Researchers in the US have developed a new way to make clean ammonia that could be 60% cheaper than using green hydrogen to decarbonise NH3 production.
Currently, ammonia is produced by combining hydrogen gas with nitrogen from the air in the Haber-Bosch process, which is extremely energy intensive.
As such, most of the focus on reducing emissions from NH3, particularly for use in fertilisers and other chemicals, has been to decarbonise the hydrogen feedstock — today made predominantly from fossil gas or coal — by producing green H2 from renewables-powered electrolysis.
However, a research team at the University of Illinois Chicago (UIC) has designed a new process of lithium-mediated ammonia synthesis (LMAS) that can run at low temperatures, which combines nitrogen gas and a hydrogen-donating fluid such as ethanol with a charged lithium electrode.
Nitrogen atoms “stick” to the lithium before combining with hydrogen to form NH3 molecules, they say.
From there, both the lithium catalyst and unused hydrogen ions are regenerated in each cycle, meaning this method does not result in the by-production of large quantities of H2 gas — one of the stumbling blocks of similar electrochemical ammonia production methods.
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LMAS could only cost $450 per tonne of NH3 if scaled up — 60% cheaper than other green ammonia production pathways, according to the research team’s lead Meenesh Singh.
Market intelligence agency S&P’s most recent monthly average prices for grey ammonia ranged from $299 to $475 per tonne, depending on where it was produced, while a tonne of green NH3 made from renewable hydrogen was calculated to cost between $750 and $888.
“The lithium-based approach can actually be found in any organic chemistry textbook. It’s very well-known” Singh said. “But making this cycle run efficiently and selectively enough to meet economically feasible targets was our contribution.”
Ethanol is currently mass-produced around the world fairly cheaply from biomass, mainly corn and sugar cane, but it can also be made from non-food matter, such as straw, switchgrass and corn stover (the husks, leaves and stalks left over once corn is harvested).
But the UIC researchers have not stated the expected lifecycle emissions footprint of LMAS compared to other green ammonia production methods.
The UIC paper about this research, published in the scientific journal ACS Applied Materials & Interfaces, has been criticised in some quarters for being overly optimistic about future cost reductions from this technology.
“There have been numerous claims of lithium-mediated electrochemical ammonia synthesis methods in the past two years in the chemistry literature by various research groups, all selling hope of low temperature, modest pressure routes to producing ammonia,” says Paul Martin of the Hydrogen Science Coalition.
“Frequently, these papers make confusing statements about important matters such as current (coulombic) efficiency, which readers sometimes confuse with the more important factor which is energy efficiency. This current paper, for instance, does not demonstrate any test result with an energy efficiency of greater than 10%, with most results a great deal below that — a figure that water electrolysis plus Haber Bosch ammonia synthesis can very easily exceed by a wide margin.
“At scale, a significant fraction of the cost of ammonia is input energy, so efficiency matters a lot. The techno-economic conclusions in the paper about reductions in total cost relative to the Haber Bosch process when fed green hydrogen are, in my opinion, greatly overstated, and are not a reflection of the results demonstrated in the laboratory. The paper shows evidence of inadequate peer review, particularly of the techno-economic claims.”
Ammonia is used today for the production of fertilisers and chemicals, and is being lined up as a major clean shipping fuel in the years to come.
This article was updated on 3 April with the comments from Paul Martin.
