Ammonia production is responsible for about one percent of greenhouse gas emissions, although this could be underestimated. That may not sound like much, but we worry a lot about the emissions of air travel, which isn’t that much larger. It's unlikely that nearly 8 billion people can live in this world without ammonia-based fertilizers, however, so a better way has to be found. Australian scientists think they're close.
Ammonia (NH3) production currently relies on the Haber-Bosch process – which won the Nobel Prize in 1918 – but is badly in need of replacing. The process uses immense amounts of energy and relies on hydrogen, usually produced from methane, some of which leaks. “Green ammonia” can be made using renewable energy and electrolysis of water, but is currently very expensive.
“The problem with using electricity to convert nitrogen directly to ammonia is that nitrogen is so stable it is very hard to get it to dissolve in water,” Dr Emma Lovell of the University of New South Wales told IFLScience. “So we took a step back and thought about how nature does it.”
Lightning turns some of the atmosphere into NOx molecules, which then get transformed into other compounds, allowing this essential element to make up part of every living thing. Lovell and colleagues started thinking about ways to replicate this.
Presumably, others have had the same idea, but Lovell thinks previous work has failed as "Expertise in plasma physics and electrochemistry don't overlap much. We were just lucky that one person working in each got a coffee together and things moved on from there."
In Energy and Environmental Science Lovell and other team members describe a plasma bubble column reactor that first converts atmospheric nitrogen into NOx like lightning, and then electrolyzes water to make hydrogen that displaces the oxygen.
Having started with a very inefficient process, Lovell told IFLScience that energy use per gram of ammonia produced has come down 100-fold. She thinks further two or three-fold improvements are possible, which might finally knock Haber-Bosch off its perch.
In addition to the environmental costs of producing ammonia, existing methods require giant manufacturing plants for efficiency, which then means shipping the product worldwide. The world was reminded of the dangers when 3,000 tonnes of ammonium nitrate fertilizer stored at Beirut port exploded last year, leaving 300,000 homeless and at least 204 dead.
Besides not requiring fossil fuels, Lovell's method can operate at a variety of scales. “The technology could be used to produce ammonia directly on site and on demand...which means we negate the need for storage and transport,” Lovell said in a statement. The team is working on making a design that could run on-farm using nothing but air, water, and a few solar panels.
NOx gasses are both locally toxic and greenhouse polluting, but Lovell told IFLScience she doubts the closed system they are building will let any leak before conversion to ammonia.
Solving one of the world's great environmental challenges might be enough for most people, but Lovell and her colleagues think they can help with an even larger one. Hydrogen carries many hopes as a way to store and transport energy from places rich in sunlight and wind to those where it is lacking. Senior author Professor Rose Amal noted, “Hydrogen is very light, so you need a lot of space to store it, otherwise you have to compress or liquify it, but liquid ammonia actually stores more hydrogen than liquid hydrogen itself.”
Recent advances in the easy splitting of ammonia to nitrogen and hydrogen raise the possibility ammonia transport could solve many of hydrogen’s problems.