Figure 1. The dielectric barrier discharge (DBD) reactor is used to produce fertilizer. Source: The University of Warwick.
Part of the project’s task is to develop the solid catalysts used in the solar process. “The problem is that this reaction results in multiple compounds because of the side reactions which are essentially unavoidable byproducts of the main reaction,” notes Rebrov.
The project’s Italian research partner is investigating possible catalysts that include titanium -dioxide-supported metal or metal oxide nanoparticles of various shapes and sizes, typically below 2 nm, and rhodium/titanium dioxide mixes.
In a packed-bed plasma reactor, the catalyst, in the form of spherical pellets, is sandwiched between two AC or pulsed high-voltage supplied electrodes. Adding catalytic promotors can increase selectivity. Currently, carbon dioxide conversion efficiency is 35–40%.
“Our task is to make the reactor and catalyst in such a configuration that it can interact with the plasma species without destroying the catalyst. It’s quite a challenge because what we are proposing is quite unusual,” explains Rebrov.
Fertilizer production involves splitting atmospheric nitrogen and water to make ammonia and nitrates. The researchers aim to use plasma energy to reduce nitrogen under ambient conditions. This would open up a sustainable pathway for production of fertilizers and energy carriers on a global scale.
Here, the researchers are testing ruthenium/carbon nanotubes and alloys of indium and gallium as catalysts, on a ruthenium/aluminium trioxide bed, in a dielectric barrier discharge reactor.
“Research is focused on process conditions in future markets such as Africa where farmers use little fertilizer, ammonia production is at a very low level, and future population growth and increase in food demand will be the most significant,” says Rebrov. He envisages a number of truck-mounted 1,000-t/yr facilities with a process operating at atmospheric pressure and about 100°C.
Five fertilizer companies have joined this part of the project as a user club and Evonik was involved with three earlier nitrogen-fixation projects.
The South Australian No-Till Farmers Association has created one important possible market. This organization promotes the technology to Australian farmers as a new concept in conservation agriculture and to help reduce soil acidification, erosion and greenhouse gas emissions. It plans to work with state governments to introduce a green fertilizer bonus for plasma-made nitrate fertilizers of about 10–15% of their cost.
“Collectives of 20 farmers could support a truck-mounted plant,” says Volker Hessel, professor with the school of chemical engineering and a sustainability and process intensification expert from the University of Adelaide, Australia.
Volker, whose department is taking part in the study, notes that different areas of Australia have access to different types of renewable energy. “For example, truck-mounted plants could use biomass energy from the forests in the north of Australia and then use solar and wind available in other parts of the country as they move down to follow the growing season.”
“Our technology should be seen as disruptive and transformative as it gives new windows of opportunity and business models to many different places,” he adds.
However, the catalyst work and future challenges include scaling up the reactor — especially in terms of flow distribution and uniformity of electric field — and then interfacing it to a sustainable downstream processing design, concludes Rebrov.
