Dimethyl ether: A promising fuel for marine engines

Graphical abstract

1 Introduction

Climate change represents one of the most significant challenges humanity faces today. Since the First Industrial Revolution, the large-scale use of fossil fuels has spurred the development of human society. However, it has also led to severe global warming, threatening human survival and sustainable development. In 2023, the 28th Conference of the Parties (COP28) to the United Nations Framework Convention on Climate Change (UNFCCC) in Dubai reached an agreement on establishing a roadmap for the “transitioning away from fossil fuels,” marking what is regarded as “the beginning of the end of fossil fuels” [1]. Currently, over 150 countries worldwide have announced plans to achieve net-zero carbon emissions or carbon neutrality by around 2050.

To achieve this goal, the world is undergoing an unprecedented green energy transition, shifting from fossil fuels to renewable energy sources. Renewable energy is transitioning from a supplementary role to becoming the primary energy source. By accelerating the deployment of renewables, fossil-based power generation will gradually be replaced, leading to electricity decarbonization and progress toward zero-carbon goals. Meanwhile, renewable energy sources, including green electricity and biomass, can be used to produce renewable green fuels, eliminating the reliance on petroleum and contributing to fuel decarbonization and net-zero carbon emissions [2,3], as shown in Fig.1.

Fig.1 Schematic diagram for production pathways of renewable green fuels.

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The maritime shipping industry is currently a key sector for the adoption of green fuels. Accounting for over 80% of global trade volume, maritime transportation emitted 1.008 billion tons of CO₂ in 2022. If the shipping industry were a country, it would rank as the sixth largest emitter of carbon dioxide in the world.

On July 7, 2023, the International Maritime Organization (IMO) adopted a new greenhouse gas (GHG) reduction strategy at the Marine Environment Protection Committee (MEPC 80) meeting. This strategy mandates a reduction of at least 40% in average CO₂ emissions per transport work by 2030, compared to 2008 levels. It also commits to the adoption of fuels with zero or near-zero GHG emissions, with the ultimate goal of achieving net-zero GHG emissions from international shipping by 2050.

On May 16, 2023, the European Union announced the legislative reform of the EU Emissions Trading System (EU ETS), focusing on the “Reform of the rules for monitoring, reporting, and verification of emissions from maritime transport”. Starting from 2024, the EU ETS will cover 50% of GHG emissions from international routes and 100% of emissions from routes within the European Economic Area. Shipowners will be required to purchase carbon allowances for 40% of their shipping emissions starting in 2024, with the percentage increasing to 70% in 2025 and 100% in 2026.

On September 22, 2023, the EU introduced the Fuel EU Maritime Regulation, which will take effect in 2025, aiming to gradually reduce the GHG intensity of fuels used in the shipping industry [1]. As a result, the global shipbuilding and maritime industries are accelerating their transition away from fossil fuels.

2 Green fuel technologies

Propulsion systems, often referred to as the “heart” of ships, play a critical role in determining the efficiency of maritime transportation, the level of environmental pollution, and GHG emissions. Currently, 99% of global marine vessels are powered by reciprocating internal combustion engines. Given the unique characteristics of maritime shipping, such as long voyage distances, large tonnage, and complex operating environments, new energy propulsion systems such as pure electric power and fuel cells face significant challenges in terms of power density, reliability, and cost. Consequently, internal combustion engines are expected to remain the primary power source for marine propulsion systems in the foreseeable future. Renewable green fuels, however, represent a fundamental pathway for achieving decarbonization and net-zero carbon emissions in maritime transportation.

Hydrogen is a widely recognized zero-carbon fuel, but its direct use on ships is hindered by challenges in storage, transportation, and safety. Therefore, converting hydrogen into hydrogen-based fuels with higher energy density and easier storage and transport, has gained widespread acceptance [4]. Since 2021, shipping giants like Maersk have driven the “green methanol fuel scheme” as a popular solution to reduce carbon emissions in the international shipping industry.

