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Understanding alternative marine fuels

Ships emit less carbon dioxide (CO2) per ton per mile than other means of transport, but there’s still considerable room for improvement on both emissions and costs. Even disregarding environmental factors, fuel accounts for about 60% of ship operating expenses, giving ship managers financial motivation to improve fuel efficiency.

The Energy Efficiency Design Index and Ship Energy Efficiency Management Plans aim to optimise the efficiency of any fuel, while companies look to alternative fuels for both better energy efficiency and lower emissions.

Considerations for alternative fuels

We have to weigh several factors when selecting an alternative fuel.

For manned ships, safety is the primary consideration. A fuel needs to be safe to produce, transport, store and use. Fuels that need high infrastructure investment for safety or are toxic to humans after short exposures, are likely to be overlooked in favour of safer fuels.

Environmental impact is a major consideration. We need to minimise emissions of greenhouse gases and pollutants including carbon dioxide, sulphur oxides (SOx), nitrous oxides (NOx), methane (CH4) and particulate matter (PM). Fuels that minimise emissions in use but produce emissions elsewhere in the life-cycle won’t address our problem. Traditional fossil fuels produce the bulk of their emissions when burned; conversely, many alternative fuels produce most of their emissions during production or transportation.

Production methods and availability are also key considerations. If alternative fuels are uneconomical, unreasonably difficult to produce, or difficult to scale they’re unlikely to be widely adopted.

Energy density and storage volume are critical when it comes to transportation and storage. Few fuels are as energy-dense, safe and easy to store as diesel, which can make loading, transportation and storage a challenge.

The final key factor to consider is whether current engines can run on the alternative fuel. Modifying engines is possible, but replacing every engine is impractical. An alternative fuel that will work in existing engines has a clear advantage over fuels that require an extensive engine redesign.

Alternative fuels available today

Three groups of alternative fuels are available today: fossil fuels, biofuels, and zero carbon fuels.

Fossil Fuels

Fossil fuels are naturally occurring carbon-based fuels, including the most common marine fuels in use today: heavy fuel oil (HFO), marine diesel oil (MDO) and marine gas oil (MGO). However, liquefied natural gas (LNG) and liquefied petroleum gas (LPG) have advantages over traditional marine fuels.

LNG and LPG don’t eliminate CO2 emissions, but they do reduce them. Their easy availability and the infrastructure already in place give LNG and LPG an advantage over other solutions.

Liquefied Natural Gas (LNG)

LNG is mainly methane. If you must use a fossil fuel, LNG has the lowest carbon content. This means it doesn’t eliminate CO2 emissions, but it does reduce them when compared to traditional fuels. LNG produces about 25% less carbon dioxide than traditional marine fuels, as well as very little SOx and NOx.

The main LNG challenges are transportation, storage, and engine and vessel modifications. At 1 atmosphere, LNG boils at -160°C, so it has to be refrigerated to remain in liquid form. Even without the need for engine modification or replacement, this presents a problem for fuel storage on a conventional vessel.

According to Wärtsilä, converting an existing engine for LNG is economically more feasible than installing new engines, and in practice, all vessels can be converted if they have room for the LNG tank.

Methane loss is a second problem. Methane is more damaging to the environment than CO2. If we don’t contain it, leaks contribute to greenhouse gas emissions. In addition, LNG can pass through the engine unburned, know as methane slip. This is a significant source of methane emissions from LNG engines.

While LNG carriers have used cargo boil-off as fuel for some time, the use of LNG as a marine fuel in other sectors is growing, particularly on cruise ships. Recent EU grants to promote the use of LNG as transportation fuel are likely to make LNG an appealing choice for shipowners and managers.

Liquefied Petroleum Gas (LPG)

LPG is any liquefied mixture of propane (C3H8) and butane (C4H10) in liquid form. Pressure tanks maintain its liquid state, which makes LPG easier to handle than LNG. For this reason, it’s been used as an alternative vehicle fuel among environmentally conscious consumers for years.

While LPG emits more CO2 than LNG does, and requires engine modification or replacement, there are already LPG terminals worldwide. If bunkering infrastructure were in place, LPG could be part of a larger solution.

