As the world shifts to low-carbon energy, finding alternatives to fossil fuels is essential. Hydrogen (H₂) stands out as a leading candidate because it can be produced in large quantities and used in many ways. Countries representing about 90% of the world's energy supply are already engaged in large-scale hydrogen projects, anticipating robust international trade between renewable-rich regions and demand hubs. However, transporting hydrogen is challenging because it has a low energy density. It needs to be either turned into a liquid at very low temperatures or compressed at high pressures, which is costly and risky due to potential leaks. 

 

A promising solution for transporting hydrogen over long distances is to convert it into ammonia using the Haber-Bosch process. This industrial process is already applied at scale, mostly to produce agricultural fertilizers, making ammonia the second most produced chemical worldwide. As an energy carrier, ammonia would offer advantages like storage at more reasonable conditions, matured transport infrastructure, and the ability to be converted back into hydrogen or burned as a fuel. Overall this strategy holds promise for developing an ammonia-based economy, with ongoing projects exploring its use in vessels and power plants. However, despite the exciting premise, environmental considerations about potential undesired emissions due to improper ammonia management or use need comprehensive exploration. 

 

Ammonia (NH₃) is a toxic gas that can pollute the air and water, harming both ecosystems and human health. The minimization of its leakages will hence be a priority. Additionally, undesired emissions of nitrogen oxides (NO_x) and nitrous oxide (N₂O) could occur during unabated or improper ammonia combustion. NO_x gases contribute to the formation of smog and acid rain and are already a global concern, especially in urban settings. N₂O, the so-called laughing gas, is a greenhouse gas around 300 times more potent than CO₂ -- nothing to laugh about! N₂O is also the leading anthropogenic contributor to stratospheric ozone depletion. In aggregate, these emissions would add another significant perturbation to the nitrogen cycle, a crucial aspect of Earth's ecosystems that has already been disrupted by agricultural activities. At the global level, it is estimated that the safe planetary boundary for nitrogen has already been crossed. 

 

The risk of harmful nitrogen emissions depends on how much ammonia is produced and how much is lost through leaks or unwanted reactions during combustion. For example, if ammonia fuel achieves a market penetration of around 5% of the current global primary energy demand, ammonia production would need to increase by around ten times compared to current levels. If then only a few percent of the nitrogen in ammonia are lost due to leakages or undesired emissions during combustion, the resulting perturbation of the global nitrogen cycle could be comparable to the global impact of fertilizers. Moreover, with a 1% nitrogen conversion of ammonia into N₂O, ammonia combustion would have a greenhouse gas footprint worse than coal. It would hence provide more climate damage than conventional fossil fuels. Minimizing these emissions will hence be a priority for the success of the ammonia economy. 

To maximize the benefit of ammonia adoption in the energy sector, it will be necessary to address, before implementation, the environmental challenges through proactive engineering measures. Identifying worst-case scenarios for ammonia systems can highlight areas of concern during development and optimization. Alternative combustion strategies, ammonia cracking, and existing technologies for converting emissions back into nitrogen offer potential solutions. There is an urgent need for early evaluation of combustion systems to mitigate emissions and further work to explore regulatory strategies to ensure optimal outcomes for ammonia fuel. We can learn from the mistakes of the past to guide the transition to an environmentally friend ammonia-based energy system.