January 2022

Biofuels, Alternative/Renewable Fuels

Comparing green fuels for marine engines—Part 1

The marine shipping industry, which accounts for 80% of global trade and 3% of annual global carbon emissions,1 must adhere to stringent International Maritime Organization (IMO) goals of reducing total annual greenhouse gas (GHG) emissions by at least 50% below 2008 levels by 2050.2

Bushiri, D., Refinery Automation Institute

The marine shipping industry, which accounts for 80% of global trade and 3% of annual global carbon emissions,1 must adhere to stringent International Maritime Organization (IMO) goals of reducing total annual greenhouse gas (GHG) emissions by at least 50% below 2008 levels by 2050.2 Essentially, GHGs—in the form of carbon dioxide (CO2) emissions—must be reduced by 40% by 2030 and by 70% by 2050 to achieve this goal. Similarly, the European Union (EU) plans to reduce its annual CO2 emissions to 40% below 2005 levels by 2050.3 To achieve these reductions, marine fuels must produce near-zero emissions at every step of their lifecycles.

Large oil companies have been pressured to undertake the GHG emissions footprint reduction. Part of this push is to make green fuels with inherently lower GHG emissions than conventional fossil fuels. The U.S. Environmental Protection Agency (EPA) defines alternative fuels to include gaseous fuels (such as hydrogen, natural gas and propane), alcohols (such as ethanol, methanol and butanol), vegetable and waste-derived oils, and electricity.4 Of these alternative fuels, LNG, hydrogen, ammonia, methanol and biofuels are considered potential green fuels for the marine industry. These alternative fuels must not only reduce emissions, but must also be transportable, storable, producible and inexpensive, and able to generate enough energy to propel large ships across the world.

However, the shipping industry primarily uses hydrocarbons—in the form of heavy fuel oil (HFO) and marine gasoil (MGO)—in its engines because they are cost effective and widely available. To adhere to emissions reductions by 2050, the Institute of Electrical and Electronics Engineers (IEEE) forecasts that at least 50% of all marine engines will run on green fuels by the mid-2040s (FIG. 1).

FIG. 1. Forecast of fuel usage in the marine industry. Source: IEEE.

Significant efforts are required to innovate pathways to use green fuels in existing or new engines and in bunkering/port infrastructures. As an initial step, an assessment of the technical performance and environmental impacts of each future green fuel is required. Multiple factors such as cost, availability, production and infrastructure vary for these fuels and may influence the feasibility of their use. A thorough investigation of these potential fuels is needed to evaluate the practicality and feasibility of each option. Do these green fuels actually reduce GHG emissions? Will they operate with the same throughput as fossil fuels? How much will they cost to implement? For green fuels to be attractive to both shipowners and fuel producers, they must correlate to the functionality of fossil fuels and not just meet the emissions requirements.

Finally, standardized engineering definitions and calculations are needed to be able to make comparisons on an equal basis, such as the use of standardized emissions lifecycle models to calculate carbon footprints.

Green fuels for marine engines

This article examines five potential green fuels for marine engines: LNG, hydrogen, ammonia, methanol and biofuels.

LNG. LNG is leading the race as a substitute green fuel. It has a relatively high energy density of 22.2 MJ/L, a rapidly developing infrastructure and is widely available. However, it is viewed as a non-sustainable transition fuel since it emits 2% more carbon during its lifecycle. LNG has the potential to be a bridge fuel to hydrogen due to similar cryogenic storage requirements.

Hydrogen. Hydrogen is an energy-dense fuel (FIG. 2) and can be produced with no GHG emissions. With an energy density of 8.5 MJ/L, liquid hydrogen can be used in both internal combustion engines (ICEs) and fuel cells. However, the journey using hydrogen sustainably is long winded. At present, gray hydrogen is produced via methane reforming, which emits GHGs comparable to HFOs. Zero-carbon-emissions hydrogen is produced from renewable-electricity-driven water electrolysis, which is more expensive than gray hydrogen production. Although hydrogen-energy dense, it has a much lower energy density vs. fossil fuels, which will result in cargo capacity limitations.

FIG. 2. Energy density of fuels per liter equivalent of fuel. Source: The Royal Society.

