New prospects for green ammonia

Converting ammonia production to a low-emission process is one of the major challenges in the fight against climate change. Ammonia, one of the world’s most widely produced chemicals, is essential for the fertiliser industry and thus for the production of plant-based food. However, its production is expensive and causes high emissions.

A research team from Forschungszentrum Jülich, TU Munich and Linde Engineering has conducted a study simulating what a reactor would need to look like in order to produce ammonia cost-effectively and in an environmentally friendly way, i.e. using renewable energies. The results have now been published in the International Journal of Hydrogen Energy.

Ammonia as a climate factor

According to the British Royal Society, global ammonia production generates around 500 million tonnes of CO₂ annually – equivalent to Germany’s annual CO₂ emissions and around 1.8 per cent of global emissions. The main reason for this is that traditional ammonia synthesis requires hydrogen, which until now has been obtained almost exclusively from fossil natural gas – a process that releases large amounts of CO₂.

Green ammonia offers a climate-friendly alternative here: the hydrogen (H₂) required is not obtained from natural gas, but from water via electrolysis. This is split into oxygen (O₂) and hydrogen using electricity from renewable energies. The hydrogen produced then reacts with nitrogen (N₂) from the air in the Haber-Bosch process to form ammonia (NH₃).

The challenge: Fast load changes

The problem: electricity from wind and solar energy is not continuously available. This means that electrolysis does not supply the same amount of hydrogen at all times, causing fluctuations in the supply to the plant. In order for an ammonia plant to respond flexibly to these fluctuations, it must be able to ramp its production up and down – in other words, it must be load-flexible.

However, conventional systems are designed for continuous, steady operation. Rapid load changes lead to significant pressure fluctuations inside the reactors and pipes. This places high mechanical stress on the components. To withstand this over the long term, the equipment would have to be built with thicker walls and more robust materials, which would significantly increase costs and make construction much more complex.

This is precisely where the new study comes in: it shows how these pressure fluctuations can be reduced with intelligent control – and thus also the requirements for the mechanical stability of the systems.

New solution for fluctuations

“For green ammonia to contribute to the fight against climate change, the costs must be competitive,” says Prof. Andreas Peschel, Director at the Institute for Sustainable Hydrogen Economy at Forschungszentrum Jülich and co-author of the study.

According to the study, this can be achieved with a new type of pressure control in what is known as an ammonia loop. This production step ensures that unused reaction gases such as hydrogen and nitrogen are fed back into the process. According to the team’s simulations, the flexible control of this loop allows load changes to be implemented at high speed and with low pressure fluctuations.

Production output can be changed by three percent within a minute – a speed that is not possible with today’s natural gas-based plants. This would mean that smaller buffers and thinner walls for plant components would suffice in future, which would reduce material costs.

The team had already presented a flexibly operable reactor type in an earlier study. Now the next step is to follow at the Jülich Research Centre: test facilities that demonstrate the new pressure control and high load dynamics under realistic conditions.

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