Ammonia is just as effective as hydrogen in zero-emission green steel production, a study has found, offering potential cost savings of about 18% when relying on imported H2.
Steel production is currently responsible for about 7% of the world’s greenhouse gas emissions, and while renewable energy can be used in electric arc furnaces to melt scrap steel for re-use, the only currently available method to remove emissions during the extraction of iron from ore (ie, iron oxide) is to use green hydrogen, rather than the coal or natural gas used today.
This is because oxygen has to be removed from the ore via a chemical reaction at the same time as it is melted, so a fuel that both reacts with oxygen and provide high-temperature heat is required to produce raw iron, in a process known as direct iron reduction.
Because of the huge amounts of hydrogen that would needed to replace fossil fuels in iron production, countries such as Germany are looking to import green H2 at scale in the coming years, most of which will arrive at European shores in the form of ammonia — which is far easier to transport across long distances than pure hydrogen (compressed or liquefied) and also has a higher energy density by volume, making it an economically efficient method of transporting H2.
But this would mean that imported ammonia would have to be cracked into hydrogen (and nitrogen) for use in green steel production — an expensive energy-intensive process (requiring about 30% of the energy stored in the H2 and the use of the rare metal, ruthenium, as a catalyst) that would add significantly to the cost of green hydrogen.
Being able to use imported ammonia directly would therefore reduce costs by about 18%, according to the study, Reducing Iron Oxide with Ammonia: A Sustainable Path to Green Steel, written by researchers from the Max Planck Institute for Iron Research and published in the journal Advanced Science.
And while burning ammonia usually results in high emissions of nitrogen oxides (NOx) — one of which (N2O) is 273 times more potent a greenhouse gas than CO2 over a 100-year period — “no formation of any ozone-destroying NOx molecules were observed during ADR [ammonia direct reduction]”, the paper says.
It seems that oxygen in the air, which would normally react with the nitrogen in ammonia to form NOx, is removed in the process in the same way that oxygen from the iron oxide is removed, leaving mainly pure nitrogen instead at temperatures of around 350°C — allowing the remaining hydrogen to work its magic — while a small proportion of the nitrogen temporarily attaches to the iron as iron nitride (Fe4N).
“Moreover, nitrogen, a nontoxic, non-greenhouse gas, as a by-product of ammonia decomposition can act as a heat carrier in a shaft furnace to maintain the reaction temperature and thus enhance the efficiency for the endothermic [ie, heat-absorbing] reduction of iron oxide with hydrogen,” the study says.
It adds that the Fe4N formation is actually a benefit to the quality of the iron produced.
“The nitride formation is another key advantage of ADR, as nitriding improves the aqueous [ie, water] corrosion resistance of iron. The nitride passivated [ie, added a thin inert layer to make the metal less reactive] the otherwise highly active reduced iron, offering a safety-critical benefit for handling and logistics.
“Otherwise, for the downstream processing of the reduced material, the porous sponge iron [ie, direct-reduced iron] is prone to re-oxidation and strong exothermic [ie, heat-producing] reactions with oxygen or moisture due to its high surface-to-volume ratio... Thus, the sponge iron produced by HyDR [hydrogen-based direct reduction] must be compacted into hot briquetted iron to reduce the porosity for shipping and handling, which is not necessary with ADR.”
In addition, after melting, the protective nitride is almost completely removed, resulting in a final material with 99.4% iron, but the advantageous structure remains, due to it having far less pores than hydrogen-derived sponge iron.
“In summary, ADR is kinetically as effective for producing green iron as HyDR at 700°C. The direct utilization of ammonia in the reduction process offers a process shortcut, alleviating the need for a preliminary ammonia cracking step into hydrogen and nitrogen,” the study explains.
“ADR provides a novel approach to deploying intermittent renewable energy for an unprecedented and disruptive technology transition toward sustainable metallurgical processes. With these benefits, it connects two of the currently most greenhouse gas intense industries (namely, steel and ammonia production industries) and opens a pathway to render them more environmentally benign and sustainable.
“At the same time, it can eliminate logistic and energetic disadvantages associated with the use of pure hydrogen, when it needs to be transported.”