Renewable Methanol as an Agent for the Decarbonization of Maritime Logistic Systems: A Review

Abstract

Background: The transition to low-carbon economies has become a global priority, particularly in sectors with high greenhouse gas emissions, such as maritime transport. Renewable fuels, especially methanol, have emerged as promising alternatives to conventional fossil fuels due to their potential to reduce carbon footprints and contribute to sustainable logistics systems. Methods: This study employs a combined qualitative and quantitative approach to assess the impact of renewable fuel production on maritime transport decarbonization. The analysis integrates economic feasibility, energy efficiency, and environmental benefits, providing a comprehensive evaluation of methanol’s role in reducing emissions. Results: Findings indicate that methanol offers significant potential for the decarbonization of maritime transport. Its relatively low production costs and high energy density position it as a viable alternative to traditional fuels. Additionally, the study highlights the substantial reduction in greenhouse gas emissions that methanol adoption could achieve, reinforcing its role in mitigating climate change effects. Conclusions: The study concludes that integrating methanol as a primary fuel in maritime transport can accelerate the sector’s decarbonization. However, successful implementation depends on supportive policy regulations and further research to optimize production and supply chain integration. The findings emphasize the strategic importance of renewable fuels in developing sustainable and resilient logistics systems.

1. Introduction

Maritime transport stands as a cornerstone of global trade, yet its environmental footprint, particularly in terms of greenhouse gas emissions, poses a significant challenge to sustainability goals that must be urgently addressed. From the perspective here presented, the reliance on fossil fuels in this sector underscores the need for innovative, low-carbon alternatives that can balance economic vitality with ecological responsibility. The global conversation around climate change has never been more urgent, with rising temperatures and growing environmental concerns driving a pressing need to shift toward a low-carbon economy [1]. Scientific consensus points to greenhouse gas emissions from human activities—especially industrial production and consumption—as the main drivers of global warming [2]. Among the many contributors, the transportation sector stands out due to its long-standing reliance on fossil fuels, which are rich in carbon and major sources of emissions [3]. As a result, decarbonizing transportation is increasingly seen as a crucial step in tackling climate change and moving toward a more sustainable future [4].

Maritime transport occupies a position of unquestionable relevance within the global economic scenario, serving as a crucial element for the circulation of goods and raw materials on an international scale [5]. This sector, which shoulders the enormous responsibility of conducting approximately 90% of global trade volume, has its significance recognized in all quarters of the economy [6]. It thus exerts a fundamental influence on supply chains and the economic development structure of numerous nations, and its significant participation on the world stage also carries substantial environmental impacts. Maritime transport stands out as a considerable source of greenhouse gas (GHG) emissions, accounting for around 3% of global emissions [7]. These figures signal a challenge of vast magnitude that needs to be addressed since GHG emissions significantly contribute to the worsening phenomenon of global climate change, the consequences of which are increasingly tangible and devastating. In this context, the international community faces the imperative need to seek effective strategies and measures aimed at the decarbonization of maritime transport [8]. This issue demands a careful and stringent approach, as it must harmonize economic development, strongly anchored in maritime transport, with environmental preservation and the mitigation of global warming [9]. Therefore, the pursuit of sustainable development that allows progress without compromising the future of forthcoming generations is one of the great contemporary challenges [10].

The transition to alternative fuels emerges as one of the most viable solutions to this challenge. Among the alternatives, renewable methanol presents itself as a promising option due to its low-carbon nature and compatibility with existing liquid fuels infrastructure [11]. Renewable methanol can be produced from various sources, such as biomass, carbon dioxide, and urban and industrial waste [12]. However, each of these sources has distinct characteristics and requires specific production methodologies. These methodologies have been the subject of numerous studies, which aim to enhance the efficiency and economic feasibility of renewable methanol production [13,14,15,16]. In light of this reality, an in-depth analysis of the different renewable methanol production methodologies is justified, and reviewing these methods will not only provide a comparative framework of currently available techniques but also identify gaps and directions for future research.

Methanol occupies a strategic position among alternative fuels due to its dual role as both a sustainable energy carrier and a means of recycling carbon dioxide. Its compatibility with existing maritime infrastructure and lower production costs compared to other alternatives make it a key candidate for addressing immediate decarbonization goals in maritime logistics.

Despite the abundance of individual studies on the various renewable methanol production methodologies, there is a gap in the scientific literature when it comes to comprehensive comparative analysis. It is essential to fill this gap to facilitate decision-making by policymakers, investors, and other stakeholders in the field of maritime transport decarbonization. Also, a clear understanding of the various renewable methanol production methodologies is a precondition for the development of effective decarbonization strategies, which will have to consider not only technical aspects but also economic, environmental, and social implications. In this perspective, this review article is justified by its contribution to the debate on maritime transport decarbonization. Thus, by reviewing the methodologies of renewable methanol production, it aims to provide an updated view of the existing literature and the practice of maritime transport decarbonization by providing a solid foundation for future decisions and research.

In this review, the paper proposes drawing a comprehensive overview of the various methodologies to produce renewable methanol, highlighting its potential in the decarbonization of maritime transport, an industry that is significantly impacted by the increasing pressures to reduce greenhouse gas emissions. The first objective is to gather and systematize the most recent knowledge in the field of renewable methanol production. To this end, methodologies will be evaluated that use biomass, carbon dioxide, and urban and industrial waste as raw materials. Subsequently, the paper aims to examine the applicability of these technologies in the maritime sector, highlighting the challenges, benefits, and potential that the use of renewable methanol can bring to the decarbonization of this sector. In addition, this paper seeks to identify and discuss the main technical, economic, and political obstacles for the broader adoption of renewable methanol in maritime transport, pointing out possible solutions and directions for future research. Finally, the paper contributes an informed and sustainable debate on the decarbonization strategies of maritime transport, highlighting the crucial role that renewable methanol can play in this context.

2. Materials and Methods

2.1. Search Criteria

Conducting a systematic review necessitates the application of consistent and comprehensive search criteria capable of capturing the entirety of the pertinent literature. This section outlines the search criteria adopted in this study, aiming to ensure an accurate and complete review of renewable methanol production methodologies for maritime transport decarbonization. The review began by selecting the databases to be consulted. To guarantee the coverage of relevant publications, the Scopus database was selected for use, chosen for its extensive coverage of the peer-reviewed literature across scientific disciplines, its robust indexing of interdisciplinary research relevant to energy and environmental studies, and its advanced search functionalities that facilitate systematic reviews. This database is recognized for its breadth and contains a variety of publications spanning multiple disciplines, making it ideal for interdisciplinary investigation. Search terms were chosen bearing in mind the necessity to encompass all relevant aspects of the study. Therefore, combinations of the following keywords were used: “methanol”, “renewable methanol”, “production”, “methodologies”, “decarbonization”, and “maritime transport”, as outlined in

Table 1. Number of documents associated with keywords related to renewable methanol and production methodologies.

Keywords	Documents
Methanol	309,817
Renewable + Methanol	4679
Renewable + Methanol + Production	2687
Renewable + Methanol + Production + Methodologies	188

and

Table 2. Number of documents focused on methanol, decarbonization, and maritime transport.

Keywords	Documents
Methanol + Decarbonization + Maritime + Transport	10

. To ensure that no pertinent study was inadvertently excluded, variations of these terms and their translations into English were also included. Inclusion criteria were delineated to ensure that only studies directly relevant to the topic were selected. Thus, for studies to be included in the review, they must (i) explicitly focus on renewable methanol production, (ii) explore one or more methanol production methodologies, and (iii) reference the application of methanol in maritime transport decarbonization or comparable contexts. Simultaneously, exclusion criteria were established with the aim of removing studies that, despite using the keywords, are not pertinent to this review’s objective. Studies were excluded if they (i) focus solely on methanol production from fossil sources, (ii) do not include specific information about production methodologies, or (iii) do not reference the use of methanol in decarbonization contexts.

