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Hydrogen and ammonia : the risk of climate-damaging leaks

Didier Hauglustaine

Physicist and CNRS Research Director

Fabien Paulot

Atmospheric Chemistry Researcher at the Geophysical Fluid Dynamics Laboratory in Princeton

Key takeaways

Green hydrogen (produced by the electrolysis of water using renewable energies) is seen by the EU as a cornerstone of the energy transition.
To move away from dependence on Russian fossil fuels, the EU wants to produce 9.6 million tonnes of green hydrogen by 2030.
Naturally abundant in the atmosphere, hydrogen is not a greenhouse gas, but its increase increases the concentration of other gases, contributing to the greenhouse effect.
The hydrogen economy relies on another gas: ammonia.
But using ammonia as an energy carrier poses major challenges in terms of nitrous oxide emissions, a potent greenhouse gas.
Numerous studies stress that we must be careful not to invest in a solution that could do more harm than good for the climate.

Green hydrogen – produced by the electrolysis of water using renewable energies – is seen by the European Union (EU) as a cornerstone of the energy transition. Since Russia invaded Ukraine, the EU has stepped up its ambitions to move away from dependence on Russian fossil fuels : by 2030, the targets have been raised to 9.6 million tonnes of green hydrogen produced in the EU, and 10 million tonnes imported (40% of which in the form of ammonia)¹. The combustion of hydrogen (H₂) produces water and nitrogen oxides, thereby avoiding the release of CO₂ – a greenhouse gas (GHG) – into the atmosphere.

By replacing fossil fuels with green hydrogen, and taking current leakage rates into account, we can reduce CO₂ emissions by 94%.

The effects of hydrogen on the climate

Hydrogen is naturally abundant in the atmosphere. It is the product of the breakdown of certain atmospheric chemical compounds and is also released during the combustion of fossil fuels, forest fires or by geological processes. Around 40% of the atmospheric concentration is due to human activities².

Hydrogen is not a greenhouse gas. “When the concentration of hydrogen changes, atmospheric chemistry is disturbed and this indirectly impacts the concentration of greenhouse gases,” explains Fabien Paulot. The main mechanism is the destruction of the hydroxyl radical (OH) by hydrogen. Hydrogen is a powerful oxidiser of methane, so its reduction increases the concentration of methane – a potent Greenhouse Gas. The increase in hydrogen concentration also increases the amount of tropospheric ozone and stratospheric water vapour, contributing to the greenhouse effect.

In its gaseous form, hydrogen can be transported over long distances in existing gas networks. However, these installations – as well as production facilities – are recording anomalies, such as the massive methane leaks observed by satellite over the last few years. Air Liquide, a hydrogen producer, estimates the loss of compressed hydrogen (in its gaseous form) at 4.2%. The figure rises to 20% for hydrogen transported in liquid form³. “Unlike methane, it is not possible to measure hydrogen by satellite,” comments Fabien Paulot. “These estimates are therefore rather uncertain. On the other hand, we believe that future technologies could reduce leakage.” Despite the fact that the rise in hydrogen increases the greenhouse effect (see box), do these leaks offset any positive effects on the energy transition ? “It seems highly unlikely,” replies Didier Hauglustaine. Along with Fabien Paulot, he co-authored a publication on the subject published in 2023 in the journal Nature Communications Earth & Environment⁴. “By replacing fossil fuels with green hydrogen, and taking current leakage rates into account, we can reduce CO₂ emissions by 94%,’ explains Didier Hauglustaine. For blue hydrogen, these figures fall to 70–80%. Even taking current uncertainties into account, hydrogen remains of great interest as a tool for reducing the impact of energy on the climate, particularly when it comes to shipping, road transport and heavy industry.”

But the hydrogen economy relies on another important gas in the value chain : ammonia. Hydrogen (H₂) can be converted into ammonia (NH₃). The latter is then either burnt to provide a direct source of energy or converted back into hydrogen by cracking. These processes have been mastered and the direct combustion of ammonia is already being used on ships. In an energy transition scenario where global warming is limited to 1.5°C, the International Renewable Energy Agency (IRENA)⁵ estimates that in 2050, hydrogen will cover 12% of the world’s energy demand. In this scenario, a quarter of the hydrogen consumed worldwide comes from international trade. What’s more, 55% is transported in the form of pure or mixed hydrogen and 45% by ship, mostly in the form of ammonia.

Ammonia, a false solution ?

Ammonia is essential to a hydrogen-based economy. But the transport of ammonia (NH₃) also presents a risk of leakage, with far more detrimental effects on the climate. Some of the compounds produced by the combustion of NH₃ are powerful greenhouse gases, such as nitrous oxide (N₂O), which has a warming potential 265 times greater than that of CO₂. In an article published in the journal PNAS in November 2023⁶, American scientists assess this risk. Since ammonia has similarities with methane, they use the same leakage rates as for methane, measured by satellite. 0.5 to 5% of ammonia could be lost to the environment in the form of reactive nitrogen. These losses can be explained by leaks, but also by the combustion of ammonia : when incomplete, this contributes to the emission of reactive nitrogen into the atmosphere. For the highest estimate (5% losses), this represents the equivalent of half of the global climate disruption currently caused by the use of nitrogen fertilisers (the equivalent of 2.3 Gt CO₂ are emitted each year, i.e. 1/5th of emissions from the agricultural sector).

In addition, undesirable reactions occur during the combustion of ammonia. Although these have been minimised by recent technologies, they still exist and generate N₂O in particular. The authors of the study in PNAS believe that this effect could completely offset the positive benefits of the energy transition, outweighing the current climate impact of fossil fuels such as coal. Even in the best-case scenario (where there would be no leakage), the team calculates that ammonia has a higher carbon footprint than wind or geothermal energy, but comparable to that of solar energy.

In 2022, another scientific team assessed the impact of a transition to ammonia to decarbonise maritime transport⁷. Their conclusion was similar : small leaks of N₂O – during combustion or transport – completely offset the climate impact of such a transition. “These estimates are the first to be made, and they include some uncertainties, because this economy is still in its infancy, so they may be overstating the case,” comments Didier Hauglustaine. “But they are crucial : they sound the alarm about ammonia, which has a significant impact on the climate.” Ammonia is an attractive solution for the maritime sector : it is relatively easy to convert an internal combustion engine to use ammonia, and manufacturers are already preparing dedicated engines. These initial studies show how important it is to be careful not to invest in false solutions that may do more harm in terms of the climate.