Lloyd Somerville, of Tapanui, asks :-
I read that with the end of oil, ammonia could be the next fuel for transport because it can be produced cheaply from renewable electricity. Can it be used in internal combustion engines?
Ian Brown, a chemist with Callaghan Innovation, responded.
While ammonia is not conventionally considered to be a fuel gas, it has long been known that anhydrous ammonia can be burned in air with the release of energy. The products of combustion are simply nitrogen gas and water vapour.
The attractions of using a carbon free fuel system are clear and any move to minimise the impact of the carbon rich exhaust gases from our existing international vehicle fleet deserves serious consideration and thorough evaluation. So what has been learned in the past and what is current thinking?
Ammonia has been trialled as a fuel for internal combustion engines since the Second World War. The primary driver for this early work was to lessen the dependence on scarce war time diesel fuel in Western Europe.
In the 1960’s the US Military undertook extensive engineering development and trialling of ammonia as a transport fuel, not just in the context of fuelling X-15 supersonic aircraft and rocket technologies but also in the understanding the basic application of ammonia fuel technologies in internal combustion engines.
By way of background, at normal temperature and atmospheric pressure ammonia is a gas but it is a liquid at higher pressures (about 10-12 times atmospheric pressure at ambient temperatures). Similar to propane or LPG, it can be stored and transported as a liquid but used as a gas.
Anhydrous ammonia is used worldwide in the nitrogen fertiliser industry and is easy to store and deliver in large quantities. A storage and delivery infrastructure of pipelines, vessels, rail and truck already exists for ammonia in many larger industrialised nations, making this one of the most transported chemicals worldwide.
So what are the positives about direct use of ammonia as a transport fuel? It is carbon-free, there is existing infrastructure in place in many countries for handling ammonia, it has a high octane rating (about 120 versus petrol at 86-93), it can be derived directly from nitrogen and hydrogen gases using the century old Haber Bosch process, it can be used in Spark Ignition and Compression Ignition engines (eg Diesel)
But there are the challenges, such as low flame speed, high heat of vaporization, high auto-ignition temperature (651 °C), narrow flammability limits (16-25% by volume in air), relatively low energy density per litre (about half that of petrol), and its toxicity.
Let’s examine a few of the points above to encourage discussion or debate: • Some of the challenges relating to combustion engine technology can be overcome by use of supercharging, high compression ratios, ignition system modifications, and partial dissociation of the fuel before induction. Ammonia will not compression ignite except at very high pressures, so a small amount of high-cetane fuel can be added to shorten the ignition delay. For example, research shows that a 5 percent biodiesel and 95 percent ammonia blend works well in farm machinery. • Although ammonia itself does not produce greenhouse gases during combustion, if the ammonia is produced from hydrogen derived from natural gas or coal (as it is in many countries) then the overall process does indeed emit greenhouse gases. • The relatively narrow flammability limits in air mean that there is reduced risk of explosion when compared with many other hydrocarbon based fuels. • The toxicity issue is interesting. The current USA Government Personal Exposure Limits for ammonia and carbon monoxide are identical (50 ppm) so the risk from exposure to unburned exhausted ammonia fuel (or fuel leakage) is notionally equivalent to that of the exhausted CO from a petrol engine. However, the human nose is a very effective detector for ammonia at this 50ppm level, whereas our nose will not detect CO at any level, let alone 50 ppm. The problem remains: a poorly tuned engine running on ammonia fuel will be just as harmful as a poorly tuned engine running on petrol. The difference is that you get to smell the ammonia a long time before it causes physiological damage.
There is another way to consider the value of ammonia as a fuel. Remember that each molecule of ammonia contains three hydrogen atoms. So what if we instead view the ammonia as a carrier for hydrogen fuel and not as a fuel in its own right. There has been much work and much hype surrounding the concept of a ‘hydrogen economy’. A number of major vehicle manufacturers have very active programmes in place to develop hydrogen fuel cell based engine technologies. A major challenge is that of cost effectively compressing and storing the hydrogen, which has an energy density (energy per unit mass) of 120 MJ/kg, over twice that of petrol. So maybe we could transport our hydrogen as ammonia and then chemically split the hydrogen from the ammonia ‘on demand’ as we use it in our fuel cell powdered engine? This is certainly the subject of current international research. Taking this even further, we could incorporate the ammonia molecules into some other stable chemical framework which would safely bond to and store this ammonia until such time as we wished to release the ammonia molecules and extract the hydrogen. We could imagine a relatively benign solid powder material looking like sand or common salt which is easily treated in situ in our vehicle - to free the ammonia and catalytically extract the hydrogen. This is precisely the thinking behind recent research at the University of Oxford, where researchers are examining materials such as sodium amide (NaNH2) as an ammonia/hydrogen carrier.
Don’t lose sight of a less obvious, but potentially vital, benefit of using ammonia/hydrogen technologies and that is energy storage. The conversion of electricity to hydrogen via wind turbines and a novel water electrolyser technology has already been developed by Callaghan Innovation and partners to demonstration level in New Zealand. This is a first giant step towards time shifting energy generation and storage to match energy demand. The integrated wind/solar/hydrogen demonstration system on Matiu-Somes Island in Wellington Harbour (see www.hylink.nz) has already converted some 3500 kWh of excess electrical energy into stored hydrogen. While it requires the long view to foresee off peak electrical energy contributing ammonia/hydrogen fuel to our vehicle fleet, you can be sure that slowly and progressively the ducks are lining up on the wider world stage, and New Zealand is part of that process.