Ammonia (NH3) contains about 17.5% hydrogen by molecular weight.
The process of cracking hydrogen from ammonia usually results in about 15% of the ammonia mass being available as useable hydrogen.
Hydrogen is usually transported as high pressure gas, at between 200 and 700 Bar in Tube Tanker Trailers. The high pressure tubes are made from high strength materials including carbon fibre. The current large Tube Tanker Trailers have a capacity of around 1100 Kg at a pressure of 500 Bar.
Ammonia is usually transported as liquid in carbon steel or stainless steel tankers at 15 to 20 Bar. There are ISO tanks which are built into a 20 foot container frame, each having a capacity of about 12,000 Kg of ammonia. A normal container trailer can accommodate two ISO containers giving a total capacity of 24,000 Kg of ammonia which translates to 3,600 Kg of hydrogen.
It is fair to say that using ammonia as a hydrogen carrier will result in 3 to 5 times as much hydrogen being transported per truckload and the capital cost of the ammonia tanks will be very much cheaper than the high pressure tube tankers. Transporting hydrogen as ammonia therefore, has the potential to dramatically reduce the transport cost component of hydrogen fuel at the filling station.
There have been fleets of ammonia tankers plying the high seas for decades. One such fleet transports 5 million tonnes of liquid ammonia around the world every year. These ships typically contain 15,000 to 25,000 tonnes of ammonia liquid in insulated steel hold tanks held at 3 kpag and -33C. There are many ports around the globe which have been using ammonia liquid loading and unloading infrastructure as well as large scale ammonia terminal storage at 3 kpag/-33C , for many decades. The efficient and safe systems and infrastructure already exist and can be greatly expanded to allow ammonia to be traded internationally as an energy commodity.
A typical ammonia liquid storage terminal comprising two 50M diameter insulated storage tanks will occupy approximately 1.5 Hectares and contain 36,000 tonnes of ammonia. Conversion of the energy contained into electricity via gas turbines will realise in excess of 60,000 MWHr of electrical output. (Based on a conversion efficiency of 28%) Conversion via cracking the hydrogen from the ammonia and feeding fuel cells will realise a higher output, probably in excess of 80,000 MWHr. All of this electrical output will be generated with zero carbon emissions provided the liquid supplied to the terminal is green ammonia. Given that the biggest conventional battery in the world has a storage capacity of 130 MWHr , this “ammonia battery” offers massive carbon free energy storage capacity in the order of 500 times that of conventional technology.
Ammonia as a hydrogen vector
Hydrogen is very difficult and expensive to store and transport. There are two main ways to improve the energy density of hydrogen, compress the gas to very high pressures of up to 700 atmospheres or liquefy it by cooling it to very low temperatures below -240C. Using ammonia as a hydrogen carrier is more efficient and cost effective than either of these options.
In the next instalment we will discuss progress and the way forward for ammonia in various energy applications.