Carbon-free fuels could have a dark side

As climate-friendly fuels, hydrogen (H 2 ) and ammonia (NH 3 ) are enticing. Because they lack carbon, they can be burned to produce nothing but environmentally benign water and nitrogen (N 2 ). But if producers do not take care to prevent leaks or incomplete combustion, researchers are now warning, the fuels could generate pollutants that could harm human health and shrink or reverse the climate benefits.

For example, one analysis finds that, under a worst case scenario, using ammonia as a fuel could have a greenhouse gas footprint as bad as burning an equivalent amount of coal. “We can’t just be hoping these things work,” says Amilcare Porporato, an environmental engineer at Princeton University and a co-author of the study, which was published last week in the Proceedings of the National Academy of Sciences . “We need to do due diligence.”

These potential side effects are too often overlooked, says Paul Wolfram, a researcher at the Joint Global Change Research Institute. “The focus is almost solely on [carbon dioxide] emissions,” he says.

Today, hydrogen and ammonia mainly come from energy-intensive, polluting processes. But they can also be made cleanly, with renewable electricity, resulting in a green fuel. Green hydrogen got a boost last month, when the U.S. Department of Energy announced $7 billion in funding to support several hubs to make it. Ammonia has an additional advantage: Unlike hydrogen, it can be liquefied at mild pressures and transported relatively easily

When ammonia leaks or isn’t burned completely, however, the nitrogen it contains can give rise to reactive nitrogen species. These compounds include nitrous oxide (N 2 O), a greenhouse gas about 273 times more potent than carbon dioxide, and other nitrogen oxides, collectively called NO x , which are notorious air pollutants. For instance, NO 2 is a key ingredient in smog and acid rain and leads to the formation of particles that can cause asthma.

Porporato and his colleagues modeled best and worst case scenarios in a society that could be producing some 1600 million tons of ammonia per year by 2060–70. (Currently, the annual ammonia market is about 180 million tons per year, and most is used for fertilizer.) They looked at scenarios in which between 0.5% and 5% of the nitrogen in ammonia was lost as reactive nitrogen compounds instead of being converted back into harmless atmospheric nitrogen. They found that emission rates of nitrous oxide in particular controlled whether ammonia would help control global warming or harm the climate as much as burning coal.

Hideaki Kobayashi, a combustion engineer developing ammonia turbines at Tohoku University, says the group’s projections are too pessimistic. His turbines, he says, burn ammonia efficiently and produce little to no nitrous oxide, while catalytic converters in the exhaust systems get rid of any nitrogen oxides. And he says regulations will help limit emissions in Japan and elsewhere.

But Wolfram worries about the potential for rogue emissions in the marine shipping industry, which is now rolling out ammonia-powered ships. In a 2022 study of the potential impacts, he and his colleagues found that switching the entire maritime shipping industry to ammonia would take about four times as much of the chemical as the market currently produces. If just 0.4% of the nitrogen in this fuel were converted to nitrous oxide, they found it would completely zero out the benefits of switching from carbon-based fuels. The shipping industry is regulated by the International Marine Organization, but it may be hard to spot leaks from individual ships, Wolfram says. “To me it seems hard to monitor and control all these emissions.”

Hydrogen also comes with problems. Leaks from pipelines and other infrastructure could indirectly lead to rising levels of methane, a strong greenhouse gas emitted by natural sources and fossil fuel production. That’s because hydrogen reacts with and depletes hydroxyl radicals, chemical species in the atmosphere that play a key role in breaking down methane.

A 2022 analysis of green hydrogen found that if leak rates are as high as 10%, its climate impact would be about half that of an equivalent amount of fossil fuels—still an improvement, but not quite living up to hydrogen’s promise. Keeping the leakage to just 1% or so could preserve the climate benefits, the study found.

To know what to expect, researchers need more data about real-world leak rates, says Ilissa Ocko, a climate scientist at the Environmental Defense Fund (EDF) who led the 2022 study. Next year, EDF will launch a monitoring campaign in Europe and North America to provide what Ocko says are the first measurements of leak rates in the field.

The ammonia and hydrogen economies are in their infancy but could soon grow up, she adds. “We need to make sure we address these issues before it becomes a problem.”