Green ammonia remains a challenge, but researchers make significant progress
The production of ammonia, essential for fertiliser production, is responsible for almost 1.5% of global carbon emissions. This needs to be greener, which is why researchers have been looking for sustainable alternatives for years. However, according to scientists at TU Delft, writing in the journal Energy & Environmental Sciences, completely green ammonia production is not within reach yet. Despite the technological challenges, the researchers remain hopeful.
The Haber-Bosch process. This is unlikely to mean much to you, and yet this chemical process has been feeding the world’s population for about 100 years, explains associate professor Ruud Kortlever. It is the method of choice for producing ammonia, and therefore fertiliser. ‘Ammonia is made up of nitrogen and hydrogen. The nitrogen is taken from the air and the hydrogen from natural gas. These substances are then brought into contact with a catalyst and heated under high pressure to a temperature of 500 degrees Celsius. This is an extremely energy-intensive process.’
This not only results in significant CO2emissions, but because of the size of the facilities required, there are only a limited number of locations in the world where these plants can be built. ‘This means that the main end product, fertiliser, also has to be shipped around the world, making it inaccessible and expensive, especially in low- and middle-income countries,’ says Kortlever. So there is an urgent need for a new production method that both reduces carbon emissions and enables local production.
Economic feasibility
For the past decade, scientists, including Kortlever, have been looking for alternatives. Electrochemical ammonia synthesis via nitrogen reduction reaction (NRR) is considered one of the most promising technologies. This process involves converting nitrogen (N₂) in to ammonia (NH₃) using electrical energy.
In their new study, Kortlever and Boaz Izelaar, who defends his thesis on this topic November 14, have carried out the most comprehensive comparison of different NRR methods to date. ‘Everyone is rushing to improve the conversion, rather than looking at the whole process and the techno-economic feasibility. We have identified the biggest challenges and what we need to focus on to make sustainable electrochemical methods competitive.’
Ruud Kortlever
Boaz Izelaar
Everyone is rushing to improve the conversion, rather than looking at the whole process and the techno-economic feasibility. We have identified the biggest challenges and what we need to focus on to make sustainable electrochemical methods competitive.
Ruud Kortlever
Not yet competitive
They looked at four different processes and compared them with the conventional Haber-Bosch process and the electrified Haber-Bosch process. They analysed the entire process, including electrochemical performance, scalability, energy consumption and economic viability. Kortlever acknowledges that the results were disappointing. ‘We concluded that none of the methods can compete with Haber-Bosch yet. At the moment, electrified Haber-Bosch is still the most appealing option. At the same time, our study clearly shows the challenges with electrochemical processes. This provides inspiration for follow-up studies to make these technologies competitive.’
Kortlever is focusing mainly on the lithium-mediated route. Under the most optimistic economic assumptions, lithium-mediated NRR is currently not feasible on a large scale. This means that Li-NRR remains of scientific interest but is not suitable for profitable large applications. ‘We are now looking for a new metal that can act as a mediator; we have our eyes on magnesium, for example, so we can significantly improve the energy efficiency of the entire process.’
Climate targets
Although the relatively new electrochemical processes are not yet competitive, Kortlever remains optimistic. However, he stresses that more attention and cooperation is needed between knowledge institutions, industry and government. Global demand for ammonia continues to rise, driven by the need for fertilisers for food production. ‘Switching to more sustainable methods is therefore essential, especially if we are to meet the 2050 climate targets,’ he concludes.