Credit: MartinLueke/Shutterstock.com | Massive ships transport billions of metric tons of goods around the globe every year, emitting greenhouse gases as they sail.
One hundred thousand giant ships sail the world’s oceans, transporting 80–90% of everything traded internationally. Most of those ships are propelled by engines that burn heavy fuel oil, the material left over from refining crude oil. This fuel’s combustion releases tons of particulates and accounts for about 3% of global greenhouse gas emissions. Large multinational organizations aim to cut emissions from shipping by 50% in the coming years, but the path to decarbonizing maritime transport is unclear. Experts are evaluating various methods to clean up ship emissions and are considering alternative fuels, including liquefied natural gas, hydrogen, ammonia, and methanol. All have pros and cons in terms of emissions, safety, feasibility, and cost.
Look at all the stuff around you. Unless you’re in the middle of the desert or somewhere else far from civilization, nearly everything you see traveled to you by sea.
“Ships bring 80–90% of most everything you want or need, or the raw materials used for making those things,” says Natasha Brown, a senior spokesperson for the London-based International Maritime Organization (IMO), a United Nations agency with 175 member states. The organization oversees shipping safety and security and is responsible for preventing water and air pollution from ships. “Mobile phones, iPads, grains for breakfast cereal, iron ore, crude oil, bananas, and avocados. All of it crosses the oceans by ship.”
Raw materials and manufactured products cross the oceans every day in enormous vessels, including bulk cargo carriers, tankers, and container ships. The variety of goods shipped across the oceans in 2019 weighed a whopping 11 billion metric tons (t), an increase of roughly 3 billion t from just a decade earlier, according to the United Nations Conference on Trade and Development.
This volume of shipping traffic burns massive amounts of fuel and produces large amounts of air pollution. “The shipping industry uses more than 300 million tons of fossil fuels every year, roughly 5% of global oil production,” says Camille Bourgeon, a specialist in air pollution and energy efficiency in the marine environment at the IMO. In 2018, global shipping activity emitted roughly 1.05 billion t of carbon dioxide into the atmosphere, accounting for about 2.9% of the total global anthropogenic CO2 emissions for that year, according to the IMO’s 2020 greenhouse gas study.
As global commerce continues to grow, transoceanic shipping will grow too. So the shipping industry and regulators see a need for change.
“We are faced with a real crisis, a real urgency here, and we need to respond,” says Morten Bo Christiansen. Christiansen is head of decarbonization at Copenhagen, Denmark–based A.P. Moller-Maersk, one of the world’s largest shipping companies.
Maersk and other shippers are working to cut emissions from shipping by at least 40% by 2030 relative to 2008 levels, in keeping with international agreements reached in 2018 by the IMO’s member states. The plan calls for cutting all greenhouse gas emissions in half by 2050 and entirely phasing out ship emissions as soon as possible in this century.
The industry hopes that by switching from standard marine fuels to greener alternatives and by boosting energy efficiency, it can cut emissions of climate-changing gases and air pollutants known to harm human health. The industry is evaluating numerous sources of energy for propelling ships, including liquefied natural gas, methanol, hydrogen, and ammonia, and it is testing demonstration vessels. But a front-runner has yet to emerge.
Heavy fuel oil (HFO) has been the fuel of choice for large ships for more than a century because it is inexpensive and energy dense—a relatively small amount can propel a ship for great distances. Also known as residual fuel oil, HFO is the gooey, tar-like residue that remains after petroleum crude has been catalytically treated (cracked) and distilled to separate lighter, more valuable fuels such as gasoline and automotive diesel. The viscous leftover, which must be heated to flow through ship engines, contains a mix of paraffins, olefins, aromatics, and asphaltenes, as well as compounds containing sulfur, nitrogen, and metals.
“Ships use so much fuel, and historically, they got away with using the worst bits, the stuff no one else wants,” says Stephen R. Turnock, a maritime engineer at the University of Southampton. “What’s more, they burned it out of sight and out of mind,” he adds, meaning they burned it in the middle of the ocean, where for many years, few people cared about noxious emissions. “It’s only when lots of ships congregate in port that people start to realize how bad these emissions are.”
The mass of goods transported internationally by ship
Percentage of internationally traded goods transported by sea.
Number of large (>100 gross tons) cargo vessels in global commercial fleet.
Amount of CO2 emitted annually from shipping. Shipping produced 3% of global human-made CO2 emissions in 2018.
Mass of fossil fuels used annually by shipping.
Sources: International Maritime Organization and United Nations Conference on Trade and Development
The shipping industry has already faced initiatives and regulations to clean their emissions. The IMO previously adopted mandatory caps on emissions of nitrogen oxides (NOx) and sulfur oxides (SOx), both of which can produce acid rain and lung-penetrating particulate matter.
In 2020, for example, a new policy came into force that lowered the maximum sulfur content of ship fuel from 3.5% by weight to 0.5%. Stricter limits, 0.1% sulfur, apply in coastal regions and other designated areas. The IMO’s NOx limits, which vary by the ship’s engine size, operating speed, and construction date, have grown increasingly stringent in the past decade and can now be as low as 2 g of emissions per KW h of energy output.
