In the rush to decarbonize, the shipping industry is exploring alternative fuels

This article was originally featured on Hakai Magazine, an online publication about science and society in coastal ecosystems. Read more stories like this at hakaimagazine.com.

For decades, the world’s commercial ships have depended on a fossil fuel so sticky and thick that it needs to be heated to around 150 °C just to get it to flow through a vessel’s innards. Heavy Fuel Oil (HFO) is one of the dirtiest fuels out there. “It’s the last step of the [oil] refining process,” says Morten Bo Christiansen, head of the energy transition team at Danish shipping giant Maersk. “You could say, the bottom of the barrel.”

Yet now, as the International Maritime Organization (IMO)—the United Nations branch tasked with managing global shipping—implements new regulations intended to force the shipping industry to cut its sulfur and carbon emissions, HFO is on the chopping block. The new rules have decision-makers like Christiansen racing to figure out which of the myriad potential fuels of the future will ultimately replace it.

Part of Christiansen’s job, alongside dozens of colleagues, is to buy the fuel that powers Maersk’s hundreds of ships. HFO and other fossil fuels, he says, are already beginning to give way to cleaner alternatives.

In mid-2023, for example, a new container ship, the US $160-million Laura Maersk, began operating in the Baltic Sea. “It looks like any other container ship,” says Christiansen. But the Laura Maersk has never run on oil. Instead, it’s powered by methanol. There are lots of different ways of producing methanol, and not all are environmentally friendly. However, when sustainably sourced, such as by capturing gas produced at landfill sites or through various processes powered by renewable energy, methanol can be significantly less polluting than fossil fuels. The Laura Maersk is already plying the waters off northern Europe, and more than 200 other ships capable of running on methanol are currently on the order books of shipyards around the world.

To meet the IMO’s newest regulations—which it designed to put the shipping industry on a path to net zero by 2050—commercial vessels will have to cut their carbon emissions by at least 30 percent in just six years, says Simon Bullock, a shipping and climate change researcher at the University of Manchester in England. Should the industry take any longer than that, he adds, it will be next to impossible for shipping companies to reduce emissions aggressively enough to meet the most ambitious regulatory targets. In a paper published in 2023, Bullock and his colleagues showed how, if things don’t move fast enough, the goals of the 2015 Paris Agreement would be in jeopardy.

To reach net zero, getting rid of HFO is a must. And, to have the greatest effect on carbon emissions, alternative fossil fuels currently being used by some ships, such as liquefied natural gas, ultimately need to go. But reengineering existing ships to use significantly cleaner fuels is not easy, and building new vessels is neither fast nor cheap. There’s also the issue of supply. For methanol, the global availability of this fuel is currently nowhere near enough to meet the shipping industry’s colossal demand.

The challenge for Christiansen and others like him, then, is to figure out how to transition vessels to alternative fuels without any major technological or logistical hiccups—and to do so in an incredibly short time.

For now, the only way forward is to diversify fuel inputs. Besides methanol, the shipping industry is exploring other non–fossil fuel options, such as hydrogen, ammonia, electricity, and nuclear power. Given how controversial the latter is—nuclear struggled to take off as a commercial-ship fuel and is used almost exclusively for military vessels—nuclear is unlikely to have any impact on shipping during the next few crucial years. Powering a large number of huge cargo ships with electricity using suites of on-board batteries is also unlikely because batteries can’t pump out enough power to compensate for their size and high price.

Ammonia, though, is a real contender. The big benefit of ammonia is that it’s an important commodity in many other industries—notably agriculture, in which millions of tonnes of it are used every year as a fertilizer. Ammonia has a solid supply chain, and the chemical is already stored at many of the world’s ports.

However, like methanol, ammonia is imperfect. For starters, it’s incredibly toxic, so accidents and spills could be ecologically disastrous. At present, no ports will allow ammonia-powered vessels to dock—a stance that forced the Green Pioneer, a proof of concept ship retrofitted to run on both ammonia and diesel, to use fossil fuels to power its visit to the COP28 conference in the United Arab Emirates last year.

And ammonia is also harder to burn than some of the alternatives. Designing more reliable, more efficient ammonia-burning engines is a huge challenge currently being tackled by companies like the Germany-based MAN Energy Solutions, which also built the methanol-burning engine on the Laura Maersk.

