Ammonia economy

Ammonia: The Fuel for the Hydrogen Economy

Hydrogen Carriers

Once a copious supply of hydrogen is established, the Hydrongen Economy could be implimented directly with hydrogen as its name implies. But our research has shown that there are clear advantages to using ammonia instead. The ammonia mollecule has 3 hydrogen atoms and 1 nitrogen atom. It contains absolutely no carbon whatsoever. It is easy to make on an industrial scale by means of the Haber process in which nitrogen from the air is combined with hydrogen at pressures of around 200 atmospheres and temperatures of the order of 500°C in the presence of an iron catalyst. And it is easy to use as a fuel. It can be reformed back into nitrogen and hydrogen so that the latter can be used in fuel cells although there are fuel cells that can operate directly from ammonia. And it can be burned directly in an internal combustion engine albeit with some modification, resulting in an exhaust of water and nitrogen. Ammonia is therefore, a good 'carrier' of hydrogen.

Hydrogen carriers are of interest because hydrogen on its own is very difficult to store and by inference, to transport. By weight, hydrogen has a far higher energy content than any other fuel. But by volume it has by far the lowest. Roughly speaking, 1Kg of hydrogen contains about as much energy as 4 liters of gasoline, but at normal atmospheric pressure, occupies a stagering 11 cubic meters! To be as energy dense by volume as gasoline we would need to compress the hydrogen to something approaching 3000 atmospheres pressure. Even with a more achievable 300 atmospheres a hydrogen tank would be 10 times the size of a gasoline tank containing the same amount of energy.

And tanks capable of withstanding that sort of pressure are very heavy, not to mention expensive. Typically a modern tank made of carbon fiber composites, would weigh some 15 to 20 times the weight of the hydrogen it conatains and weigh more than the equivelent gasoline tank. This puts paid to hydrogen's advantage of being the most energy dense fuel by weight.

And it gets worse. Hydrogen has the smallest atomic size of all elements and as a result, has a nasty habit of seeping into and through the walls of the tank, weakening thier structure and posing a significant risk of structural failure. Clearly with 300 atmospheres of highly flamable gas involved, such an outcome is unwelcome. Tanks can be lined with a material less suseptible to hydrogen infusion and this certainly helps but only up to a point. Compressed hydrogen tanks will likely need regular inspection and are likely to have limited lifetimes.

Some car manufacturers have experimented with liquid hydrogen stored in a dewar. But it takes a lot of energy, about 40 percent of the fuel's energy content, to refrigerate hydrogen to the -253°C needed to liquify it. And of course, the hydrogen in the tank won't stay cold and liquid all by itself. While dewars are very effective at keeping out the heat, they are not perfect and heat from the outside slowly creeps in causing the hydrogen to steadily boil off. It either has to be vented which adds to the wastage or the tank has to incorporate a cryostat which is a very expensive extra. In our view, liquid hydrogen for applications other than taking men to the moon, is completely impractical.

The problems posed by compression are less than those posed by liqification making compression a possible albeit expensive option for road transport where weight was not critical. The tank size would be mitigated if we use fuel cells instead of an internal combustion engine as the greater efficiency would require less fuel for the same distance. And we could accept a shorter range. But compression is a much less plausible for aircraft. Implimentation of the hydrogen economy directly by using hydrogen is not really a practical proposition. Hence the interest in hydrogen carriers.

The Ammonia economy

Ammonia suffers from none of the storage and transportation problems posed by hydrogen. Insted of needing tanks capable of withstanding enormous pressure, ammonia is a liquid at ordinary temperatures when pressurized to a mere 8 atmospheres and there is no tendancy for ammonia to pervade the walls of the tank. This is comparable to LPG and means we can use cheap, reliable tanks that last and do not require regular intensive inspection.

Converting hydrogen to ammonia does involve an energy cost that is higher than the cost of compression but only a fraction of that of liquifaction. This matters but if you have a carbon free source of hydrogen that is economic, this energy cost is acceptable if it results in a fuel that is cheap to transport and store. Another drawback is that compared to gasoline, an ammonia tank would occupy nearly 3 times the volume and be twice the weight. A headache for car designers yes but a blazing migraine no. A serious challenge for aircraft designers certainly, but a plauability rather than an inplausibility. The hydrogen economy was never going to come for free but the drawbacks of ammonia are in our view an acceptable price to pay for carbon neutrality. And compared to compressed hydrogen or the even worse liquid hydrogen, ammonia is a dream.

The hazards of ammonia

Ammonia has a very nasty smell so small leaks are easily detected but larger leaks are a serious threat to health. Concentrations of 5,000 parts per million are lethal. Large leaks arising from such as a car accident could easily kill the occupants as well as kill or very seriously injure, passers by.

Ammonia sounds scary but nor would you want to be to close to a gasoline tank that was about to be involved in an accident and rupture. The reason we never think of this is because decades have passed since being in a car accident commonly resulted in people getting cooked. Modern cars are designed to prevent tank rupture even in the most serious accidents and the strategy works.

It is actually more meaningful to compare ammonia not to gasoline but to LPG as both are ligufied gasses under pressure. Tank rupture is clearly seen as being a very small risk by authorities charged with ensuring safty. If this were not the case there would not be LPG cars. In the event of a rupture, LPG is not going to poison anyone as ammonia could, but on the other hand, ammonia is much less likely to turn people into kebabs. With an ignition temperature in the region of 700°C, ammonia outside an engine, is essentially inflamable.

Ammonia fueled internal combustion engines

The real future of ammonia as a fuel almost certainly lies with fuel cells that can either operate directly with ammonia or a mixture of nitrogen and hydrogen obtained by craking ammonia. However these are some way off and require scientific breakthroughs to become economic and as allways, it is prudent to avoid the assumption such breakthroughs will eventually be forthcoming. Fortunately ammonia can be burned in an IC engine and this allows us to take advantage of the huge legacy of pre-exiting cars with IC engines and the manufacturing infrastructure that builds them. Ideally the IC engine would be built from scratch to a design specifically optimized for ammonia but existing IC engines can be converted with satisfactory results. Please click here for a more detailed discussion on the ammonai engine.

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