A Comprehensive Multicriteria Evaluation Approach for Alternative Marine Fuels

Abstract

In the last decade, shipping decarbonization has accelerated rapidly in response to the regulatory framework. Shifting toward alternative marine fuel options is the subject of extensive study from stakeholders and researchers. This study attempts to propose a decision support model for alternative fuel evaluation. The decision-making process is multidimensional, comprising economic, technical, environmental, and social aspects, and has been carried out with the aid of the outranking multicriteria methodology, Promethee II. The approach is based on a comprehensive list of 11 criteria and 25 sub-criteria, covering all the crucial aspects. The weighting criteria process postulates the viewpoints of six stakeholder categories, including all the stakeholders’ preferences: shipowners, fuel suppliers, industry and engine manufacturers, academics, banks and the public. The results demonstrated that although LNG, MGO and HFO are classified in the highest positions, there are renewable options that also appear in high-ranking positions in most categories and especially among academics, banks, the public and in the combined case scenario. The commercially available options of drop-in biofuels, bio and e-LNG, fossil and bio methanol were ranked in these high positions. This approach offers insight into the assessment and selection of alternative marine fuel options, providing an incentive for strategic planning.

1. Introduction

Maritime transportation constitutes a crucial part of the transportation sector and the global economy, with an over 80% share of the volume of commodities transported by sea [1]. In 2018, 2.89% of the global anthropogenic CO₂ emissions was due to emissions from the shipping sector [2]. International shipping emits 70% of global shipping energy emissions and, supposing that it was a state, it would have the sixth or seventh largest CO₂ emissions [3]. The International Maritime Organisation (IMO) has proposed ambitious strategies to decrease GHG emissions from international shipping during this century [2]. Initially, the first strategy, Resolution MEPC.304(72), included initial targets to shrink CO₂ emissions per transport work, with a minimum 40% and 70% until 2030 and 2050, respectively, based on emissions from 2008. Furthermore, GHG emissions have to be reduced by at least 50% by 2050 compared to 2008 [4]. Shipping GHG emissions are going to be increased between 50% and 250% by 2050 (compared to 2008 levels) if no actions are taken [3]. In July 2023, the IMO adopted the IMO GHG Strategy from Ships (Resolution MEPC.377(80)), which includes the use of zero/near-zero GHG emission technologies, fuels and energy sources for a minimum 5–10% of the energy used by international shipping until 2030. GHG emissions from international shipping should become net zero by or around 2050. Furthermore, the current Strategy recalls the 2018 Strategy and might be replaced in the future from a revised IMO GHG Strategy in 2028 [5].

Additionally, the European Commission (EC), in 2021, introduced the ‘Fit for 55′ legislative package, mandating a reduction of 55% of GHG emissions by 2030. The shipping sector is also affected by this European package, as it will be included in the European emission trading system (EU ETS), applying to all vessels exceeding 5.000 gross tonnage (GT) and covering 100% of intra-European Economic Community (EEC) emissions as well as 50% of extra-EEC emissions. The FuelEU Maritime Proposal set a stepwise limit for reducing the carbon content of the maritime fuel, and the European energy taxation directive (EU ETD) also set a gradually increasing minimum tax for maritime fuels [6,7,8,9].

A shift to low- or zero-carbon fuels seems to be mandatory to comply with the above-mentioned IMO and EU targets. The decarbonization of the shipping sector encompasses a range of possible and innovative alternative technical and operational measures. Liquified Natural Gas (LNG), methanol, biofuels, hydrogen, ammonia and electricity are discussed worldwide as promising alternative fuel options. Each marine fuel option has to face its own specific challenges for its adoption. New buildings will accelerate the compliance process.

In response to the legislation requirements, there has been a growing trend in the literature to compare and evaluate the alternative fuel options. Ampah et al. reviewed 583 papers, published between 2000 and 2020 in the field of alternative marine fuels, demonstrating the growth of interest in the field. One of the research gaps, they concluded, is that most of their examined studies only considered the effect on emission reductions from their proposed measures. The types of proposed measures mainly covered technical aspects, such as hull design, power and propulsion system, energy sources and operational optimization [10]. However, fuel option selection requires a multidimensional approach and appropriate tools of evaluation to aid in decision making. Although multicriteria decision-making techniques have been popular over decades in the field of supplier selection, there are only a few research studies employing MCDM methods for alternative fuel selection, considering economic, technical, environmental and social aspects. Furthermore, only a part of these published studies (Deniz and Zincir [11], Hansson et al. [12,13], Mandic et al. [14] and Yang et al. [15]) included the alternative fuels of methanol, hydrogen, ammonia, bio and e-fuels in their assessment. Thus, the development of a comprehensive integrated evaluation framework that aligns with independent shipping stakeholders seems to be a demanding challenge.

The aim of the current paper is to present an in-depth evaluation process of alternative marine fuels by identifying a variety of criteria. This approach assesses 16 alternative fuel options, also taking into consideration the current fossil fuels, HFO (Heavy Fuel Oil) and MGO (Marine Gas Oil) and adopting a set of 25 significant and coherent key parameters. As the fuel selection problem has a multidimensional nature, a multicriteria analysis was assumed for the assessment process, covering the economical, technical, environmental and social aspects of the problem.