Ammonia. With an energy density of 11.5 MJ/L, liquid ammonia is another viable option that can be used in existing engines and bunkering facilities. Used either as a hydrogen carrier or as a fuel, green ammonia can have zero emissions within its lifecycle when produced with green hydrogen or directly via electrochemical cells. However, just like hydrogen, ammonia is less energy dense than conventional fossil fuels. It is also highly toxic and requires strict guidelines.

Methanol. Methanol has a high energy density of 16 MJ/L. It gained popularity due its availability and easy storability. Renewable methanol (i.e., bio-methanol or e-methanol) has the potential of reducing GHG emissions by 65%–95%. However, it is less dense than traditional fuels, and renewable methods are significantly more expensive than conventional fossil-fuel-derived methanol.

Biofuels. Biofuels are the most researched alternative fuel. Several marine engines and fueling infrastructures are compatible with biofuels, with minimum modifications. Bio-derived fuels (e.g., biodiesel) show potential. The major challenge with the use of biofuels in the marine industry is the significant increase in production required to satisfy demand, which will necessitate more land reuse and the elimination of vegetation. The sustainability of biofuels is put into question when the lifecycle analysis and carbon offset are considered. GHG emissions reductions are hindered by land-use changes and other upstream emissions.

These green fuels have the potential to reduce carbon emissions. However, when the exact quantities are presented alongside the lifecycle analysis, their viability is questioned. The primary challenges are that green fuels have lower energy densities vs. conventional fossil fuels. There are also diverse issues regarding availability, port and bunker infrastructure, engine development challenges, cost, supply and safety. The lifecycle emissions further highlight the misconception that people have about green fuels and their emissions reductions.

As a possible solution, a multistage strategy must be developed. Since infrastructure exists for LNG, biofuels and conventional methanol, these fuels can be viable as short-term solutions (5 yr–10 yr). Corporations should start moving away from LNG, gray methanol and biofuels in the medium term (10 yr–15 yr) and begin focusing on converting LNG engines/facilities to use green hydrogen fuel and converting biofuels/methanol to e-methanol. As a long-term solution (15 yr–20 yr), corporations should invest in advancing green ammonia technology. Since ammonia infrastructure already exists and can be used in existing engines/bunking facilities, green ammonia has the capability to be used as a sustainable fuel for the marine industry, while also ensuring zero GHG emissions. It would also be beneficial for diverse fuel alternatives to continue being used to provide flexibility to the industry. Therefore, in the long term, most ships should be fueled by ammonia, while smaller ships can be fueled by hydrogen and existing methanol ships should continue operations using e-methanol.

Statement of the problem

The questions that the marine industry are battling with is which green fuel is the most feasible alternative for short-term and long-term sustainability, and is it realistic? This article provides a review analyzing the feasibility and practicality of several green fuels. First, it is important to consider the source/feedstock and production mechanism used to produce the fuel—the amount of energy required to produce the fuel and the relative cost of production. Then, the practicality of each fuel is investigated based on the port and bunker infrastructure required, fuel engine development, projection of the correlation between fossil fuels, the fuel’s suitability among long-range vessels, supply issues and the associated risks involved. The short-term and long-term initiatives associated with each green fuel are analyzed through an assessment of current and projected technological projects, research and development initiatives and projects, and competing industries. Finally, the overall feasibility outlining the benefits of each fuel and the lifecycle analysis of GHG reduction of each fuel are provided as recommendations.

Part 1 of this article will examine LNG, hydrogen and ammonia. Part 2, to be published in the February issue, will examine methanol and biofuels.


The following are the results and findings for LNG, hydrogen and ammonia.


As a preliminary requirement, an alternative marine fuel should contain less carbon than the existing hydrocarbons used in the industry. LNG is considered an interim potential solution, since it contains less carbon per unit of energy.5 When burned during combustion, LNG will release less CO2 vs. conventional fuels. For years, LNG has been used as a secondary fuel through combusting boil-off gas from LNG cargoes.6 Due to the IMO’s sulfur standard regulations, LNG could act as a fuel replacement. Many companies in the marine industry considered switching to LNG-fueled engines, since they emit less than 0.1% of the fuel-equivalent threshold.7

LNG is produced via the hydraulic fracturing extraction of natural gas, which is then liquefied and stored under cryogenic conditions (–160°C). The main source of natural gas is primarily underground reserves, but some companies utilize biogas. Natural gas in its liquid state is about 600 times smaller in volume than in its gaseous state.8 Therefore, natural gas is liquefied to enable easy global transportation.

