Future fuels: Ammonia and hydrogen

Determined to raise the level of debate around which fuels will power transportation in the not-too-distant future, Jorgen Pedersen asks if ammonia and hydrogen can, or should be, used to help meet our Net Zero goals

It has taken quite a lot of research to dive into ammonia/hydrogen fuels and what they could mean for our ecology and how they could potentially be used for our future transport needs.  While, as this article indicates, there is a lot happening in this field, it doesn’t seem to be discussed as widely as battery electric or hydrogen technologies. 

Ammonia-based fuel systems have been used in transport applications since the 1940s, implemented due to traditional fuel shortages during the Second World War.  Based on the research I have undertaken, I believe it’s true to say that it seems to have huge potential, but there will be challenges in using it as a regular transport fuel source.  However, putting that into perspective, that’s no different to all other future fuels currently either being used or considered, including battery electric power options. 

This short paper will identify the pros, cons, and opportunities that ammonia-based fuels could offer.

What is ammonia, and how does it differ from hydrogen?

Hydrogen, known as H on the periodic table, is generally known as H2 when referring to hydrogen fuel systems. But why? Where does the 2 come from? Hydrogen only has one electron in its outermost shell and requires two electrons to be stable; therefore, two hydrogen atoms bond and share electrons to achieve stability.

Ammonia, known as NH3, comprises one nitrogen atom and three hydrogen atoms. In short, hydrogen is as volatile as ammonia isn’t. Ammonia has a thick flame, a very small stoichiometric value, and can be quite difficult to maintain a flame without introducing additional chemicals or swirl burners (more on this later).

Most of us would have experienced the power and volatility of hydrogen as part of our first-year chemistry lessons when we heard the distinctive loud pop of ignited hydrogen. If not, then we must all have heard of the Hindenburg. By comparison, ammonia is very stable, and consequently potentially much safer.

The relative advantages of ammonia hydrogen fuels

Hydrogen has a low molecular size, which makes it difficult to capture and retain. It can be stored as liquid hydrogen if stored at either ~125-150 PSI or at -253°C. While this is possible, the opportunity for catastrophic failure is acute.  Ammonia in its natural state is extremely stable and is used in a multitude of different daily activities ranging from cleaning to agriculture, as an antimicrobial in processed food production, as a refrigerant, and as an effective NOx reducer when combined with Selective Catalytic Reduction (SCR), which essentially provides a mechanism to scrub NOx out of waste gases and as such is used in a large proportion of thermal power applications.

It's therefore important to point out that like hydrogen, ammonia if burnt like a traditional fuel while it is carbon-friendly, it does produce NOx. Though it must also be identified that through the use of selective catalytic reduction NOx emissions can be limited to as low as 10 parts per million (ppm). But unlike hydrogen, ammonia is extremely stable and has a very similar boiling temperature and condensation pressure point to that of propane, making it as easy to store and ship as propane, which has been used for a variety of applications for decades.

Ammonia can also be used in Solid Oxide Fuel Cells (SOFC), which are somewhat similar to the fuel cells that can be used by hydrogen, using a conversion/cracking process to generate electricity. With ammonia, unsurprisingly, this is a more complicated process and requires an operating temperature of 500-1000°C depending on the process and the thickness of the electrolytic film used for energy conversion. But with current advances, SOFCs can now be developed to provide power sources for portable electronic devices

Ammonia can also be used in a fuel cell using a proton exchange membrane (PEM) to crack ammonia into hydrogen. Based on research by Birmingham University, this seems to be more efficient than the SOFC method of cracking hydrogen.

Ammonia can also be used as a stable fuel that can once delivered be cracked in a reactor to create hydrogen on demand and then be used in a standard hydrogen fuel cell. This has already been delivered by a company called Amogy based out of NY, which has developed a completely contained and scalable ammonia-to-hydrogen solution that has been trialled in everything from drones (5KW) to tractors (100KW) to lorries (300KW) to one of the first ammonia powered tugboats (1MW).

Ammonia is the second most manufactured chemical with about 180 million tonnes produced each year. We are therefore already well-versed in the handling, shipping, and distribution of ammonia, which avoids the complexity of handling hydrogen.