The rest of this paper is organized as follows: Section 2 presents the examined alternative marine fuels. Section 3 displays a literature review in the field and a brief presentation of the proposed methodology, including the used multicriteria methodology and the weighting criteria process. Section 4 analyzes the criteria used and determines their values, while in Section 5, the results from the implementation of the evaluation methodology are presented, followed by Section 6 with this research’s conclusions.

2. Alternative Marine Fuels

Globally, there are several potential marine fuel options as viable solutions to oil-based fuels to aid the shipping industry in achieving the future emission reduction targets. In this study, the examined marine fuels are divided into three categories: (a) commercially available fuel options, (b) fuels in the demonstration phase and (c) fuels under development. HFO and MGO are used as baseline options, given that they are the current dominant fuels in international shipping. A brief general description of the examined alternative fuels is given below:

Liquified Natural Gas (LNG) is the most prolific and commercially available fuel. The main energy source for LNG is natural gas, composed of methane, liquefied at −162 °C, at atmospheric pressure. As a renewable replacement for LNG, bio-LNG has a much lower carbon footprint than other fossil fuels or biofuels. It is made by processing organic waste, such as animal waste or municipal waste. LNG could also be produced synthetically with the power to gas process. This process includes hydrogen production from water, using a renewable electricity source (wind, solar or other option) or it can be processed into methane by adding non-fossil carbon dioxide obtained from carbon capture. E-LNG is interchangeable with LNG and is able to be utilized in existing infrastructure. Nearly all the current LNG production is from natural gas [16].

Methanol is mainly used to produce chemicals, like formaldehyde, plastics, and acetic acid. It is produced from carbon sources, such as natural gas, coal, biomass, and even CO₂. About 65% of methanol production is currently based on natural gas reformation (grey methanol), while the rest (35%) is largely based on coal gasification (brown methanol) [17]. In this study, grey and brown methanol is referred to as fossil methanol. Blue methanol is produced using blue hydrogen in combination with carbon capture technology. Biomass feedstocks, such as forestry and agricultural waste or biogas from landfill and municipal solid waste, can be used as the raw materials for biomethanol production. Green e-methanol is obtained with hydrogen production from renewable electricity sources or with the carbon capture process. It has the advantage of being liquid in ambient conditions and so there is no need for refrigeration or pressurization for transport and storage. Its bunkering process is similar to HFO, and only minor modifications are necessary to existing infrastructure, being already available in some ports [17].

Biofuels can be made from a variety of feedstocks and can be used as drop-in fuels with minimal alterations to the existing equipment. In some cases, an alternative fuel may not be useable in its 100% pure form and may require ‘blending’ to produce a drop-in solution [16]. Advanced biofuels are produced from specific feedstocks with no indirect land use change (ILUC). In this study, the examined biofuels are as follows: HTL (hydrothermal liquefaction) fuel oil, pyrolysis fuel oil, HVO and FAME and their respective feedstocks of energy crops, lignocellulosic biomass, oil crops, waste oils and fats. Although crop-based feedstocks, like palm oil and soybean, are widely available, their use in Europe is limited due to the policy of ILUC. Lignocellulosic biomass, such as forestry and agricultural residues as well as woody and grassy energy crops, seems to have greater future potential, whereas waste oils and fats in the maritime sector have to face competition from other transport sectors. Biofuels’ sustainability depends mostly on the type of feedstock [16]. Hydrothermal liquefaction (HTL) is direct thermochemical conversion of wet biomass into bio-crude at 300–350 °C and 10–25 MPa. Pyrolysis is a thermochemical conversion of biomass to bio-oil. HVO (Hydrotreated Vegetable Oil) is produced through a hydrotreating process, also called hydroprocessing, in order to remove sulphur, oxygen, and nitrogen. Fatty Acid Methyl Ester (FAME), known as biodiesel, is a prevalent biofuel in the EU. Nowadays, waste, used cooking oils and animal fats are the main feedstocks for FAME, and transesterification is the used chemical process [18].

Hydrogen can be produced from both fossil and renewable sources. Each year, almost 95% of global hydrogen production comes from gas and coal (grey hydrogen) [16]. Green hydrogen is hydrogen through water electrolysis fueled by renewable-based electricity. Hydrogen can also be produced from biomass as a biofuel. The production process refers to the steam reformation of methane (biogas) obtained from the anaerobic digestion of organic waste. The choice of an alternative production pathway for hydrogen is determined to a large extent by the local availability of the energy source [16]. It can be utilized in internal combustion engines or fuel cells. Experience from LNG in shipping could be useful, given the similarity of the known technology of cryogenic conditions. A key barrier to the liquefaction of hydrogen is the low temperature needed, −253 °C. Hydrogen facilities have to increase approximately more than 220-times to reach the current LNG facilities, becoming widespread in world trade [19].