Many shippers have met the more stringent regulations by switching from standard supplies of marine fuels to cleaner, costlier ones with less sulfur or by blending very-low-sulfur fuels with others to achieve the needed purity. In some cases, ship operators comply with the regulations by switching to less-polluting fuels just in certain coastal regions.
But other shippers continue using standard fuels and treat engine exhaust chemically to remove the oxides of nitrogen and sulfur. NOx forms at the high temperatures typical of marine engines as air, which is mostly dinitrogen, reacts with fuel to drive combustion. More than 95% of nitrogen oxides in the exhaust can be removed readily via a process known as selective catalytic reduction, which is widely used to control NOx emissions from diesel trucks and cars. In the process, a reducing agent such as an aqueous urea solution is added to the stream of exhaust gas. The exhaust species and reducing agent then react on the surface of a catalyst to produce dinitrogen and water.
Many ships strip sulfur and other contaminants from engine emissions using exhaust-gas scrubbers. These devices work by reacting acidic exhaust gases with an alkaline scrubbing material. The technology can reduce ship emissions of SOx to mandated levels. But scrubbers are a source of controversy because some ships discharge the spent scrubbing medium into the oceans, polluting those waters.
Key players in the shipping industry agree that the most promising path to further clean up emissions is by moving away entirely from traditional HFO and toward alternative fuels. The alternative fuels that the industry is considering all have advantages and disadvantages, which experts are weighing.
Liquefied natural gas (LNG) tops the list of nontraditional fuels currently used in commercial ships, including some large container vessels. The number of LNG-fueled ships has climbed in the past decade from tens to hundreds, and more have been ordered.
LNG typically consists of roughly 90% methane, several percent ethane, and a mixture of other light alkanes. Suppliers cool the gas mixture to cryogenic temperatures (–162 °C) to liquefy the fuel so it can be stored in nonpressurized containers, occupying about one six-hundredth of the volume of the gas.
In several ways, LNG would be an improvement over HFO. For example, switching from HFO to LNG could reduce SOx emissions by 99%, NOx emissions by 80%, and CO2 emissions by as much as 20%. LNG also produces relatively little particulate matter. A key disadvantage of LNG is that it consists primarily of methane, which has a far higher global warming potential than CO2—86 times as high, by some estimates. So even small gas leaks during production, refueling, or use could result in a relative increase in greenhouse gas emissions.
Other drawbacks include the large capital investment required for LNG-compatible engines, fuel tanks, and new infrastructure for refueling ships in port, a process known as bunkering. In addition, some industry watchers question the suitability of investing further in LNG because it would extend the use of carbon-based fuels for at least another 20 years, which is a common life span for large ships.
Maritime engineers, naval architects, and other shipping experts are busy evaluating other alternative fuels and ways to achieve low-emission or zero-emission shipping. These analyses consider the raw materials, production methods, performance, and overall life cycles of fuels, in addition to other factors, such as the economics of shipping.
In one study, Joanne Ellis and Martin Svanberg of SSPA Sweden, a ship research and testing center, together with colleagues at Luleå University of Technology, evaluated renewable methanol as a shipping fuel (Renewable Sustainable Energy Rev. 2018, DOI: 10.1016/j.rser.2018.06.058).
Methanol is currently produced mainly via the catalytic conversion of synthesis gas, a mixture of carbon monoxide and hydrogen obtained from reforming natural gas or from coal gasification. But methanol has also been produced from many types of solid and liquid biomass feedstocks, including agricultural and forest residues and farming and poultry waste.
Switching to methanol made from these biomass sources could lower emissions from shipping and reduce the industry’s overall environmental impact, Ellis says. She also notes that although many of the technologies needed to produce methanol from these biosources have not been commercialized at the large scale needed for the shipping industry, they have proved feasible at the pilot or demonstration scale.
Methanol offers advantages over some alternative fuels. Because methanol is a liquid that is stored, transported, and used at ambient temperature, implementing it as a shipping fuel would be more straightforward than switching to cryogenic LNG or gaseous fuels such as hydrogen.
“Renewable methanol is a technically viable option to reduce emissions from shipping,” and there are no major challenges with potential supply chains, Ellis says. She adds that there are economic barriers, including capital investments and the fact that biomethanol currently costs more than conventional fuels, “but they do not seem to be insurmountable.”
Hydrogen is often touted as a clean fuel because water is its only combustion product. The way the fuel is made, however, greatly affects its ecofriendliness. Selma Atilhan and Mahmoud M. El-Halwagi of Texas A&M University and coworkers analyzed hydrogen as a shipping fuel, assessing the environmental impact of the fuel based on its production method.
The group classified hydrogen in three ways. They labeled it as gray when it was made by reforming natural gas or other fossil fuels, which is the case for about 95% of global hydrogen production. Blue refers to standard hydrogen production when carbon emissions are captured, stored, or used. Green corresponds to hydrogen made from renewable feedstocks using a renewable source of energy throughout production.
