Renewable ammonia: the future of fuels?

By Douglas R. MacFarlane | January 16, 2023

Screenshot of YouTube video describing how an ammonia economy could work. Image courtesy of Monash Ammonia Project https://www.youtube.com/watch?v=lSYCac7KD8o&t=29s

Renewable ammonia: the future of fuels?

By Douglas R. MacFarlane | January 16, 2023

Half a decade or so ago, it seemed that the time had arrived for the hydrogen economy. But when the engineers of the world began to consider the details of using vast supplies of renewables to generate hydrogen in quantities that have never been produced previously and moving it safely around the world, they saw a daunting prospect, for a variety of reasons. These included difficulties in using existing bulk carriers and pipelines for hydrogen transportation as well as the safety aspects of the need to store large amounts of hydrogen in transport hubs. The research community then began to cast around for alternatives, compounds that could carry hydrogen but were safer and more straightforward to transport in large quantities over great distances. Ammonia rapidly emerged as a leading candidate.

The possibility of using ammonia—a compound of one nitrogen atom and three hydrogen atoms—as a hydrogen carrier was encouraged by the demonstration of technologies that could be used to split the ammonia molecule back into its constituents as needed at the point of use. The advantage of ammonia over hydrogen involves its ease of handling and transportation in bulk. Systems for moving ammonia are well established. This is not the case with hydrogen, which poses corrosion challenges with respect to steel pipelines and other containers. Among energy-importing countries, Japan in particular has been clear about its preference for a hydrogen carrier such as ammonia as part of its energy mix, beginning before the end of this decade.

But ammonia has gained increasing attention not just as a carrier for hydrogen, but as a fuel in its own right.

Just in the last three or four years, as it became understood that large quantities of ammonia could be part of energy supplies of the future, a nearly century-old technology re-emerged. Ammonia can be used to fuel relatively traditional internal combustion engines, with minor modifications similar to those involved in converting a gasoline engine to use liquified petroleum gas. In this arrangement, ammonia is stored in the fuel tank in liquid form, under pressure. From there, the fuel expands into fuel lines and enters the engine as a fuel-air mixture that burns only slightly more sluggishly than gasoline. This engine technology is attracting substantial attention, particularly in regard to large marine engines.[1]Ammonia’s prospects as a major transportation fuel were highlighted when the International Maritime Organization released plans that began to focus on ammonia as a shipping fuel of the future.

Liquid ammonia has an energy density that is around half that of gasoline, so it is unlikely to become a long-haul jet fuel. But for other forms of heavy transport, it is a good fit in circumstances where batteries cannot provide the needed range.[2]

Ammonia also offers a pathway to power production in energy-importing regions. Japan has demonstrated co-firing of coal-fired power stations with ammonia, offering a way to transition toward renewables as substantial quantities of renewable ammonia become available. Mitsubishi and others have also developed gas turbine generators that can run on ammonia—an extremely attractive form of high-efficiency power generation from ammonia in the future.

Like hydrogen, ammonia has great potential as a no-carbon-emissions alternative to fossil fuels. In principle, the main product of burning ammonia in an engine is nitrogen, a gas that constitutes some 78 percent of the atmosphere and is, in itself, not a greenhouse gas. However, as is true for all engines, it is difficult to avoid the over-oxidation of the fuel, producing nitrogen-oxide exhaust gases, which cause health problems and accelerate climate change. Fortunately, there is a well-known technology for dealing with nitrogen oxides (NOx) that is commonplace in buses and trucks throughout the world: injecting a small, carefully metered amount of an ammonia compound—usually urea, but ammonia itself is workable—into an exhaust stream and causing a reaction with NOx gases that produces only nitrogen and water.

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Possible pathways for increased ammonia production

The traditional approach to ammonia production involves a century-old technology known as the Haber-Bosch process, which uses large amounts of energy to convert natural gas and water to ammonia and carbon dioxide. Where green hydrogen (H2) is available in large quantities—and the quantities that are required to fuel a modern-day Haber-Bosch plant require a multi-gigawatt source of power—the process can become renewables-driven, so carbon dioxide emissions are greatly reduced or even eliminated. Demonstration plants and plans for large installations are under development in various parts of the world; particularly attractive are regions where both renewables and access to pipelines and international shipping ports are readily available.

Ammonia absorption illustration, circa 1906
An early experiment with ammonia, from the 1906 book, Elementary Chemistry Lessons.
Image courtesy of Wikimedia Commons

The combined, renewable H2 + Haber-Bosch process for producing ammonia is both capital and energy intensive, and the research community is looking towards more direct approaches to ammonia production from renewables. In principle, this could be carried out in an electrolysis cell.

Thermodynamically, it certainly looks feasible. Unfortunately, if water is the main electrolyte solvent—which is highly desirable—avoiding hydrogen production as a reaction in parallel to the production of ammonia becomes a huge challenge; a mixture of hydrogen and ammonia is not a particularly valuable outcome.[3] At this point in time, the holy grail of electro-reduction of nitrogen to ammonia in water solution remains elusive. Advanced atomistic simulations, coupled with artificial intelligence and machine learning, may in the future offer a path towards this goal by providing a way to rapidly screen families of catalysts and explore synergistic mixtures of active materials, perhaps inspired by the iron–molybdenum structure that performs nitrogen fixation in plants.