Ammonia constitutes the basic product in chemical industries and especially in the production of fertilizers. Although it is a carbon-free fuel, its application is currently limited. It has the advantage of being used directly as marine fuel or as a hydrogen carrier (ammonia is converted back to hydrogen for combustion) [20]. Ammonia can be produced through three pathways based on the energy source used: natural gas (fossil grey or blue ammonia), renewable ammonia taking advantage of solar photovoltaics and wind (green ammonia) and from residual biomass and municipal waste (bio-ammonia) [16]. Renewable ammonia is chemically identical to fossil-based ammonia. It can be characterized as a versatile fuel as it can be stored in liquid form at atmospheric pressure at −33 °C or at ambient temperature and at least 8 bar and can be used in internal combustion engines, gas turbines and fuel cells [21]. The conversion of the existing ammonia tankers to ammonia-fueled ships could be applicable in the short term, as the issue of fuel availability from ports ceases to exist. Fossil-based ammonia will perform a transitional role as a short-term solution in decarbonization, whereas renewable NH₃ is predicted to have a dominant role in future markets. Although renewable ammonia is able to displace conventional fuels, its use can increase nitrogen oxide emissions, NO_X, nitrous oxide and N₂O, and an aftertreatment technology is obligatory. Furthermore, ammonia is a hazardous toxic chemical [21].

3. Materials and Methods

3.1. Literature Overview

Currently, the shipping industry has a number of possible low- and zero-carbon marine fuels available to meet IMO 2030 and 2050 emission reduction goals. The selection of each fuel option has its own special characteristics, composing a multicriteria decision-making problem with a finite set of criteria comparing stakeholders’ priorities [22]. Thus, the comparative evaluation of fuel options needs a rigorous decision support framework able to incorporate the different preferences of the stakeholder groups. Multicriteria methods can solve problems with conflicting and multiple objectives, expressed by the decision makers and stakeholders.

The topic of making marine fuels greener has been investigated by various researchers. Studies are mainly divided into investigations of the feasibility of alternative technologies and literature reviews mapping the research in respective domains. As a preliminary step, an extensive literature review was implemented. Although there are numerous studies in the field of shipping decarbonization, the number of publications dealing with the multicriteria evaluation of marine fuels is only twelve, as shown in

Table 1. Literature review—multicriteria applications.

	Authors	Method	Evaluation Alternatives
1	Ren J. and Lützen M., 2015 [23]	Fuzzy AHP and VIKOR	Low sulphur fuel, Scrubber and LNG
2	Deniz C. and Zincir B., 2016 [11]	AHP	Methanol, Ethanol, LNG and Hydrogen.
3	Ren J. and Liang H., 2017 [24]	Fuzzy logarithmic least squares and fuzzy TOPSIS	Methanol, LNG and Hydrogen
4	Ren J. and Lützen M., 2017 [22]	Dempster-Shafer theory and a trapezoidal fuzzy AHP	LNG, Nuclear and Wind power
5	Hansson J. et al., 2019 [12]	AHP	LNG, LBG, Methanol from NG, Renewable methanol, Hydrogen, HVO and HFO
6	Kim A.R. and Seo Y.-J., 2019 [25]	Fuzzy AHP	Low sulphur fuels, Scrubbers and LNG
7	Hansson J. et al., 2020 [13]	AHP	NG-NH3, Elec-NH3, LNG
8	Luciana (Marcu) Τ.A. et al., 2021 [26]	AHP	LNG and oil gas
9	Mandic Ν. et al., 2021 [14]	AHP and SAW	Biofuels, LNG, Hydrogen, LPG, Batteries
10	Carvalho F. et al., 2021 [27]	Qualitative analysis	Alternative fuels’ production pathways
11	Moshiul A.M. et al., 2023 [28]	TOPSIS	Criteria assessment
12	Yang Z. et al., 2023 [15]	AHP and q-ROLPBM (q-Rung Orthopair Linguistic Partition Bonferroni mean)	E-fuel, Solar fuel, Biofuel, E-biofuel

. Their ranking results are utilized as a base for comparison with the obtained results from this study.

An overview of the paper’s content is summarized below:

Ren J. and Lützen M., 2015 [23], combined Fuzzy AHP and VIKOR to validate three alternative technologies (low-sulphur fuel, scrubber, and LNG), resulting in LNG as the most viable option for long-term use.

Deniz C. and Zincir B., 2016 [11], qualitatively compared methanol, ethanol, LNG and hydrogen with eleven criteria using the AHP methodology, based on given points from five experts. According to their assessment process, LNG was placed in front of the three examined alternative fuel options, followed by hydrogen and closing with methanol and ethanol.

Ren J. and Liang H., 2017 [24], applied fuzzy logarithmic least squares for the weights’ calculation and fuzzy TOPSIS for the assessment of three alternative marine fuels, methanol, LNG and hydrogen, taking into consideration 11 criteria (including environmental, economic, technological and social). This resulted in a similar classification to that of Deniz and Zincir (2016) [11].

Ren J. and Lützen M., 2017 [22], combined Dempster–Shafer theory and a trapezoidal fuzzy AHP for the sustainability assessment of nuclear power, LNG and wind energy as possible energy resources for shipping, prioritizing nuclear power as a sustainable alternative for shipping.