Another search criterion employed was a time limit. Considered studies published from the year 2000 onward were considered. This decision was made based on the understanding that the technological and scientific advancements of the past two decades have rendered many methods and discoveries prior to this period obsolete. Also, decarbonization policies and a focus on renewable energy sources have gained more significance in this century, making more recent studies more pertinent to this review. To ensure the quality of the review, only studies published in peer-reviewed journals were considered and original articles were prioritized over other literature reviews to avoid redundancies and ensure an original view of the data. The selection of journals and keywords was guided by the objective of capturing a broad yet focused spectrum of research relevant to renewable methanol production and its application in maritime transport decarbonization. Journals were implicitly included via the Scopus database, which indexes a wide range of peer-reviewed publications across disciplines such as energy, environmental science, and engineering, ensuring comprehensive coverage of interdisciplinary studies. The keyword sets (e.g., “methanol”, “renewable methanol”, “decarbonization”, and “maritime transport”) were iteratively developed based on an initial scoping review of seminal works in the field and refined to balance specificity (e.g., “methanol + decarbonization + maritime + transport”) with broader relevance (e.g., “renewable + methanol + production”). Additional keywords, such as “Biomass Syngas Conversion to Methanol” and “Methanol from CO₂ for Maritime Transport”, were incorporated to address emerging methodologies and applications identified during preliminary searches, ensuring the inclusion of cutting-edge research.

To ensure comprehensive coverage, additional keywords such as “Biomass Syngas Conversion to Methanol”, “Photocatalytic and Electrocatalytic Conversion of CO₂ to Methanol”, and “Methanol from CO₂ for Maritime Transport” were included. This adjustment aimed to address potential gaps identified in the original search methodology.

2.2. Data Selection and Extraction

The data selection and extraction stage is crucial in conducting any systematic review. It involves the thorough collection and analysis of relevant information identified in the selected studies, aiming to answer the proposed research questions. This process requires a systematic and consistent approach to ensure the replicability and validity of the review’s conclusions. In the present review, data extraction was carried out by two independent reviewers, in accordance with the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines [17]. This collaborative method serves to mitigate potential biases and ensure the quality of collected data. For each selected study, the following data were extracted: author(s), year of publication, study objective, explored renewable methanol production methodology, main findings and conclusions, as well as study limitations, which are analyzed and presented in Section 3. These pieces of information were stored in a data matrix created in Excel software, allowing easy comparison and analysis of the studies.

In the data extraction phase, special attention was given to renewable methanol production methodology. This emphasis reflects the importance of methanol in the decarbonization of maritime transport and the need to understand the efficacy and feasibility of various production methodologies. The information gathered in this stage served as the basis for the comparative evaluation of methodologies presented in Section 4. Throughout the entire data extraction process, the reviewers strived to maintain an open approach, acknowledging the diversity and complexity of the studies in question. They endeavored to fully comprehend each study, considering it within its specific context and avoiding oversimplifications that could distort the findings and conclusions.

2.3. Bibliometric Analysis Methodology

Aria and Cuccurullo introduced an innovative methodology for scientific mapping analysis through the ‘bibliometrix’ R package [18]. Their methodology relies on the collection and processing of bibliographic data to construct matrices of co-citation, co-authorship, and keyword co-occurrence, which are pivotal for network analysis and knowledge mapping in the scientific field. The ‘bibliometrix’ tool explores data analysis with a systematic and quantitative view of the literature through various metrics, such as correspondence analysis, cluster analysis, and factor analysis, enabling us to identify and visualize key trends, authors, sources, and thematic evolution within a scientific domain.

While the bibliometric analysis methodology employed in this review is established and robust, it is acknowledged that it is no longer considered innovative as of 2024. Recent advancements, such as the use of neural networks for the literature search and systematic mapping in fields like artificial intelligence and computational chemistry, offer promising alternatives for handling large datasets. Incorporating these approaches in future work may enable a more comprehensive and dynamic exploration of the field.

3. Bibliometric Analysis and State-of-the-Art

3.1. First Set of Keywords

3.1.1. Trends and Impact in Renewable Methanol Research

The bibliometric analysis presented in this section is supported by a comprehensive overview of key studies, summarized in

Table 3. Comprehensive summary of key studies on renewable methanol and maritime decarbonization (2008–2024).

Author(s)	Methodology/Focus	Key Findings	Identified Gaps
Taarning et al. (2008) [19]	Biomass (catalysts)	Demonstrated viability of gold catalysts for converting biomass into chemicals like methanol.	Lack of life cycle assessment (LCA) for maritime applications.
Strazza et al. (2010) [20]	Methanol in fuel cells (SOFCs)	Assessed environmental benefits of methanol-fueled SOFCs for auxiliary power on ships.	Limited analysis on large-scale implementation costs.
Kim et al. (2011) [21]	CO2 (solar thermal energy)	Showed feasibility of producing methanol from CO2 using solar thermal energy.	High energy requirements limit economic feasibility.
Sarma et al. (2013) [22]	Bio-hydrogen from glycerol	Found crude glycerol viable for hydrogen production, supporting biofuel waste management.	Limited direct application to methanol production or maritime use.
García-Moreno et al. (2014) [23]	Waste fish oil (biodiesel)	Optimized biodiesel production from waste fish oil, with potential methanol integration.	Need to evaluate environmental and economic impacts at large scale.
Rivarolo et al. (2014) [24]	Biomass and CO2 (distribution networks)	Analyzed distribution networks for methanol and other fuels in regional scenarios.	Focused on regional case studies; lacking global maritime extrapolation.
Sánchez et al. (2015) [25]	Biomass (Jatropha oils)	Optimized biodiesel production using renewable catalysts and alcohols.	Limited data for direct maritime sector application.
Verma and Sharma (2016) [26]	Methanol/ethanol in biodiesel	Compared methanol and ethanol effects on Karanja biodiesel, optimizing transesterification.	Limited focus on maritime-specific applications.
Roh et al. (2016) [27]	CO2 (conversion processes)	Proposed techno-economic methods for converting CO2 into methanol.	High energy requirements remain a challenge.
Sánchez et al. (2016) [28]	Biomass (jojobyl alcohols)	Enhanced biodiesel and alcohol production using renewable catalysts in a pressurized reactor.	Limited scalability data for maritime fuel use.
Dasireddy & Likozar (2019) [29]	CO2 (advanced catalysts)	Improved Cu/ZnO/Al2O3 catalysts for CO2 conversion to methanol.	Limited data on catalyst stability at industrial scale.
Ampah et al. (2021) [30]	Bibliometric review (alternative fuels)	Highlighted rising focus on methanol, ammonia, and hydrogen for maritime decarbonization.	Limited integration of economic analyses across fuel options.
Watanabe et al. (2022) [31]	Biofuels (drop-in and hydrogen-based)	Assessed climate change impacts of biofuels in European maritime transport up to 2050.	Regional focus limits applicability to global maritime contexts.
Yakubson (2022) [32]	Hydrogen and other fuels	Evaluated hydrogen, ammonia, methanol, and synthetic kerosene for decarbonization potential.	Broad scope lacks detailed methanol-specific maritime analysis.
Sevim and Zincir (2022) [33]	Biodiesel/renewable diesel	Discussed drop-in fuels’ compatibility with existing maritime systems.	Limited exploration of methanol-specific benefits or challenges.
Khan et al. (2023) [34]	Clean liquid fuels	Explored potential of liquid fuels like methanol for transportation decarbonization.	Limited focus on maritime-specific implementation barriers.
Ramsay et al. (2023) [35]	Future fuels and emissions	Outlined transition needs for low/zero-emission fuels, including methanol.	Broad overview lacks detailed methanol production analysis.
Elkafas et al. (2023) [36]	Clean propulsion (hydrogen, methanol)	Identified hydrogen PEMFC as best for short-distance ferries, with methanol as an option.	Limited cost-benefit analysis for methanol in broader maritime use.
Bertagna et al. (2023) [37]	Methanol in electric cruise ships	Analyzed methanol’s impact on all-electric cruise ship design.	Emerging interest; lacks long-term operational data.
Parris et al. (2024) [38]	Methanol as marine fuel	Reviewed methanol’s economic and environmental viability for GHG reduction.	Recent study; limited data on large-scale adoption feasibility.
deManuel-López et al. (2024) [39]	Port regulations (Iberian context)	Analyzed port roles in supporting decarbonization, including methanol infrastructure needs.	Regional focus; lacks global port network implications.

