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|Ammonia: The Fuel for the Hydrogen Economy|
Why Ammonia Works and Hydrogen Won't
Once a copious supply of hydrogen is established, the Hydrogen Economy could be implimented directly with hydrogen as the term implies. But there are very clear advantages to using ammonia instead. The ammonia mollecule comprises 3 hydrogen atoms and 1 nitrogen atom and absolutely no carbon whatsoever. It is produced 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 bar 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 readily reformed back to nitrogen and hydrogen by such as the sodium amide reaction (see below) making it a very practical feedstock for fuel cells. More significantly, it can be burned directly in IC engines albeit with some modification, resulting in an exhaust of water and nitrogen. Ammonia is therefore, a good 'carrier' of hydrogen.
Hydrogen on its own by contrast, is very difficult to store and by inference, to transport. By weight, hydrogen has the highest energy content of any 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 practical 300 atmospheres a hydrogen tank would be 10 times the size of an energy equivelent gasoline tank. And tanks capable of withstanding that sort of pressure are very heavy, not to mention expensive. Carbon fiber composites offer a much better specific strength (strength to weight ratio) then steel but a carbon fiber tank would still outweigh the hydrogen it contains by a factor of 15 to 20 making it heavier than the equivelent gasoline tank. This completely demolishes hydrogen's advantage of being the most energy dense fuel by weight. To make matters 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 their structure and posing a significant risk of structural failure. Clearly with 300 atmospheres of highly flamable gas involved, such an outcome is unwelcome. Lining the tank with a material less suseptible to hydrogen infusion helps but only up to a point. Compressed hydrogen tanks need regular inspection and 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 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 slowly boil off. It either has to be vented which adds to the wastage or the tank has to incorporate a cryostat which would be an expensive extra. In it clear that liquid hydrogen for applications other than taking men to the moon, is completely impractical.
One form of hydrogen storage that has shown promise is adsorbtion of hydrogen by certain materials such as certain metal hydrides or even activated carbon. Adsorbtion is where mollecules (in this case of hydrogen gas) adhere to a surface. The extent to which this happens depends on the material selected as the adsorber, the surface area and factors such as temperature and pressure. The basic idea is to pump in the hydrogen under pressure (much less than 300 atm though) and then during operation, create steadily less favourable temperatures and presures such that the hydrogen liberates itself from the adhering surface. This approach at least solves the problem of high presures forcing hydrogen to seep through the tank walls as the pressure is less. But it does little to improve the weight with the best adsorbers still only managing to store something like 5 percent of hydrogen by weight. And nor does it do much to speed up trips to the gas station as controlling the temperature of the tank severely limits the rate at which the tank can be charged.
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. 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. And you are still left with a tank roughly three times the volume and twice the weight of the gasoline equivelent. The hydrogen economy was never going to come for free but the overall costs of the ammonia route are the lowest on offer and well within what is practical. For car designers the higher volume and weight of the tank is an issue but not much of one. It is a more serious challenge for aircraft designers certainly, but with ammonia as the carrier, hydrogen powered flight is a plauability rather than an inplausibility.
Ammonia is also of comparable safety to gasoline. It has a very nasty and strong smell so small leaks are easily detected but on the downside, larger leaks are a serious threat to health. Concentrations of 5,000 parts per million are lethal. Large leaks from a tank ruptured in a car accident could easily kill the car's occupants as well as kill or seriously injure, passers by. But on the upside, the toxic threat dissipates quickly as ammonia is lighter than air and vents upwards rapidly. It won't cremate people either. With a slow flame speed, a narrow flamability range of 16 to 25 percent by volume with air and an auto-ignition temperature of 651°C, ammonia is essentially inflamable. Ammonia even comes with off-the-shelf convienence. Ammonia is an important industrial chemical with established infrastructure that worldwide produces, stores and transports upwards of 150 million tons of ammonia each year.
Driving on Ammonia: Critique of Ammonia Fueled IC Engines
One could argue that 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 cracking ammonia. However these require further scientific breakthroughs to become economic and it is far from clear if such breakthroughs will ever be forthcoming. Fortunately ammonia can be burned in an IC engine and while ideally, the IC engine would be built from scratch to a design specifically optimized for ammonia, existing IC engines can be converted with satisfactory results.
Ammonia's characteristics pose significant challenges to its use as a fuel. The high autoignition temperature is just slightly higher than the temperatures generally created by the compression stroke in a typical diesel engine. Diesels have typical compression ratio of between 15:1 and 20:1.and engines at the top end of this range could only reliably employ compression ignition if the gases in the induction manifold had an elevated temperature. The autoignition temperature suggests a spark ignition regime.