Hansson J. et al., 2019 [12], assessed seven alternate marine fuels (LBG, fossil and renewable methanol, fossil and electric hydrogen, HVO and HFO with scrubbers). The alternative fuels were compared through pairwise comparisons with regard to four main categories of criteria, economic, technical, environmental and social, and 10 sub-criteria, based on the preferences of Swedish stakeholders. LNG and HFO were classified at the highest levels, followed by fossil methanol and biofuels. Meanwhile, the evaluation based on the Swedish government expressed priority in renewable marine fuels, renewable hydrogen and renewable methanol, whereas, in 2020, Hansson J. et al. [13] attempted to evaluate the prospects of ammonia compared to LNG, MGO, hydrogen, HVO, LBG and methanol, including 10 criteria and using AHP. The weights of the criteria were retrieved from shipowners, fuel producers, engine manufacturers and Swedish government authorities. They observed that ammonia has restricted potential for large-scale applications, as issues remain to be solved.

Kim A.R. and Seo Y.-J., 2019 [25], used fuzzy AHP to evaluate three existing alternatives for emission reductions, low-sulphur fuels, scrubbers and LNG-powered vessels for Korean shipping companies.

Luciana (Marcu) Τ.A. et al., 2021 [26], used 6 criteria and AHP methodology to assess LNG and oil gas.

Mandic Ν. et al., 2021 [14], proposed the application of AHP and Simple Additive Weighting (SAW) for the alternative marine fuel assessment in coastal shipping. Biofuels, LNG, hydrogen, LPG and batteries were prioritized using 10 criteria covering environmental, technological and economical aspects, and the selected study area was Croatia. Electric propulsion stands out from all the alternatives, and the ranking order is differentiated according to the stakeholder groups.

Carvalho F. et al., 2021 [27], developed a qualitative analysis for ranking 14 fuel production options for the Brazilian maritime trade. The analysis incorporated 9 criteria, including technical, economic and environmental. The drop-in fuels dominated in their results followed by bio-methanol and bio-LNG, whereas green hydrogen and green ammonia were the least-promising alternatives for Brazil.

Moshiul A.M. et al., 2023 [28], used the multicriteria technique, TOPSIS, to assess the most important criteria for the selection of fuel alternatives by prioritizing the preferences of shipowners and shipping companies’ management of Singapore firms. The criteria assessment process included 15 factors and 77 subfactors, considering technical aspects, technology status, policies, economic, environmental and socio-political aspects. Their assessment indicated technological aspects, technology status, expenditure, ecosystem impact and health and safety as the most crucial criteria.

Yang Z. et al., 2023 [15], evaluated four alternative low-carbon fuel production pathways (e-fuel, solar fuel, biofuel, e-biofuel), using AHP and the q-ROLPBM operator. The evaluation process was carried out with 13 criteria, including economic, environmental, technical and social, for the United Kingdom. Their research indicated e-fuel and e-biofuel as the most promising production pathways.

The above review concludes that the existing literature for comparison and evaluation of multiple alternative fuels is limited. In the majority of papers, the evaluation includes 2–3 fuel options, and only Hansson J. et al. [12,13] and Mandic Ν. et al. [14] deal with 5–7 fuels, incorporating a manageable set of 10 criteria, while Carvalho F. et al. [27] and Moshiul A.M. et al. [28] assess fuel production pathways and evaluation criteria, respectively. A broader range of fuel options incorporating a broader range of criteria will provide additional insight in the obtained rankings. In the present study, 16 fuel options are considered and assessed through 25 criteria, which also constitute the novelty of the presented methodological framework. In addition, the multicriteria method Promethee II is applied for the first time to a fuel option evaluation.

3.2. Methods

3.2.1. Multicriteria Evaluation Methodology

Each multicriteria approach has its own advantages and disadvantages, and the choice of the appropriate one depends on the nature of the problem. As can be seen in the literature review, the majority of existing studies are based on the AHP method, where all the criteria and alternatives must be compared by the decision maker/user in a pairwise process, which might be impossible in cases with many criteria.

Outranking methods have been developed rapidly during the last few decades, as they incorporate the characteristic of allowing incomparability between a finite number of alternatives and a conflicting set of criteria [29,30]. Electre (elimination and choice translating reality) and Promethee (Preference Ranking Organization Method for Enrichment Evaluation) are the most commonly used outranking multicriteria techniques [31]. TOPSIS (technique for order preference by similarity to ideal solutions) is another option to Electre methodology, based on the comparison of Euclidean distances of alternatives.

The multicriteria methodology Promethee II was adopted for the assessment process in this research, taking advantage of the pairwise comparison of the alternatives and their final ranking as an output of the process, without the involvement of the decision maker in the process of extracting the results. A multicriteria preference index is formulated for each alternative action X (named “Alternative Fuels”). The importance of each criterion is expressed by a weight. The preference functions, V-type and usual type, for quantitative and qualitative criteria, respectively, were selected for the calculation of the preference index. Furthermore, a preference threshold was considered, whereas the indifference threshold was ignored. The alternative fuels were sorted by a positive or negative flow, Φ⁺(Χ) and Φ⁻(Χ), where X is the alternative fuel. The positive flow, “Φ⁺(Χ)”, indicates how the alternative X outranks all the others, and the negative flow, “Φ⁻(Χ)”, indicates a preference among all other alternatives compared to alternative X. The net outranking flow, Φ(Χ), determines an overall score for each alternative [30,31].