In 2023, European shipowners, such as Compagnie Maritime Belge (CMB), began promoting the “ammonia fuel scheme,” generating notable market interest. Shipbuilding and engine manufacturing companies have started developing engines and vessels that use green methanol and ammonia as fuel, spurring the global production and preparation of green fuels.

Green methanol, also known as renewable methanol, is produced using renewable energy, hydrogen, and carbon sources. This process involves electrolyzing water with green electricity to produce hydrogen, which is then combined with carbon monoxide (CO) and carbon dioxide (CO₂)—derived from biomass gasification or CO₂ from carbon capture technologies—to synthesize methanol [5,6]. The use of methanol in marine engines relies primarily on dual-fuel technology, which combines methanol with diesel. This engine technology has become relatively mature in recent years [7], with major global ports such as Rotterdam, Gothenburg, Singapore, and Shanghai announcing plans to initiate methanol bunkering operations.

Renewable ammonia, also known as green ammonia, is produced using renewable energy, hydrogen, and nitrogen sources. Hydrogen is generated via water electrolysis powered by green electricity, and nitrogen is obtained from air separation to produce ammonia through a synthesis process [4,8]. The “zero-carbon” attribute of ammonia has garnered significant attention. However, ammonia engine technology is still in development. When used as a fuel, ammonia poses several challenges, including a high ignition energy requirement, slow flame propagation, poor combustion stability, and difficulties in achieving stable and rapid combustion. Additionally, ammonia combustion can lead to high emissions of NO_x, N₂O and unburned ammonia [8,9]. Ammonia is toxic, and comprehensive safety regulations for its onboard use are still lacking.

Internal combustion engines are typically categorized into two types: spark-ignition and compression-ignition engines, as shown in Fig.2. Spark-ignition engines use high-octane fuels such as gasoline, natural gas, methanol, and ammonia. In contrast, marine engines are typically compression-ignition engines, which rely on high-cetane fuels such as diesel and heavy fuel oil. To use high-octane fuels like natural gas, methanol, or ammonia in compression-ignition marine engines, due to their poor compression-ignition characteristics, dual-fuel systems are required. In these systems, diesel fuel is often used to ignite natural gas, methanol, or ammonia. Dual-fuel systems require two independent fuel supply and injection systems, which increase manufacturing costs, system complexity, and operational and maintenance costs. Furthermore, because diesel fuel is still used in the process, it is difficult to achieve net zero carbon emissions.

Fig.2 Basic principle of internal combustion engines.

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3 Dimethyl ether as a new green marine fuel

This paper proposes a new fuel for marine engines: dimethyl ether (DME), with the chemical formula CH₃–O–CH₃. DME is the simplest ether compound and is recognized for being both non-toxic and environmentally benign. Its chemical structure, featuring a high oxygen content, enables smoke-free combustion, making it an ideal candidate for clean fuel applications. DME has a high cetane number, typically in the range of 55–60, which makes it ideal for use in a compression ignition (CI) marine engine. A summary of its physicochemical properties is provided in Tab.1.

Tab.1 Physicochemical properties of different combustion engine fuels

	Diesel	Methanol	Ammonia	Dimethyl ether
Molecular formula	–	${\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}$	${\mathrm{N}\mathrm{H}}_3$	${\mathrm{C}\mathrm{H}}_3{\mathrm{O}\mathrm{C}\mathrm{H}}_3$
Storage status/Normal status	Liquid/Liquid	Liquid/Liquid	Liquid/Gas	Liquid/Gas
Storage density/Density at normal temperature and pressure/(kg·m–3)	830–850	785	600/0.73	688/1.97
Boiling point at normal pressure/°C	180–370	64.7	–33.5	–24.8
Saturated vapor pressure at 20°C/bar	–	–	8.6	5.0
Low heating value (LHV, MJ·kg–1)	45	20	18.8	28.4
Octane number	–	112	130	61
Cetane number	40–55	–	–	55–60
Flammability limit (in the air, vol%)	0.6–7.5	6.7–36	15–28	3–18.6
Ignition temperature/K	527	737	930	621
Laminar burning velocity/(cm·s–1)	40–50	36	7	47.5
Toxicity	–	Toxic	Toxic	None

