Hit or miss: Ammonia as a climate-friendly fuel

In the quest for a clean shipping fuel, ammonia is emerging as an unexpected hero. Made from air and water using renewable electricity, it promises near-zero emissions. According to the IMO and IEA, ammonia's future potential is significant—but there are some caveats.

Ammonia (NH3) could turn out to play a key role as an energy carrier in the decarbonisation of energy systems. The idea, in principle, is simple and brilliant: synthesise ammonia from air and water using renewable electricity, and release the energy in combustion engines or fuel cells with close to zero emissions. It’'s a renewable and carbon-free fuel, with technology and infrastructure already partly in place as the world already uses about 180 million tonnes of ammonia annually, mostly for fertilisers.

Major organisations such as the International Energy Agency (IEA) and the International Maritime Organization (IMO) are now betting on ammonia as a main energy carrier for maritime shipping, and to some extent also for co-firing with coal in power plants. It is also possibly a future aviation fuel. Ammonia-fuelled ships are already on the market, and by 2050 the IEA’s Net Zero Roadmap (2023) envisions almost half of global shipping to be ammonia-powered. Future energy use of ammonia could land somewhere around 1,500 million tonnes annually, almost ten times today’s fertiliser use.

The question now is how low the emissions will really be. Two broad issues come to mind.

First, will the ammonia be fossil-free? Today’s ammonia production is based on fossil fuels with the carbon emittedand generates large emissions of CO2. To achieve a climate benefit, today’s so-called brown ammonia needs to be replaced by blue ammonia (still based on fossil fuels, but with carbon capture and storage) or by green ammonia (produced from renewable electricity). Technically, this can be done, but actual implementation and upscaling remains to be seen. Without decarbonised ammonia production, the system would just become a complicated and costly way of using fossil energy.

Second, what are the use-phase emissions? Real-world ammonia combustion is not emission-free but and exhaust gases contain some unburnt ammonia as well as nitrogen oxides (NOx) and nitrous oxide (N2O). The nitrous oxide is particularly troubling as ammonia fuel is meant to be a climate solution: if just 0.4% of fuel nitrogen is converted to nitrous oxide, this emission alone causes the equivalent global warming as a fossil-fuelled system.

What emissions should we expect, then? The answer is that we don’t really know. From an engineering perspective, ammonia is actually not a great fuel: it is difficult to ignite, and it burns slower and colder than fossil fuels. Compared to fossil fuels, there is much less experience burning ammonia, and technology is now therefore evolving rapidly. Research shows that emission rates of reactive nitrogen can vary enormously, from almost zero up to several percent of fuel nitrogen, depending on many factors including design of the combustion chamber, the fuel mix, the fuel-to-oxygen ratio, and the operating conditions (e.g., full or reduced load). On one hand, this is good news: it means that technical research and development can mitigate reactive nitrogen emissions. On the other hand, it means that laboratory evidence of low emissions cannot be assumed to hold generally: there is an urgent need to improve monitoring of actual emissions in real-world applications.

Research and development are accelerating in both public and private organiszations to close the many research gaps. Key issues from a sustainability perspective include emissions and mitigation in normal use as well as in accidents such as ammonia spills; life cycle assessments of “well-to-wake” emissions; and the potential for market upscaling and possible price effects on the agri-food system which depends on ammonia for fertiliser production.

In the regulatory landscape, discussions are ongoing but no major actor has yet taken on a systems-oriented approach to ensure climate benefit and limit reactive nitrogen emissions. The IMO’s Marine Environment Protection Committee (MEPC) has recently moved forward with guidelines for well-to-wake life cycle assessments, a useful step forward, which but which urgently needs to be complemented by actual life-cycle assessments as well as monitoring and regulation.

In summary, ammonia shows some promise as a low-emission energy carrier, but it does not automatically lead to climate benefit and it also risks increasing emissions of ammonia and nitrogen oxides. As industry and international organisations already seem to be set on ammonia, research and regulation on environmental aspects need to accelerate to eliminate unacceptable side effects and ensure the climate benefit that would motivate ammonia as an energy carrier in the first place.

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