3.2.2. Weighting Criteria

Elicitation of weights is always a challenging phase in the decision-making process and it is crucial to reflect all the possible preferences. Simos technique is an indirect weighting methodology, aiding in the expression of preferences, even for stakeholders unfamiliar with decision-making methodologies. The initial Simos approach was extended to face robustness issues, creating the revised Simos approach [32].

In this study, a weight was assigned to each criterion, and the process was carried out according to the revised Simos approach. The criteria weights were categorized in seven different strategies obtained through literature review and interviews/workshops with shipping-related stakeholders. Stakeholders are categorized into six groups associated with their expertise: (i) shipowners, (ii) fuel suppliers, (iii) industry-engine manufacturers, (iv) academics, (v) banks and (vi) public. The seventh category, “combined case” scenario, reflects the combined viewpoints of all stakeholders for the criteria and is estimated as the weighted geometric mean for each criterion. All the participants were informed of the content of the research. A 10-point Likert scale was used to capture each criterion’s importance. As can be seen in

Table 2. Criteria weights (%).

Criteria Weights (%)		Shipowners	Fuel Suppliers	Industry—Engine Manufacturer	Academics	Banks	Public	Combined Case
Capex	C1	13.8	4.6	8.9	4.6	17	6.2	8.1
Opex	C2	3.1	1.6	7.1	3.1	7.3	6.2	3.9
Fuel Cost	C3	15.3	13.6	10.7	12.1	8.9	6.2	12.4
Fuel Availability	C4	15.3	16.5	14.4	9.1	13.8	6.2	14.5
Adaptability	C5	10.8	12.1	7.1	10.6	5.7	6.2	8.1
Commercial effects	C6	9.2	3.1	3.5	1.6	4.1	6.2	3.8
Risk assessment	C7	12.3	7.6	5.3	7.6	15.4	15	10.3
Emissions reduction	C8	6.2	10.6	12.6	16.5	12.2	16.8	14.5
Fuel properties	C9	4.7	9.1	12.6	13.6	2.5	6.2	6
Regulation	C10	7.7	15.1	16.2	15.1	10.6	13.3	16.7
Job creation	C11	1.6	6.1	1.6	6.1	2.5	11.5	1.7

, there are some similarities among the stakeholder groups, as the importance given to the following criteria is obvious: “emissions reduction”, “regulation”, “fuel availability” and the economic criteria, “Capex” and “fuel cost”.

4. Criteria

The literature review, in Section 3.1, highlighted that the evaluation criteria are usually grouped into four main aspects: economic, technical, environmental and social. In this study, 25 sub-criteria were derived as the most frequently used indicators and were categorized into 11 main criteria and the 4 above-mentioned groups (Figure 1).

4.1. Economic Indicators

Economic indicators (

Table 3. The values of the economic indicators [9,12,13,16,33,34].

Fuels/Criteria		Capex 1 ($/kW) 2	Opex 3 ($/MWh) 2	Fuel Cost 4
				Current Fuel cost ($/GJ Fuel) 5	Potential Cost Reduction
HFO		4800–7300	5	5–12	Low
MGO		4500–7040	5	12–14	Low
Commercially available options
LNG	LNG	5100–7710	9	7–10	Low
	Bio-LNG			8.5–28.5	Medium
	e-LNG			23–110	High
In demonstration phase
Methanol	Fossil	4700–7180	6	4–31	Low High (for CCS) Medium High
	Blue			21–237
	Bio			22–35
	e-methanol			58–463
Biofuels (Drop-in)	HTL fuel oil	4500–7040	5	51–98	Medium
	Pyrolysis fuel oil			31–45
	HVO			24–39
	FAME			20–35
Under Development
Hydrogen	Grey or Blue	6500–12,040	11	11–26	High (for CCS)
	Green			16–33	High
	Bio			20–54	High (for gasification)
Ammonia	Fossil (or blue)	5200–11,400	9–11	16–27	High (for CCS)
	Green			23–27	High (for electrolysers and renewable energy)
	Bio			20–54	High

¹ Capex: includes the cost for onboard infrastructure per engine capacity, ² in 2015 dollars (2015 is selected as a reference year common for all fuel options based on the data retrieved by the literature review process), ³ Opex: fuel cost in not included, ⁴ The sub-criteria of “Fuel cost” do not include potential carbon taxes, ⁵ in 2021 dollars (2021 is selected as a reference year common for all fuel options based on the data retrieved by the literature review process).

) can be broken down into (a) capital cost for propulsion (Capex): this includes the expenditures of propulsion and related system components per installed engine capacity (such engines’ cost, fuel tanks, pipelines, gas alarm systems, fuel processors, etc.); (b) operational cost (Opex): this includes crew cost, maintenance and insurance cost but excludes fuel cost [12,13]; (c) fuel cost: the expense of the fuel price is divided into two subcriteria, the current fuel cost (based on fuel prices of 2021) and the potential reduction for future cost, according to the prediction of IMO [9,16,33,34].