In August 2018, BW LPG announced plans to convert twelve of their LPG carriers to a LPG-fuelled propulsion system. Conversion of the first two Isle of Man flagged vessels under DNV’s new LPG notation is scheduled for summer 2020. The Isle of Man considers the design to be as safe as methane (LNG) under the International Gas Code.


A biofuel is biomass converted into a liquid or gas fuel. We can produce biofuels from a wide variety of sources and processes, and they often mimic naturally-occurring fossil fuels.

The challenge with biofuels is ensuring that they don’t cause more problems than they solve. Many biofuels rely on food crops as a source material, which can lead to problems of indirect land use change. If biofuel crops are more profitable than food crops, food shortages could result. If vast areas of farmland change to growing a single crop for biofuel production, loss of biodiversity is unavoidable.

Using alternative sources of biomass, such as by-products of food production or marine biomass, could avoid this problem if they are available in sufficient quantities. However, this approach can be difficult to scale.

While biofuels can improve carbon and greenhouse gas emissions when compared with traditional fuels, many biofuels produced from crops rely heavily on synthetic fertilisers and herbicides. This can lead to nutrient and sediment pollution.


Given hydrogen and CO2, methane is straightforward to make. Because it’s chemically identical to LNG, it’s compatible with current LNG propulsion technology.

Synthetic methane natural gas (SNG), bio-methane and liquid biogas (LBG) all fall under the umbrella of biogas. Carbon-neutral synthetic or bio-methane made using renewable energy is carbon-neutral, but more expensive to produce.

In June 2020, ESL shipping’s iron ore ship MS Viikki became the first ship in Finland to be refuelled with 100% renewable biogas. The biogas was supplied by Gasum, a a forerunner in sustainable Nordic energy solutions and circular economy.


Biodiesel includes several products that can be substituted for marine diesel oil (MDO) or marine gasoil (MGO). Hydrotreated vegetable oil (HVO), biomass-to-liquids (BTL), fatty acid methyl ester (FAME), liquid biogas (LBG), and straight vegetable oil (SVO) are all forms of biodiesel. In the same vein, straight vegetable oil (SVO) can replace HFO with minimal or no engine modification

Biodiesel from a range of sources is becoming more widely available as a marine fuel. Combined with the fact that it can be used in standard diesel engines, it’s growing in popularity as a marine fuel. The problem is that large-scale production is unsustainable using current methods.

As early as 2012, Maersk and the US Navy were already testing algal biodiesel; more recent trials include the UECC car carrier Autosky and the Norden’s tanker Nord Highlander. To comply with IMO 2020, the process of blending biodiesel with MGO or MDO to reduce its sulfur content is growing in popularity. This gives the biofuel industry a chance to scale up production, while shipping companies get to know the available biofuel options.

Alcohol (Methanol and Ethanol)

Alcohol was around for a very long time before we started using it as marine fuel. The two alcohols that are useful as fuels are methanol (CH3OH) and ethanol (C2H5OH). Methanol has the lowest carbon content and  highest hydrogen content of any liquid fuel, but both options comply with the most stringent emission control measures.

Methanol and ethanol are simple to produce from a variety of sources, including coal, natural gas, biomass, or even directly from CO2 and hydrogen. They have similar properties to LNG but are easier to handle. Modifying existing engines, bunkering and storage infrastructure to handle alcohol costs less than converting engines and infrastructure to handle LNG and LPG.

The downside is that alcohols are more expensive to produce than LNG and LPG, bunkering facilities are limited, and they’re highly flammable and toxic. Because there’s no existing infrastructure for storage and handling, use of pure alcohol as a fuel would require extensive infrastructure investment.

In 2016, Waterfront Shipping Company Ltd., Mitsui O.S.K. Lines, Ltd., Marinvest/Skagerack Invest, and Westfal-Larsen Management ordered seven 50,000 DWT dual-fuel ships that can run on methanol, HFO, MDO and MGO. In 2018, several of those companies, along with IINO Kaiun Kaisha, Ltd. and the NYK Group ordered another four.