There are very few if any reports of successful practical ammonia production by direct electrolysis. Fortunately, an alternative reaction pathway exists. It involves the use of an intermediate component and is referred to as the mediated electrochemical nitrogen reduction reaction. Such a reaction involving lithium as the intermediate component is in fact well known in the lithium-battery field; a piece of lithium metal will react rapidly with nitrogen from the atmosphere, initially producing a blue sheen on the metal. The compound formed is lithium nitride, which is also well known to react with water, or an acid, to form ammonia. Recent reports have demonstrated that this process can be used to generate ammonia on a continuous basis.

In this kind of work, a key performance metric is the efficiency by which electrons are converted into ammonia molecules. Simple chemistry requires three electrons per ammonia molecule, and if this ratio is achieved, this is referred to as 100 percent Faradaic efficiency. Achieving 100 percent efficiency has been considered to be a vital step in this technology development. Very recent work from our group at Monash University has demonstrated for the first time this required level of efficiency. Building on this development, we formed a spin-out company, Jupiter Ionics, in 2021 to scale up the technology.

The remaining challenge in this mediated approach is the energy cost of the process. Since it involves a pathway through lithium nitride, the energy input required is governed by this component, and the net result is that the energy cost is substantial. Current estimates place this at around 25 percent energy efficiency; by comparison, renewable ammonia produced by a green H2 + Haber-Bosch process is likely to end up at around 40 percent to 50 percent energy efficiency overall. On the other hand, the electrolysis process is capable of being implemented on a highly distributed scale that easily can be turned down to match the intermittency of renewables, in contrast to the Haber-Bosch process, which only achieves such efficiency at very large scale—thousands of tons per day—and is very sensitive to receiving a constant supply of hydrogen. This creates the prospect of ammonia fuel generation and storage at the wind or solar farm level.

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Why the market favors increased production and use of ammonia

Today, ammonia’s primary use is for fertilizers, and the agricultural sector will certainly be the first consumers of renewable ammonia, its value in this market being considerably greater than its likely value as a fuel. And the farming community is particularly excited by the potential for distributed, localized production that could shorten supply routes and ensure timely delivery.

But as technologies for producing ammonia using renewable energy mature, production could expand, allowing ammonia to make its place—and perhaps a leading place—as an easily transported fuel for use in the effort to decarbonize transportation around the world and thereby make major strides toward the global goal of net-zero carbon emissions.

 

Endnotes

[1] For example, MAN Energy Solutions, among others, is developing a fuel-flexible, marine, two-stroke engine that will run on ammonia.

[2] Despite the lower energy density, ammonia and the turbines that it could drive are nonetheless attracting attention as a fuel and engine for short- to medium-haul air travel, offering a solution to the conundrum of how to deliver the required quantities of energy, in any form, into major airports in Europe and North America for their aircraft refueling needs.

[3] The solution would be an electro-catalyst on the cathode that is very active in absorbing N2 onto its surface, allowing reduction to happen and then readily releasing the ammonia produced to refresh the surface; but it must also be very antagonistic towards absorbing water or protons and reducing them to hydrogen. Hundreds of research papers have been devoted to this concept over the last three or four years, fuelled by a number of government programs that were prepared to fund a very wide range of projects in the hope that something might work. Unfortunately, many of these reports claim exciting results, but in fact are only detecting ammonia derived from impurities or the equipment used, because the amounts of ammonia produced are very small and the methods of analysis for ammonia are very sensitive. This has even caused some commentators to wonder if this was growing into another “cold fusion” episode. Fortunately, other branches of science devoted to understanding nitrogen fixation in plants have well-developed methods for ensuring genuine results and rejecting false-positives, and these are slowly being adopted in this renewable ammonia field. At the same time reports are beginning to appear refuting the more impressive claims from that earlier period.

Disclosure statement

Doug MacFarlane is Chief Scientific Officer of, and a shareholder in, Jupiter Ionics Pty Ltd.

Funding

The author is grateful for funding from the Australian Renewable Energy Agency under contract 2018-RND009.

 

 

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Dennis G Morell
1 year ago

While I generally appreciate the potential of ammonia, I believe any article about it must point out its dangers. As a retired chemist I have had several intersections with ammonia and most have been poor. 1. It is an amazing solvent and if any elastomer in an ammonia system is incorrect it will leak and can lead to catastrophic results, 2. The olfactary perception range is very wide. When my group was using ammonia in a high pressure system we had ammonia monitors, but one young chemical engineer would march into my office and announce to me that we had… Read more »

Nigel F
Nigel F
1 year ago

Wouldn’t synthesising methane from Hydrogen and CO2 be an alternative option? Methane can be used in existing natural gas infrastructure and can be used as a basis for producing more complex hydrocarbons.