The relative disadvantages of ammonia hydrogen fuels

While the common elements of ammonia, namely nitrogen and hydrogen, can both be extracted greenly, with hydrogen being electrolytically obtained from water using green energy and nitrogen being extracted from air, the Haber-Bosch (most commonly used) process of creating ammonia does generate some CO2. This could be recaptured using proven carbon capture technologies.

If being used in a direct combustion capacity, an ammonia flame is unstable, but when mixed with methane and passed through a swirl burner, the flame can be stabilised, and the energy extracted. This process does require a second fuel to support stabilisation, but this is technology that continues to be worked on and is likely to be overcome in the shorter term.

Ammonia-fuelled transport

The earliest examples of ammonia being used as a fuel are from the 19th century when locomotives in the UK and streetcars in New Orleans used ammonia as their energy source. While references to these can be found there is little information on the exact technologies used.  During the Second World War, Belgium, suffering a diesel embargo, adapted about 100 buses to run on liquid ammonia.  The buses travelled over 100,000 km without incident.

Since then, there have been forays into the benefits of ammonia as a fuel most notably by the US military in the 1960s that converted a single-cylinder engine to run on ammonia but concluded that while hydrocarbons were available ammonia provided little benefit.

And of course, while not a true transport option, the X15 (the fastest aircraft ever built) that reached speeds of Mach 6.8 was largely powered by ammonia.  

Notable mentions

With a realisation that battery electric power will not meet current demands particularly for heavy goods transport research into the capabilities and industrial uses of ammonia-based fuels has continued at pace. Fortescue and Deutsche Bahn are in partnership to modify diesel locomotives to run on both hydrogen and ammonia-based fuels.

The University of Birmingham has undertaken a simulation study of three different ammonia-based technologies along a 318 km rail track from Felixstowe to Hams Hall.

MAN Energy Solutions has developed a two-stroke ammonia engine for maritime uses. While this is only a prototype, MAN seems confident that the technology created can be used not only in new engines but also to retrofit existing engines.

IHI Power Systems has developed their engines to run on ammonia, achieving a 95%+ ammonia fuel mixture ratio and producing near-zero nitrous oxide and unburnt ammonia emissions.

Is there a downside to ammonia-based fuels?

There are two options currently available to use ammonia as a future fuel – direct burn as part of an internal combustion engine (ICE), but as mentioned this does produce some NOx which in turn would need to be captured.  The other option is to crack ammonia back to its component parts and then use the hydrogen produced as part of an in-line process to provide hydrogen to an internal fuel cell.  

I believe that there will be some additional hurdles to overcome in terms of flame/burn stability, but I also think those issues have largely been put to bed now which is why one is now seeing a number of maritime applications being tested and introduced.

Conclusion

While there are inefficiencies with using ammonia as a fuel source, there are also clear inefficiencies with all other future-based fuels, including battery electric technologies. When we approached the topic of ammonia as a future fuel, I confess to being quite sceptical, but the more research that has been undertaken, the more it seemed to fill the gap that conventional hydrocarbon fuels were leaving, particularly for heavy goods vehicles, maritime applications and perhaps even rail applications

As with all things, cost will inevitably be forefront in our minds, and like all new and evolving technologies including battery power, costs will start to reduce in line with streamlining production and as technology becomes more mainstream.  However, given the capability to extract hydrogen from ammonia through in vehicle cracking solutions (the term given to breaking long chain hydrocarbons into more useful molecules) it’s only a matter of time before ammonia-based fuels reaches a cost point and is introduced as a mainstream fuel source. 

Taking into account some of the technological issues that have had to be overcome to get to the point where ammonia-based engines are starting to make an appearance, I was truly surprised to find out that not only has ammonia-based transport been around for more than a century; they were used as a reliable transport mode in Belgium during the Second World War.  Given the difficulties current technology has had in overcoming flame consistency with sensors and computer technology that currently exists, there is perhaps something we might learn from these earlier examples.

My own view is that much more research needs to be undertaken on all future fuels to determine the total pollution impact from cradle to grave, which must include non-fuel-specific particulates. If that was undertaken, I’m sure that the current winning technologies might be perceived in a somewhat poorer light.

Lastly, while it’s inevitable to have an opinion as to which technology is better among several options, my personal view is that we must not only explore all avenues but also need a selection of future fuels to meet future transport demands. No one technology fills all the gaps that are required to meet all our long-term transport needs.