Why alternative fuels like green ammonia matter
Patrik Ölund, SVP and Head of Ovako Group R&D, explains why switching to ammonia as a fuel presents new challenges for the traditional steel grades used in internal combustion engines, and why our new generation of lean alloy steels could be the answer.
Second only to power generation, transportation is one of the largest sources of global greenhouse gas emissions. Mobility from land, air, and sea pumps out more than 8 billion metric tons of carbon dioxide equivalent (GtCO₂e) into the atmosphere each year (source: Statista).
The urgent need to decarbonize transport systems requires finding fossil-free ways to power them. The direct electrification of cars is of course a very significant contribution. However, at the current stage of technology, electric vehicles (EVs) are not the best solution for some applications. That means sectors such as large trucks and shipping will probably still rely on internal combustion engines (ICEs) well into the future.
Much progress has been made in boosting the efficiency of ICEs that rely on fossil fuels. Even so, they will always emit CO₂. This is where alternative E-fuels will play an important part, in the form of e-hydrogen, e-methane, and e-methanol.
The projected demand for E-fuels is huge. According to an analysis undertaken by Rolls-Royce Power Systems, 20,000 terawatt-hours of fuel-based energy will be needed in 2050. That is equivalent to 2 trillion liters of diesel.
One promising alternative fuel is ammonia (NH₃), derived from the electrolysis of water powered by renewable energy, hence the term “green ammonia.” It is easy to store and transport, and when burned, it emits only nitrogen gas and water.
Materials challenges when using ammonia as an alternative fuel
Stress corrosion cracking (SCC)
- Stress corrosion cracking (SCC) - this is the formation and growth of cracks in steel in a corrosive environment. It can lead to unexpected and sudden failure of normally ductile metals when subjected to tensile stress, especially at elevated temperature. The chemical environment that causes SCC for a particular alloy is often one which is only mildly corrosive to the metal. Hence, metal parts with severe SCC can appear bright and shiny, while being filled with microscopic cracks. This makes it common for SCC to go undetected prior to catastrophic failure. The susceptibility of steel to SCC increases with the presence of oxygen, stress levels and the strength of the grade used.
Nitridation
- Nitridation – Nitridation of steel when exposed to ammonia involves the diffusion of nitrogen into the matrix, leading to the formation of nitride layers on the surface. These hard and brittle nitride layers can compromise the mechanical properties of the component.
Wet corrosion and pitting attack
- Wet corrosion – the presence of oxygen and chlorides can result in localized pitting attack. This is the formation of small, deep cavities or pits on the surface of the steel. In contrast to uniform corrosion, which affects the entire surface evenly, pitting corrosion is highly localized. It can penetrate deeply into the material, often going unnoticed until catastrophic failure occurs.
Lean alloy design could be the answer
Stainless steel might appear to be the obvious solution for applications exposed to ammonia fuel. But it is costly due to the expensive alloying elements such as nickel, molybdenum and chromium. Furthermore, chlorides, oxygen and ammonia species such as ammonium nitrate, ammonium sulfate and ammonium hydroxide can break down the passive film that gives stainless steel its corrosion resistant properties. It also has limited fatigue resistance under repeated cyclic loading.
This is why a special focus for Ovako’s R&D team is to investigate the design of lean alloys that enable us to reduce the alloying content while maintaining corrosion resistance. The aim is to find a cost-effective and sustainable solution to bridge the gap between carbon steel and high-alloy steels, with the goal to make it possible to use ammonia as a fuel for ICEs.
Lean Cr-Al alloy
Currently, a particular area of interest for us is in developing aluminum (Al) - alloyed steels that are resistant to corrosion and stress cracking in ammonia environments. Our test programs have helped us to establish that a key design factor is to have chromium (Cr) content greater than 4% by weight, with an atomic ratio between Cr:Al greater than 1.
This approach promotes the formation of a compact, dense protective oxide surface layer with improved stability to prevent local breakdown. Initial results have been encouraging with the test alloys having a more positive pitting potential than conventional steels, showing that they are more resistant to pitting corrosion, because it takes a stronger oxidative environment to break down the passive layer.
A new Corrosion Resistance Index
An interesting development of our work on Cr-Al alloys has been the development of a new Corrosion Resistance Index (CRI) to help predict performance where the traditional PREN (pitting resistance equivalent number) index that predicts corrosion resistance does not apply.
The two most common ways to calculate the PREN are: PREN = Cr + 3.3Mo + 16N and PREN = Cr + 3.3Mo + 30N, where Cr, Mo, and N are chromium, molybdenum, and nitrogen concentration in percentage by weight. But they do not take account of the beneficial effect of Al.
Our findings are that aluminum can enhance the corrosion resistance of chromium‐containing steel with a coefficient of 5.2. Therefore, we have created a new formula that takes account of the presence of Cr and Al:
CRI = Cr + 3.3Mo + 16N + 5.2Al.
For more information on this new CRI please see this paper: https://doi.org/10.1002/maco.202314266
Ammonia can fertilize a transport revolution
The process to produce ammonia on an industrial scale was developed in the early years of the last century. Used as a fertilizer, it revolutionized agriculture. Now it promises to revolutionize transportation as a promising carbon-free fuel, particularly for hard-to-decarbonize sectors, thanks to its ease of transport and storage. There are some specific materials challenges that must be solved for this to happen, and Ovako’s lean alloy steels are leading the way.