, which details the authors, publication years, methodologies, findings, and gaps across the reviewed literature. The analysis of the dataset, spanning from 2008 to 2023, provides a view of the progress and structure of academic output in this particular field of study. With a total of 36 papers published across 18 different sources, this dataset exhibits an annual growth rate of 13%, calculated based on the increase in publications within the Scopus dataset from 2008 to 2023, reflecting growing research interest in renewable methanol and related fields. The average age of the documents, at approximately 6 years, suggests that most of the research is recent. With an average of 50 citations per document, it demonstrates that these works had a significant impact on the scientific community, highlighting the relevance and influence of the subject. In terms of content, the thematic diversity is underscored by the use of 562 Keywords Plus (ID) and 138 Author’s Keywords (DE), demonstrating the breadth of topics covered and the methodological and conceptual approaches employed by researchers. This thematic variety is essential for fostering interdisciplinary dialogue and for addressing the complexity of the field of study in a holistic manner. Regarding authorship, a total of 146 authors contributed to the analyzed set of documents, with only one document being single-authored. These data highlight the collaborative nature of research, with an average of 4.5 co-authors per document. However, the absence of international co-authorship might indicate a geographical concentration of research or the need to expand collaboration networks to include diverse perspectives and expertise from different regions of the world.

The publication trends in the field of renewable energy and conversion, as well as the valorization of biomass and sustainable technologies, can be analyzed through the distribution of articles by source (Table 3). The journal “Renewable Energy” leads with a total of seven articles, and analyzing the aim and scope of the source, this predominance suggests a strong concentration of studies aimed at exploring and optimizing renewable energy sources. The journals “Energy Conversion and Management” and “Fuel”, with four articles each, indicating a considerable focus on energy conversion and the search for more efficient and less polluting fuels. “Applied Energy” and “Journal of Cleaner Production”, with three publications each, may underscore the relevance of research directed towards the practical application of energy solutions and clean production, respectively. Publications in the journals “Biomass Conversion and Biorefinery” and “Energy”, each with two articles, may highlight the importance given to biomass conversion into renewable energy sources and the broad study of the energy sector. The other journals, with one article each, include a variety of titles ranging from specific aspects of sustainable chemistry, as in “ChemSusChem”, to broader approaches to energy and environment, as seen in “Energy and Environmental Science”. This diversity of sources might illustrate the inherent multidisciplinarity of the field in study, where technological innovation, environmental sustainability, and economic viability intertwine.

The analysis of the data also reveals a trend in the citation of articles related to renewable energy and energy conversion. The article by Taarning et al. [19], published in ChemSusChem, leads with a total of 288 citations and an annual average of 16.94 citations. Following this, the study by Kim et al. [21], published in Energy and Environmental Science, with a total of 203 citations and an annual average of 14.50, emphasizes the importance of technological development in harnessing solar energy for methanol production from CO₂. Dasireddy and Likozar [29], in Renewable Energy, present a normalized TC of 2.46, highlighting the rapid growth of scientific interest in the work, which discusses recent advances in renewable energy, with a total of 144 citations and a rate of 24.00 citations per year. Other articles, including those by Verma and Sharma [26] in Fuel, and Strazza et al. [20] in Applied Energy, had significant impacts on the discussion about optimized biodiesel production and the application of fuel cells in ships.

The analysis of the word frequency reveals significant focus areas in the fields of renewable energy, energy conversion, and biodiesel production. The term “methanol” appears most frequently (47 occurrences), highlighting its importance as an alternative fuel in research aimed at decarbonization and sustainable energy production. The word “biodiesel” (18 occurrences) and expressions related to its production, such as “transesterification” (15 occurrences) and “biodiesel production” (9 occurrences), indicate a consolidated research area focused on finding sustainable alternatives to fossil fuels. The methodology of “response surface methodology” (17 occurrences) emerges as a predominant analytical tool for process optimization. “Carbon dioxide” (16 occurrences) underlines the attention given to the capture and reuse of CO₂, emphasizing the role of research in mitigating greenhouse gas emissions. Energy efficiency is another relevant theme, as indicated by the words “energy efficiency” (eight occurrences) and “optimization” (eight occurrences), reflecting the ongoing need to improve the performance and sustainability of energy systems. The reference to “renewable energy resources” (seven occurrences) and related terms emphasizes the research direction toward exploring and maximizing the use of renewable energy sources. The focus on “catalysis”, “catalyst”, and “catalysts” (totaling 21 occurrences) demonstrates the significance of chemistry and chemical engineering in optimizing reactions for the production of alternative fuels. “Renewable energy source” and related variations, totaling 29 occurrences, underline the strategic orientation of research towards sustainability and independence from fossil fuels. The mention of economic analyses and “economic analysis” (six occurrences) highlights the importance of economic viability in the transition to renewable energies.

The thematic analysis of the documents reveals patterns and areas of focus around environmentally sustainable technologies, energy efficiency, and the production of alternative fuels (Figure 1). “Carbon dioxide” leads in occurrences within the cluster, showing high centrality among the keywords. Methanol production-related subjects are also identified as areas of significant interest, reflecting the ongoing search for more sustainable and efficient energy processes and systems. The emphasis on “renewable energy resources” underscores the importance assigned to sustainable energy sources in the global energy transition. Economics, costs, and market and sensitivity analyses are recognized for their relevance in assessing the viability and performance of energy projects, suggesting a holistic approach that integrates economic considerations into the development of sustainable energy solutions. Interestingly, catalysis appears as a distinct cluster, reflecting its critical importance in facilitating efficient chemical reactions for the production of sustainable fuels and chemicals.

The observed lack of international co-authorship may reflect the local and regional interests that dominate research in this field. Additionally, commercial and geopolitical factors, such as intellectual property concerns and resource allocation priorities, might contribute to this trend. An analysis of author affiliations highlights a concentration of studies in regions with established maritime industries, suggesting the need for broader collaboration to incorporate diverse perspectives.

To provide a systematic overview of the reviewed articles,

Table 4. Summary of articles on renewable methanol production methodologies.

Author(s)	Production Methodology	Key Findings	Identified Gaps
Taarning et al. (2008) [19]	Biomass (catalysts)	Demonstrated the viability of gold catalysts for converting biomass into chemicals such as methanol.	Lack of life cycle assessment (LCA) for maritime applications.
Kim et al. (2011) [21]	CO2 (solar thermal energy)	Presented feasibility for producing methanol from CO2 using solar thermal energy.	High energy requirements for the process, limiting its economic feasibility.
Dasireddy & Likozar (2019) [29]	CO2 (advanced catalysts)	Analyzed improvements in Cu/ZnO/Al2O3 catalysts for CO2 conversion into methanol.	Limited data on catalyst stability at an industrial scale.
Strazza et al. (2010) [20]	Methanol in fuel cells (SOFCs)	Assessed environmental benefits of using methanol in fuel cells for auxiliary systems on ships.	Limited analysis on large-scale implementation costs.
Sánchez et al. (2015) [25]	Biomass (Jatropha oils)	Optimized biodiesel production using different alcohols and renewable catalysts.	Limited data for direct application in the maritime sector.
Rivarolo et al. (2014) [24]	Biomass and CO2	Analyzed distribution networks for methanol and other fuels in regional scenarios.	Focused on regional case studies, with no extrapolation to the global maritime context.
García-Moreno et al. (2014) [23]	Waste fish oil	Demonstrated high efficiency in biodiesel production from waste, with potential integration into methanol.	Need to evaluate environmental and economic impacts at a large scale.
Roh et al. (2016) [27]	CO2 (conversion processes)	Proposed innovative methods for converting CO2 into methanol using techno-economic approaches.	High energy requirements remain a challenge.

summarizes key findings and gaps. This systematization facilitates the identification of trends and research opportunities in renewable methanol production methodologies. Figure 2 illustrates the number of articles published annually within the Scopus dataset, highlighting a consistent 13% growth rate over the 15-year period.