4.2. Technical Indicators

This category assesses fuel availability, adaptability of technology, commercial effects of the adoption of the alternative fuel option and their performance in the case of a hazard. More specifically, fuel availability includes the following sub-criteria: (i) production technology readiness, representing the existing level of adequacy of the production technology and the necessary processes, and (ii) raw material availability, meaning the current availability of feedstocks and energy sources [16]. Τhe technology feasibility of the alternative fuels, as regards the onboard procedures of bunkering, storage, processing, conversion and propulsion, is examined through the criterion of “Adaptability” (it is expressed using the TRL score, too). It should be noted that TRL score describes the stage of development of a technology and is measured on a scale from 0 (idea/concept stage) to 9 (full commercial application of technology). In general, alternative fuel systems are more feasible when new building ships, from their application to existing ships [11]. The combination of technological maturity with the growing demand for alternative fuels will have a direct impact on increasing the availability of bunkering infrastructure and operating ships.

The criterion of “Commercial effects” describes the impact of the use of the alternative fuel in a ship’s operation and is divided in two sub-criteria: (i) bunkering intervals and (ii) volumetric energy density. Bunkering intervals range from hours to months, depending on the selection of the alternative fuel, and affect the ship route and its bunkering plan [35,36]. Furthermore, energy density should be considered for the different types of fuel, as higher volumetric energy density requires less space for onboard storage of the fuel and, consequently, a higher cargo loading capacity for the ship. LNG is about one-third of the volumetric energy density of diesel, and liquid hydrogen, methanol and ammonia are around 40–50% of LNG, whereas biodiesel is the closest to diesel [16,36,37,38].

The last indicator for this category is “Risk assessment”. “Together in Safety” [39], a non-regulatory shipping industry safety coalition, carried out a risk-ranking process for different hazard scenarios. The examined scenarios included possible events that might occur in the daily operations of a vessel: navigation (loss of maneuverability, motion at sea, etc.), external events (ship collision, ignition), ship operations other than bunkering (crew change, system components etc.), bunkering (misalignment of the bunkering stations, loss of control etc.), and fuel preparation, use and monitoring (loss of control). The “Risk assessment” sub-criteria are measured based on the performance of the alternative fuels in the examined scenarios. The highest score means the best performance. The values for all technical indicators are shown in

Table 4. The values of technical indicators [11,16,35,36,37,38,39].

Fuels/Criteria		Fuel Availability		Adaptability 1				Commercial Effects		Risk Assessment 2
		Production Technology (TRL)	Raw Material Availability	Bunkering	Storage on Board	Processing and Conversion	Propulsion	Bunkering Intervals	Volumetric Energy Density (MJ/L)	Navigation	External Events	Ship Operations	Bunkering
HFO		9	Widely	9	9	9	9	Months	39.8	13/13	4/4	6/6	12/12
MGO		9	Widely	9	9	9	9	Months	38.4	13/13	4/4	6/6	12/12
Commercially available options
LNG	LNG	9	Widely	9	9	9	9	Weeks	20.6	8/13	2/4	1/6	12/12
	Bio-LNG	7–8	Constrained	9	8	9	9
	e-LNG	6–7	Constrained	9	8	9	9
In demonstration phase
Methanol	Fossil	9	Widely	9	7	7	6–7	Weeks	15.7	12/13	2/4	3/6	3/12
	Blue	6	Constrained	9	7	7	6–7
	Bio	8	Constrained	5.3	7	7	6–7
	e-	7	Constrained	5.3	7	7	6–7
Biofuels (Drop-in)	HTL fuel oil	5–6	Widely	9	9	9	9	Months	33	13/13	4/4	6/6	12/12
	Pyrolysis fuel oil	8	Widely	9	9	9	9
	HVO	9	Widely	9	9	9	9
	FAME	9	Constrained	9	9	9	9
Under Development
Hydrogen	Grey or Blue	9	Widely	3	6	2	5–7	Hours/Days	8.51	7/13	2/4	1/6	12/12
	Green	7–8	Constrained	3	6	2	5–7
	Bio	6–7	Constrained	3	6	2	5–7
Ammonia	Fossil (or blue)	9	Widely	3.7	7	4.5	2–7	Weeks	15.7	7/13	1/4	1/6	1/12
	Green	5–6	Constrained	3.7	7	4.5	2–7
	Bio	5–6	Constrained	3.7	7	4.5	2–7

¹ Bunkering: it is assumed the average in terms of the technology readiness for equipment, procedures and fuel quality standards. Storage on Board: the maximum value was taken into account among the alternative storage options: structure tank, membrane containment system, IMO type A, B and C tank. Processing and Conversion: In case of ammonia as a fuel, it is obtained the average of the score for internal combustion engines and fuel cells. Propulsion: includes ICE 2-stroke, 4-stroke, main auxiliary boilers and reformers. All the values for the biofuels have been adopted from the values for Bio-diesel. ² The score for each category and fuel is calculated based on the risk ranking of the examined scenarios. The score for drop-in biofuels has been assumed the same as conventional fuels.

.

