Hydrogen and ammonia: the risk of climate-damaging leaks

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%.

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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 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.

Anaïs Marechal