3.1.2. Advances in Methanol Production and Maritime Applications

The evolution of fuels for maritime transport decarbonization has been a focal point of academic research, reflecting a significant shift towards sustainable and renewable energy sources over the past few years. Strazza et al. [20], in their pioneering work, conducted a comparative life cycle assessment (LCA) of methanol-fueled solid oxide fuel cells (SOFCs) as auxiliary power systems onboard ships, highlighting methanol’s potential for significant environmental improvements. This study underscored the environmental benefits of fuel cells, especially when fed with bio-methanol, showcasing a promising pathway towards reducing maritime emissions. In a similar vein, Taarning et al. [19] explored the aerobic oxidation of furfural and hydroxymethylfurfural over gold catalysts, emphasizing the shift from conventional petrochemical feedstocks to biomass feedstocks for the production of commodity base chemicals. This transition is seen as important for stabilizing chemical prices and aligns with the broader goals of sustainable transportation fuels development. Sánchez et al. [25] provided an insightful comparative study on the production of esters from Jatropha oil using different short-chain alcohols, optimizing the transesterification reaction. This research highlighted the environmental and economic benefits of utilizing renewable biomass, such as n-butanol, for biodiesel production, thereby contributing to the decarbonization of maritime transport. Rivarolo et al. [24] assessed the distribution network of hydro-methane, methanol, oxygen, and carbon dioxide in Paraguay, exploring the economic and strategic viability of different transportation technologies for biofuels. Their study offered a methodological approach to choosing the most cost-effective delivery mode for each product, thereby supporting the transition to renewable energy sources in the maritime sector. Furthering the investigation into bio-hydrogen production, Sarma et al. [22] examined the effect of different crude glycerol components on hydrogen production by Enterobacter aerogenes NRRL B-407. Their findings pointed to the feasibility of utilizing crude glycerol, a by-product of biodiesel production, as a substrate for clean hydrogen production, thus presenting an innovative solution to waste management in biofuel production processes. Verma and Sharma [26] conducted a comparative analysis of the effect of methanol and ethanol on Karanja biodiesel production, optimizing the transesterification process. Their work contributed to understanding the optimization of biofuel production processes, emphasizing the potential of ethanol as a renewable alternative for biodiesel production. Lastly, García-Moreno et al. [23] focused on optimizing biodiesel production from waste fish oil, assessing the impact of transesterification parameters on biodiesel yield and properties. This study aligns with the objective of utilizing waste products for energy production, marking a significant step towards sustainable biofuel production. Kim et al. [21] and Roh et al. [27] further expanded the scope of research into CO₂ conversion processes, exploring innovative methods for methanol production from CO₂ using solar thermal energy and developing feasible CO₂ conversion processes. These studies underscore the importance of technological innovation in achieving decarbonization goals in maritime transport. Sánchez et al. [28] investigated the enhancement of jojobyl alcohols and biodiesel production using a renewable catalyst in a pressurized reactor, showcasing the potential for high-efficiency biofuel production methods.

3.2. Second Set of Keywords

3.2.1. Emerging Research on Maritime Decarbonization Fuels

The analysis of the dataset, covering the period from 2021 to 2024, reveals significant insights into academic output in a specific field of study. With an annual growth rate of 25.99%, there’s a notable increase in the volume of documents produced, totaling 10 documents distributed among different types, including articles, book chapters, conference papers, and reviews. This growth underscores a rising interest and expansion of research in the area addressed. The dataset originates from 10 different sources, such as journals and books, indicating a diversity of mediums for research dissemination. The average age of the documents is 1.3 years, suggesting that the body of literature analyzed is relatively recent and, possibly, aligned with current trends and developments in the field of study. The average number of citations per document is 18.1, reflecting the impact, albeit in an exploratory phase, of these works within the academic community.

Content analysis of the documents reveals the use of 88 “Keywords Plus” and 49 “Author’s Keywords”, demonstrating the diversity of themes and sub-themes explored in the documents. This aspect may indicate a rich variety of approaches and perspectives within the study area, contributing to a broader and more multifaceted understanding of the subject. Regarding authorship, a total of 38 authors were identified, with only one author responsible for single-authored documents. These data point to a prevalence of collaborative works, corroborated by the existence of a single single-authored publication and an average of 3.8 co-authors per document. Furthermore, a 50% rate of international co-authorship highlights cross-national collaboration, reflecting the importance of global integration in advancing academic research. The types of documents include four articles, one book chapter, one conference paper, and four reviews. This distribution suggests a balanced contribution between original research and synthetic works, like reviews, which play a crucial role in consolidating existing knowledge and identifying research gaps.

The distribution of articles by source, as per the exported data, provides a comprehensive insight into the dissemination of academic research related to electrical systems for vehicles and transportation electrification, as well as sustainability and clean technologies. Each of the listed sources has published one article, indicating a diversification of publication channels and, consequently, a broad dissemination of the generated knowledge in this area of study. The “2023 IEEE International Conference on Electrical Systems for Aircraft, Railway, Ship Propulsion and Road Vehicles and International Transportation Electrification Conference, ESARS-ITEC 2023” stands out as a forum for presenting technological advances and innovations in electrical systems for various modes of transport, emphasizing the importance of academic and professional events in fostering dialogue and collaboration between researchers and the industry. Journals like “Energies”, “Energy Conversion and Management”, “Energy, Environment, and Sustainability”, “Journal of Cleaner Production”, “Journal of Marine Science and Application”, “Journal of Marine Science and Engineering”, “Renewable and Sustainable Energy Reviews”, “Russian Journal of Applied Chemistry”, and “Sustainability (Switzerland)” represent significant academic publications covering a wide range of topics related to energy, sustainability, and environmentally friendly technologies. The inclusion of articles in these journals underlines the relevance of sustainability and energy transition themes across various fields, from applied chemistry and clean production to marine engineering and reviews of renewable and sustainable energies. This spectrum of publications reflects the interdisciplinary nature of research in transportation electrification and clean energy technologies, encompassing technical, environmental, management, and public policy aspects. Moreover, the variety of sources suggests a broad academic and professional recognition of the importance of addressing sustainability challenges and energy efficiency in multiple contexts and scales (

Table 5. Diversification of publication sources in maritime transport decarbonization research (2021–2024).

Sources	Articles
2023 IEEE International Conference on Electrical Systems for Aircraft, Railway, Ship Propulsion and Road Vehicles and International Transportation Electrification Conference, ESARS-ITEC 2023	1
Energies	1
Energy Conversion and Management	1
Energy, Environment, and Sustainability	1
Journal of Cleaner Production	1
Journal of Marine Science and Application	1
Journal of Marine Science and Engineering	1
Renewable and Sustainable Energy Reviews	1
Russian Journal of Applied Chemistry	1
Sustainability	1

).

The analysis of the exported data related to the citation of academic articles in the scope of decarbonization of maritime transport reveals significant information about their impact and relevance within the scientific community. The paper by Ampah et al. [30] published in the Journal of Cleaner Production stands out with a total of 148 citations and an annual average of 37 citations, reflecting strong resonance within the field of study on alternative marine fuels and decarbonization. This work serves as a benchmark, establishing a norm of total citations (TCs) per year of 1.00. Following this highlight, the article by Watanabe et al. [31] in Energy Conversion and Management has a total of 16 citations and an average of 5.33 TC per year. Khan et al. [34] has a total of eight citations, with an average of four TC per year and a normalized TC of 2.29, indicating a rapid increase in interest from the scientific community, considering the recency of its publication in Renewable and Sustainable Energy Reviews. The works of Sevim and Zincir [33], Ramsay et al. [35], and Elkafas et al. [36], published respectively in Energy, Environment, and Sustainability, Journal of Marine Science and Application, and Journal of Marine Science and Engineering, receive more moderate recognition, with total citations ranging from one to three and a normalized TC below 1. The study by Bertagna et al. [37], presented at the IEEE conference, received one citation, reflecting a possible emerging interest from the academic community in electrical solutions for the decarbonization of maritime transport. The articles by Yakubson [32], Parris et al. [38], and deManuel-López et al. [39] have not yet received citations, which can be attributed to the fact that they are more recent publications and have not yet had the opportunity to be widely disseminated or recognized in the scientific literature.

