The Shipping Industry Looks for Green Fuels

by Mitch Jacoby (Chemical & Engineering News) Transporting goods by sea contributes significantly to global carbon emissions. Switching from traditional petroleum-based marine fuels to low-carbon alternatives could drastically shrink shipping’s climate impact — … 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 (International Maritime Organization’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.

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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 shipping industry has already faced initiatives and regulations to clean their emissions. The IMO previously adopted mandatory caps on emissions of nitrogen oxides (NO_x) and sulfur oxides (SO_x), 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 NO_x 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.

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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.

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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.

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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.

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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).

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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.

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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 NO_x 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).

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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. READ MORE

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