping industry must adopt measures to achieve the ambitious goals of the IMO’s strategy for reducing GHG emissions. In this study, a 154,000-ton shuttle tanker was used as the research object, and the design of three types of mainstream dual-fuel power schemes, that is, LNG–MDO, LPG–MDO, and MeOH–MDO, was prioritized. An in-depth analysis was focused on the carbon emissions profile and GWP of different dual-fuel power generation scenarios, providing a more environmentally friendly dual-fuel power solution for the main engine of ships. Furthermore, the results provide an effective analytical argument for the development and revision of the corresponding key IMO guidelines, which is of great significance for the decision-making process of the IMO in regulating and guiding the sustainable development of the global shipping industry.

Developed by the IMO, the EEDI standard mainly quantifies the carbon emissions of marine fuel during the operational phase, and the carbon emissions generated during fuel production cannot be considered. However, especially for certain fuels such as MeOH (coal) in this study, the emission factors during the production phase are much higher than those during the combustion phase, resulting in more carbon emissions. This reveals the limitation of the EEDI of only considering carbon emissions during the operational phase. Meanwhile, when low-carbon fuels are used as the primary fuel, the EEDI cannot accurately quantify their impact on carbon emission reduction. In contrast, LCA can be applied to evaluate the carbon control potential of low-carbon fuels quantitatively throughout the well-to-wake phase while also considering the environmental impact of additional GHGs. When benchmarked against conventional MDO’s EEDI, LNG can achieve up to approximately 27.24% reduction in carbon emission potential. Comparatively, when evaluated relative to conventional MDO’s GWP, MeOH biomass demonstrates a maximum decarbonization potential of about 58.48%. Therefore, with the increasing implementation of carbon control policies in the shipbuilding and dismantling phases in various countries, the carbon emission analysis model considering the full life cycle will have a significantly important role. In addition, because of the fundamental similarities between the EEXI and EEDI in terms of calculation formulae and assessment objectives, the methodological ideas and analyses of this study are equally applicable to existing ships (i.e., ships under the EEXI framework). For small vessels such as ferries and yachts that fall outside IMO regulatory jurisdiction, this study methodology can still be effectively applied to assess their environmental impacts through LCA, without being limited by ship type.

The main research results provide basic support for the revision of carbon emission factors in the relevant regulations of the shipping industry, such as the EEDI, that is to say, not only the carbon emissions in the combustion stage of fuel oil, but also the carbon emissions in the extraction, refining, and production stages of fuel oil should be considered, especially in the important period of transformation from traditional fuels to low-carbon and zero-carbon fuels. However, this study has certain limitations. First, although LCA was conducted for the three types of low-carbon fuels, zero-carbon fuels and their manufacturing processes were not considered. Second, although emission factor data provided by the internationally authoritative IPCC database were used, actual main engine emission tests were not conducted; thus, the conclusions of the comparative analysis may contain discrepancies. Therefore, zero-carbon fuels should be considered, and the reliability of the method provided in this study should be verified experimentally.