The production of green DME is very similar to that of methanol, and primarily follows two main pathways [10]. The first pathway synthesizes DME from hydrogen, which is produced via water electrolysis powered by green electricity, combined with CO derived from biomass gasification. The second pathway involves hydrogen produced via water electrolysis and CO₂ captured from various sources, such as industrial carbon capture, biomass combustion, or emerging technologies like direct air capture. Additionally, DME can be produced by dehydrating methanol, utilizing existing methanol production infrastructure and commercial-scale processes.

From both a fuel applicability and environmental perspective, methanol, DME, and ammonia are considered as carbon-neutral or zero-carbon fuels. However, since methanol and ammonia engines still require diesel fuel for ignition, in particular, pure diesel operation is required at low-load ship operational conditions, marine engines using these fuels cannot achieve zero carbon emissions. In contrast, DME, with its excellent compression-ignition properties, eliminates the need for a separate ignition source, allowing for single-fuel operation with a simpler and more reliable fuel system. More importantly, DME enables the achievement of net-zero carbon emissions.

From an economic standpoint, the cost of green methanol, DME, and ammonia fuels is primarily influenced by the price of green electricity. With the rapid development of photovoltaic and wind power technologies, the cost of green electricity continues to decline. Moreover, global carbon constraints, such as the EU’s carbon tariffs, carbon allowances, and carbon-related incentives, are expected to reduce the green premium of renewable green fuels over time.

Regarding fuel safety, methanol is classified as a toxic, ammonia as highly toxic, and DME as non-toxic and environmentally friendly. In summary, compared to methanol and ammonia, DME offers unique advantages in terms of engine compatibility, environmental performance, and safety. Therefore, it is believed that DME has the potential to become a more competitive green fuel option for marine engines.

4 Research on DME as fuel for CI engine and vehicle

Researches on DME as a fuel were initiated in 1995, with contributions from institutions such as Denmark Technical University, Haldor Topsoe A/S, Navistar, AVL, and AMOCO, which investigated its use in CI engines [11–13]. In China, research on DME as an alternative fuel for CI engine and vehicles has been conducted by the authors at Shanghai Jiao Tong University. This research has focused on various aspects, including DME fuel injection systems, spray characteristics, combustion processes, engine performance, emissions, engine reliability, and the development of DME-tolerant seal materials [14–17]. Key technical challenges, such as low-NO_x combustion, material compatibility, lubrication, and wear issues associated with DME, have been addressed, ensuring the reliability and safety of DME-powered engines. These studies have shown that DME is a promising fuel for achieving more efficient and cleaner engine operation.

As a result, China’s first DME-powered city bus in 2003, followed by a DME-powered city bus fleet in 2009, was developed. The world’s first commercial application of DME in public transportation was realized in 2010 on Shanghai’s Route 147, a project spearheaded by Shanghai Jiao Tong University in collaboration with the Shanghai Automotive Industry Corporation and Shanghai Diesel Engine Company, as shown in Fig.3 and Fig.4. These milestones have laid a solid foundation for the potential application of DME fuel in marine engines.

Fig.3 China’s first DME-powered city bus.

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Fig.4 World’s first commercial application of DME-powered city bus.

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5 Conclusions

In alignment with the global carbon neutrality goal, the shipping industry is undergoing a green energy transformation, moving away from fossil fuels. DME, which can be produced from green hydrogen, biomass, or carbon dioxide captured through carbon capture technologies, offers a promising alternative. DME is non-toxic and environmentally benign, and possesses a high cetane number, making it an ideal fuel for compression ignition marine engines. As a result, DME is considered one of the most promising and suitable marine engine fuels to replace traditional diesel fuel.

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Competing Interests

Zhen Huang is the Editor-in-Chief of Frontiers in Energy, who was excluded from the peer-review process and all editorial decisions related to the acceptance and publication of this article. Peer-review was handled independently by the other editors to minimise bias.

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