4.3. Environmental Indicators

The main reason to select alternative marine fuels is the emissions’ reductions in order to comply with the regulation targets. In this study, a lifecycle perspective (WTW-Well to Wake) was considered for the quantification of GHG emission reduction indicators, including the stages of fuel production, distribution and transport as well as the final consumption from the ship [16,17,40,41]. The percentage values of emission reductions are related to HFO, as a reference case (

Table 5. The values of environmental indicators [16,17,40,41].

		Emissions Reduction
Fuels/Criteria		Relative GHG	Relative SOx	Relative NOx	Relative PM
HFO		0%	0%	0%	0%
MGO		0%	0%	0%	0%
Commercially available options
LNG	LNG	−15%	−100%	−80%	−100%
	Bio-LNG	−80%
	e-LNG	−80%
In demonstration phase
Methanol	Fossil	+29%	−99%	−60 to −80%	−95%
	Blue	−42% to −60%
	Bio	−85% to −91%
	e- methanol	−58% to −94%
Biofuels (Drop-in)	HTL fuel oil	−80% to −82%	−100% (assuming low sulphur in feedstock)	Uncertain (depends on fuel properties)	~0%
	Pyrolysis fuel oil	−77% to −80%			~0%
	HVO	−53% to −89%	−100%	0% to −20%	Generally reduced
	FAME	−53% to −89%	−99% to −100%	0%
Under Development
Hydrogen	Grey or Blue	−22% (blue) +70% (grey)	0% (ICE)—100% (FC)	0% (ICE)—100% (FC)	−100%
	Green	−87%	−100%	−100%
	Bio	Highly	Moderate 1	Moderate 1
Ammonia	Fossil (or blue)	~−14%	0% (ICE)—100% (FC)	0% (ICE)—100% (FC)	−100%
	Green	~−77%	−100%	−100%
	Bio	Highly	Moderate 1	Moderate 1,2

¹ It has been considered in line with the respective conventional fuel and depending on the used propulsion system, as at the time of preparing the manuscript there is not available quantitative data for bio-hydrogen and bio-ammonia. ² Ammonia can be used in modified ICEs or FCs. The combustion of NH₃ produces N₂O and in case of ICE an aftertreatment of removing N₂O is necessary [2].

).

4.4. Social Indicators

In the literature, the social pillar represents safety factors and economic growth at a local level (

Table 6. The values of social indicators [10,11,16,28,37,38,40].

		Fuel Properties—Safety			Regulation 1	Job Creation 1
Fuels/Criteria		Flammability (vol%) 2	Toxicity	Corrosiveness	Existing Regulation	New Jobs
HFO		1–6	Non-toxic	Non-corrosive	5	0
MGO		0.7–5	Non-toxic	Non-corrosive	5	0
Commercially available options
LNG	LNG	5–15 (Methane)	Non-toxic	Non-corrosive	5	1
	Bio-LNG					5
	e-LNG					4
In demonstration phase
Methanol	Fossil	6–36	Acutely-toxic	Corrosive	4	1
	Blue			Corrosive		2
	Bio			Corrosive (upon degradation)		5
	e-methanol			Corrosive		4
Biofuels (Drop-in)	HTL fuel oil	0.6–7.5	Non-toxic	Corrosive (upon degradation)	5	1
	Pyrolysis fuel oil					1
	HVO					1
	FAME					1
Under Development
Hydrogen	Grey or Blue	4–75	Non-toxic	Non-corrosive	3	2
	Green					4
	Bio					5
Ammonia	Fossil (or blue)	15–25	Very toxic	Corrosive	2	2
	Green					4
	Bio					5

¹ The criteria “Existing regulation” and “job creation” are presented in the range of 0–5, where 0 represents the lowest value and 5 the highest. ² Flammability limits in air (vol%): show the range of vapour concentrations of a certain chemical, over which a flammable mixture gas or vapour in air can be ignited at 25 °C and atmospheric pressure.

). Safety factors include the fuel properties (such as flammability, toxicity and corrosiveness) and the regulatory compliance, which is one of the most crucial criteria, highly rated in almost all the stakeholders’ preferences. The flammability limit is an indicator of the required amount of a fuel to be burnt in the air volumetrically. Indicatively, hydrogen burns easily with the widest flammability limits [11]. The criterion “Regulation” is quantified in a range of 0–5, after studying the existing regulations, standards and guides from numerous organizations: IMO, ISO, Class Society, SGMF, European Committee for Standardization (CEN) and Methanol Institute. Additionally, for the social aspect, the production process of the alternative marine fuels could play a leading role in creating new jobs compared to conventional fuels, generating the indicator “Job creation”. Obviously, existing conventional fuel options are not able to create new job opportunities opposed to bio-fuels and e-fuels, which have the highest potential [10,16,28,37,38,40].

5. Results and Discussion

The results generated through the multicriteria evaluation process are displayed in Figure 2. The evaluation process also included HFO and MGO as the current baseline fuel options for the purpose of comparison. The examined biofuels, HTL, pyrolysis fuel oil, HVO and FAME, were grouped together in the evaluation process, called “Drop-in” fuels, as their values are common in almost all the indicators.