The frequency of keywords in the data reflects the trends and focuses within the field of research in maritime transport decarbonization. The word “methanol” occurs most frequently, being mentioned five times, which indicates its relevance as an alternative fuel in recent studies. This may signal the growing interest in sustainable methanol production methods and their potential uses in maritime applications. The words “decarbonisation” and “greenhouse gases”, each with four occurrences, emphasize the centrality of efforts to reduce greenhouse gas emissions in maritime transport. The terminology reflects a consonance with global climate change mitigation goals and the search for effective strategies to achieve decarbonization targets. “Ammonia”, “climate change”, “decarbonization”, “maritime transport”, “methanol fuels”, and “ships”, with three mentions each, indicate fields of study that are recognizably interconnected and essential to understanding the technical, environmental, and regulatory dimensions of the energy transition in the maritime sector. The presence of terms such as “climate change” emphasizes the connection of research with broader environmental concerns, while “maritime transport” and “ships” focus on the scope of the practical applications of decarbonization strategies. The term “current”, mentioned twice, may indicate a discussion around current practices and emerging technologies, highlighting a turning point where the current state is challenged by new developments. This lexical analysis suggests an academic narrative that is deeply rooted in contemporary environmental concerns and focused on the search for innovative solutions for maritime transport. The emphasis on “methanol” and other alternative fuels shows a potentially viable path to reducing greenhouse gas emissions and meeting the growing demands for sustainable transportation methods.

The data analysis on keyword centrality and clustering related to research in decarbonization of maritime transport. The centrality of a word in keyword co-occurrence networks can be an indicator of its importance within the set of documents analyzed. “Methanol” stands out as the most frequently occurring word (five times) and the most central in the data presented, with the highest values of betweenness centrality, closeness centrality, and PageRank. This indicates that methanol is not only a frequently discussed topic but also serves as an important linking point among other topics within the cluster named “methanol”, underlining its central role in the discussion about alternative fuels for the decarbonization of maritime transport. The words “decarbonisation” and “greenhouse gases”, with four occurrences each, also exhibit significant centrality values, showing that they are strongly interconnected themes within the methanol discussion. These terms are critical to understanding the context and goals of research in sustainable fuels. “Ammonia”, “climate change”, “decarbonization”, “maritime transport”, “methanol fuels”, and “ships”, with three mentions each, suggest that these concepts are closely related in the decarbonization debate, forming a cohesive set of topics within the same cluster. The centrality value of these words, though less than that of methanol, still reflects a prominent position in the academic dialogue. The term “current” appears with two occurrences and has the lowest centrality, which may indicate that it is a complementary topic within the general discussion and not a central axis of investigation. This analysis reveals the research network structure where methanol acts as a core aggregator of discussions on emission reduction in maritime transport. The presence of keywords associated with methanol and significant centrality measures for terms related to greenhouse gases and climate change reflect an integrated focus on environmental sustainability and the search for solutions to the challenges of the maritime sector.

The work by Ampah et al. [30], titled “Reviewing Two Decades of Cleaner Alternative Marine Fuels: Towards IMO’s Decarbonization of the Maritime Transport Sector”, published in the Journal of Cleaner Production in 2021, is the most cited, with a total of 148 citations and an average of 37 citations per year, representing an indicator of total citations per year of 1.000. This paper occupies a prominent position, reflected by its high value in PageRank, suggesting substantial impact in the area of study on alternative fuels and the decarbonization of the maritime sector. Following is the work by Watanabe et al. [31], “Drop-in and Hydrogen-Based Biofuels for Maritime Transport: Country-Based Assessment of Climate Change Impacts in Europe up to 2050”, in the periodical Energy Conversion and Management in 2022. With 16 citations and an average of 5.333 TC per year, this paper also has a high value of normalized total citations (NTCs), indicating strong relevance in the discussion about the assessment of climate change impacts and biofuels on European maritime transport. Khan et al. [34], with the paper “Potential of Clean Liquid Fuels in Decarbonizing Transportation—An Overlooked Net-Zero Pathway?” published in Renewable and Sustainable Energy Reviews in 2023, have accumulated eight citations and an average of four TC per year. This demonstrates significant interest from the scientific community, reinforced by its PageRank. The work presented by Bertagna et al. [37] at the IEEE conference in 2023, “Impact of Fuel Switch to Methanol on the Design of an All Electric Cruise Ship”, records one citation to date, reflecting an emerging interest in the role of methanol and the design of electric cruise ships. The remaining articles, including those published in Energies; Energy, Environment, and Sustainability; Journal of Marine Science and Application; Journal of Marine Science and Engineering; Russian Journal of Applied Chemistry; and Sustainability (Switzerland), range from zero to three citations, indicating that they may be more recent publications or with a reach still developing within the academic community.

3.2.2. Alternative Fuels for Sustainable Maritime Transport

The evolution of fuels for the decarbonization of maritime transport is a subject of growing interest and importance within the global context of greenhouse gas emission reductions. Recent studies have been investigating viable alternatives to replace traditional fossil fuels with more sustainable options. Yakubson [32] highlights hydrogen as an efficient direction for the global economy’s decarbonization, including maritime transport, evaluating the possibility and efficiency of using hydrogen, ammonia, methanol, and synthetic kerosene. Elkafas et al. [36] examine the feasibility of clean propulsion systems on short-distance ferries, identifying hydrogen-powered PEMFC as the best option for zero-emission retrofitting. Ampah et al. [30] conduct a bibliometric review on clean alternative marine fuels, pointing to an increasing attention to fuels like methanol, ammonia, and hydrogen. Sevim and Zincir [33] discuss biodiesel and renewable diesel as drop-in fuels for the decarbonization of maritime transport, highlighting the advantage of their compatibility with existing fuel systems. Parris et al. (2024) [38] review methanol as an effective marine fuel for greenhouse gas reduction, emphasizing its economic and environmental viability. Ramsay et al. [35] provide an outlook on future fuels and their associated emissions, highlighting the need for a transition to alternative fuels with zero or lower GHG emissions. Watanabe et al. [31] assess the climate change impacts of drop-in and hydrogen-based biofuels in European maritime transport, showing different mitigation potentials depending on the fuel type and production country. Khan et al. [34] discussed the potential of clean liquid fuels in decarbonizing transportation, emphasizing the importance of sustainable solutions for internal combustion engine vehicles. deManuel-López et al. [39] analyze the role of Iberian ports in regulations on the decarbonization of maritime transport, pointing to the need for port adaptation to new alternative fuel demands. Bertagna et al. [37] investigate the impact of switching to methanol on the design of an all-electric cruise ship, showing technical and economic considerations for transitioning to this fuel. These studies indicate a clear direction toward adopting alternative fuels such as hydrogen, ammonia, methanol, biodiesel, and renewable diesel in the maritime sector. Each fuel presents specific advantages and challenges to be overcome, emphasizing the need for a multifaceted approach to achieve decarbonization in this critical sector. Implementing these solutions requires not only technological innovation but also regulatory support, adaptation of port infrastructure, and investments in research and development.

4. Maritime Transportation and Decarbonization

4.1. Greenhouse Gas Emissions (GHGs) in Maritime Transportation

In an era where climate change increasingly becomes a global concern, maritime transport, as one of the most vital industries worldwide, assumes an ambiguous stance. On one hand, maritime transportation remains an inextricable part of the global economy, handling approximately 80% of worldwide commerce in terms of volume and over 70% in terms of value [40]. On the other hand, it signifies a substantial source of greenhouse gas (GHG) emissions, an escalating environmental concern. The global fleet of vessels is a considerable source of GHG emissions, contributing around 2.5% of global CO₂ emissions [41]. It is estimated that if maritime transportation were a country, it would be the sixth largest CO₂ emitter in the world, behind powerful economies like China, the United States, India, Russia, and Japan [42]. Projections indicate that the sector’s emissions are set to rise significantly in the coming decades due to global trade growth unless substantial mitigation measures are undertaken [43]. The burning of fossil fuels in ship engines generates GHG emissions, including CO₂, CH₄, and N₂O [44]. CO₂ is by far the most emitted gas, accounting for over 98% of the sector’s GHG emissions [45]. Maritime transportation’s CO₂ emissions have remained relatively stable in recent years, though they have grown significantly over the past decades due to global trade expansion [30,46]. Beyond CO₂ emissions, maritime transportation is also accountable for a significant share of harmful atmospheric pollutants’ emissions, like sulfur oxides (SO_x) and nitrogen oxides (NO_x), that bear implications for both human health and air quality [47,48,49].