In terms of energy requirement, the production process extraction and liquification is energy intensive. It requires a relatively high initial capital investment, which involves exploration, drilling, piping to a coastal liquification plant and the liquification process. In addition, shipowners can expect to pay approximately $5 MM more for an LNG-fueled vessel than one that is run off conventional marine fuels.9 LNG engines require a larger investment than installing scrubbers on ship, which is another alternative solution to adhere to the IMO requirement. However, an offset of the capital investment is expected, due to fuel cost advantages—LNG is the most cost-efficient fossil fuel, once its production infrastructure is secure.10

However, LNG has a lower energy density than diesel. The Alternative Fuels Data Center reports that LNG has a specific energy density of 21,240 Btu/lb and a mass density of 3.49 lb/gal, while low-sulfur diesel has a specific energy density of 18,122 Btu/lb and a mass density of 7.09 lb/gal.11 While the specific energy density is slightly comparable, the large difference in mass densities means that diesel has nearly twice the amount of energy/gal when compared to LNG. Ships fueled by LNG also require more space for fuel tanks, which may limit cargo capacity.

All major shipping companies are either developing, or have already launched, a variety of engines that run on LNG. These technologies include a two-stroke engine and a dual-fuel, slow-speed engine that operates on LNG as the primary fuel and on diesel as a secondary fuel.12 A variety of engines can use LNG or natural gas. These include steam engines, lean-burn spark-ignition engines, low-pressure injection dual-fuel (LPDF) engines, high-pressure injection dual-fuel (HPDF) engines and gas turbines.

Although LNG emits 20%–30% less carbon during combustion, LNG has a large carbon footprint. Pavlenko et al. analyzed LNG’s lifecycle when used in different engines.13 Since LNG is mostly methane, each engine releases unburned methane due to incomplete combustion. Fugitive methane is a more potent GHG than CO2, and its emissions are more detrimental to the environment. To perform a complete lifecycle analysis, the upstream and downstream emissions had to be evaluated. Pavlenko et al. evaluated the upstream emissions of LNG using GHS, regulated emissions and energy use in transportation (GREET) modeled by the Argonne National Laboratory. Downstream emissions were sourced from available company data sets. As shown in FIG. 3, LNG has higher upstream emissions than conventional fuels such as MGO, very-low-sulfur fuel oil (VLSFO) and HFO. These high emissions are a result of high methane emissions due to leakage that occurs during extraction, processing and transport. In addition, the liquification process contributes to upstream emissions. Although LNG has lower downstream emissions than conventional fuels, LNG has a higher total GHG emissions rate when the upstream emissions and methane slips are accounted for, irrespective of engine types.

FIG. 3. Lifecycle GHG emissions by engine and fuel type.5


According to the International Energy Agency (IEA), hydrogen is a potential carbon-zero fuel alternative. With byproducts like water and steam, hydrogen can eliminate pollutants from the transportation industry. Hydrogen is also abundant in the environment and can be produced from diverse sources. Its use in the transportation sector is still in the primary phase. Hydrogen became popular with the use of fuel-cell-powered vehicles. Using hydrogen fuel cells integrated with an electric motor is about three times more efficient than gasoline-powered ICEs.14 Hydrogen is used in light-duty, fuel-cell vehicles; however, can it be used to propel ships across the ocean?

One challenge with using hydrogen as a fuel is its production process. Readily available substances, such as organic matter, water and hydrocarbons, can be used as a source of hydrogen.14 FIG. 4 depicts the relevant sources of hydrogen and their associated names. The most common source of hydrogen is methane/natural gas, which is called gray hydrogen. Steam methane reforming is used to convert methane to hydrogen through the process of synthesis gas generation, hydrogen generation and gas purification. CO2 is the primary dilutant removed during the purification process. Similarly, brown hydrogen is hydrogen that is produced from coal, using the same steam methane reforming (SMR) process.