But ammonia's low flame speed suggest a compression ignition regime. Unless the ignition and combustion can be completed in one go, the piston can outpace the flame during the power stroke, leaving unburnt fuel in the cylinder to be expelled by the exhaust stroke. This is both polluting and wasteful of fuel. Flame speeds for any given fuel depend on the mix of air to fuel. If the mixture is too lean the flame speed can be so low that energy released by burning cannot match the rate of cooling as the piston decends. When this happens, the power stroke fails and the engine stalls. Ammonia's low flame speed limits how low the fuel concentration can be which in turn limits the range of power outputs of the engine. This is less of a problem in applications such as CHP or electic car range extension but is a major problem in a conventional car. It particularly affects the engine's ability to tick over.
But compression ignition is not easy to get right with ammonia. It has a critical temperature of 132.4°C which means that above this temperature it is a gas no matter how high the pressure. Since the injector is screwed directly into the cylinder head controlling its temperature and that of any ammonia within it, is difficult to say the least. It will start out cold, warm up to an operating temperature that could be below the critical - but probably not reliably below it. In any event the vapor pressure of ammonia varies dramatically with temperature. This means the most practical method of injecting ammonia into the cylinder is as a gas at all times. But since the pressure in the cylinder at the top of the compression stroke is very high (~100 bar), the ammonia injectors would have to include a pre-heater and be fed using a very high pressure pump.
An engine running on ammonia would need to have the compression ratio of a diesel but use spark ignition and introduce the fuel in the induction manifold in the same way that LPG is introduced. That is easy if you are building an engine from scratch but otherwise not. The fuel introduction part is easy as it the same as an LPG conversion except that the tank must be able to withstand 25 atmosphere (ammonia has a higher vapor pressure than LPG) and steel pipes are to be used instead of copper as the latter would rapidly corrode. Then to convert a deisel you have to disconnect and seal off the injectors, drill holes for spark plugs and add the ignition circuit to fire them. In most cases that is simply not practical. A gasoline engine is easier as the flame speed problem due to the lower compression ratio can be mitigated by having multiple spark plugs per clylinder. That is easier as the ignition circuit is already in place but still not practical.
By far the best approach to a IC engine conversion is to reform part of the ammonia stream back into hydrogen and nitrogen so that the fuel introduced to the cylinders is a mix of hydrogen and ammonia. Hydrogen has a lower auto-ignition temperature of 500°C, a much faster flame speed and a greater flamability range starting as low as 4 percent by volume compared to ammonia's 16 percent. In simple terms, as long as we ensure the gases entering the cylinders is at least 4 percent hydrogen, the whole mix will be embued with the lower auto-ignition temperature and faster flame speed. Ammonia ignites at 651°C because that is the temperature at which it dissassociates into nitrogen and hydrogen. And one way of reformng ammonia into nitrogen and hydrogen is to heat it to this temperature. But it can be achieved at a much lower and much more practical 400°C by using the sodium amide reaction. We found a very good research paper on this reaction which is set within the context of ammonia as a hydrogen carrier fuel. Its authors are William I. F. David, Joshua W. Makepeace, Samantha K. Callear, Hazel M. A. Hunter, James D. Taylor, Thomas J. Wood and Martin O. Jones. You can view it here
Ammonia with Turbo/Super Chargers, Intercoolers and Variable Valve Regimes
Turbochargers and superchargers boost an engine's power by compressing the air in the intake manifold so that each stroke processes more air and fuel. It effectively increases the engine capacity. But it also slightly raises the temperature of the air in the intake manifold. This slight rise in temperature translates into a big rise in the temperature at the top of the compression stroke and the peak flame temperature when the fuel is burned. In a spark ignition engine the elevated compression temperature could cause knocking but even in a compression ignition engine, the higher peak flame temperature could cause other damage. Therefore an intercooler is used to mitigate the rise in temperature in the intake manifold.
If you have converted a gasoline engine with a compression ratio of 10:1 or less, with ammonia there is a lot of scope for the engine to accept an elevated temperature in the intake manifold. The intercooler can be either bypassed or set to cut in at significantly higher temperature than for gasoline. But if you have converted a diesel or are otherwise using a 15:1 or higher compression ratio, the ammonia air mixture could pre-ignite without intercooling.
Some advanced engines use a variable valve regime to close off the intake valve at some point before the intake stroke is complete. This effectively reduces the compression ratio and the power rating of the engine but gives a noticable improvement in fuel efficiency. As the expansion ratio is unchanged this is now greater than the compression ratio. The exhaust temperature and pressure is lower so the exhaust contains less energy and so less energy goes to waste. Such engines are commonly heavily supercharged to mitigate the reduced power output per unit capacity and freguently have higher would be compression ratios. Ammonia conversions work exactly the same in such engines as they do in engines lacking this regime.