Although the maritime transport sector is more efficient in terms of GHG emissions per ton–kilometer transported compared to road and air transportation, it still faces significant challenges in reducing its GHG emissions [50]. Given the long lifespan of vessels and the current dependence on fossil fuels, the decarbonization of the maritime sector will be a lengthy and complex process. This complexity is related to the international nature of maritime transport, meaning that decarbonization solutions need to be agreed upon and implemented at a global level. Additionally, the size and scale of maritime transportation also pose barriers to the rapid introduction of new technologies and fuels. However, this does not mean that decarbonization of the maritime sector is a lost cause. Quite the contrary, there is growing awareness and effort towards finding efficient and sustainable solutions for GHG emissions reduction in this vital industry. To address the issue of GHG emissions in maritime transport, various strategies have been proposed, including improving ship energy efficiency, implementing more stringent regulations on GHG emissions, and transitioning to low-carbon fuels [7,51,52]. Among the low-carbon fuels, renewable methanol is gaining increasing attention due to its potential to reduce GHG emissions and facilitate the transition towards a low-carbon economy [53]. However, it is crucial to note that the transition to renewable methanol as marine fuel will not be a magic solution. The production, transportation, and use of renewable methanol also have environmental implications that need to be carefully evaluated.

4.2. Concepts and Policies for Decarbonization

Decarbonization represents a sustainability goal that seeks to minimize the global dependence on fossil fuels and consequently curtail CO₂ and other GHG emissions [4]. This process is predicated on the transition to renewable energy sources, enhancement of energy efficiency, and the application of carbon capture and storage technologies [54]. Global attention to the necessity of decarbonization has been growing, particularly with respect to maritime transport, which, while being one of the most efficient forms of transport in terms of emissions per ton of cargo, is still accountable for a substantial share of global GHG emissions [30,55,56,57]. As a global industry, maritime transport confronts a series of challenges in achieving decarbonization, which include the requirement for new technologies, infrastructures, regulations, and behavioral shifts. Decarbonization policies have become an area of escalating focus worldwide, and numerous strategies are being explored to reduce carbon emissions within the maritime transport sector [58].

The development and implementation of effective decarbonization policies necessitate understanding that transitioning to a low-carbon economy will bring about significant changes in current practices and structures [59]. These changes encompass the need to develop new technologies and infrastructures, invest in research and development, create mechanisms to incentivize the adoption of sustainable practices, and encourage international cooperation. The International Maritime Organization (IMO) has played a pivotal role in promoting decarbonization policies within the maritime transport sector [57]. The IMO’s initial strategy for reducing GHG emissions, adopted in 2018, laid down ambitious targets for the industry, including reducing GHG emissions per transport work by at least 40% by 2030, compared to 2008 levels, and aspiring to phase out these emissions entirely by the end of the century [60]. Moreover, the IMO has been encouraging the adoption of technical and operational measures to enhance ship energy efficiency, such as the implementation of ship energy efficiency management plans, the establishment of energy efficiency indicators, and the promotion of more efficient ship propulsion technologies and designs [61]. However, effectively decarbonizing maritime transport will require more than merely improving ship efficiency. It will necessitate a joint and coordinated effort from all stakeholders, including governments, industry, academia, and civil society, to promote the adoption of alternative low-carbon fuels, develop suitable refueling infrastructures, and implement regulations that encourage sustainable practices.

4.3. The Role of Methanol in Maritime Transport Decarbonization

Methanol has emerged as a promising solution to the challenge of decarbonizing maritime transport, a sector that is critical to the global economy yet also contributes significantly to global greenhouse gas emissions [62]. To understand the potential role of methanol in this context, it is essential to explore the unique characteristics of this compound and how it can be used to mitigate the environmental impact of maritime transport. Methanol, also known as methyl alcohol, is a colorless, volatile, and flammable liquid with a wide range of applications in various sectors, from the production of formaldehyde to its use as a solvent or antifreeze [63]. However, it is its ability to be used as fuel that makes it particularly relevant for the discussion on decarbonization. Methanol can be produced from a variety of sources, including natural gas, coal, and various forms of biomass. Additionally, it is possible to produce methanol from CO₂, offering an opportunity to “recycle” this potent greenhouse gas [64,65].

In maritime transport, methanol can be used as a “drop-in” fuel, meaning that it can directly replace fossil fuels in existing engines and infrastructures, with some modifications [66,67]. This makes the transition to methanol relatively straightforward and cost-effective compared to other fuel alternatives [68]. Moreover, methanol has a reasonably high energy content and a higher energy density than other alternative fuels, such as hydrogen [69]. This makes it well-suited for long-distance maritime transport, where fuel energy density is a key factor [70]. The combustion of methanol also results in significantly lower emissions of atmospheric pollutants compared to conventional fossil fuels [71]. For instance, methanol combustion produces no sulfur, resulting in zero sulfur oxide (SO_x) emissions, a harmful atmospheric pollutant [72]. Similarly, nitrogen oxide (NO_x) and particle emissions are also considerably reduced. For these reasons, methanol is often considered a “clean fuel” [73]. The use of renewable methanol, produced from sustainable and renewable carbon sources, can further help reduce the GHG emissions from maritime transport. While the production of methanol itself may involve CO₂ emissions, these can be captured and reused in the production process if carbon capture and utilization (CCU) techniques are employed [74]. Furthermore, if the energy used in methanol production is sourced from renewables, the GHG emissions can be further reduced [75].

Although this review focuses on renewable methanol, it is important to situate it within the broader context of alternative fuels, such as hydrogen and ammonia. Hydrogen offers zero-emission potential at the point of use and high energy efficiency but presents storage and distribution challenges due to its low energy density and cryogenic requirements. Ammonia, a carbon-free fuel, shows potential for maritime applications, particularly when produced from renewable sources. However, it faces issues such as toxicity and the need for significant modifications to existing ship engines. Methanol, by comparison, benefits from compatibility with current infrastructure, lower toxicity, and relatively simpler handling. These factors position methanol as a practical intermediate solution for decarbonizing maritime logistics while other technologies mature.

5. Methodologies for the Production of Renewable Methanol

5.1. Methanol Produced from Biomass

5.1.1. Biomass Gasification

Biomass gasification is a technique that converts biomass into synthesis gas (syngas), a mixture of carbon monoxide and hydrogen [76]. This gas is then converted into methanol through a catalytic process known as methanol synthesis [77]. The gasification process begins with the drying of biomass, thus removing the water content and rendering it suitable for gasification. Subsequently, the biomass is subjected to high temperatures in the presence of a gasifying agent, typically oxygen, steam, or a mixture of both [78]. This step promotes the breaking down of chemical bonds in the biomass and the formation of synthesis gas. The efficiency of the biomass gasification process is highly dependent on variables such as the type of biomass used, the gasifying agent, temperature, and pressure conditions, as well as the type of catalyst used in methanol synthesis [79].

5.1.2. Biomass Pyrolysis

Biomass pyrolysis is another technique that can be utilized for methanol production. It differs from gasification in that the biomass is thermally decomposed in the absence of oxygen, thereby avoiding combustion [80]. The result of pyrolysis is a mixture of gases, liquids (known as bio-oil), and solids (char) [81]. For the production of methanol, the resultant gases, rich in carbon monoxide and hydrogen, are collected and subjected to methanol synthesis, akin to the process that occurs in biomass gasification [82]. Pyrolysis presents advantages in terms of flexibility, as it can be optimized to produce a broader range of products beyond methanol, such as liquid hydrocarbons and combustible gases [83]. However, this process requires careful handling of solid and liquid by-products to ensure its sustainability. Although biomass gasification and pyrolysis are technically feasible, each presents its own challenges [84]. Issues related to energy efficiency, waste management, economic viability, and environmental impact must be carefully considered to determine the best option in each scenario.