Zero Carbon Fuels


We can produce hydrogen in several ways, including from natural gas, water electrolysis, and breakdown of ammonium. Unfortunately, some methods just move carbon production upstream. Currently, we produce nearly all hydrogen from natural gas, which requires electricity. Common forms of electricity generation release CO2. While we could use renewable energy, it’s something to consider in any plans to use hydrogen as a fuel. In addition, when we break natural gas down to create hydrogen it releases even more CO2.

On a brighter note, hydrogen is a building block for a range of fuels. It can be used directly as a liquified gas, combined with CO2 to produce methane, known as power-to-gas (PtG), or combined with CO2 to produce a liquid fuel similar to diesel, known as power-to-liquid (PtL). Power-to-liquid and power-to-gas are known collectively as power-to-fuel (PtF)

As a fuel, hydrogen produces zero CO2 emissions. The problem is that, depending on its state, hydrogen’s energy density is 4-8 times lower than traditional fuels. To liquify hydrogen it has to be kept at -253°C at atmospheric pressure. It’s also highly flammable and will ignite at just 4% oxygen, making it difficult to transport safely.

If we could store and transport hydrogen in another form, for example in natural gas, water or ammonium, it would be easier than transporting raw hydrogen. If we then convert it back to hydrogen on board, we could avoid or minimise several of hydrogen’s problems.


As a fuel in an internal combustion engine, ammonia has zero CO2 emissions in use. It can be used for both internal combustion engines and fuel cells, and is easier to store and transport than compressed hydrogen. Because it’s among the most common industrial chemicals, it’s readily available, although there’s currently no marine bunkering infrastructure.

Like most alternative fuels, ammonia has a much lower energy density than traditional fuel oils. It’s twice as heavy and needs three times the space for the same amount of energy. Ammonia is also caustic and toxic, so handling, transport and storage facilities need to be adapted to ensure safety. Despite that, it’s no more dangerous than petrol.

From an environmental perspective, there’s an extra hurdle to overcome: industrial ammonia production emits more CO2 than any other chemical-making reaction. While scientists are making progress towards cleaner production methods, we’re not there yet.

Alternative marine energy sources

Nuclear power

Nuclear power is clean and reliable, and the ability to go for long periods without refuelling is helpful in remote areas. Russia’s nuclear-powered icebreakers benefit from those advantages, giving them a clear advantage in Arctic regions. To offset this, the US Coast Guard is planning a new generation of nuclear-powered icebreakers.

The challenges of disposal of used fuel and the crew training required mean that only governments are pursuing nuclear power as a viable fuel alternative.

Fuel cell systems

In a fuel cell, a continuous supply of fuel and an oxidising agent meet and react producing electricity and heat. Depending on the type of fuel cell, they can be about 60% efficient. Compared to internal combustion engines, they’re quiet, cool, and vibration-free.

German, Greek and US submarines use hydrogen fuel cells, which have been proved to be reliable. However, ammonia fuel cells are growing in popularity. In early 2020, Equinor announced their plan to convert the LNG-powered supply vessel Viking Energy to run on ammonia fuel cells. This EU-funded project is due to be completed in 2024. They plan to test the fuel cells over long distances, and hope that ammonia will meet 60 to 70% of the power requirements on board. The remaining power will be delivered by LNG.

Looking ahead

In 2020, no single alternative fuel is ready to replace all traditional marine fuels across the industry. Despite that, the industry is coalescing around a few solutions. Dual and multi-fuel vessels are growing in popularity. These ships can run on traditional fuel oils, biofuel, and one or more other fuel options.

Since we don’t know which alternative fuel solution will win out, dual and multi-fuel vessels give the flexibility to take advantage of future economic and technological shifts.

Nic Gardner

Nic Gardner is a Maritime Technology Analyst at Thetius. She is a master mariner who holds an unlimited UK CoC and has seagoing experience on capesize bulk carriers, ro-pax ferries, sail training ships, hospital ships, general cargo tramp ships, container ships and fisheries protection boats. When she is not at sea, Nic writes about a range of topics including technology and the maritime industry. Nic is also the author of “Merchant Navy Survival Guide: Survive & thrive on your first ship”, a book to give aspiring seafarers the knowledge and tools they need to make a success of their first trip to sea.

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