FIG. 4. The different types of hydrogen production.15

Although SMR is the cheapest way to produce hydrogen, this process emits a significant amount of carbon, resulting in emitting CO2 as a byproduct.15 SMR emits about 10 t of CO2/t of natural gas, while coal produces 19 t of CO2/t of coal.16 To minimize the emissions associated with brown hydrogen, carbon capture technologies are added to the SMR purification step to create blue hydrogen. However, this process relies on fossil fuel sources, which still have a carbon footprint. Hydrogen produced via water electrolysis has the potential to eliminate emissions from this production step. Water electrolysis uses electricity to separate hydrogen and oxygen from water. Electrolysis powered by fossil-fuel-sourced grid electricity is called blue hydrogen, while electrolysis powered by renewable energy is referred to as green hydrogen. While green hydrogen is the most environmentally friendly way of producing hydrogen, it is the most expensive option. Another production option—turquoise hydrogen—is still in the research phase. Turquoise hydrogen is produced through the process of pyrolysis, which converts methane to pure solid carbon and hydrogen.

For hydrogen to be a reasonable fuel alterative, green hydrogen should be the primary type used in engines or fuel cells. Significant investments are required in renewable energy technology. At present, only 3.9% of the hydrogen produced comes from water electrolysis.15 Intervention is needed to reduce the cost of green hydrogen, so that it may be used as a fuel. Fortunately, the cost of green hydrogen has reduced by approximately 50% since 2015 and is forecast to continue to decrease as more projects focus on renewable energy.16 Hydrogen production will also need to increase significantly to meet the demands of the marine industry. Most produced hydrogen is used as a component in the chemical industry and in oil refineries.

Like LNG, hydrogen has a lower energy density than conventional marine fuels. Hydrogen has an energy density of 8.5 MJ/L, which is about 15% less than the energy density of diesel.16,17 Therefore, ship capacity becomes a significant disadvantage for hydrogen’s use in the marine industry. To store the same amount of cargo on board will require fuel tanks to be seven times larger than diesel tanks.16 Hydrogen could be ideal for smaller ships or for shorter trips that have frequent access to bunkering stations. Larger ships would need hydrogen storage that takes up less space on the ship (e.g., ammonia or liquid hydrogen).

Hydrogen must also be stored at high cryogenic conditions (–253°C), requiring expensive bunkering facility requirements and stringent safety procedures. According to the IEA, the cost infrastructure is offset by the cost of fuel when calculated at a 15 yr–20 yr lifespan.17 Once the fuel cost becomes competitive, hydrogen may be a viable fuel alternative in the future.

Hydrogen can be used directly as a fuel in an ICE and in fuel cells to generate electricity. Dual-fuel ICEs are a possible route for hydrogen; whereby, MGO can be used when the hydrogen inventory on ships runs out.15 However, there is presently no hydrogen fuel engine commercially available. There are also no federal regulations for design considerations for the use of hydrogen as a marine fuel, and, therefore, there is no formal approval for its commercial use. The U.S. Coast Guard may approve alternative design proposals for companies on a case-by-case basis.18 The knowledge gained from the use of LNG may bridge to developing design solutions for hydrogen-fueled vessels. Overall, there are many projects geared toward developing hydrogen technologies. For example, the U.S. Department of Energy (DOE) has funded many projects that focus on overcoming hydrogen’s shortfalls.