5.2. Methanol Produced from Carbon Dioxide

5.2.1. Water Electrolysis and CO₂ Conversion

The process of water electrolysis and CO₂ conversion, known as “Power-to-Methanol”, consists of two main steps: the electrolysis of water to produce H₂, and the methanol synthesis reaction, in which hydrogen is combined with CO₂ to form methanol (CH₃OH) [85]. In the first step, water is split into its constituent components, hydrogen and oxygen, by means of an electric current [86]. It is an endothermic process, i.e., it requires energy to occur. Ideally, this energy comes from renewable sources, such as wind, solar, or hydroelectric, to minimize the carbon footprint of the process. In the second step, the produced hydrogen reacts with CO₂ in the presence of a catalyst to form methanol [87]. This reaction is exothermic, i.e., it releases energy. The CO₂ used can be obtained from industrial emission sources, contributing to the mitigation of greenhouse gas emissions. However, this methodology presents significant challenges. The overall energy efficiency of the process is one of the main ones, given the amount of energy required for water electrolysis. Additionally, the selection of a suitable catalyst for the methanol synthesis reaction is of utmost importance and is the subject of intense research to improve selectivity and reaction rate [88].

5.2.2. Artificial Photosynthesis

Artificial photosynthesis, on the other hand, seeks to mimic the natural process of plants converting CO₂ and sunlight into chemical energy [89]. In this context, the idea is to use solar energy to convert CO₂ directly into methanol in a single step [90]. Different approaches are being explored in this field, such as the use of photocatalysts and photoelectrochemical cells [91]. Photocatalysts are materials that absorb sunlight and transfer energy to CO₂, encouraging the chemical reaction to form methanol [92]. Photoelectrochemical cells, on the other hand, operate similarly to solar cells, but instead of generating electricity, they produce methanol from CO₂ [93]. Artificial photosynthesis, although promising, is still in the early stages of development. The main challenges include the efficiency of converting sunlight into chemical energy, the selectivity of the reaction to produce methanol over other products, and the stability and cost of the catalysts.

5.3. Methanol Produced from Urban and Industrial Waste

The use of urban and industrial waste for the production of renewable methanol represents a strategic solution for waste management and clean energy production, contributing to the decarbonization of maritime transport [94]. This approach has the advantage of turning an environmental problem into a valuable resource, bringing us closer to the concept of a circular economy. Municipal solid waste (MSW) is composed of various materials, including organic matter, plastics, paper, glass, and metals. Many of these materials possess potential energy that can be recovered [95]. To do this, MSW undergoes a process of gasification, where the material is converted into gas under controlled conditions of temperature and pressure [96]. The produced gas, known as syngas, is a mixture of CO and H₂, which can be transformed into methanol through the gas synthesis process. Industries produce a variety of waste, many of which contain carbon-rich materials. These, like MSW, can be gasified to produce syngas and subsequently methanol. However, industrial waste presents additional challenges, such as the presence of contaminants that can interfere with the gasification process or make it environmentally harmful [97]. Therefore, appropriate prior treatment of these wastes is essential. The production of methanol from urban and industrial waste holds significant potential for reducing GHG emissions. However, the viability of this approach depends on various factors, including the availability and composition of the waste, the efficiency of the gasification process, and the waste collection and processing infrastructure [98].

There are technological and logistical challenges that need to be overcome for the production of methanol from waste to be widely adopted [99]. Waste storage and transportation require suitable infrastructures, as well as the population’s and industry’s adherence to waste separation and disposal practices [100]. The development of cleaner and more efficient gasification technologies is a rapidly growing field of research. Nonetheless, the production of methanol from urban and industrial waste offers a promising path towards maritime transport decarbonization. This approach provides a sustainable alternative to the use of fossil fuels and contributes to efficient waste management, bringing us closer to a circular economy model where waste is seen as a resource, not a problem [101].

5.4. Comparative Evaluation of Methodologies

Beginning with the production of methanol from biomass, it is important to underscore the efficiency of this approach, given that biomass stands as an abundant renewable resource. However, there is an urgent need to consider sustainability in the cultivation and extraction of biomass. Gasification and pyrolysis are the most utilized methods, both being highly efficient in methanol production, albeit with their own limitations. Gasification, for example, requires high temperatures and pressures, whereas pyrolysis may result in potentially hazardous by-products. The production of methanol from carbon dioxide emerges as a highly sustainable and ecologically correct option, considering it assists in reducing greenhouse gas emissions. Yet, current technologies such as water electrolysis and CO₂ conversion still bear significant energy and, consequently, economic costs. Artificial photosynthesis, despite being a promising field, is still in the initial stages of research and development, limiting its application at an industrial scale at present. Lastly, the production of methanol from urban and industrial waste emerges as a potentially viable solution. This method is effective in waste utilization, contributing to a circular economy; however, the quality and consistency of the produced methanol may vary depending on the composition of the waste utilized. In terms of energy efficiency, methanol production from biomass and waste appears to be more efficient than production from carbon dioxide. Nevertheless, the environmental impact of methanol production from biomass and waste could be greater, depending on waste management and the origin of the biomass. On the other hand, methanol production from carbon dioxide holds the potential to be more sustainable in the long term, as it could assist in reducing the concentration of greenhouse gases in the atmosphere. From an economic perspective, the production costs of renewable methanol are a critical issue. Currently, the production of methanol from biomass and waste is generally cheaper, but the production of methanol from carbon dioxide holds the potential to become more economical as technologies evolve and become more efficient.

To provide a quantitative perspective,

Table 6. Comparison of key metrics for biodiesel and methanol production from CO₂ and biomass. The table evaluates carbon footprint, energy efficiency, economic feasibility, and technological maturity for each pathway.

Metric	Biodiesel	Methanol (CO2)	Methanol (Biomass)
Carbon Footprint	Moderate	Low	Low–Moderate
Energy Efficiency	High	Moderate	High
Economic Feasibility	High	Low	Moderate
Technological Maturity	High	Low	Moderate–High

compares the carbon footprint, energy efficiency, production scale, and economic feasibility of renewable methanol against biodiesel and conventional marine fuels. The life cycle assessment (LCA) of methanol shows an average reduction of greenhouse gas emissions by 60–70% compared to fossil fuels, contingent on the production method. For instance, CO₂-derived methanol achieves the lowest carbon footprint but remains costlier due to current technology limitations. These findings underscore the potential trade-offs between economic viability and environmental sustainability.

Among the discussed methods, biomass gasification is the most technologically mature, with a higher production efficiency. However, its scalability is constrained by biomass availability and potential impacts on land use. CO₂ conversion, while highly sustainable, remains economically challenging due to high energy requirements for water electrolysis. Urban and industrial waste offer a promising circular economy approach, yet the variability in waste composition affects process consistency. These findings highlight the importance of tailoring production methods to regional and industrial contexts to achieve scalability and sustainability.

6. Challenges and Opportunities in Renewable Methanol Production

6.1. Energy Efficiency

Energy efficiency can be a pivotal concept in assessing the various methodologies for renewable methanol production, as it signifies the ability to proficiently transform input resources (be they biomass, CO₂, water, or energy) into methanol as the end product [66]. This aspect gains particular importance when contemplating the sustainability and economic viability of these methodologies. It is imperative to clarify that the energy efficiency of methanol production hinges on a number of factors, including the technology employed, operating conditions, and the attributes of the input resources [102]. Therefore, this analysis should be perceived as a broad appraisal of patterns and trends, rather than an absolute comparison among different technologies.

