The maritime industry is responsible for about 3% of global greenhouse gas (GHG) emissions from human activities [
1], and the pace of their reduction in recent years has yet to be considered satisfactory by many stakeholders. Accordingly, governments and regulators have adopted new ambitious targets to remain on track with the Paris Agreement goals. Various strategies for reducing carbon dioxide (
) and other GHGs are currently being discussed. One primary pathway for GHG reduction in the shipping industry involves transitioning to alternative, low-carbon fuels. Among the most emerging options are bio-diesel and bio-oils, bio-methane, methanol, ammonia, and hydrogen. Each of these alternatives to fossil fuels has distinct characteristics, advantages, and limitations. At present, no single alternative fuel can fully meet the needs of the entire maritime industry due to factors such as availability, supply chain infrastructure, and price competitiveness.
This paper presents a multi-parametric methodology for assessing the feasibility of utilizing alternative fuels on cargo vessels, focusing on technical, environmental, and economic aspects. The technical analysis is centred on the ship’s propulsion system and the general arrangement design with the following main objectives: to define a propulsion plant and to find suitable spaces for the fuel tanks and preparation system. The methodology includes the IMO and the Classification Society requirements for low-flashpoint fuels. The environmental assessment consists of several key performance indicators (KPIs) for the examination of GHG emissions, including the IMO indexes (, ) and the new FuelEU directive approach (Well-to-Wake). Additionally, the environmental analysis includes the equivalent carbon dioxide () emissions related to the transported cargo unit, a key index to foster the development of more Eco-friendly logistics. The preliminary economic evaluation comprises estimating capital expenditure (CapEx) and operational expenditure (OpEx) related to the propulsion and fuel systems.
The shipping sector traditionally relies on fossil fuels, particularly residual fuels such as heavy fuel oil (HFO) and distillates like marine diesel oil (MDO). Several alternatives are under scrutiny and, for the majority, the environmental and economic impacts are linked to the production methods and feedstock. Bio-diesel is an alternative fuel currently used in shipping, with Fatty Acid Methyl Esters (FAMEs) being the most common type [
4]. To be considered sustainable, FAMEs must be produced from renewable feedstock. However, the limited availability of these sources means that bio-diesel is not widely available [
4]. Another option to replace fossil fuels is bio-oils, which can be produced from a more extensive variety of sources than bio-diesel, giving them long-term potential in the shipping sector. Bio-oils have yet to be commercially available due to criticality in production and low technology readiness [
4]. Like bio-oils, bio-gases can be obtained from various biological and waste products; their chemical composition strongly depends on the source they come from [
5]. Hydrogen has the most significant
reduction potential. However, its role as an alternative to fossil fuels is currently limited by complex storage and distribution, and high costs, especially when produced from green sources [
6]. Ammonia is attractive as a marine fuel for its status as a zero-carbon fuel when produced from renewable sources. The high toxicity and corrosive nature of this chemical are critical issues for its use as marine fuel [
7]. Methanol properties have led to a recent growth of interest in its use as an alternative marine fuel [
8]; thus, it has been selected for testing the methodology in the case study. Methanol can be produced using various sources, including fossil feedstock but also from biomass, urban garbage, wood residues, and other ecological products [
9]. The different methanol types are brown methanol from coal, grey methanol from natural gas, blue methanol from fossil and ecological raw materials, and green methanol from renewable sources. Each pathway corresponds to a specific GHG footprint. The GHG emissions from the transportation sector can be divided into two categories: Well-to-Tank (WtT) and Tank-to-Wake (TtW); the Well-to-Wake (WtW) emissions include the entire fuel emission chain. The WtT emissions are attributable to raw material supply, fuel production, transport, storage onshore, and bunkering; the TtW emissions account for the combustion processes onboard. TtW emissions are stoichiometric; for methanol, about 69.1 g
/MJ is the figure [
10], irrespective of the production pathway. The WtT emissions highlight the differences in the various production methods and feedstock. The various methanol production pathways also influence the costs and availability of this chemical. Grey methanol is widely produced worldwide, and its price is competitive with traditional marine fuels. In contrast, green methanol is a small reality with only 0.2 Mt/year of global production [
11], and consequently with higher costs. Methanol’s physical and environmental characteristics have captured the attention of the maritime industry. Beginning in 2015 with the Stena Germanica conversion, the use of methanol on board has risen exponentially, arriving at a total number of 230 vessels in operation and on order for delivery by 2028 [
12]; bulk carrier vessels represent only a tiny percentage of the total. The growing order book of methanol-fuelled vessels demonstrates the increasing interest in using this alternative fuel. However, the container shipping giants mainly drive the ongoing fuel transition, with little or no influence in the other transport sectors. The challenge to decarbonizing shipping is to spread innovations by finding sustainable solutions for the different types and sizes of ships. The academic and research sectors are collaborating with the maritime industry to find alternative solutions for the decarbonization of the global fleet. Stolz et al. [
13] studied the techno-economic suitability of various innovative fuels such as methanol, ammonia, and methane for European bulk cargo ships. Deniz and Zincir [
14] and Ammar [
15] followed an environmental–economic approach: the former compared different fuel types, including methanol, and the latter applied the methanol as fuel on board a cellular container ship, used as a case study. Karvounis et al. [
16] investigated the techno-economic–environmental performance of various power plants for cargo vessels, including hybrid solutions, using ammonia as a green fuel. Maloberti and Zaccone [
17] developed an optimization algorithm to minimize the GHG emission of a hybrid system installed on board a touristic ferry. Additionally, the researchers have begun to study autonomous surface vessels driving this sector towards more energy-efficient ships with minor operational expenditures [
18].
Compared to the existing literature, the primary innovation of this study lies in its multidisciplinary assessment of ship design, which incorporates the technical, ecological, and economic perspectives essential for supporting fuel transition projects. Additionally, a significant feature of this work is the introduction of innovative indexes used as KPIs, which allow for the completion of feasibility analysis alongside traditional parameters. The selection of a small-sized bulk carrier as a case study is also noteworthy; this contrarian choice highlights the challenges posed by limited spaces during fuel conversion, impacting the ship’s design, operations, and safety. The methodology presented in this work provides a theoretical foundation for future applications and developments of optimization models. The rest of the paper is structured as follows:
Section 2 describes the methodology,
Section 3 describes the case study,
Section 4 shows the results, and
Section 5 is dedicated to the conclusions.