The marine fuel rankings present certain similarities among the stakeholder groups. LNG, MGO and HFO are classified in the highest positions in almost all the examined stakeholder categories. The high rating of these fuels is due to their widespread availability, which resulted in their good performance on several criteria, reflecting the current state of the shipping sector, unlike other new marine fuels, which are still in the development phase. When focusing on alternative marine fuels, there are renewable options that also appear in high-ranking positions in the majority of categories and especially among academics, banks, the public and the combined case scenario. In these high positions, commercially available options of drop-in biofuels, bio and e-LNG, and fossil and bio methanol are included. Bio and green ammonia registered the lowest scores in all stakeholder groups due to their high costs in the economic indicators, low adaptability and low performance in risk assessment, as well as the lack of existing regulation. This observation aligns with the conclusions of Hansson et al., 2020 [13], for the use of ammonia. Gradual decarbonization of the current fossil-based ammonia plants with the co-production of renewable hydrogen, replacing a percentage use of natural gas, should be stimulated at an early stage, as well as fostering the development of new production plants. The hydrogen options remain in intermediary positions, with a small lead in conventional hydrogen production.

Figure 3 shows the impact of each criterion in the classification process for the five highest-ranked fuel options for all stakeholder groups, separately. In the shipowners’ graph, the lines of MGO and HFO almost overlap, and their highest performance occurred mainly due to their high performance in technical indicators, while Bio-LNG is distinguished for its values in environmental indicators. A similar influence is observed in the fuel suppliers’ group and industry-engine manufacturers’ group, who prioritized existing regulation, fuel cost, availability and emissions’ reductions. On the contrary, the environmental indicators had a key role for the classification of academics and the public. Fossil methanol is the new entry in the banks group because of the relatively good performance in risk assessment, fuel availability and emission reductions.

Although the existing literature does not include the same alternatives of marine fuels in the multicriteria evaluation, the current classification is in line with the findings of Hansson et el. 2019 and 2020 [12,13]. The common point with the rest of the manuscripts is the dominant role of LNG in the classification. Deviations among the findings of the published studies could also be observed due to the different decision-making methodologies used as well as due to diverse influences from the stakeholders during the weighting process.

The ranking process is obviously influenced by the selection of the set of criteria. The number and range of criteria selected are crucial factors in minimizing the risk of an inaccurate outcome. In this study, the chosen criteria cover a broad range of key aspects, along with the expression of viewpoints by diverse groups of stakeholders. The different views of stakeholders regarding the importance of the criteria also serve as a kind of sensitivity analysis for the obtained results. The importance and performance of the examined fuels might also be differentiated due to new policies or the further development of existing technologies.

Some limitations should be considered in the context of the obtained findings in the current study. The main limitation is that alternative shipping propulsion systems are not integrated in the examined fuel options. Specialization of the engine’s types (for instance, ICE or FC), technical parameters of the current and possible future applications of mainstream marine engines and the possible aftertreatment technology could lead to distinct values for certain criteria, such as emission reductions for the case of ammonia. Accordingly, future research might be enriched by specifying the evaluation process for different types of ships (deep-sea, short-sea shipping, coastal shipping).

6. Conclusions

This study developed a holistic evaluation framework, incorporating four sustainability aspects, economic, technical, environmental and social, as well as six stakeholders’ views. The variations among the stakeholder group priorities resulted in different classifications of the examined fuel options. The decision-making process through the proposed methodology has the advantage of flexibility and the ability to examine a variety of criteria at the same time as considering the preferences of many decision makers or stakeholders. According to shipowners, engine manufacturers and fuel suppliers, MGO, LNG and HFO are top ranked, followed by drop-in fuels, while, based on academics, banks and the public, drop-in fuels, bio-LNG, e-LNG and MGO are ranked first, followed by LNG, HFO and MGO. The ranking, which came from a combined case scenario, is a mixture of the above-mentioned outcomes.

The contribution of this research is demonstrated by the multidimensional evaluation of alternative fuel options, incorporating a plethora of criteria for more accurate results, from the perspective of six stakeholder group preferences. The proposed framework may serve as a baseline for decision makers/stakeholders to endorse strategies for existing ships and newbuilds. It is remarkable that the criteria of fuel cost, fuel availability and regulation gain a high priority for the majority of stakeholders. Fuel cost is an uncertain parameter, especially for fuel options that are currently under development, which may heavily influence the outcome.

Further research could focus on the introduction of the factor of carbon tax in the formulation of the fuel cost and the willingness of shipping stakeholders to pay their emissions and up to what amount. For instance, according to the report of IRENA and AEA [21], the cost gap between conventional ammonia and renewable ammonia could be bridged by a carbon tax up to USD 150/ton of CO₂. Furthermore, a thorough forecast of the price and availability of renewable fuels would contribute decisively to the results. Importance should also be given to the social criterion of job creation, as while the creation of new jobs is usually considered, a possible simultaneous reduction in existing jobs has not been examined yet. The feasibility of alternative fuel options is still a long way off, and further research and assessment are required, especially for deep-sea shipping. Although bio-fuels are primarily of interest, competition from the demand of other sectors will influence their applicability in the maritime sector. All the renewable fuel options require support and initiatives for long-term use throughout their supply chain, from the production phase to the selected propulsion system.