The production of methanol from biomass, for instance, involves the conversion of organic matter (such as agricultural waste, wood, or algae) into methanol through processes like gasification or pyrolysis [103]. The energy efficiency of these processes relies heavily on the quality and features of the biomass. For instance, biomass with a high moisture content might require additional energy for drying prior to gasification, thereby decreasing the overall energy efficiency of the process [104]. Conversely, some processes might leverage the residual thermal energy from gasification for electricity generation, improving the overall system’s energy efficiency.

The production of methanol from carbon dioxide, either through water electrolysis and CO₂ conversion or artificial photosynthesis, is another area of keen interest. These processes harbor the potential to be highly energy efficient, especially if the electrical energy used for electrolysis is sourced from renewable origins. However, these technologies are still in nascent stages of development and face significant challenges, such as the need for efficient catalysts and long-term stability of conversion systems.

The production of methanol from urban and industrial waste may be an appealing option in terms of energy efficiency, especially considering that the waste that would otherwise be discarded is here transformed into a valuable resource. However, similar to biomass, the energy efficiency of this process heavily depends on the characteristics of the waste, as well as the technology used for its conversion.

6.2. Costs and Economic Viability

The transition towards a more sustainable economy, backed by the production of renewable methanol, not only poses technical challenges but also carries significant financial implications [105]. The cost of producing renewable methanol is highly variable and depends on a range of factors, including the production methodology employed, geographical location, availability and cost of raw materials and energy inputs, as well as broader economic factors, such as the state of the energy market and prevailing government policies [106]. Broadly speaking, the direct costs associated with the production of renewable methanol can be divided into capital costs and operational costs [21]. Capital costs encompass the investment required for the construction and installation of production infrastructure, while operational costs cover ongoing expenses for maintenance, operation, raw materials, and energy [107]. At present, the production of methanol from renewable sources may be more costly than conventional production from fossil fuels, mainly due to the high capital costs of the technologies involved [108]. However, these costs are expected to decrease as the technologies evolve and achieve a larger scale of production [109]. Operational costs, on the other hand, are heavily influenced by the prices of raw materials and energy inputs. For example, the cost of methanol produced from biomass can vary considerably depending on the availability and price of biomass in the specific location [110]. Similarly, the cost of methanol produced from carbon dioxide is influenced by electricity prices, as this process requires significant energy [21]. The economic viability of renewable methanol production is also strongly influenced by government policies [111]. Tax incentives and subsidies for renewable energy production, for instance, can significantly improve the economic viability of renewable methanol production [112]. Conversely, the absence of such policies can make renewable methanol production economically challenging. Furthermore, the competitiveness of renewable methanol in the energy market also plays a crucial role in its economic viability [113]. If renewable methanol can be sold at a competitive price relative to other fuels, this will enhance its economic viability. However, if the price of renewable methanol is significantly higher than that of conventional fuels, this may pose a barrier to its adoption [114].

The economic viability of methanol production depends on regional energy prices, policy incentives, and technological maturity. Current estimates place the cost of renewable methanol at USD 350–USD 700 per ton, compared to USD 100–USD 250 per ton for conventional methanol. Advances in renewable energy integration and process optimization are expected to lower these costs over time, making methanol more competitive.

6.3. Environmental Impacts and Sustainability

The environmental impact and sustainability of this process are not confined merely to GHG emissions. Additional considerations encompass, but are not limited to, land use efficiency, water usage, waste generation, and impacts on biodiversity [115]. Biomass gasification and pyrolysis, methods utilized for producing methanol from biomass, involve the use of natural resources, notably arable lands. These processes demand substantial amounts of raw material, often grown on lands that could be purposed for agriculture [116]. This raises questions about land use efficiency and the potential impact on food security. Furthermore, the cultivation of biomass, particularly monocultures, may have detrimental effects on biodiversity [117].

Water usage is another relevant environmental consideration. Producing methanol from CO₂, whether through water electrolysis and CO₂ conversion or artificial photosynthesis, requires water [118]. This is a finite resource, and many regions of the world are already grappling with water scarcity. Therefore, it is important to assess the water balance of renewable methanol production in different regional contexts and develop strategies to minimize water usage.

Waste generation is another environmental issue because producing methanol from urban and industrial waste is a promising strategy for resource utilization but also involves the treatment and management of this waste [119]. Additionally, the quality of methanol produced from these processes needs to be considered, as there may be contaminants that render the methanol unsuitable for certain applications [120].

Regarding biodiversity, any activity that involves intensive exploitation of natural resources can have negative impacts [121]. For example, intensive biomass cultivation can lead to soil degradation and habitat loss, adversely affecting local fauna and flora. Hence, the assessment of impacts on biodiversity should be an integral part of the sustainability analysis of renewable methanol production [122]. It is important to underline that sustainability is a complex issue that cannot be evaluated merely in terms of environmental impact [123]. Other dimensions of sustainability, such as economy and society, should also be considered. For instance, renewable methanol production may create jobs and boost local economic development, but it can also lead to inequality if the benefits are not distributed equitably [124]. An appropriate policy framework is necessary to ensure that renewable methanol production is carried out sustainably. Public policies have a crucial role to play in setting environmental standards, encouraging innovation, and promoting social justice [106].

7. Conclusions

In this review, diverse methodologies available for the production of renewable methanol were explored, underscoring its critical relevance to the decarbonization of maritime transport—a sector vital to global trade yet challenged by significant greenhouse gas emissions. The main findings affirm that, despite obstacles to large-scale implementation, renewable methanol stands out as a promising and sustainable solution to reduce the maritime industry’s carbon footprint. This study offers a synthesis of current knowledge, serving as a valuable resource for a wide audience: researchers in renewable energy and chemical engineering seeking to innovate production technologies, maritime industry stakeholders exploring sustainable fuel alternatives, policymakers shaping regulations to advance decarbonization objectives, and environmental scientists assessing the broader sustainability implications of such transitions. Each production method examined—biomass gasification and pyrolysis, carbon dioxide conversion through water electrolysis and artificial photosynthesis, and the utilization of urban and industrial waste—presents unique strengths, such as high energy efficiency or contributions to a circular economy, alongside challenges like economic feasibility and technological readiness that demand targeted solutions. The bibliometric analysis reveals a dynamic field, with a 13% annual increase in publications from 2008 to 2023 within the Scopus dataset, signaling growing scientific and practical interest in renewable methanol as a decarbonization agent. This trend, illustrated in Figure 2, highlights the urgency and momentum behind sustainable maritime solutions. However, significant gaps persist, necessitating focused research and action. To bridge these, specific recommendations are proposed for key groups. Chemists and catalysis researchers should prioritize the development of more efficient, stable catalysts for CO₂ conversion and biomass gasification, addressing limitations in selectivity and scalability noted in works like Dasireddy and Likozar (2019) [29]. Technologists and engineers are encouraged to enhance energy efficiency in processes such as water electrolysis and waste gasification, tackling high energy demands identified in studies by Kim et al. (2011) and Roh et al. (2016) [21,27]. Environmental scientists should undertake comprehensive life cycle assessments (LCAs) tailored to maritime applications, building on gaps highlighted by Taarning et al. (2008) and Strazza et al. (2010) [19,20], to fully evaluate methanol’s environmental impact. Meanwhile, policymakers and industry practitioners must collaborate to establish supportive regulations, incentives, and infrastructure—such as bunkering facilities for methanol—to overcome economic barriers and facilitate adoption, aligning with the International Maritime Organization’s targets of a 40% emissions reduction by 2030 and a phase-out by century’s end. This review consolidates the state-of-the-art and charts a strategic path forward for maritime decarbonization. The potential of renewable methanol lies in its compatibility with existing infrastructure, its capacity to recycle carbon dioxide, and its adaptability to diverse feedstocks, positioning it as a bridge to a low-carbon future. Yet, realizing this potential hinges on overcoming technical, economic, and environmental hurdles through interdisciplinary collaboration. Public policies will play a pivotal role, driving innovation via research and development funding and fostering market viability through subsidies and standards. By addressing these challenges, renewable methanol can accelerate the transition to sustainable maritime logistics, balancing economic imperatives with ecological stewardship. This work thus provides both a foundation for understanding current capabilities and a call to action for stakeholders to collectively advance a resilient, decarbonized maritime sector.