Global hydrogen production from fossil sources currently generates 830 million t of CO2 equivalents (CO2 eq) per year, a unit that accounts for other greenhouse gases. Hydrogen can be made renewably, however, by splitting water using energy from solar power, wind, nuclear power, hydropower, and other sources. Plants that will generate hundreds of metric tons of hydrogen per day via some of these routes are under construction.
The analysis led the Texas A&M researchers to conclude that liquid hydrogen is a top choice for substantially cutting carbon emissions, but the fuel needs to be green.
The team found that gray liquid hydrogen is reasonably cost effective (close to one-fourth the cost of green hydrogen), and it produces almost no carbon emissions when combusted to propel the ship. But during hydrogen production, gray hydrogen’s carbon footprint, 120–155 g CO2 eq per megajoule of energy contained in the fuel, exceeds that of heavy fuel oil, about 90 g CO2 eq/MJ. The production of blue liquid H2 can have a lower carbon footprint (40–90 g CO2 eq/MJ) depending on carbon-capture technology and other factors. The carbon footprint of green liquid H2 can be as low as 4.6 and 11.7 g CO2 eq/MJ for hydrogen made with wind and solar energy, respectively, making it a promising shipping fuel (Curr. Opin. Chem. Eng. 2021, DOI: 10.1016/j.coche.2020.100668).
Back at Southampton, Turnock and maritime engineering colleagues Charles J. McKinlay and Dominic A. Hudson conducted a detailed analysis of hydrogen, ammonia, methanol, and other fuels. The team finds that although these compounds can be burned in internal combustion engines, using the compounds in fuel cells instead would extract the greatest amount of energy from the fuels and provide the potential for generating emission-free electricity.
Are fuel cells up to the challenge? Could they be used to power a large container ship? Thomas T. Petersen, a manager with Ballard Power Systems Europe, a major fuel-cell manufacturer, thinks so. “The problem is not the technology but the fuel supply infrastructure. It is simply not there yet, and it will take a while before quantities are readily available to bunker a large container vessel.”
And using fuel cells requires vessels with electrically powered propulsion systems. Electrified ships are less common than internal combustion engine ships, but many are sailing across the globe. Most of those electric ships derive their power from lithium-ion batteries, which may be unsuitable for long-distance shipping because of their size, weight, and charging needs.
In terms of fuels to pair with the fuel cells, ammonia has a few advantages: it’s a carbon-free material, it can be conveniently stored and used as a liquid under mild conditions, and it isn’t flammable. But as McKinlay points out, ammonia isn’t harmless—it can form NOx and atmospheric particulate matter. And it is highly toxic and corrosive.
The Southampton researchers note that hydrogen is often thought to be too low in volumetric energy density for shipping, meaning that storing sufficient fuel quantities on board would take up too much space, leaving little room for cargo. But according to their analysis, cryogenic liquid storage of hydrogen is a viable option. The team concludes that hydrogen is the leading candidate to support zero-emission large-scale shipping in the future (Int. J. Hydrogen Energy 2021, DOI: 10.1016/j.ijhydene.2021.06.066).
Numerous demonstration ships powered by nontraditional energy sources sail the seas today, generating real-world data and experience for industry and regulators to learn from. Most of these ships are relatively small, but larger ones are on the way. Yara Birkeland is a Norwegian ship powered by lithium-ion batteries. It runs between three ports in Norway and can transport up to 120 standard containers. A Japanese tanker named E5 also runs on Li-ion batteries and delivers fuel to cargo ships in Tokyo Bay.
Hydra is a Norwegian ferry that can carry 80 automobiles and nearly 300 passengers. It can run on batteries, fuel cells powered by liquid hydrogen, or both. Other demonstration ships operating on alternative fuels include a biomethanol-fueled pilot boat that’s part of a Swedish project called Fastwater, and Hydroville, a small, private, hydrogen-powered shuttle operating in Antwerp, Belgium.
Maersk plans to take alternative shipping fuels to a whole other level. The shipping giant recently announced that in the first quarter of 2024, it will begin operating eight methanol-fueled oceangoing container vessels capable of transporting 16,000 standard containers each. Most of today’s largest ships have a capacity of roughly 10,000 to 20,000 containers. The new ships will replace older, less efficient ones, reducing annual CO2 emissions by 1 million t, according to Ole Graa Jakobsen, head of Maersk’s fleet technology. He’s quick to put that number in perspective. In 2020, Maersk’s global fleet emitted 33 million t. “That’s 1 down and 32 million to go,” he says. Maersk plans to add four more of these ships to its fleet in 2025, bringing the total CO2 emission reduction to 1.5 million t, or 4.5% of Maersk’s 2020 fleet emissions.
“We are at the start of a long voyage to decarbonize shipping,” the IMO’s Bourgeon says as he surveys the flurry of activity that’s beginning to move the shipping industry from conventional fuels to environmentally friendlier ones. “These ships are huge and they use a vast amount of fuel. This won’t be an easy task, but we don’t have a choice.”