Lars Tingbjerg Danielsen, the promotion manager at MAN Energy Solutions, explains that ammonia has a high ignition temperature—it needs to be above 650 °C before it lights—and so must be burned alongside a secondary pilot fuel. Its flame is fickle, too. Compared with methanol, ammonia’s flame speed (how fast the flame expands as the fuel combusts) is six times slower. If the engine spins too quickly, combustion will falter, allowing ammonia to slip out of the engine. Given its toxicity, this would be a significant contamination risk.

Luckily, giant two-stroke ship engines turn relatively slowly compared with smaller engines, says Danielsen, so it’s easier to get ammonia to sustain combustion within them. His firm is currently using high-speed cameras to study the ignition of the fuel.

Danielsen adds that determining the most effective fuel depends, in part, on how a given ship is used. Methanol supply facilities are expensive to build, so methanol-fueled ships might be best suited to repetitive routes with fixed ports of call. According to Maersk’s research, ammonia is and will likely remain cheaper than methanol, he says, so it may make sense to use it for the biggest ships, which require the most energy.

Maersk, for its part, is interested in burning ammonia. “We certainly expect it to be in the fuel mix in the future,” says Christiansen. However, neither Maersk nor any other major shipping company currently has ammonia-burning ships, and Christiansen declined to confirm whether Maersk has plans to announce any such orders this year. The Yara Eyde, the world’s first ammonia-powered cargo ship, is expected to be launched in 2026, and about a dozen ammonia-burning vessels are on order globally.

At present, the shipping industry is not overly keen on Stephen Turnock’s favorite alternative fuel: hydrogen. But Turnock, a maritime engineer at the University of Southampton in England, likes hydrogen because, as with ammonia, it can be created with renewable energy, yet burning it produces nothing but steam. (Ammonia combustion creates a mix of nitrogen gas and water—not too bad—while burning methanol yields carbon dioxide and water.)

Yet the industry is wary of working with something so hard to handle. Hydrogen evaporates at -235 °C—colder than the surface of the moon at night—so compressing it into a liquid to put into a ship’s fuel tank requires a lot of energy. And because hydrogen molecules are so incredibly tiny (even by nanoscopic chemical standards), hydrogen tends to leak through the smallest of cracks.

Maersk conducted a pilot study with hydrogen, says Christiansen, and found that using it would be more expensive than methanol and ammonia. And that’s saying something because methanol is itself three times more expensive than conventional ship fuels, he says.

Because ammonia actually contains hydrogen—ammonia is one nitrogen atom and three hydrogen atoms—there’s also the opportunity for a hybrid approach: pump a ship’s fuel tanks full of ammonia and then chemically convert it into hydrogen on board. It’s a relatively inefficient conversion that wastes a lot of energy, but this process would, ultimately, allow for propulsion with zero carbon emissions. Amogy, a start-up headquartered in New York, is working toward testing a tugboat powered by this hybrid approach later this year.

Yet in the rush to find cleaner fuels, Bullock, the shipping and climate researcher, cautions against overlooking all the other ways the industry can slash its emissions. “The sector has to put a greater focus on the other things it can do,” he says. For instance, so-called wind-assisted technology (a truly ancient ship propulsion method) can reduce the effort required from a ship’s main engine no matter what it’s burning.

Stricter regulations could also play a role in driving down shipping’s carbon emissions, Bullock argues. Ports could refuse entry to ships that don’t have a high enough energy-efficiency rating based on a scale introduced by the IMO last year, for example. Christiansen at Maersk says he backs proposals from some within the shipping industry for a Green Balance Mechanism that would raise the cost of polluting fuels to subsidize greener alternatives.

But there is hardly any time left for shipping companies to make big changes. The industry only has a few years to cut its carbon emissions by nearly one-third—largely because it has kept its “head in the sand” for too long, says Turnock.

Christiansen remains optimistic. If shipping can transform itself, it will be harder for other industries, such as aviation, to delay their own green transition. “Hey, if these shipping guys can get this done,” he says, “what’s your excuse?”

This article first appeared in Hakai Magazine and is republished here with permission.