Hydrogen’s lifecycle analysis as a fuel is dependent on the source of production. Hydrogen production emits 60 MMtpy of CO2, primarily from brown/gray hydrogen production, according to the U.S. EPA. Brown/gray hydrogen and HFO emit roughly the same amount of carbon.16 Like LNG, gray hydrogen has high upstream emissions that results from the extraction of natural gas. This includes methane leakage during the extraction process, along with methane slips during transportation and SMR processes. Blue hydrogen captures approximately 90% of the CO2 emitted during the SMR process.19 However, this option still involves major upstream emissions. Howarth et al. conducted a study on blue hydrogen and found that it emits 9%–25% less CO2 than gray hydrogen. However, since methane is used to power carbon capture technology, fugitive methane emissions are much greater than gray hydrogen. Howarth et al. concluded that the lifecycle GHG emissions of blue hydrogen are 20% times greater than with burning natural gas or coal. Turquoise hydrogen is still being researched, but it has the potential of minimizing carbon emissions from the hydrogen fuel lifecycle. The process produces solid carbon instead of CO2, which eliminates the need for carbon capture. The solid carbon can be used for other applications. However, methane is used as the feedstock and produces significant upstream emissions. These emissions can be reduced by using renewable energy to drive pyrolysis.20 Green hydrogen is the optimal solution, since it does not produce emissions during production or combustion. However, emissions associated with renewable electricity production are not considered.


While some may think of ammonia as a hydrogen storage medium, ammonia can be used as a fuel in marine engines. In recent years, ammonia received a lot of attention as a potential future carbon-zero alternative fuel for the transportation industry. For example, Kang and Holbrook evaluated the feasibility of using ammonia as a fuel in light-duty cars.21 Marine shipping companies are also investing in ammonia-fueled vessels and engines.

Ammonia is a beneficial fuel alternative, since combustion produces only nitrogen and water. It is already shipped globally in huge quantities for the fertilizer industry. Ammonia is a liquid at room temperature and at moderate pressures, meaning it is relatively easy to store, and, despite common misconceptions, ammonia can be relatively safe to use.

Anhydrous ammonia is primarily produced from nitrogen and hydrogen via the Haber-Bosch process. There are different types of ammonia: gray/brown, blue, turquoise and green. The lifecycle emissions and prices are also dependent on the hydrogen production. Nitrogen is separated from air, using a low-energy technology. Additional research is being conducted on novel ways to produce ammonia. Biological nitrogen fixation is a potential source for green ammonia. It uses the nitrogenase enzyme to catalytically convert atmospheric nitrogen to anhydrous ammonia.22 Electrochemical cells can also be used to convert water and nitrogen to green ammonia, which can eliminate the need for a separate hydrogen production step. Renewable energy may be used in this process to ensure zero carbon emissions.

With a density of 11.5 MJ/L, ammonia is twice as dense as hydrogen and more dense than liquid hydrogen.21 However, ammonia is less dense than diesel and requires less fuel tank capacity than hydrogen. Since ammonia is a liquid fuel at room temperature and has similar properties to propane, it can be used in existing bunkering facilities. Minor changes to materials on vessels would be needed to ensure safety on board. Material compatibility requirements for shipping ammonia are well known and can be easily implemented at bunkering facilities and on marine vessels.

Regarding ammonia supplies, global ammonia production would have to significantly increase to meet global fuel demand. All the raw materials are readily available in the environment to meet the demand; however, large amounts of electricity will be required.23 More bunkering facilities must be built to accommodate the increased production.

The associated risk with using ammonia is its toxic and corrosive nature. While ammonia is a toxic chemical requiring strict safety precautions, it is less flammable than hydrogen and LNG. It has the potential to emit nitrous oxide, which can be eliminated with the use of a catalyst to favor the reaction that produces atmospheric nitrogen and water as combustion products.

Corporations are in a race to develop different engines to facilitate the use of ammonia as a fuel. These technologies include two-stroke and four-stroke ammonia engines, as well as ammonia fuel cells that will convert ammonia to hydrogen to produce electricity. One major shortfall of ammonia is its inability to ignite quickly. Secondary fuels, such as hydrogen or conventional fuels, could solve this issue in a dual-fuel ICE. Another solution is to develop a spark-ignited gas engine to facilitate the combustion of ammonia.

Part 2

Part 2 will be published in the February issue. HP


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  3. Kallas, S., “Roadmap to a Single European Transport Area—Towards a Competitive and Resource-Efficient Transport System,” European Commission, 2011.
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  20. Florence School of Regulation, “Between Green and Blue: A Debate on Turquoise Hydrogen,” March 18, 2021, online: https://fsr.eui.eu/between-green-and-blue-a-debate-on-turquoise-hydrogen
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