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.
Yes. 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, making it clean provided the ammonia is produced from renewable sources.
Ammonia has been trialled as a fuel for internal combustion engines since the Second World War when diesel was scarce. In the 1960’s the US Military undertook extensive engineering development and trialling of ammonia as a fuel: for supersonic aircraft, rockets, and internal combustion engines. Similar to propane or LPG, it can be stored and transported as a liquid under pressure but used as a gas.
Ammonia is used worldwide in the manufacture of fertilisers, synthetic fibres, explosives, and for refrigeration. Infrastructure for delivering large quantities already exists in industrialised nations.
It has a high octane rating (about 120 versus petrol at 86-93), and 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 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.
Research shows that a 5 percent biodiesel and 95 percent ammonia blend works well in farm machinery. 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 levels for ammonia are the same as for carbon monoxide (50ppm) but the human nose will smell this level of ammonia from a poorly tuned engine whereas it cannot smell carbon monoxide at all.
Each molecule of ammonia contains three hydrogen atoms. A lot of development is being put into hydrogen fuel cell based engine technologies. So why not transport the hydrogen as ammonia and then chemically split the hydrogen from the ammonia ‘on demand’ as we use it in our fuel cell powdered engine? We could go further and incorporate the ammonia chemically into solid grains making it even safer to transport.
The conversion of electricity to hydrogen, via wind turbines and a novel water electrolyser technology, has already been developed by Callaghan Innovation and partners. The integrated wind/solar/hydrogen demonstration system on 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 New Zealand is part of that process.