Researchers produce 'green' ammonia

Chemical engineers at UNSW Sydney and The University of Sydney have developed a method to make ‘green’ ammonia from air, water and renewable electricity, without the high temperatures, high pressure and huge infrastructure currently needed to produce it.

The new production method — described in a laboratory-based proof-of-concept study published in Energy and Environmental Science — has the potential to play a role in the global transition towards a hydrogen economy, where ammonia is increasingly seen as a solution to the problem of storing and transporting hydrogen energy.

The fossil-fuel-powered production of ammonia has been energy intensive since large-scale manufacturing began in the early 1900s, requiring temperatures higher than 400°C and pressures greater than 200 atm.

Co-author Dr Emma Lovell from UNSW’s School of Chemical Engineering said the traditional way to make ammonia — known as the Haber–Bosch process — is only cost-effective when produced on a massive scale, due to the huge amounts of energy and expensive materials required.

“The current way we make ammonia via the Haber–Bosch method produces more CO₂ than any other chemical-making reaction,” she explained.

“In fact, making ammonia consumes about 2% of the world’s energy and makes 1% of its CO₂, which is a huge amount if you think of all the industrial processes that occur around the globe.”

Dr Lovell said that, in addition to the big carbon footprint left by the Haber–Bosch process, producing millions of tonnes of ammonia in centralised locations means even more energy is required to transport it around the world, not to mention the hazards that go with storing large amounts of it in one place.

She and her colleagues therefore looked at how to produce it cheaply, on a smaller scale, using renewable energy.

“The way that we did it does not rely on fossil fuel resources, nor emit CO₂,” Dr Lovell said.

“And once it becomes available commercially, the technology could be used to produce ammonia directly on site and on demand — farmers could even do this on location using our technology to make fertiliser — which means we negate the need for storage and transport. And we saw tragically in Beirut recently how potentially dangerous storing ammonium nitrate can be.

“So if we can make it locally to use locally, and make it as we need it, then there’s a huge benefit to society as well as the health of the planet.”

Converting atmospheric nitrogen to ammonia

ARC DECRA Fellow and co-author Dr Ali (Rouhollah) Jalili said trying to convert atmospheric nitrogen (N₂) directly to ammonia using electricity has posed a challenge to researchers for the last decade due to the inherent stability of N₂ that makes it difficult to dissolve and dissociate.

Dr Jalili and his colleagues devised proof-of-concept lab experiments that used plasma (a form of lightning made in a tube) to convert air into an intermediary known among chemists as NO_x — either NO₂- (nitrite) or NO₃- (nitrate). The nitrogen in these compounds is much more reactive than N₂ in the air.

“Working with our University of Sydney colleagues, we designed a range of scalable plasma reactors that could generate the NO_x intermediary at a significant rate and high energy efficiency,” he said.

“Once we generated that intermediary in water, designing a selective catalyst and scaling the system became significantly easier. The breakthrough of our technology was in the design of the high-performance plasma reactors coupled with electrochemistry.”

Professor Patrick Cullen, who led the University of Sydney team, added, “Atmospheric plasma is increasingly finding application in green chemistry. By inducing the plasma discharges inside water bubbles, we have developed a means of overcoming the challenges of energy efficiency and process scaling, moving the technology closer to industrial adoption.”

Solving storage issues

Scientia Professor Rose Amal, Co-Director of ARC Training Centre for Global Hydrogen Economy, said the team’s ‘green’ method of ammonia production could solve the problem of storage and transport of hydrogen energy.

“Hydrogen is very light, so you need a lot of space to store it, otherwise you have to compress or liquify it,” Professor Amal said.

“But liquid ammonia actually stores more hydrogen than liquid hydrogen itself. And so there has been increasing interest in the use of ammonia as a potential energy vector for a carbon-free economy.”

Professor Amal says ammonia could potentially be made in large quantities using the new green method ready for export.

“We can use electrons from solar farms to make ammonia and then export our sunshine as ammonia rather than hydrogen.

“And when it gets to countries like Japan and Germany, they can either split the ammonia and convert it back into hydrogen and nitrogen, or they can use it as a fuel.”

The team will work on commercialising this breakthrough, and is seeking to form a spin-out company to take its technology from laboratory scale into the field.

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